Change search
Refine search result
1234567 1 - 50 of 911
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1. Herbert, Corentin
    et al.
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bouchet, Freddy
    Atmospheric Bistability and Abrupt Transitions to Superrotation: Wave-Jet Resonance and Hadley Cell Feedbacks2020In: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 77, no 1, p. 31-49Article in journal (Refereed)
    Abstract [en]

    Strong eastward jets at the equator have been observed in many planetary atmospheres and simulated in numerical models of varying complexity. However, the nature of the transition from a conventional state of the general circulation, with easterlies or weak westerlies in the tropics, to such a superrotating state remains unclear. Is it abrupt or continuous? This question may have far-reaching consequences, as it may provide a mechanism for abrupt climate change in a planetary atmosphere, both through the loss of stability of the conventional circulation and through potential noise-induced transitions in the bistability range. We study two previously suggested feedbacks that may lead to bistability between a conventional and a superrotating state: the Hadley cell feedback and a wave-jet resonance feedback. We delineate the regime of applicability of these two mechanisms in a simple model of zonal acceleration budget at the equator. Then we show using numerical simulations of the axisymmetric primitive equations that the wave-jet resonance feedback indeed leads to robust bistability, while the bistability governed by the Hadley cell feedback, although observed in our numerical simulations, is much more fragile in a multilevel model.

  • 2.
    Li, Xiang-Yu
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Swedish e-Science Research Centre, Sweden; University of Colorado Boulder, USA.
    Brandenburg, Axel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado Boulder, USA.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    Haugen, Nils E. L.
    Mehlig, Bernhard
    Rogachevskii, Igor
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel.
    Condensational and Collisional Growth of Cloud Droplets in a Turbulent Environment2020In: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 77, no 1, p. 337-353Article in journal (Refereed)
    Abstract [en]

    We investigate the effect of turbulence on the combined condensational and collisional growth of cloud droplets by means of high-resolution direct numerical simulations of turbulence and a superparticle approximation for droplet dynamics and collisions. The droplets are subject to turbulence as well as gravity, and their collision and coalescence efficiencies are taken to be unity. We solve the thermodynamic equations governing temperature, water vapor mixing ratio, and the resulting supersaturation fields together with the Navier-Stokes equation. We find that the droplet size distribution broadens with increasing Reynolds number and/or mean energy dissipation rate. Turbulence affects the condensational growth directly through supersaturation fluctuations, and it influences collisional growth indirectly through condensation. Our simulations show for the first time that, in the absence of the mean updraft cooling, supersaturation-fluctuation-induced broadening of droplet size distributions enhances the collisional growth. This is contrary to classical (nonturbulent) condensational growth, which leads to a growing mean droplet size, but a narrower droplet size distribution. Our findings, instead, show that condensational growth facilitates collisional growth by broadening the size distribution in the tails at an early stage of rain formation. With increasing Reynolds numbers, evaporation becomes stronger. This counteracts the broadening effect due to condensation at late stages of rain formation. Our conclusions are consistent with results of laboratory experiments and field observations, and show that supersaturation fluctuations are important for precipitation.

  • 3.
    Hartung, Kerstin
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e‐Science Research Centre, Sweden; Ludwig‐Maximilians‐Universität München, Germany.
    Shepherd, Theodore G.
    Hoskins, Brian J.
    Methven, John
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e‐Science Research Centre, Sweden.
    Diagnosing topographic forcing in an atmospheric dataset: The case of the North American Cordillera2020In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 146, no 726, p. 314-326Article in journal (Refereed)
    Abstract [en]

    It is well known from modelling studies that surface topography influences the large-scale atmospheric circulation and that several model biases are associated with incorrect representation of topography. The textbook explanation of topographic effects on large-scale circulation appeals to the theoretical relationship between surface forcing and vortex stretching along trajectories in single-layer models. The goal of this study is to design and use a simple diagnostic of the large-scale forcing on the atmosphere when air is passing over topography, directly from atmospheric fields, based on this theoretical relationship. The study examines the interaction of the atmosphere with the North American Cordillera and samples the flow by means of trajectories during Northern Hemisphere winter. We detect a signal of topographic forcing in the atmospheric dataset, which, although much less distinct than in the theoretical relationship, nevertheless exhibits a number of expected properties. Namely, the signal increases with latitude, is usually stronger upslope than downslope, and is enhanced if the flow is more orthogonal to the mountain ridge, for example during periods of positive Pacific-North American index (PNA). Furthermore, a connection is found between an enhanced signal of topographic forcing downslope of the North American Cordillera and periods of more frequent downstream European blocking.

  • 4.
    Hieronymus, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Interannual Variability of the Overturning and Energy Transport in the Atmosphere and Ocean During the Late Twentieth Century with Implications for Precipitation and Sea Level2020In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 33, no 1, p. 317-338Article in journal (Refereed)
    Abstract [en]

    The overturning circulations in the atmosphere and ocean transport energy from the tropics to higher latitudes and thereby modulate Earth's climate. The interannual variability in the overturning over the last 40 years is found to be dominated by two coupled atmosphere-ocean modes. The first is related to the meridional motion of the intertropical convergence zone and the second to El Nino. Both modes have a strong influence on the sea level variability in the tropical Indo-Pacific Ocean. The interannual variability of the cross-equatorial energy transport is dominated by the first mode, and the variability is larger in the Indo-Pacific Ocean than in the Atlantic Ocean or the atmosphere. Our results suggest an important role of oceanic energy transport in setting precipitation patterns in the tropics and a key role of the Indo-Pacific Ocean as a climate modulator.

  • 5. Hochman, Assaf
    et al.
    Alpert, Pinhas
    Kunin, Pavel
    Rostkier-Edelstein, Dorita
    Harpaz, Tzvi
    Saaroni, Hadas
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology . Uppsala University, Sweden.
    The dynamics of cyclones in the twentyfirst century: the Eastern Mediterranean as an example2020In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 54, no 1-2, p. 561-574Article in journal (Refereed)
    Abstract [en]

    The Mediterranean region is projected to be significantly affected by climate change through warming and drying. The Eastern Mediterranean (EM) is particularly vulnerable since the bulk of the precipitation in the region is associated with a specific circulation pattern, known as Cyprus Low (CL). Here, we study the influence of increased greenhouse gases on the average properties and dynamics of CLs, using a regional semi-objective synoptic classification. The classification is applied to NCEP/NCAR reanalysis data for the present day (1986-2005) as well as to eight CMIP5 models for the present day and for the end of the century (2081-2100; RCP8.5). This is complemented by a dynamical systems analysis, which is used to investigate changes in the dynamics and intrinsic predictability of the CLs. Finally, a statistical downscaling algorithm, based on past analogues, is applied to eighteen rain stations over Israel, and is used to project precipitation changes associated with CLs. Significant changes in CL properties are found under climate change. The models project an increase in CL meridional pressure gradient (0.5-1.5 hPa/1000 km), which results primarily from a strong increase in the pressure over the southern part of the study region. Our results further point to a decrease in CL frequency (- 35%, as already noted in an earlier study) and persistence (- 8%). Furthermore, the daily precipitation associated with CL occurrences over Israel for 2081-2100 is projected to significantly reduce (- 26%). The projected drying over the EM can be partitioned between a decrease in CL frequency ( 137 mm year(-1)) and a reduction in CL-driven daily precipitation ( 67 mm year(-1)). The models further indicate that CLs will be less predictable in the future.

  • 6.
    Gumbel, Jörg
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Megner, Linda
    Stockholm University, Faculty of Science, Department of Meteorology .
    Christensen, Ole Martin
    Stockholm University, Faculty of Science, Department of Meteorology . Chalmers University of Technology, Sweden.
    Ivchenko, Nickolay
    Murtagh, Donal P.
    Chang, Seunghyuk
    Dillner, Joachim
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekebrand, Terese
    Giono, Gabriel
    Hammar, Arvid
    Hedin, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson, Bodil
    Stockholm University, Faculty of Science, Department of Meteorology .
    Krus, Mikael
    Li, Anqi
    McCallion, Steven
    Olentšenko, Georgi
    Pak, Soojong
    Park, Woojin
    Rouse, Jordan
    Stegman, Jacek
    Stockholm University, Faculty of Science, Department of Meteorology .
    Witt, Georg
    Stockholm University, Faculty of Science, Department of Meteorology .
    The MATS satellite mission - gravity wave studies by Mesospheric Airglow/Aerosol Tomography and Spectroscopy2020In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 20, no 1, p. 431-455Article in journal (Refereed)
    Abstract [en]

    Global three-dimensional data are a key to understanding gravity waves in the mesosphere and lower thermosphere. MATS (Mesospheric Airglow/Aerosol Tomography and Spectroscopy) is a new Swedish satellite mission that addresses this need. It applies space-borne limb imaging in combination with tomographic and spectroscopic analysis to obtain gravity wave data on relevant spatial scales. Primary measurement targets are O-2 atmospheric band dayglow and nightglow in the near infrared, and sunlight scattered from noctilucent clouds in the ultraviolet. While tomography provides horizontally and vertically resolved data, spectroscopy allows analysis in terms of mesospheric temperature, composition, and cloud properties. Based on these dynamical tracers, MATS will produce a climatology on wave spectra during a 2-year mission. Major scientific objectives include a characterization of gravity waves and their interaction with larger-scale waves and mean flow in the mesosphere and lower thermosphere, as well as their relationship to dynamical conditions in the lower and upper atmosphere. MATS is currently being prepared to be ready for a launch in 2020. This paper provides an overview of scientific goals, measurement concepts, instruments, and analysis ideas.

