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  • 1. Balsley, Ben
    et al.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology.
    On the scale-dependence of the gradient Richardson number in the residual layer2008In: Boundary-Layer Meteorology, Vol. 127, p. 57-72Article in journal (Refereed)
    Abstract [en]

    We present results of a technique for examining the scale-dependence of the gradient Richardson number, Ri, in the nighttime residual layer. The technique makes use of a series of high-resolution, in situ, vertical profiles of wind speed and potential temperature obtained during CASES-99 in south-eastern Kansas, U.S.A. in October 1999. These profiles extended from the surface, through the nighttime stable boundary layer, and well into the residual layer. Analyses of the vertical gradients of both wind speed, potential temperature and turbulence profiles over a wide range of vertical scale sizes are used to estimate profiles of the local Ri and turbulence structure as a function of scale size. The utility of the technique lies both with the extensive height range of the residual layer as well as with the fact that the sub-metre resolution of the raw profiles enables a metre-by-metre ‘sliding’ average of the scale-dependent Richardson number values over hundreds of metres vertically. The results presented here show that small-scale turbulence is a ubiquitous and omnipresent feature of the residual layer, and that the region is dynamic and highly variable, exhibiting persistent turbulent structure on vertical scales of a few tens of metres or less. Furthermore, these scales are comparable to the scales over which the Ri is less than or equal to the critical value of Ric of 0.25, although turbulence is also shown to exist in regions with significantly larger Ri values, an observation at least consistent with the concept of hysteresis in turbulence generation and maintenance. Insofar as the important scale sizes are comparable to or smaller than the resolution of current models, it follows that, in order to resolve the observed details of small Ri values and the concomitant turbulence generation, future models need to be capable of significantly higher resolutions.

  • 2. Balsley, Ben
    et al.
    Svensson, Gunilla
    Stockholm University.
    Tjernström, Michael
    Stockholm University.
    On the scale-dependence of the gradient Richardson number in the residual layer2007In: Boundary-Layer Meteorology, Vol. 127, p. 57-72Article in journal (Refereed)
  • 3.
    Bengtsson, Lisa
    et al.
    Sveriges meteorologiska och hydrologiska institut (SMHI), Norrköping.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Källén, Erland
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Large-Scale Dynamical Response to Subgrid-Scale Organization Provided by Cellular Automata2011In: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, no 12, p. 3132-3144Article in journal (Refereed)
    Abstract [en]

    Due to the limited resolution of numerical weather prediction (NWP) models, sub-grid scale physical processes are parameterized, and represented by grid-box means. However, some physical processes are better represented by a mean and its variance, a typical example being deep convection, with scales varying from individual updraughts to organized meso-scale systems. In this study, we investigate, in an idealized setting, whether a cellular automaton (CA) can be used in order to enhance sub-grid scale organization by forming clusters representative of the convective scales, and yield a stochastic representation of sub-grid scale variability. We study the transfer of energy from the convective to the larger atmospheric scales through nonlinear wave interactions. This is done using a shallow water (SW) model initialized with equatorial wave modes. By letting a CA act on a finer resolution than that of the SW model, it can be expected to mimic the effect of, for instance, gravity wave propagation on convective organization. Employing the CA-scheme allows to reproduce the observed behaviour of slowing down equatorial Kelvin modes in convectively active regions, while random perturbations fail to feed back on the large-scale flow. The analysis of kinetic energy spectra demonstrates that the CA sub-grid scheme introduces energy back-scatter from the smallest model scales to medium scales. However, the amount of energy back-scattered depends almost solely on the memory time scale introduced to the sub-grid scheme, whereas any variation in spatial scales generated does not influence the energy spectra markedly.

  • 4.
    Bengtsson, Lisa
    et al.
    Sveriges meteorologiska och hydrologiska institut (SMHI), Norrköping.
    Tijm, Sander
    Vána, Filip
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Impact of flow-dependent horizontal diffusion on resolved convectionin AROME.2012In: Journal of Applied Meteorology and Climatology, ISSN 1558-8424, E-ISSN 1558-8432, Vol. 51, no 1, p. 54-67Article in journal (Refereed)
    Abstract [en]

    Horizontal diffusion in numerical weather prediction models is, in general, applied to reduce numerical noise at the smallest atmospheric scales. In convection-permittingmodels, with horizontal grid spacing on the order of 1–3 km, horizontal diffusion can improve themodel skill of physical parameters such as convective precipitation. For instance, studies using the convection-permitting Applications of Research to Operations at Mesoscale model (AROME) have shown an improvement in forecasts of large precipitation amounts when horizontal diffusion is applied to falling hydrometeors. The nonphysical nature of such a procedure is undesirable, however. Within the current AROME, horizontal diffusion is imposed using linear spectral horizontal diffusion on dynamicalmodel fields. This spectral diffusion is complemented by nonlinear, flow-dependent, horizontal diffusion applied on turbulent kinetic energy, cloud water, cloud ice, rain, snow, and graupel. In this study, nonlinear flowdependent diffusion is applied to the dynamical model fields rather than diffusing the already predicted falling hydrometeors. In particular, the characteristics of deep convection are investigated. Results indicate that, for the same amount of diffusive damping, the maximum convective updrafts remain strong for both the current and proposed methods of horizontal diffusion. Diffusing the falling hydrometeors is necessary to see a reduction in rain intensity, but amore physically justified solution can be obtained by increasing the amount of damping on the smallest atmospheric scales using the nonlinear, flow-dependent, diffusion scheme. In doing so, a reduction in vertical velocity was found, resulting in a reduction in maximum rain intensity.

  • 5. Bocquet, Florence
    et al.
    Balsley, Ben
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Comparing Estimates of Turbulence Based on Near-Surface Measurements in the Nocturnal Stable Boundary Layer2011In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 138, no 1, p. 43-60Article in journal (Refereed)
    Abstract [en]

    Tethered Lifting System (TLS) estimates of the dissipation rate of turbulent kinetic energy (epsilon) are reasonably well correlated with concurrent measurements of vertical velocity variance (sigma(2)(w)) obtained from sonic anemometers located on a nearby 60-m tower during the CASES-99 field experiment. Additional results in the first 100m of the nocturnal stable boundary layer confirm our earlier claim that the presence of weak but persistent background turbulence exists even during the most stable atmospheric conditions, where e can exhibit values as low as 10(-7) m(2) s(-3). We also present a set of empirical equations that incorporates TLS measurements of temperature, horizontal wind speed, and e to provide a proxy measurement for sigma(2)(w) at altitudes higher than tower heights.

  • 6. Bosveld, Fred C.
    et al.
    Baas, Peter
    Steeneveld, Gert-Jan
    Holtslag, Albert A. M.
    Angevine, Wayne M.
    Bazile, Eric
    de Bruijn, Evert I. F.
    Deacu, Daniel
    Edwards, John M.
    Ek, Michael
    Larson, Vincent E.
    Pleim, Jonathan E.
    Raschendorfer, Matthias
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part B: Results and Process Understanding2014In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 152, no 2, p. 157-187Article in journal (Refereed)
    Abstract [en]

    We describe and analyze the results of the third global energy and water cycle experiment atmospheric boundary layer Study intercomparison and evaluation study for single-column models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL; the diagrams include the results of a number of sensitivity runs performed with one of the models. The models are characterized in terms of thermal coupling to the soil, longwave radiation and turbulent mixing. It is shown that differences in longwave radiation schemes among the models have only a small effect on the simulations; however, there are significant variations in downward radiation due to different boundary-layer profiles of temperature and humidity. The differences in modelled thermal coupling to the land surface are large and explain most of the variations in 2-m air temperature and longwave incoming radiation among models. Models with strong turbulent mixing overestimate the boundary-layer height, underestimate the wind speed at 200 m, and give a relatively large downward sensible heat flux. The result is that 2-m air temperature is relatively insensitive to turbulent mixing intensity. Evening transition times spread 1.5 h around the observed time of transition, with later transitions for models with coarse resolution. Time of onset in the morning transition spreads 2 h around the observed transition time. With this case, the morning transition appeared to be difficult to study, no relation could be found between the studied processes, and the variation in the time of the morning transition among the models.