  • 7. Hochman, Assaf
    et al.
    Alpert, Pinhas
    Harpaz, Tzvi
    Saaroni, Hadas
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology . Uppsala University, Sweden.
    A new dynamical systems perspective on atmospheric predictability: Eastern Mediterranean weather regimes as a case study2019In: Science Advances, E-ISSN 2375-2548, Vol. 5, no 6, article id eaau0936Article in journal (Refereed)
    Abstract [en]

    The atmosphere is a chaotic system displaying recurrent large-scale configurations. Recent developments in dynamical systems theory allow us to describe these configurations in terms of the local dimension-a proxy for the active number of degrees of freedom-and persistence in phase space, which can be interpreted as persistence in time. These properties provide information on the intrinsic predictability of an atmospheric state. Here, this technique is applied to atmospheric configurations in the eastern Mediterranean, grouped into synoptic classifications (SCs). It is shown that local dimension and persistence, derived from reanalysis and CMIP5 models' daily sea-level pressure fields, can serve as an extremely informative qualitative method for evaluating the predictability of the different SCs. These metrics, combined with the SC transitional probability approach, may be a valuable complement to operational weather forecasts and effective tools for climate model evaluation. This new perspective can be extended to other geographical regions.

  • 8. Havnes, Ove
    et al.
    Antonsen, Tarjei
    Baumgarten, Gerd
    Hartquist, Thomas W.
    Biebricher, Alexander
    Fredriksen, Ashild
    Friedrich, Martin
    Hedin, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    A new method of inferring the size, number density, and charge of mesospheric dust from its in situ collection by the DUSTY probe2019In: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 12, no 3, p. 1673-1683Article in journal (Refereed)
    Abstract [en]

    We present a new method of analyzing measurements of mesospheric dust made with DUSTY rocket-borne Faraday cup probes. It can yield the variation in fundamental dust parameters through a mesospheric cloud with an altitude resolution down to 10 cm or less if plasma probes give the plasma density variations with similar height resolution. A DUSTY probe was the first probe that unambiguously detected charged dust and aerosol particles in the Earth's mesosphere. DUSTY excluded the ambient plasma by various biased grids, which however allowed dust particles with radii above a few nanometers to enter, and it measured the flux of charged dust particles. The flux measurements directly yielded the total ambient dust charge density. We extend the analysis of DUSTY data by using the impact currents on its main grid and the bottom plate as before, together with a dust charging model and a secondary charge production model, to allow the determination of fundamental parameters, such as dust radius, charge number, and total dust density. We demonstrate the utility of the new analysis technique by considering observations made with the DUSTY probes during the MAXIDUSTY rocket campaign in June-July 2016 and comparing the results with those of other instruments (lidar and photometer) also used in the campaign. In the present version we have used monodisperse dust size distributions.

  • 9. Kodama, C.
    et al.
    Stevens, B.
    Mauritsen, Torsten
    Stockholm University, Faculty of Science, Department of Meteorology .
    Seiki, T.
    Satoh, M.
    A New Perspective for Future Precipitation Change from Intense Extratropical Cyclones2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 21, p. 12435-12444Article in journal (Refereed)
    Abstract [en]

    Extratropical cyclones, major contributors to precipitation in the midlatitudes, comprise mesoscale fronts and fine-scale convective storms. Intense oceanic cyclones pose natural hazards, making reliable projections of their changes with global warming of great interest. Here, we analyze the first ever global climate simulations to resolve such mesoscale dynamics of extratropical cyclones. The present-day structure, frequency, and precipitation of the oceanic extratropical cyclones compare well with reanalyses and new satellite datasets that resolve the multiscale cloud-precipitation system. Simulated precipitation from intense oceanic cyclones increases at a rate of 7%/K-1, following Clausius-Clapeyron, with warming. The same scaling is apparent also in the interhemispheric contrast, suggesting that the latter could serve as a predictor of the former. Projected changes in precipitation from intense oceanic cyclones with warming may thus be testable using a reliable global observation network of precipitation in the present day.

  • 10. Sedlar, Joseph
    et al.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    A Process-Based Climatological Evaluation of AIRS Level 3 Tropospheric Thermodynamics over the High-Latitude Arctic2019In: Journal of Applied Meteorology and Climatology, ISSN 1558-8424, E-ISSN 1558-8432, Vol. 58, no 8, p. 1867-1886Article in journal (Refereed)
    Abstract [en]

    Measurements from spaceborne sensors have the unique capacity to fill spatial and temporal gaps in ground-based atmospheric observing systems, especially over the Arctic, where long-term observing stations are limited to pan-Arctic landmasses and infrequent field campaigns. The AIRS level 3 (L3) daily averaged thermodynamic profile product is widely used for process understanding across the sparsely observed Arctic atmosphere. However, detailed investigations into the accuracy of the AIRS L3 thermodynamic profiles product using in situ observations over the high-latitude Arctic are lacking. To address this void, we compiled a wealth of radiosounding profiles from long-term Arctic land stations and included soundings from intensive icebreaker-based field campaigns. These are used to evaluate daily mean thermodynamic profiles from the AIRS L3 product so that the community can understand to what extent such data records can be applied in scientific studies. Results indicate that, while the mid- to upper-troposphere temperature and specific humidity are captured relatively well by AIRS, the lower troposphere is susceptible to specific seasonal, and even monthly, biases. These differences have a critical influence on the lower-tropospheric stability structure. The relatively coarse vertical resolution of the AIRS L3 product, together with infrared radiation through persistent low Arctic cloud layers, leads to artificial thermodynamic structures that fail to accurately represent the lower Arctic atmosphere. These thermodynamic errors are likely to introduce artificial errors in the boundary layer structure and analysis of associated physical processes.

  • 11.
    Bulatovic, Ines
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Savre, J.
    Riipinen, Ilona
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Tampere University of Technology, Finland.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Aerosol Indirect Effects in Marine Stratocumulus: The Importance of Explicitly Predicting Cloud Droplet Activation2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 6, p. 3473-3481Article in journal (Refereed)
    Abstract [en]

    Climate models generally simulate a unidirectional, positive liquid water path (LWP) response to increasing aerosol number concentration. However, satellite observations and large-eddy simulations show that the LWP may either increase or decrease with increasing aerosol concentration, influencing the overall magnitude of the aerosol indirect effect (AIE). We use large-eddy simulation to investigate the LWP response of a marine stratocumulus cloud and its dependence on different parameterizations for obtaining cloud droplet number concentration (CDNC). The simulations confirm that the LWP response is not always positiveregardless of CDNC treatment. However, the AIE simulated with the model version with prescribed CDNC is almost 3 times larger compared to the version with prognostic CDNC. The reason is that the CDNC in the prognostic scheme varies in time due to supersaturation fluctuations, collection, and other microphysical processes. A substantial spread in simulated AIE may thus arise simply due to the CDNC treatment. Plain Language Summary Our poor understanding of aerosol-cloud-radiation interactions (aerosol indirect effects) results in a major uncertainty in estimates of anthropogenic aerosol forcing. In climate models, the cloud water response to an increased aerosol number concentration may be especially uncertain as models simplify, or do not account for, processes that affect the cloud droplet number concentration and the total amount of cloud water. In this study, we employ large-eddy simulation to explore how different model descriptions for obtaining the number concentration of cloud droplets influences the cloud water response of a marine stratocumulus cloud and thus the simulated aerosol indirect effect. Our simulations show a qualitatively similar cloud water response regardless of model description: the total amount of cloud water increases first and then decreases with increasing aerosol concentration. However, the simulated aerosol indirect effect is almost 3 times as large when the number concentration of cloud droplets is prescribed compared to when it is dependent on the calculated supersaturation and other microphysical processes such as collisions between cloud droplets. Our findings show that a relatively simple difference in the treatment of the number concentration of cloud droplets in climate models may result in a significant spread in the simulated aerosol indirect effect.

  • 12. Yu, Xiaolong
    et al.
    Garabato, Alberto C. Naveira
    Martin, Adrian P.
    Buckingham, Christian E.
    Brannigan, Liam
    Stockholm University, Faculty of Science, Department of Meteorology .
    Su, Zhan
    An Annual Cycle of Submesoscale Vertical Flow and Restratification in the Upper Ocean2019In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 49, no 6, p. 1439-1461Article in journal (Refereed)
    Abstract [en]

    Numerical simulations suggest that submesoscale turbulence may transform lateral buoyancy gradients into vertical stratification and thus restratify the upper ocean via vertical flow. However, the observational evidence for this restratifying process has been lacking due to the difficulty in measuring such ephemeral phenomena, particularly over periods of months to years. This study presents an annual cycle of the vertical velocity and associated restratification estimated from two nested clusters of meso- and submesoscale-resolving moorings, deployed in a typical midocean area of the northeast Atlantic. Vertical velocities inferred using the nondiffusive density equation are substantially stronger at submesoscales (horizontal scales of 1-10 km) than at mesoscales (horizontal scales of 10-100 km), with respective root-mean-square values of 38.0 +/- 6.9 and 22.5 +/- 3.3 m day(-1). The largest submesoscale vertical velocities and rates of restratification occur in events of a few days' duration in winter and spring, and extend down to at least 200 m below the mixed layer base. These events commonly coincide with the enhancement of submesoscale lateral buoyancy gradients, which is itself associated with persistent mesoscale frontogenesis. This suggests that mesoscale frontogenesis is a regular precursor of the submesoscale turbulence that restratifies the upper ocean. The upper-ocean restratification induced by submesoscale motions integrated over the annual cycle is comparable in magnitude to the net destratification driven by local atmospheric cooling, indicating that submesoscale flows play a significant role in determining the climatological upper-ocean stratification in the study area.