  • 7.
    Engström, Anders
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson, Johannes
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    The Importance of Representing Mixed-Phase Clouds for Simulating Distinctive Atmospheric States in the Arctic2014In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, no 1, p. 265-272Article in journal (Refereed)
    Abstract [en]

    Observations from the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) suggest that the Arctic Basin is characterized by two distinctly different preferred atmospheric states during wintertime. These states appear as two peaks in the frequency distribution of surface downwelling longwave radiation (LWD), representing radiatively clear and opaque conditions. Here, the authors have investigated the occurrence and representation of these states in the widely used ECMWF Interim Re-Analysis (ERA-Interim) dataset. An interannually recurring bimodal distribution of LWD values is not a clearly observable feature in the reanalysis data. However, large differences in the simulated liquid water content of clouds in ERA-Interim compared to observations are identified and these are linked to the lack of a radiatively opaque peak in the reanalysis. Using a single-column model, dynamically controlled by data from ERA-Interim, the authors show that, by tuning the glaciation speed of supercooled liquid clouds, it is possible to reach a very good agreement between the model and observations from the SHEBA campaign in terms of LWD. The results suggest that the presence of two preferred Arctic states, as observed during SHEBA, is a recurring feature of the Arctic climate system during winter [December–March (DJFM)]. The mean increase in LWD during the Arctic winter compared to ERA-Interim is 15 W m−2. This has a substantial bearing on climate model evaluation in the Arctic as it indicates the importance of representing Arctic states in climate models and reanalysis data and that doing so could have a significant impact on winter ice thickness and surface temperatures in the Arctic.

  • 8.
    Frey, Lena
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bender, Frida A.-M.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Cloud albedo changes in response to anthropogenic sulfate and non-sulfate forcings in CMIP5 models2017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 14, p. 9145-9162Article in journal (Refereed)
    Abstract [en]

    The effects of different aerosol types on cloud albedo are analysed using the linear relation between total albedo and cloud fraction found on a monthly mean scale in regions of subtropical marine stratocumulus clouds and the influence of simulated aerosol variations on this relation. Model experiments from the Coupled Model Intercomparison Project phase 5 (CMIP5) are used to separately study the responses to increases in sulfate, non-sulfate and all anthropogenic aerosols. A cloud brightening on the month-to-month scale due to variability in the background aerosol is found to dominate even in the cases where anthropogenic aerosols are added. The aerosol composition is of importance for this cloud brightening, that is thereby region dependent. There is indication that absorbing aerosols to some extent counteract the cloud brightening but scene darkening with increasing aerosol burden is generally not supported, even in regions where absorbing aerosols dominate. Month-to-month cloud albedo variability also confirms the importance of liquid water content for cloud albedo. Regional, monthly mean cloud albedo is found to increase with the addition of anthropogenic aerosols and more so with sulfate than non-sulfate. Changes in cloud albedo between experiments are related to changes in cloud water content as well as droplet size distribution changes, so that models with large increases in liquid water path and/or cloud droplet number show large cloud albedo increases with increasing aerosol. However, no clear relation between model sensitivities to aerosol variations on the month-to-month scale and changes in cloud albedo due to changed aerosol burden is found.

  • 9. Goosse, Hugues
    et al.
    Kay, Jennifer E.
    Armour, Kyle C.
    Bodas-Salcedo, Alejandro
    Chepfer, Helene
    Docquier, David
    Jonko, Alexandra
    Kushner, Paul J.
    Lecomte, Olivier
    Massonnet, Francois
    Park, Hyo-Seok
    Pithan, Felix
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Vancoppenolle, Martin
    Quantifying climate feedbacks in polar regions2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 1919Article in journal (Refereed)
    Abstract [en]

    The concept of feedback is key in assessing whether a perturbation to a system is amplified or damped by mechanisms internal to the system. In polar regions, climate dynamics are controlled by both radiative and non-radiative interactions between the atmosphere, ocean, sea ice, ice sheets and land surfaces. Precisely quantifying polar feedbacks is required for a process-oriented evaluation of climate models, a clear understanding of the processes responsible for polar climate changes, and a reduction in uncertainty associated with model projections. This quantification can be performed using a simple and consistent approach that is valid for a wide range of feedbacks, offering the opportunity for more systematic feedback analyses and a better understanding of polar climate changes.