  • 13. Valsangkar, Akash Anil
    et al.
    Monteiro, Joy Merwin
    Stockholm University, Faculty of Science, Department of Meteorology .
    Narayanan, Vidya
    Hotz, Ingrid
    Natarajan, Vijay
    An Exploratory Framework for Cyclone Identification and Tracking2019In: IEEE Transactions on Visualization and Computer Graphics, ISSN 1077-2626, E-ISSN 1941-0506, Vol. 25, no 3, p. 1460-1473Article in journal (Refereed)
    Abstract [en]

    Analyzing depressions plays an important role in meteorology, especially in the study of cyclones. In particular, the study of the temporal evolution of cyclones requires a robust depression tracking framework. To cope with this demand we propose a pipeline for the exploration of cyclones and their temporal evolution. This entails a generic framework for their identification and tracking. The fact that depressions and cyclones are not well-defined objects and their shape and size characteristics change over time makes this task especially challenging. Our method combines the robustness of topological approaches and the detailed tracking information from optical flow analysis. At first cyclones are identified within each time step based on well-established topological concepts. Then candidate tracks are computed from an optical flow field. These tracks are clustered within a moving time window to distill dominant coherent cyclone movements, which are then forwarded to a final tracking step. In contrast to previous methods our method requires only a few intuitive parameters. An integration into an exploratory framework helps in the study of cyclone movement by identifying smooth, representative tracks. Multiple case studies demonstrate the effectiveness of the method in tracking cyclones, both in the northern and southern hemisphere.

  • 14.
    Fitch, Anna C.
    Stockholm University, Faculty of Science, Department of Meteorology . Pacific Northwest National Laboratory, Washington, USA.
    An Improved Double-Gaussian Closure for the Subgrid Vertical Velocity Probability Distribution Function2019In: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 76, p. 285-304Article in journal (Refereed)
    Abstract [en]

    The vertical velocity probability distribution function (PDF) is analyzed throughout the depth of the lower atmosphere, including the subcloud and cloud layers, in four large-eddy simulation (LES) cases of shallow cumulus and stratocumulus. Double-Gaussian PDF closures are examined to test their ability to represent a wide range of turbulence statistics, from stratocumulus cloud layers characterized by Gaussian turbulence to shallow cumulus cloud layers displaying strongly non-Gaussian turbulence statistics. While the majority of the model closures are found to perform well in the former case, the latter presents a considerable challenge. A new model closure is suggested that accounts for high skewness and kurtosis seen in shallow cumulus cloud layers. The well-established parabolic relationship between skewness and kurtosis is examined, with results in agreement with previous studies for the subcloud layer. In cumulus cloud layers, however, a modified relationship is necessary to improve performance. The new closure significantly improves the estimation of the vertical velocity PDF for shallow cumulus cloud layers, in addition to performing well for stratocumulus. In particular, the long updraft tail representing the bulk of cloudy points is much better represented and higher-order moments diagnosed from the PDF are also greatly improved. However, some deficiencies remain owing to fundamental limitations of representing highly non-Gaussian turbulence statistics with a double-Gaussian PDF.

  • 15.
    Hutchinson, David K.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Coxall, Helen K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    de Boer, Agatha M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Arctic closure as a trigger for Atlantic overturning at the Eocene-Oligocene Transition2019In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 10, article id 3797Article in journal (Refereed)
    Abstract [en]

    The Eocene-Oligocene Transition (EOT), approximately 34 Ma ago, marks a period of major global cooling and inception of the Antarctic ice sheet. Proxies of deep circulation suggest a contemporaneous onset or strengthening of the Atlantic meridional overturning circulation (AMOC). Proxy evidence of gradual salinification of the North Atlantic and tectonically driven isolation of the Arctic suggest that closing the Arctic-Atlantic gateway could have triggered the AMOC at the EOT. We demonstrate this trigger of the AMOC using a new paleoclimate model with late Eocene boundary conditions. The control simulation reproduces Eocene observations of low Arctic salinities. Subsequent closure of the Arctic-Atlantic gateway triggers the AMOC by blocking freshwater inflow from the Arctic. Salt advection feedbacks then lead to cessation of overturning in the North Pacific. These circulation changes imply major warming of the North Atlantic Ocean, and simultaneous cooling of the North Pacific, but no interhemispheric change in temperatures.

  • 16. Browny, Nicola Jane
    et al.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Pemberton, Per
    Arctic Ocean Freshwater Dynamics: Transient Response to Increasing River Runoff and Precipitation2019In: Journal of Geophysical Research - Oceans, ISSN 2169-9275, E-ISSN 2169-9291, Vol. 124, no 7, p. 5205-5219Article in journal (Refereed)
    Abstract [en]

    Simulations from a coupled ice-ocean general circulation model are used to assess the effects on Arctic Ocean freshwater storage of changes in freshwater input through river runoff and precipitation. We employ the climate response function framework to examine responses of freshwater content to abrupt changes in freshwater input. To the lowest order, the response of ocean freshwater content is linear, with an adjustment time scale of approximately 10years, indicating that anomalies in Arctic Ocean freshwater export are proportional to anomalies in freshwater content. However, the details of the transient response of the ocean depend on the source of freshwater input. An increase in river runoff results in a fairly smooth response in freshwater storage consistent with an essentially linear relation between total freshwater content and discharge of excess freshwater through the main export straits. However, the response to a change in precipitation is subject to greater complexity, which can be explained by the localized formation and subsequent export of salinity anomalies which introduce additional response time scales. The results presented here suggest that future increases in Arctic Ocean freshwater input in the form of precipitation are more likely to be associated with variability in the storage and release of excess freshwater than are increases in freshwater input from river runoff. Plain Language Summary This paper shows that the Arctic Ocean adjusts to changes in freshwater input over time scales of about one decade. How much of the added freshwater is stored in the Arctic depends, however, on how the freshwater enters the ocean. If it arrives as additional river runoff, the response in Arctic freshwater storage is relatively smooth and predictable. If it falls, instead, as increased precipitation, the response is less easy to predict because it is complicated by interactions between the ocean and sea ice. This is important because the part of the freshwater that is not stored in the Arctic Ocean is exported to the North Atlantic, where it can affect the global ocean circulation.

  • 17. Olonscheck, Dirk
    et al.
    Mauritsen, Thorsten
    Stockholm University, Faculty of Science, Department of Meteorology .
    Notz, Dirk
    Arctic sea-ice variability is primarily driven by atmospheric temperature fluctuations2019In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 12, no 6, p. 430-434Article in journal (Refereed)
    Abstract [en]

    The anthropogenically forced decline of Arctic sea ice is superimposed on strong internal variability. Possible drivers for this variability include fluctuations in surface albedo, clouds and water vapour, surface winds and poleward atmospheric and oceanic energy transport, but their relative contributions have not been quantified. By isolating the impact of the individual drivers in an Earth system model, we here demonstrate that internal variability of sea ice is primarily caused directly by atmospheric temperature fluctuations. The other drivers together explain only 25% of sea-ice variability. The dominating impact of atmospheric temperature fluctuations on sea ice is consistent across observations, reanalyses and simulations from global climate models. Such atmospheric temperature fluctuations occur due to variations in moist-static energy transport or local ocean heat release to the atmosphere. The fact that atmospheric temperature fluctuations are the key driver for sea-ice variability limits prospects of interannual predictions of sea ice, and suggests that observed record lows in Arctic sea-ice area are a direct response to an unusually warm atmosphere.

  • 18.
    Tjernström, Michael
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . National Centre for Atmospheric Research, Mesoscale and Microscale Laboratory, USA.
    Shupe, Matthew D.
    Brooks, Ian M.
    Achtert, Peggy
    Prytherch, John
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sedlar, Joseph
    Stockholm University, Faculty of Science, Department of Meteorology . University of Colorado Boulder, USA.
    Arctic Summer Airmass Transformation, Surface Inversions, and the Surface Energy Budget2019In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 32, no 3, p. 769-789Article in journal (Refereed)
    Abstract [en]

    During the Arctic Clouds in Summer Experiment (ACSE) in summer 2014 a weeklong period of warm-air advection over melting sea ice, with the formation of a strong surface temperature inversion and dense fog, was observed. Based on an analysis of the surface energy budget, we formulated the hypothesis that, because of the airmass transformation, additional surface heating occurs during warm-air intrusions in a zone near the ice edge. To test this hypothesis, we explore all cases with surface inversions occurring during ACSE and then characterize the inversions in detail. We find that they always occur with advection from the south and are associated with subsidence. Analyzing only inversion cases over sea ice, we find two categories: one with increasing moisture in the inversion and one with constant or decreasing moisture with height. During surface inversions with increasing moisture with height, an extra 10-25 W m(-2) of surface heating was observed, compared to cases without surface inversions; the surface turbulent heat flux was the largest single term. Cases with less moisture in the inversion were often cloud free and the extra solar radiation plus the turbulent surface heat flux caused by the inversion was roughly balanced by the loss of net longwave radiation.