  • 10.
    Graversen, Rune Grand
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Mauritsen, Thorsten
    Stockholm University, Faculty of Science, Department of Meteorology.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology.
    Källén, Erland
    Stockholm University, Faculty of Science, Department of Meteorology.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology.
    Reply: Communications arising2008In: Nature, Vol. 455, p. E4-E5Article in journal (Refereed)
  • 11.
    Hartung, Kerstin
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-sciene Research Centre, Sweden.
    Shepherd, Theodore
    Hoskins, Brian
    Methven, John
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-sciene Research Centre, Sweden.
    Diagnosing topographic forcing in an atmospheric dataset: the case of the North American CordilleraManuscript (preprint) (Other academic)
  • 12.
    Hartung, Kerstin
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    Kjellström, Erik
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Resolution, physics and atmosphere–ocean interaction – How do they influence climate model representation of Euro-Atlantic atmospheric blocking?2017In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 69, no 1, article id 1406252Article in journal (Refereed)
    Abstract [en]

    Atmospheric blocking events are known to locally explain a large part of climate variability. However, despite their relevance, many current climate models still struggle to represent the observed blocking statistics. In this study, simulations of the global climate model EC-Earth are analysed with respect to atmospheric blocking. Seventeen simulations map the uncertainty space defined by the three-model characteristics: atmospheric resolution, physical parameterization and complexity of atmosphere–ocean interaction, namely an atmosphere coupled to an ocean model or forced by surface data. Representation of the real-world statistics is obtained from reanalyses ERA-20C, JRA-55 and ERA-Interim which agree on Northern Hemisphere blocking characteristics. Blocking events are detected on a central blocking latitude which is individually determined for each simulation. The frequency of blocking events tends to be underestimated relative to ERA-Interim over the Atlantic and western Eurasia in winter and overestimated during spring months. However, only few model setups show statistically significant differences compared to ERA-Interim which can be explained by the large inter-annual variability of blocking. Results indicate slightly larger biases relative to ERA-Interim in coupled than in atmosphere-only models but differences between the two are not statistically significant. Although some resolution dependence is present in spring, the signal is weak and only statistically significant if the physical parameterizations of the model are improved simultaneously. Winter blocking is relatively more sensitive to physical parameterizations, and this signal is robust in both atmosphere-only and coupled simulations, although stronger in the latter. Overall, the model can capture blocking frequency well despite biases in representing the mean state of geopotential height over this area. Blocking signatures of geopotential height are represented more similar to ERA-Interim and only weak sensitivities to model characteristics remain.

  • 13.
    Hartung, Kerstin
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    Struthers, Hamish
    Deppenmeier, Anna-Lena
    Hazeleger, Wilco
    An EC-Earth coupled atmosphere-ocean single-column model (AOSCM) for studying coupled marine and polar processesManuscript (preprint) (Other academic)
  • 14. Holtslag, A. A. M.
    et al.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Baas, P.
    Basu, S.
    Beare, B.
    Beljaars, A. C. M.
    Bosveld, F. C.
    Cuxart, J.
    Lindvall, Jenny
    Stockholm University, Faculty of Science, Department of Meteorology .
    Steeneveld, G. J.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Van de Wiel, B. J. H.
    STABLE ATMOSPHERIC BOUNDARY LAYERS AND DIURNAL CYCLES: Challenges for Weather and Climate Models2013In: Bulletin of The American Meteorological Society - (BAMS), ISSN 0003-0007, E-ISSN 1520-0477, Vol. 94, no 11, p. 1691-1706Article in journal (Refereed)
    Abstract [en]

    The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2-m temperature and 10-m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, the authors review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art as well as findings and recommendations from three intercomparison studies held within the Global Energy and Water Exchanges (GEWEX) Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear-sky conditions. For this purpose, single-column versions of weather and climate models have been compared with observations, research models, and large-eddy simulations. The intercomparison cases are based on observations taken in the Arctic, Kansas, and Cabauw in the Netherlands. From these studies, we find that even for the noncloudy boundary layer important parameterization challenges remain.