  • 19.
    Bender, Frida A.-M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Frey, Lena
    Stockholm University, Faculty of Science, Department of Meteorology .
    McCoy, Daniel T.
    Grosvenor, Daniel P.
    Mohrmann, Johannes K.
    Assessment of aerosol-cloud-radiation correlations in satellite observations, climate models and reanalysis2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 52, no 7-8, p. 4371-4392Article in journal (Refereed)
    Abstract [en]

    Representing large-scale co-variability between variables related to aerosols, clouds and radiation is one of many aspects of agreement with observations desirable for a climate model. In this study such relations are investigated in terms of temporal correlations on monthly mean scale, to identify points of agreement and disagreement with observations. Ten regions with different meteorological characteristics and aerosol signatures are studied and correlation matrices for the selected regions offer an overview of model ability to represent present day climate variability. Global climate models with different levels of detail and sophistication in their representation of aerosols and clouds are compared with satellite observations and reanalysis assimilating meteorological fields as well as aerosol optical depth from observations. One example of how the correlation comparison can guide model evaluation and development is the often studied relation between cloud droplet number and water content. Reanalysis, with no parameterized aerosol–cloud coupling, shows weaker correlations than observations, indicating that microphysical couplings between cloud droplet number and water content are not negligible for the co-variations emerging on larger scale. These observed correlations are, however, not in agreement with those expected from dominance of the underlying microphysical aerosol–cloud couplings. For instance, negative correlations in subtropical stratocumulus regions show that suppression of precipitation and subsequent increase in water content due to aerosol is not a dominating process on this scale. Only in one of the studied models are cloud dynamics able to overcome the parameterized dependence of rain formation on droplet number concentration, and negative correlations in the stratocumulus regions are reproduced.

  • 20. Grygalashvyly, Mykhaylo
    et al.
    Eberhart, Martin
    Hedin, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Strelnikov, Boris
    Lübken, Franz-Josef
    Rapp, Markus
    Löhle, Stefan
    Fasoulas, Stefanos
    Khaplanova, Mikhail
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology .
    Vorobeva, Ekaterina
    Atmospheric band fitting coefficients derived from a self-consistent rocket-borne experiment2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 2, p. 1207-1220Article in journal (Refereed)
    Abstract [en]

    Based on self-consistent rocket-borne measurements of temperature, the densities of atomic oxygen and neutral air, and the volume emission of the atmospheric band (762 nm), we examined the one-step and two-step excitation mechanism of O-2 (b(1)Sigma(+)(g)) for nighttime conditions. Following McDade et al. (1986), we derived the empirical fitting coefficients, which parameterize the atmospheric band emission O-2 (b(1)Sigma(+)(g) - X-3 Sigma(-)(g)) (0, 0). This allows us to derive the atomic oxygen concentration from nighttime observations of atmospheric band emission O-2 (b(1)Sigma(+)(g) - X-3 Sigma(-)(g)) (0, 0). The derived empirical parameters can also be utilized for atmospheric band modeling. Additionally, we derived the fit function and corresponding coefficients for the combined (one-and two-step) mechanism. The simultaneous common volume measurements of all the parameters involved in the theoretical calculation of the observed O-2 (b(1)Sigma(+)(g) - X-3 Sigma(-)(g)) (0, 0) emission, i.e., temperature and density of the background air, atomic oxygen density, and volume emission rate, is the novelty and the advantage of this work.

  • 21. Naakka, Tuomas
    et al.
    Nygård, Tiina
    Vihma, Timo
    Sedlar, Joseph
    Stockholm University, Faculty of Science, Department of Meteorology . University of Colorado Boulder, USA.
    Graversen, Rune
    Atmospheric moisture transport between mid-latitudes and the Arctic: Regional, seasonal and vertical distributions2019In: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 39, no 6, p. 2862-2879Article in journal (Refereed)
    Abstract [en]

    Horizontal moisture transport has a manifold role in the Arctic climate system as it distributes atmospheric water vapour and thereby shapes the radiative and hydrological conditions. Moisture transport between the Arctic and the mid-latitudes was examined based on ERA-Interim reanalysis. The meridional net transport is only a small part of the water vapour exchange between the Arctic and mid-latitudes and does not give a complete view of temporal and spatial variations in the transport. Especially near the surface, most of the northwards moisture transport is balanced by the southwards transport, and therefore the meridional net moisture transport at 60 degrees-70 degrees N peaks approximately at 100 hPa higher altitude than the northwards and southwards moisture transports. The total moisture transport (sum of absolute northwards and southwards moisture transports) has a much larger seasonal variation than the net transport (mean meridional transport), and the strength of the total transport is related to atmospheric humidity rather than the wind field. Strong individual moisture transport events contribute to a large part of the northwards moisture transport. This is consistent with the result that the net moisture transport is essentially generated by temporal variations of moisture fluxes. The moisture transport due to stationary zonal variation in the mass flux mostly defines the spatial distribution of the meridional moisture transport. The seasonal cycle of the net moisture transport is related to the seasonal cycle of transient eddy moisture transport but inter-annual variations of the net moisture transport are largely influenced by the stationary eddy moisture transport.

  • 22. Eberhart, Martin
    et al.
    Löhle, Stefan
    Strelnikov, Boris
    Hedin, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Khaplanov, Mikhail
    Stockholm University, Faculty of Science, Department of Meteorology .
    Fasoulas, Stefanos
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lübken, Franz-Josef
    Rapp, Markus
    Atomic oxygen number densities in the mesosphere-lower thermosphere region measured by solid electrolyte sensors on WADIS-22019In: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 12, no 4, p. 2445-2461Article in journal (Refereed)
    Abstract [en]

    Absolute profiles of atomic oxygen number densities with high vertical resolution have been determined in the mesosphere-lower thermosphere (MLT) region from in situ measurements by several rocket-borne solid electrolyte sensors. The amperometric sensors were operated in both controlled and uncontrolled modes and with various orientations on the foredeck and aft deck of the payload. Calibration was based on mass spectrometry in a molecular beam containing atomic oxygen produced in a microwave discharge. The sensor signal is proportional to the number flux onto the electrodes, and the mass flow rate in the molecular beam was additionally measured to derive this quantity from the spectrometer reading. Numerical simulations provided aerodynamic correction factors to derive the atmospheric number density of atomic oxygen from the sensor data. The flight results indicate a preferable orientation of the electrode surface perpendicular to the rocket axis. While unstable during the upleg, the density profiles measured by these sensors show an excellent agreement with the atmospheric models and photometer results during the downleg of the trajectory. The high spatial resolution of the measurements allows for the identification of small-scale variations in the atomic oxygen concentration.

  • 23. Faranda, Davide
    et al.
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology .
    Vannitsem, Stephane
    Attractor dimension of time-averaged climate observables: insights from a low-order ocean-atmosphere model2019In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 71, no 1, article id 1554413Article in journal (Refereed)
    Abstract [en]

    The ocean and atmosphere have very different characteristic timescales and display a rich range of interactions. Here, we investigate the sensitivity of the dynamical properties of the coupled atmosphere-ocean system when time-averaging of the trajectories of the original system is performed. We base our analysis on a conceptual model of the atmosphere-ocean dynamics which allows us to compute the attractor properties for different coupling coefficients and averaging periods. When the averaging period is increased, the attractor dimension initially shows a non-monotonic behaviour, but ultimately decreases for windows longer than 1year. The analysis of daily, monthly and annual instrumental and reconstructed indices of oceanic and atmospheric circulation supports our results. This has important implications for the analysis and interpretation of long climate timeseries with a low temporal resolution, but also for the possible convergence of climate observables subjected to long time-averages towards attractors close to hyperbolicity.

  • 24.
    Hannachi, Abdelwaheb
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Iqbal, Waheed
    Stockholm University, Faculty of Science, Department of Meteorology . Finish Meteorological Institute, Finland.
    Bimodality of hemispheric winter atmospheric variability via average flow tendencies and kernel EOFs2019In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 71, no 1, article id 1633847Article in journal (Refereed)
    Abstract [en]

    The topic of the existence of planetary winter circulation regimes has gone through a long debate. This article contributes to this debate by investigating nonlinearity in a 3-level quasi-geostrophic model and the Japanese JRA-55 reanalysis. The method uses averaged flow tendencies and kernel principal component (PC) analysis. Within two-dimensional (2D) kernel PCs the model reveals two fixed (or stationary) points. The probability density function (PDF) within this space is strongly bimodal where the modes match the regions of low tendencies in consistency with low-order conceptual models. The circulation regimes represent respectively zonal and blocked flows. Application to daily winter northern hemisphere sea level pressure and 500-hPa geopotential height yields strong bimodal PDFs. The modes represent respectively polar highs and lows with signatures of North Atlantic Oscillation. A clear climate change signal is observed showing a clear reduction (increase) of occurrence probability of polar high (low), translating into an increase of probability of zonal flow. Relation of the climate change signal to the polar amplification hypothesis is discussed.

  • 25. Mandal, Samiran
    et al.
    Sil, Sourav
    Pramanik, Saikat
    Arunraj, Kondetharayil Soman
    Stockholm University, Faculty of Science, Department of Meteorology . National Institute of Ocean Technology Chennai, India.
    Jena, Basanta Kumar
    Characteristics and evolution of a coastal mesoscale eddy in the Western Bay of Bengal monitored by high-frequency radars2019In: Dynamics of atmospheres and oceans (Print), ISSN 0377-0265, E-ISSN 1872-6879, Vol. 88, article id 101107Article in journal (Refereed)
    Abstract [en]

    Evolution of a coastal cyclonic eddy has been investigated using surface current observations from high-frequency radar (HFR) along the western Bay of Bengal (BoB) near Andhra Pradesh coast during October-December 2015. The HFRS tracked the genesis of the cyclonic eddy from early October, which persisted throughout November and dissipated after mid-December within the shelf-slope regions of HFR domain along the western boundary of BoB. A vector-based technique has been adapted to detect and track the mesoscale cyclonic eddy. The eddy is observed to propagate with a mean speed of similar to 0.27m s(-1) (23.36 km day(-1)). It is asymmetric in nature with an average radius of similar to 90 (80) km along the eastern and western (northern and southern) sides of the cyclonic eddy aligned along-shelf. The eddy has been characterized based on the Eulerian parameters; normalized vorticity (similar to 0.75), divergence (similar to 0.20), strain (similar to 0.25), and Okubu-Weiss (OW) parameter (-0.7 x 10(-9)s(-2)). Positive vorticity and divergence, along with lower strain at the eddy center, justify the cyclonic eddy. The negative values of OW parameter show good agreement with the eddy-cores detected. Kinematics show that the Rossby number (R-0) varies in the range 0.6-1.2, depicting that the cyclonic eddy is associated with mesoscale dynamical features and matches perfectly with the geostrophic balance. The eddyinduced upwelling signatures are observed from the subsurface temperature and salinity structures. The upwelling is well supported by positive (similar to 11 x 10(-7) N m(-3)) wind stress curl during November. The sea surface temperature (SST), surface chlorophyll-alpha concentration as well as sea surface salinity (SSS) associated with the cyclonic eddy, show the advection of warm waters from the open ocean and low saline cold waters from the coastline. This study reveals that the eddy evolved due to baroclinic instability, well indicated by positive values of T2 (rate of conversion of mean potential energy to eddy potential energy) and lower values of Brunt-Vaisala Frequency (N-2), whereas growth and intensification of the eddy are attributed to barotropic instability, supported by the positive values of T4 (the conversion of mean kinetic energy to eddy kinetic energy).

  • 26.
    Monteiro, Joy Merwin
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Characterization of Extreme Wet-Bulb Temperature Events in Southern Pakistan2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 17-18, p. 10659-10668Article in journal (Refereed)
    Abstract [en]

    We study extreme wet-bulb temperature (T-w) events in a regional hot spot of high T-w values in southwest Pakistan's Indus Valley. We focus on the May-June premonsoon months and employ a combination of station data, ERA-Interim reanalysis, and trajectory calculations to analyze the extreme events. We find that the events are associated with anomalous influx of marine air from the Arabian Sea. The air flows in a shallow layer below 950 hPa and is heated and moistened by diabatic processes over land, arriving at the hot spot region with very high T-w. We surmise that surface evaporation within the strip of well-irrigated land flanking the Indus River plays a major role in generating these extreme T-w values. Our results imply that studies of extreme T-w events in future, warmer climates must pay close attention to fine-scale diabatic processes within the boundary layer, especially evaporation over land.

  • 27. Vassel, Maiken
    et al.
    Ickes, Luisa
    Stockholm University, Faculty of Science, Department of Meteorology .
    Maturilli, Marion
    Hoose, Corinna
    Classification of Arctic multilayer clouds using radiosonde and radar data in Svalbard2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 7, p. 5111-5126Article in journal (Refereed)
    Abstract [en]

    Multilayer clouds (MLCs) occur more often in the Arctic than globally. In this study we present the results of a detection algorithm applied to radiosonde and radar data from an 1-year time period in Ny-Alesund, Svalbard. Multilayer cloud occurrence is found on 29% of the investigated days. These multilayer cloud cases are further analysed regarding the possibility of ice crystal seeding, meaning that an ice crystal can survive sublimation in a subsaturated layer between two cloud layers when falling through this layer. For this we analyse profiles of relative humidity with respect to ice to identify super-and subsaturated air layers. Then the sublimation of an ice crystal of an assumed initial size of r = 400 mu m on its way through the subsaturated layer is calculated. If the ice crystal still exists when reaching a lower supersaturated layer, ice crystal seeding can potentially take place. Seeding cases are found often, in 23% of the investigated days (100% includes all days, as well as non-cloudy days). The identification of seeding cases is limited by the radar signal inside the subsaturated layer. Clearly separated multilayer clouds, defined by a clear interstice in the radar image, do not interact through seeding 29% of the investigated days). There are various deviations between the relative humidity profiles and the radar images, e.g. due to the lack of ice-nucleating particles (INPs) and cloud condensation nuclei (CCN). Additionally, horizontal wind drift of the radiosonde and time restriction when comparing radiosonde and radar data cause further deviations. In order to account for some of these deviations, an evaluation by manual visual inspection is done for the non-seeding cases.

  • 28. Retsch, M. H.
    et al.
    Mauritsen, Thorsten
    Stockholm University, Faculty of Science, Department of Meteorology . Max Planck Institute for Meteorology, Germany.
    Hohenegger, C.
    Climate Change Feedbacks in Aquaplanet Experiments With Explicit and Parametrized Convection for Horizontal Resolutions of 2,525 Up to 5 km2019In: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 11, no 7, p. 2070-2088Article in journal (Refereed)
    Abstract [en]

    Earth's equilibrium climate sensitivity (ECS) is the long-term response to doubled atmospheric CO2 and likely between 1.5 and 4.5 K. Conventional general circulation models do not convincingly narrow down this range, and newly developed nonhydrostatic models with relatively fine horizontal resolutions of a few kilometers have thus far delivered diverse results. Here we use the nonhydrostatic ICON model with the physics package normally used for climate simulations at resolutions as fine as 5 km to study the response to a uniform surface warming in an aquaplanet configuration. We apply the model in two setups: one with convection parametrization employed and one with explicit convection. ICON exhibits a negative total feedback independent of convective representation, thus providing a stable climate with an ECS comparable to other general circulation models, though three interesting new results are found. First, ECS varies little across resolution for both setups but runs with explicit convection have systematically lower ECS than the parametrized case, mainly due to more negative tropical clear-sky longwave feedbacks. These are a consequence of a drier mean state of about 6% relative humidity for explicit convection and less midtropospheric moistening with global warming. Second, shortwave feedbacks switch from positive to negative with increasing resolution, originating foremost in the tropics and high latitudes. Third, the model shows no discernible high cloud area feedback (iris effect) in any configuration. It is possible that ICON's climate model parametrizations applied here are less appropriate for cloud resolving scales, and therefore, ongoing developments aim at implementing a more advanced prognostic cloud microphysics scheme.

  • 29.
    Strandberg, Gustav
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Kjellström, Erik
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Climate Impacts from Afforestation and Deforestation in Europe2019In: Earth Interactions, ISSN 1087-3562, E-ISSN 1087-3562, Vol. 23, no 1, article id 1Article in journal (Refereed)
    Abstract [en]

    Changes in vegetation are known to have an impact on climate via biogeophysical effects such as changes in albedo and heat fluxes. Here, the effects of maximum afforestation and deforestation are studied over Europe. This is done by comparing three regional climate model simulations-one with present-day vegetation, one with maximum afforestation, and one with maximum deforestation. In general, afforestation leads to more evapotranspiration (ET), which leads to decreased near-surface temperature, whereas deforestation leads to less ET, which leads to increased temperature. There are exceptions, mainly in regions with little water available for ET. In such regions, changes in albedo are relatively more important for temperature. The simulated biogeophysical effect on seasonal mean temperature varies between 0.5 degrees and 3 degrees C across Europe. The effect on minimum and maximum temperature is larger than that on mean temperature. Increased (decreased) mean temperature is associated with an even larger increase (decrease) in maximum summer (minimum winter) temperature. The effect on precipitation is found to be small Two additional simulations in which vegetation is changed in only one-half of the domain were also performed. These simulations show that the climatic effects from changed vegetation in Europe are local. The results imply that vegetation changes have had, and will have, a significant impact on local climate in Europe; the climatic response is comparable to climate change under RCP2.6. Therefore, effects from vegetation change should be taken into account when simulating past, present, and future climate for this region. The results also imply that vegetation changes could be used to mitigate local climate change.

  • 30. Saupe, Erin E.
    et al.
    Farnsworth, Alexander
    Lunt, Daniel J.
    Sagoo, Navjit
    Stockholm University, Faculty of Science, Department of Meteorology .
    Pham, Karen V.
    Field, Daniel J.
    Climatic shifts drove major contractions in avian latitudinal distributions throughout the Cenozoic2019In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 116, no 26, p. 12895-12900Article in journal (Refereed)
    Abstract [en]

    Many higher level avian clades are restricted to Earth's lower latitudes, leading to historical biogeographic reconstructions favoring a Gondwanan origin of crown birds and numerous deep subclades. However, several such tropical-restricted clades (TRCs) are represented by stem-lineage fossils well outside the ranges of their closest living relatives, often on northern continents. To assess the drivers of these geographic disjunctions, we combined ecological niche modeling, paleoclimate models, and the early Cenozoic fossil record to examine the influence of climatic change on avian geographic distributions over the last similar to 56 million years. By modeling the distribution of suitable habitable area through time, we illustrate that most Paleogene fossil-bearing localities would have been suitable for occupancy by extant TRC representatives when their stem-lineage fossils were deposited. Potentially suitable habitat for these TRCs is inferred to have become progressively restricted toward the tropics throughout the Cenozoic, culminating in relatively narrow circumtropical distributions in the present day. Our results are consistent with coarse-scale niche conservatism at the clade level and support a scenario whereby climate change over geological timescales has largely dictated the geographic distributions of many major avian clades. The distinctive modern bias toward high avian diversity at tropical latitudes for most hierarchical taxonomic levels may therefore represent a relatively recent phenomenon, overprinting a complex biogeographic history of dramatic geographic range shifts driven by Earth's changing climate, variable persistence, and intercontinental dispersal. Earth's current climatic trajectory portends a return to a megathermal state, which may dramatically influence the geographic distributions of many range-restricted extant clades.