  • 15. Jung, Thomas
    et al.
    Gordon, Neil D.
    Bauer, Peter
    Bromwich, David H.
    Chevallier, Matthieu
    Day, Jonathan J.
    Dawson, Jackie
    Doblas-Reyes, Francisco
    Fairall, Christopher
    Goessling, Helge F.
    Holland, Marika
    Inoue, Jun
    Iversen, Trond
    Klebe, Stefanie
    Lemke, Peter
    Losch, Martin
    Makshtas, Alexander
    Mills, Brian
    Nurmi, Pertti
    Perovich, Donald
    Reid, Philip
    Renfrew, Ian A.
    Smith, Gregory
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tolstykh, Mikhail
    Yang, Qinghua
    ADVANCING POLAR PREDICTION CAPABILITIES ON DAILY TO SEASONAL TIME SCALES2016In: Bulletin of The American Meteorological Society - (BAMS), ISSN 0003-0007, E-ISSN 1520-0477, Vol. 97, no 9, p. 1631-+Article in journal (Refereed)
    Abstract [en]

    The polar regions have been attracting more and more attention in recent years, fueled by the perceptible impacts of anthropogenic climate change. Polar climate change provides new opportunities, such as shorter shipping routes between Europe and East Asia, but also new risks such as the potential for industrial accidents or emergencies in ice-covered seas. Here, it is argued that environmental prediction systems for the polar regions are less developed than elsewhere. There are many reasons for this situation, including the polar regions being (historically) lower priority, with fewer in situ observations, and with numerous local physical processes that are less well represented by models. By contrasting the relative importance of different physical processes in polar and lower latitudes, the need for a dedicated polar prediction effort is illustrated. Research priorities are identified that will help to advance environmental polar prediction capabilities. Examples include an improvement of the polar observing system; the use of coupled atmosphere-sea ice-ocean models, even for short-term prediction; and insight into polar-lower latitude linkages and their role for forecasting. Given the enormity of some of the challenges ahead, in a harsh and remote environment such as the polar regions, it is argued that rapid progress will only be possible with a coordinated international effort. More specifically, it is proposed to hold a Year of Polar Prediction (YOPP) from mid-2017 to mid-2019 in which the international research and operational forecasting communites will work together with stakeholders in a period of intensive observing: modeling, prediction, verification, user engagement, and educational activities.

  • 16.
    Karlsson, Johannes
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Consequences of poor representation of Arctic sea-ice albedo and cloud-radiation interactions in the CMIP5 model ensemble2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 16, p. 4374-4379Article in journal (Refereed)
    Abstract [en]

    Clouds significantly influence the Arctic surface energy budget and a realistic representation of this impact is a key for proper simulation of the present-day and future climate. Considerable across-model spread in cloud variables remains in the fifth phase of Coupled Model Intercomparison Project ensemble and partly explains the substantial across-model spread in the surface radiative effect of the clouds. In summer, the extensive model differences in sea-ice albedo, which sets the potential of the cloud-albedo effect, are strongly positively correlated to their cloud radiative effect. This indicates that the model's sea-ice albedo not only determines the amount, but also the sign of its cloud radiative effect. The analysis further suggests that the present-day annual amplitude of sea-ice cover depends inversely on the model's sea-ice albedo. Given the present-day across-model spread in sea-ice albedo and coverage, a transition to a summer ice-free Arctic ocean translates to a model-span of increased surface shortwave absorption of about 75 W m(- 2).

  • 17.
    Karlsson, Johannes
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    The simulation of Arctic clouds and their influence on the winter surface temperature in present-day climate in the CMIP3 multi-model dataset2011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 36, no 3-4, p. 623-635Article in journal (Refereed)
    Abstract [en]

    We investigate the influence of clouds on the surface energy budget and surface temperature in the sea-ice covered parts of the ocean north of the Arctic circle in present-day climate in nine global climate models participating in the Coupled Model Intercomparison Project phase 3, CMIP3. Monthly mean simulated surface skin temperature, radiative fluxes and cloud parameters are evaluated using retrievals from the extended AVHHR Polar Pathfinder (APP-x) product. We analyzed the annual cycle but the main focus is on the winter, in which large parts of the region experience polar night. We find a smaller across-model spread as well as better agreement with observations during summer than during winter in the simulated climatological annual cycles of total cloudiness and surface skin temperature. The across-model spread in liquid and ice water paths is substantial during the whole year. These results qualitatively agree with earlier studies on the present-day Arctic climate in GCMs. The climatological ensemble model mean annual cycle of surface cloud forcing shows good agreement with observations in summer. However, during winter the insulating effect of clouds tends to be underestimated in models. During winter, most of the models as well as the observations show higher monthly mean total cloud fractions, associated with larger positive surface cloud forcing. Most models also show good correlation between the surface cloud forcing and the vertically integrated ice and liquid cloud condensate. The wintertime ensemble model mean total cloud fraction (69%) shows excellent agreement with observations. The across-model spread in the winter mean cloudiness is substantial (36-94%) however and several models significantly underestimate the cloud liquid water content. If the two models not showing any relationship between cloudiness and surface cloud forcing are disregarded, a tentative across-model relation exists, in such a way that models that simulate large winter mean cloudiness also show larger surface cloud forcing. Even though the across-model spread in wintertime surface cloud forcing is large, no clear relation to the surface temperature is found. This indicates that other processes, not explicitly cloud related, are important for the simulated across-model spread in surface temperature.