  • 31. McCoy, Daniel T.
    et al.
    Field, Paul R.
    Elsaesser, Gregory S.
    Bodas-Salcedo, Alejandro
    Kahn, Brian H.
    Zelinka, Mark D.
    Kodama, Chihiro
    Mauritsen, Thorsten
    Stockholm University, Faculty of Science, Department of Meteorology . Max Planck Institute for Meteorology, Germany.
    Vanniere, Benoit
    Roberts, Malcolm
    Vidale, Pier L.
    Saint-Martin, David
    Voldoire, Aurore
    Haarsma, Rein
    Hill, Adrian
    Shipway, Ben
    Wilkinson, Jonathan
    Cloud feedbacks in extratropical cyclones: insight from long-term satellite data and high-resolution global simulations2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 2, p. 1147-1172Article in journal (Refereed)
    Abstract [en]

    A negative extratropical shortwave cloud feedback driven by changes in cloud optical depth is a feature of global climate models (GCMs). A robust positive trend in observed liquid water path (LWP) over the last two decades across the warming Southern Ocean supports the negative shortwave cloud feedback predicted by GCMs. This feature has been proposed to be due to transitions from ice to liquid with warming. To gain insight into the shortwave cloud feedback we examine extratropical cyclone variability and the response of extratropical cyclones to transient warming in GCM simulations. Multi-Sensor Advanced Climatology Liquid Water Path (MAC-LWP) microwave observations of cyclone properties from the period 1992-2015 are contrasted with GCM simulations, with horizontal resolutions ranging from 7 km to hundreds of kilometers. We find that inter-cyclone variability in LWP in both observations and models is strongly driven by the moisture flux along the cyclone's warm conveyor belt (WCB). Stronger WCB moisture flux enhances the LWP within cyclones. This relation-ship is replicated in GCMs, although its strength varies substantially across models. It is found that more than 80% of the enhancement in Southern Hemisphere (SH) extratropical cyclone LWP in GCMs in response to a transient 4K warming can be predicted based on the relationship between the WCB moisture flux and cyclone LWP in the historical climate and their change in moisture flux between the historical and warmed climates. Further, it is found that that the robust trend in cyclone LWP over the Southern Ocean in observations and GCMs is consistent with changes in the moisture flux. We propose two cloud feedbacks acting within extratropical cyclones: a negative feedback driven by Clausius-Clapeyron increasing water vapor path (WVP), which enhances the amount of water vapor available to be fluxed into the cyclone, and a feedback moderated by changes in the life cycle and vorticity of cyclones under warming, which changes the rate at which existing moisture is imported into the cyclone. Both terms contribute to increasing LWP within the cyclone. While changes in moisture flux predict cyclone LWP trends in the current climate and the majority of changes in LWP in transient warming simulations, a portion of the LWP increase in response to climate change that is unexplained by increasing moisture fluxes may be due to phase transitions. The variability in LWP within cyclone composites is examined to understand what cyclonic regimes the mixed-phase cloud feedback is relevant to. At a fixed WCB moisture flux cyclone LWP increases with increasing sea surface temperature (SST) in the half of the composite poleward of the low and decreases in the half equatorward of the low in both GCMs and observations. Cloud-top phase partitioning observed by the Atmospheric Infrared Sounder (AIRS) indicates that phase transitions may be driving increases in LWP in the poleward half of cyclones.

  • 32.
    Li, Xiang-Yu
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Swedish e-Science Research Centre, Sweden; University of Colorado, USA.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden; National Center for Atmospheric Research, USA.
    Brandenburg, Axel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA.
    Haugen, Nils E. L.
    Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 1, p. 639-648Article in journal (Refereed)
    Abstract [en]

    Condensational growth of cloud droplets due to supersaturation fluctuations is investigated by solving the hydrodynamic and thermodynamic equations using direct numerical simulations (DNS) with droplets being modeled as Lagrangian particles. The supersaturation field is calculated directly by simulating the temperature and water vapor fields instead of being treated as a passive scalar. Thermodynamic feedbacks to the fields due to condensation are also included for completeness. We find that the width of droplet size distributions increases with time, which is contrary to the classical theory without supersaturation fluctuations, where condensational growth leads to progressively narrower size distributions. Nevertheless, in agreement with earlier Lagrangian stochastic models of the condensational growth, the standard deviation of the surface area of droplets increases as t(1/2). Also, for the first time, we explicitly demonstrate that the time evolution of the size distribution is sensitive to the Reynolds number, but insensitive to the mean energy dissipation rate. This is shown to be due to the fact that temperature fluctuations and water vapor mixing ratio fluctuations increase with increasing Reynolds number; therefore the resulting supersaturation fluctuations are enhanced with increasing Reynolds number. Our simulations may explain the broadening of the size distribution in stratiform clouds qualitatively, where the mean updraft velocity is almost zero.

  • 33.
    Broman, Lina
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Benze, Susanne
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology .
    Christensen, Ole Martin
    Stockholm University, Faculty of Science, Department of Meteorology .
    Randall, Cora E.
    Common volume satellite studies of polar mesospheric clouds with Odin/OSIRIS tomography and AIM/CIPS nadir imaging2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 19, p. 12455-12475Article in journal (Refereed)
    Abstract [en]

    Two important approaches for satellite studies of polar mesospheric clouds (PMCs) are nadir measurements adapting phase function analysis and limb measurements adapting spectroscopic analysis. Combining both approaches enables new studies of cloud structures and microphysical processes but is complicated by differences in scattering conditions, observation geometry and sensitivity. In this study, we compare common volume PMC observations from the nadir-viewing Cloud Imaging and Particle Size (CIPS) instrument on the Aeronomy of Ice in the Mesosphere (AIM) satellite and a special set of tomographic limb observations from the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite performed over 18 d for the years 2010 and 2011 and the latitude range 78 to 80 degrees N. While CIPS provides preeminent horizontal resolution, the OSIRIS tomographic analysis provides combined horizontal and vertical PMC information. This first direct comparison is an important step towards co-analysing CIPS and OSIRIS data, aiming at unprecedented insights into horizontal and vertical cloud processes. Important scientific questions on how the PMC life cycle is affected by changes in humidity and temperature due to atmospheric gravity waves, planetary waves and tides can be addressed by combining PMC observations in multiple dimensions. Two- and three-dimensional cloud structures simultaneously observed by CIPS and tomographic OSIRIS provide a useful tool for studies of cloud growth and sublimation Moreover, the combined CIPS/tomographic OSIRIS dataset can be used for studies of even more fundamental character, such as the question of the assumption of the PMC particle size distribution. We perform the first thorough error characterization of OSIRIS tomographic cloud brightness and cloud ice water content (IWC). We establish a consistent method for comparing cloud properties from limb tomography and nadir observations, accounting for differences in scattering conditions, resolution and sensitivity. Based on an extensive common volume and a temporal coincidence criterion of only 5 min, our method enables a detailed comparison of PMC regions of varying brightness and IWC. However, since the dataset is limited to 18 d of observations this study does not include a comparison of cloud frequency. The cloud properties of the OSIRIS tomographic dataset are vertically resolved, while the cloud properties of the CIPS dataset is vertically integrated. To make these different quantities comparable, the OSIRIS tomographic cloud properties cloud scattering coefficient and ice mass density (IMD) have been integrated over the vertical extent of the cloud to form cloud albedo and IWC of the same quantity as CIPS cloud products. We find that the OSIRIS albedo (obtained from the vertical integration of the primary OSIRIS tomography product, cloud scattering coefficient) shows very good agreement with the primary CIPS product, cloud albedo, with a correlation coefficient of 0.96. However, OSIRIS systematically reports brighter clouds than CIPS and the bias between the instruments (OSIRIS -CIPS) is 3.4 x 10(-6) sr(-1) (+/- 2.9x 10(-6) sr(-1)) on average. The OSIRIS tomography IWC (obtained from the vertical integration of IMD) agrees well with the CIPS IWC, with a correlation coefficient of 0.91. However, the IWC reported by OSIRIS is lower than CIPS, and we quantify the bias to -22 g km(-2) (+/- 14 g km(-2)) on average.

  • 34. Murray, Michelle
    et al.
    Soh, Wuu Kuang
    Yiotis, Charilaos
    Batke, Sven
    Parnell, Andrew C.
    Spicer, Robert A.
    Lawson, Tracy
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wright, Ian J.
    Purcell, Conor
    McElwain, Jennifer C.
    Convergence in Maximum Stomatal Conductance of C-3 Woody Angiosperms in Natural Ecosystems Across Bioclimatic Zones2019In: Frontiers in Plant Science, ISSN 1664-462X, E-ISSN 1664-462X, Vol. 10, article id 558Article in journal (Refereed)
    Abstract [en]

    Stomatal conductance (g(s)) in terrestrial vegetation regulates the uptake of atmospheric carbon dioxide for photosynthesis and water loss through transpiration, closely linking the biosphere and atmosphere and influencing climate. Yet, the range and pattern of g(s) in plants from natural ecosystems across broad geographic, climatic, and taxonomic ranges remains poorly quantified. Furthermore, attempts to characterize g(s) on such scales have predominantly relied upon meta-analyses compiling data from many different studies. This approach may be inherently problematic as it combines data collected using unstandardized protocols, sometimes over decadal time spans, and from different habitat groups. Using a standardized protocol, we measured leaf-level g(s) using porometry in 218 C-3 woody angiosperm species in natural ecosystems representing seven bioclimatic zones. The resulting dataset of 4273 g(s) measurements, which we call STraits (Stomatal Traits), was used to determine patterns in maximum g(s) (g(smax)) across bioclimatic zones and whether there was similarity in the mean g(smax) of C3 woody angiosperms across ecosystem types. We also tested for differential g(smax) in two broadly defined habitat groups - open-canopy and understory-subcanopy - within and across bioclimatic zones. We found strong convergence in mean g(smax) of C3 woody angiosperms in the understory-subcanopy habitats across six bioclimatic zones, but not in open-canopy habitats. Mean g(smax) in open-canopy habitats (266 +/- 100 mmol m(-2) s(-1)) was significantly higher than in understory-subcanopy habitats (233 +/- 86 mmol m(-2) s(-1)). There was also a central tendency in the overall dataset to operate toward a g(smax) of similar to 250 mmol m(-2) s(-1). We suggest that the observed convergence in mean g(smax) of C3 woody angiosperms in the understory-subcanopy is due to a buffering of g(smax) against macroclimate effects which will lead to differential response of C3 woody angiosperm vegetation in these two habitats to future global change. Therefore, it will be important for future studies of g(smax) to categorize vegetation according to habitat group.

  • 35.
    Mauritsen, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Max Planck Institute for Meteorology, Germany.
    Bader, Jurgen
    Becker, Tobias
    Behrens, Jorg
    Bittner, Matthias
    Brokopf, Renate
    Brovkin, Victor
    Claussen, Martin
    Crueger, Traute
    Esch, Monika
    Fast, Irina
    Fiedler, Stephanie
    Stockholm University, Faculty of Science, Department of Meteorology .
    Flaeschner, Dagmar
    Gayler, Veronika
    Giorgetta, Marco
    Goll, Daniel S.
    Haak, Helmuth
    Hagemann, Stefan
    Hedemann, Christopher
    Hohenegger, Cathy
    Ilyina, Tatiana
    Jahns, Thomas
    Jimenez-de-la-Cuesta, Diego
    Jungclaus, Johann
    Kleinen, Thomas
    Kloster, Silvia
    Kracher, Daniela
    Kinne, Stefan
    Kleberg, Deike
    Lasslop, Gitta
    Kornblueh, Luis
    Marotzke, Jochem
    Matei, Daniela
    Meraner, Katharina
    Mikolajewicz, Uwe
    Modali, Kameswarrao
    Moebis, Benjamin
    Muellner, Wolfgang A.
    Nabel, Julia E. M. S.
    Nam, Christine C. W.
    Notz, Dirk
    Nyawira, Sarah-Sylvia
    Paulsen, Hanna
    Peters, Karsten
    Pincus, Robert
    Stockholm University, Faculty of Science, Department of Meteorology . Max Planck Institute for Meteorology, Germany; University of Colorado Boulder, USA; Earth System Research Lab, USA.
    Pohlmann, Holger
    Pongratz, Julia
    Popp, Max
    Raddatz, Thomas Jurgen
    Rast, Sebastian
    Redler, Rene
    Reick, Christian H.
    Rohrschneider, Tim
    Schemann, Vera
    Schmidt, Hauke
    Schnur, Reiner
    Schulzweida, Uwe
    Six, Katharina D.
    Stein, Lukas
    Stemmler, Irene
    Stevens, Bjorn
    von Storch, Jin-Song
    Tian, Fangxing
    Voigt, Aiko
    Vrese, Philipp
    Wieners, Karl-Hermann
    Wilkenskjeld, Stiig
    Winkler, Alexander
    Roeckner, Erich
    Developments in the MPI-M Earth System Model version 1.2 (MPI-ESM1.2) and Its Response to Increasing CO22019In: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 11, no 4, p. 998-1038Article in journal (Refereed)
    Abstract [en]

    A new release of the Max Planck Institute for Meteorology Earth System Model version 1.2 (MPI-ESM1.2) is presented. The development focused on correcting errors in and improving the physical processes representation, as well as improving the computational performance, versatility, and overall user friendliness. In addition to new radiation and aerosol parameterizations of the atmosphere, several relatively large, but partly compensating, coding errors in the model's cloud, convection, and turbulence parameterizations were corrected. The representation of land processes was refined by introducing a multilayer soil hydrology scheme, extending the land biogeochemistry to include the nitrogen cycle, replacing the soil and litter decomposition model and improving the representation of wildfires. The ocean biogeochemistry now represents cyanobacteria prognostically in order to capture the response of nitrogen fixation to changing climate conditions and further includes improved detritus settling and numerous other refinements. As something new, in addition to limiting drift and minimizing certain biases, the instrumental record warming was explicitly taken into account during the tuning process. To this end, a very high climate sensitivity of around 7 K caused by low-level clouds in the tropics as found in an intermediate model version was addressed, as it was not deemed possible to match observed warming otherwise. As a result, the model has a climate sensitivity to a doubling of CO2 over preindustrial conditions of 2.77 K, maintaining the previously identified highly nonlinear global mean response to increasing CO2 forcing, which nonetheless can be represented by a simple two-layer model.

  • 36. Vichi, Marcello
    et al.
    Eayrs, Clare
    Alberello, Alberto
    Bekker, Anriëtte
    Bennetts, Luke
    Holland, David
    de Jong, Ehlke
    Joubert, Warren
    MacHutchon, Keith
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology . Uppsala University, Sweden.
    Mojica, Jhon F.
    Onorato, Miguel
    Saunders, Clinton
    Skatulla, Sebastian
    Toffoli, Alessandro
    Effects of an Explosive Polar Cyclone Crossing the Antarctic Marginal Ice Zone2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 11, p. 5948-5958Article in journal (Refereed)
    Abstract [en]

    Antarctic sea ice shows a large degree of regional variability, which is partly driven by severe weather events. Here we bring a new perspective on synoptic sea ice changes by presenting the first in situ observations of an explosive extratropical cyclone crossing the winter Antarctic marginal ice zone (MIZ) in the South Atlantic. This is complemented by the analysis of subsequent cyclones and highlights the rapid variations that ice-landing cyclones cause on sea ice: Midlatitude warm oceanic air is advected onto the ice, and storm waves generated close to the ice edge contribute to the maintenance of an unconsolidated surface through which waves propagate far into the ice. MIZ features may thus extend further poleward in the Southern Ocean than currently estimated. A concentration-based MIZ definition is inadequate, since it fails to describe a sea ice configuration which is deeply rearranged by synoptic weather. Plain Language Summary The extent of Antarctic sea ice is characterized by large regional variations that are in stark contrast with the alarming decreasing trends found in the Arctic. This is partly due to the presence of severe weather events, like extratropical cyclones travelling through the Southern Ocean and reaching the marginal ice zone (MIZ). The MIZ is a region where the ocean, atmosphere, and sea ice processes are closely interlinked. We provide direct evidence of how winter polar cyclones rearrange the MIZ and how their effects extend into the ice-covered region as far as the Antarctic continent. We present the first observations of large ice drift, ice concentration, and temperature changes as an explosively deepening cyclone crosses the MIZ. This case study is complemented by analysis of subsequent but more frequent storms that confirms how storminess in the Southern Ocean maintains a sea ice surface that is less compact, more mobile, and more extended than previously anticipated. Our results urge the scientific community to revise the current definition of the MIZ and improve its representation in models to better include the role of polar cyclones in detecting Antarctic sea ice trends.

  • 37. Jiménez-de-la-Cuesta, Diego
    et al.
    Mauritsen, Thorsten
    Stockholm University, Faculty of Science, Department of Meteorology .
    Emergent constraints on Earth's transient and equilibrium response to doubled CO2 from post-1970s global warming2019In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 12, no 11, p. 902-905Article in journal (Refereed)
    Abstract [en]

    Future global warming is determined by both greenhouse gas emission pathways and Earth's transient and equilibrium climate response to doubled atmospheric CO2. Energy-balance inference from the instrumental record typically yields central estimates for the transient response of around 1.3 K and the equilibrium response of 1.5-2.0 K, which is at the lower end of those from contemporary climate models. Uncertainty arises primarily from poorly known aerosol-induced cooling since the early industrialization era and a temporary cooling induced by evolving sea surface temperature patterns. Here we present an emergent constraint on post-1970s warming, taking advantage of the weakly varying aerosol cooling during this period. We derive a relationship between the transient response and the post-1970s warming in Coupled Model Intercomparison Project Phase 5 (CMIP5) models. We thereby constrain, with the observations, the transient response to 1.67 K (1.17-2.16 K, 5-95th percentiles). This is a 20% increase relative to energy-balance inference stemming from previously neglected upper-ocean energy storage. For the equilibrium climate sensitivity we obtain a best estimate of 2.83 K (1.72-4.12 K) contingent on the temporary pattern effects exhibited by climate models. If the real world's surface temperature pattern effects are substantially stronger, then the upper-bound equilibrium sensitivity may be higher than found here.

  • 38. Lucas, Natasha S.
    et al.
    Grant, Alan L. M.
    Rippeth, Tom P.
    Polton, Jeff A.
    Palmer, Matthew R.
    Brannigan, Liam
    Stockholm University, Faculty of Science, Department of Meteorology .
    Belcher, Stephen E.
    Evolution of Oceanic Near-Surface Stratification in Response to an Autumn Storm2019In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 49, no 11, p. 2961-2978Article in journal (Refereed)
    Abstract [en]

    Understanding the processes that control the evolution of the ocean surface boundary layer (OSBL) is a prerequisite for obtaining accurate simulations of air-sea fluxes of heat and trace gases. Observations of the rate of dissipation of turbulent kinetic energy (epsilon), temperature, salinity, current structure, and wave field over a period of 9.5 days in the northeast Atlantic during the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS) are presented. The focus of this study is a storm that passed over the observational area during this period. The profiles of epsilon in the OSBL are consistent with profiles from large-eddy simulation (LES) of Langmuir turbulence. In the transition layer (TL), at the base of the OSBL, epsilon was found to vary periodically at the local inertial frequency. A simple bulk model of the OSBL and a parameterization of shear driven turbulence in the TL are developed. The parameterization of epsilon is based on assumptions about the momentum balance of the OSBL and shear across the TL. The predicted rate of deepening, heat budget, and the inertial currents in the OSBL were in good agreement with the observations, as is the agreement between the observed value of epsilon and that predicted using the parameterization. A previous study reported spikes of elevated dissipation related to enhanced wind shear alignment at the base of the OSBL after this storm. The spikes in dissipation are not predicted by this new parameterization, implying that they are not an important source of dissipation during the storm.