  • 18.
    Karlsson, Johannes
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    The simulation of Arctic clouds and their influence on thewinter surface temperature in present-day climate in theCMIP3 multi-model datasetManuscript (preprint) (Other academic)
  • 19.
    Karlsson, Johannes
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Cardoso, Sambingo
    Teixeira, Joao
    Subtropical cloud regime transitions: boundary layerdepth and cloud-top height evolution2010In: Journal of Applied Meteorology and Climatology, ISSN 1558-8424, E-ISSN 1558-8432, Vol. 49, no 9, p. 1845-1858Article in journal (Refereed)
    Abstract [en]

    In this study, the mean and variability of boundary layer height (BLH) are analyzed along a transect in the eastern Pacific Ocean for the summer of 2003 using BLH estimates based on the height of the main relative humidity (RH) inversion and the height of low cloud tops (CTH). The observations and the regional and global model data have been prepared in the context of the Global Energy and Water Cycle Experiment (GEWEX) Cloud System Study (GCSS) Pacific Cross-Section Intercomparison (GPCI). The GPCI transect covers the transition from a stratocumulus-topped marine boundary layer (MBL) off the coast of California to a trade cumulus-topped, less-well-defined, MBL, and finally to the deep-convection regions in the intertropical convergence zone (ITCZ). The Atmospheric Infrared Sounder (AIRS) and the Multiangle Imaging Spectroradiometer (MISR) have been used to derive observational records of the two BLH estimates. Analyses from the ECMWF are also used in the study. Both BLH estimates in the models, the ECMWF analysis, and the observations agree on a southward vertical growth of the MBL along the GPCI transect in the stratocumulus region. Away from the region typically associated with extensive cloud cover, the two BLH estimates depict different evolutions of the MBL. In most models, the height of the main RH inversion decreases southward from; similar to 18 degrees N, reaching a minimum at the ITCZ, whereas the height of the RH inversion in the ECMWF analysis and a few of the models is fairly constant all the way to the ITCZ. As a result of insufficient vertical resolution of the gridded dataset, the AIRS data only manage to reproduce the initial growth of the BLH. The median-model CTH increases from the stratocumulus-topped MBL to the ITCZ. In contrast, the observed MISR CTHs decrease southward from 20 degrees N to the ITCZ, possibly indicative of the fact that in these regions MISR manages to capture a variety of cloud tops with a mean that is below the subsidence inversion while the models and the ECMWF analysis mainly simulate CTHs corresponding to the height of the subsidence inversion. In most models and in the ECMWF analysis, the height of the main RH inversion and the CTH tend to coincide in the northern part of the GPCI transect. In the regions associated with trade cumuli and deep convection there is a more ambiguous relation between the two BLH estimates. In this region, most of the models place the CTH above the main RH inversion. The ECMWF analysis shows a good agreement between the BLH estimates throughout the transect.