  • 39. Akperov, Mirseid
    et al.
    Rinke, Annette
    Mokhov, Igor I.
    Semenov, Vladimir A.
    Parfenova, Mariya R.
    Matthes, Heidrun
    Adakudlu, Muralidhar
    Boberg, Fredrik
    Christensen, Jens H.
    Dembitskaya, Mariya A.
    Dethloff, Klaus
    Fettweis, Xavier
    Gutjahr, Oliver
    Heinemann, Günther
    Koenigk, Torben
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Koldunov, Nikolay
    Laprise, René
    Mottram, Ruth
    Nikiéma, Oumarou
    Sein, Dmitry
    Sobolowski, Stefan
    Winger, Katja
    Zhang, Wenxin
    Future projections of cyclone activity in the Arctic for the 21st century from regional climate models (Arctic-CORDEX)2019In: Global and Planetary Change, ISSN 0921-8181, E-ISSN 1872-6364, Vol. 182, article id 103005Article in journal (Refereed)
    Abstract [en]

    Changes in the characteristics of cyclone activity (frequency, depth and size) in the Arctic are analyzed based on simulations with state-of-the-art regional climate models (RCMs) from the Arctic-CORDEX initiative and global climate models (GCMs) from CMIP5 under the Representative Concentration Pathway (RCP) 8.5 scenario. Most of RCMs show an increase of cyclone frequency in winter (DJF) and a decrease in summer (JJA) to the end of the 21st century. However, in one half of the RCMs, cyclones become weaker and substantially smaller in winter and deeper and larger in summer. RCMs as well as GCMs show an increase of cyclone frequency over the Baffin Bay, Barents Sea, north of Greenland, Canadian Archipelago, and a decrease over the Nordic Seas, Kara and Beaufort Seas and over the sub-arctic continental regions in winter. In summer, the models simulate an increase of cyclone frequency over the Central Arctic and Greenland Sea and a decrease over the Norwegian and Kara Seas by the end of the 21st century. The decrease is also found over the high-latitude continental areas, in particular, over east Siberia and Alaska. The sensitivity of the RCMs' projections to the boundary conditions and model physics is estimated. In general, different lateral boundary conditions from the GCMs have larger effects on the simulated RCM projections than the differences in RCMs' setup and/or physics.

  • 40.
    Scher, Sebastian
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Uppsala University, Sweden.
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology .
    Generalization properties of feed-forward neural networks trained on Lorenz systems2019In: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 26, no 4, p. 381-399Article in journal (Refereed)
    Abstract [en]

    Neural networks are able to approximate chaotic dynamical systems when provided with training data that cover all relevant regions of the system's phase space. However, many practical applications diverge from this idealized scenario. Here, we investigate the ability of feed-forward neural networks to (1) learn the behavior of dynamical systems from incomplete training data and (2) learn the influence of an external forcing on the dynamics. Climate science is a real-world example where these questions may be relevant: it is concerned with a non-stationary chaotic system subject to external forcing and whose behavior is known only through comparatively short data series. Our analysis is performed on the Lorenz63 and Lorenz95 models. We show that for the Lorenz63 system, neural networks trained on data covering only part of the system's phase space struggle to make skillful short-term forecasts in the regions excluded from the training. Additionally, when making long series of consecutive forecasts, the networks struggle to reproduce trajectories exploring regions beyond those seen in the training data, except for cases where only small parts are left out during training. We find this is due to the neural network learning a localized mapping for each region of phase space in the training data rather than a global mapping. This manifests itself in that parts of the networks learn only particular parts of the phase space. In contrast, for the Lorenz95 system the networks succeed in generalizing to new parts of the phase space not seen in the training data. We also find that the networks are able to learn the influence of an external forcing, but only when given relatively large ranges of the forcing in the training. These results point to potential limitations of feed-forward neural networks in generalizing a system's behavior given limited initial information. Much attention must therefore be given to designing appropriate train-test splits for real-world applications.

  • 41.
    Johansson, Jana H.
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Salter, Matthew E.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Acosta Navarro, Juan Camilo
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, E. Douglas
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Cousins, Ian T.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Global transport of perfluoroalkyl acids via sea spray aerosol2019In: Environmental Science: Processes & Impacts, ISSN 2050-7887, E-ISSN 2050-7895, Vol. 21, no 4, p. 635-649Article in journal (Refereed)
    Abstract [en]

    Perfluoroalkyl acids (PFAAs) are persistent organic pollutants found throughout the world's oceans. Previous research suggests that long-range atmospheric transport of these substances may be substantial. However, it remains unclear what the main sources of PFAAs to the atmosphere are. We have used a laboratory sea spray chamber to study water-to-air transfer of 11 PFAAs via sea spray aerosol (SSA). We observed significant enrichment of all PFAAs relative to sodium in the SSA generated. The highest enrichment was observed in aerosols with aerodynamic diameter < 1.6 mm, which had aerosol PFAA concentrations up to similar to 62 000 times higher than the PFAA water concentrations in the chamber. In surface microlayer samples collected from the sea spray chamber, the enrichment of the substances investigated was orders of magnitude smaller than the enrichment observed in the aerosols. In experiments with mixtures of structural isomers, a lower contribution of branched PFAA isomers was observed in the surface microlayer compared to the bulk water. However, no clear trend was observed in the comparison of structural isomers in SSA and bulk water. Using the measured enrichment factors of perfluorooctanoic acid and perfluorooctane sulfonic acid versus sodium we have estimated global annual emissions of these substances to the atmosphere via SSA as well as their global annual deposition to land areas. Our experiments suggest that SSA may currently be an important source of these substances to the atmosphere and, over certain areas, to terrestrial environments.

  • 42.
    Johansson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute (SMHI), Sweden.
    Devasthale, Abhay
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    L'Ecuyer, Tristan
    How Does Cloud Overlap Affect the Radiative Heating in the Tropical Upper Troposphere/Lower Stratosphere?2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 10, p. 5623-5631Article in journal (Refereed)
    Abstract [en]

    Characterizing two-layer cloud systems has historically been difficult. These systems have a strong radiative impact on the composition of and the processes in the upper troposphere-lower stratosphere (UTLS). Using 4 years of combined spaceborne lidar and radar observations, the radiative impact of two-layer cloud systems in the tropical UTLS is characterized, and its sensitivity to the properties of top- and bottom-layer clouds is further quantified. Under these overlapping cloud conditions, the bottom-layer clouds can fully suppress the radiative heating caused by high clouds in the UTLS, by inducing strong longwave cooling. If the vertical separation between the layers is <4 km, the radiative heating of the high cloud changes sign from positive to negative. Furthermore, the radiative effect at the top of the atmosphere is investigated, and it is found that the characteristic net warming by cirrus with ice water path <50 g/m(2) is suppressed in the two-layered system.

  • 43.
    Scher, Sebastian
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology . Uppsala University, Sweden.
    How Global Warming Changes the Difficulty of Synoptic Weather Forecasting2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 5, p. 2931-2939Article in journal (Refereed)
    Abstract [en]

    Global warming projections point to a wide range of impacts on the climate system, including changes in storm track activity and more frequent and intense extreme weather events. Little is however known on whether and how global warming may affect the atmosphere's predictability and thus our ability to produce accurate weather forecasts. Here, we combine a state-of-the-art climate and a state-of-the-art ensemble weather prediction model to show that, in a business-as-usual 21st century setting, global warming could significantly change the predictability of the atmosphere, defined here via the expected error of weather predictions. Predictability of synoptic weather situations could significantly increase, especially in the Northern Hemisphere. This can be explained by a decrease in the meridional temperature gradient. Contrarily, summertime predictability of weekly rainfall sums might significantly decrease in most regions. Plain Language Summary Due to the chaotic nature of the atmosphere, it is impossible to make weather forecasts that are completely accurate. Therefore, all weather forecasts are inherently uncertain to a certain degree. However, this uncertainty-and thus the difficulty of making good forecastsis not the same for all forecasts. This opens up the highly important question whether global warming will affect the difficulty of weather forecasts. Due to the enormous socioeconomic importance of accurate weather forecasts, it is essential to know whether climate change adaption policies also need to take into account potential changes in the difficulty and accuracy of weather forecasts. We show that in a warmer world, it will be easier to predict fields such as temperature and pressure. Contrarily, it will be harder to make accurate precipitation forecasts, which might strongly affect both disaster prevention and rainfall-dependent industries such as the energy sector, all of which heavily rely on accurate precipitation forecasts. Additionally, we show that the uncertainty of predictions of pressure fields is to a large extent controlled by fluctuations in the temperature difference between the North Pole and the equator. This is a new and important insight into the fundamentals of weather forecast uncertainty.

  • 44. de Vries, Hylke
    et al.
    Scher, Sebastian
    Stockholm University, Faculty of Science, Department of Meteorology .
    Haarsma, Rein
    Drijfhout, Sybren
    van Delden, Aarno