  • 20.
    Karlsson, Johannes
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Rodhe, Henning
    Stockholm University, Faculty of Science, Department of Meteorology .
    Cloud radiative forcing of subtropical low level clouds in global models2008In: Climate Dynamics, Vol. 30, no 7-8, p. 779-788Article in journal (Refereed)
    Abstract [en]

    Simulations of subtropical marine low clouds and their radiative properties by nine coupled oceanatmosphere climate models participating in the fourth assesment report (AR4) of the intergovernmental panel on climate change (IPCC) are analyzed. Satellite observations of cloudiness and radiative fluxes at the top of the atmosphere (TOA) are utilized for comparison. The analysis is confined to the marine subtropics in an attempt to isolate low cloudiness from tropical convective systems. All analyzed models have a negative bias in the low cloud fraction (model mean bias of –15%). On the other hand, the models show an excess of cloud radiative cooling in the region (model mean excess of 13 W m–2). The latter bias is shown to mainly originate from too much shortwave reflection by the models clouds rather than biases in the clear-sky fluxes. These results confirm earlier studies, thus no major progress in simulating the marine subtropical clouds is noted. As a consequence of the combination of these two biases, this study suggests that all investigated models are likely to overestimate the radiative response to changes in low level subtropical cloudiness.

  • 21.
    Kleman, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Rodhe, Henning
    Stockholm University, Faculty of Science, Department of Meteorology .
    Destouni, Georgia
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Gustafsson, Örjan
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Rubbat förtroende för forskarna2010In: Svenska Dagbladet, ISSN 1101-2412, no 25 majArticle in journal (Other (popular science, discussion, etc.))
  • 22.
    Koenigk, Torben
    et al.
    SMHI.
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, RuneGrand
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson, Johannes
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Willén, Ulrika
    SMHI.
    Wyser, Klaus
    SMHI.
    Arctic climate change in 21st century CMIP5 simulations with EC-Earth2012In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 40, no 11-12Article in journal (Refereed)
    Abstract [en]

    The Arctic climate change is analyzed in anensemble of future projection simulations performed withthe global coupled climate model EC-Earth2.3. EC-Earthsimulates the twentieth century Arctic climate relativelywell but the Arctic is about 2 K too cold and the sea icethickness and extent are overestimated. In the twenty-firstcentury, the results show a continuation and strengtheningof the Arctic trends observed over the recent decades,which leads to a dramatically changed Arctic climate,especially in the high emission scenario RCP8.5. Theannually averaged Arctic mean near-surface temperatureincreases by 12 K in RCP8.5, with largest warming in theBarents Sea region. The warming is most pronounced inwinter and autumn and in the lower atmosphere. The Arcticwinter temperature inversion is reduced in all scenarios anddisappears in RCP8.5. The Arctic becomes ice free inSeptember in all RCP8.5 simulations after a rapid reductionevent without recovery around year 2060. Taking intoaccount the overestimation of ice in the twentieth century,our model results indicate a likely ice-free Arctic inSeptember around 2040. Sea ice reductions are most pronouncedin the Barents Sea in all RCPs, which lead to themost dramatic changes in this region. Here, surface heatfluxes are strongly enhanced and the cloudiness is substantiallydecreased. The meridional heat flux into theArctic is reduced in the atmosphere but increases in theocean. This oceanic increase is dominated by an enhancedheat flux into the Barents Sea, which strongly contributes tothe large sea ice reduction and surface-air warming in thisregion. Increased precipitation and river runoff lead to morefreshwater input into the Arctic Ocean. However, most ofthe additional freshwater is stored in the Arctic Ocean whilethe total Arctic freshwater export only slightly increases.

  • 23. Kumar, Vijayant
    et al.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Holtslag, A. A. M.
    Meneveau, Charles
    Parlange, Marc B.
    Impact of Surface Flux Formulations and Geostrophic Forcing on Large-Eddy Simulations of Diurnal Atmospheric Boundary Layer Flow2010In: Journal of Applied Meteorology and Climatology, ISSN 1558-8424, E-ISSN 1558-8432, Vol. 49, no 7, p. 1496-1516Article in journal (Refereed)
    Abstract [en]

    The impact of surface flux boundary conditions and geostrophic forcing on multiday evolution of flow in the atmospheric boundary layer (ABL) was assessed using large-eddy simulations (LES). The LES investigations included several combinations of surface boundary conditions (temperature and heat flux) and geostrophic forcing (constant, time varying, time and height varying). The setup was based on ABL characteristics observed during a selected period of the Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99) campaign. The LES cases driven by a constant geostrophic wind achieved the best agreement with the CASES-99 observations specifically in terms of daytime surface fluxes and daytime and nighttime profiles. However, the nighttime fluxes were significantly overestimated. The LES cases with the surface temperature boundary condition and driven by a time-and height-varying geostrophic forcing showed improved agreement with the observed nighttime fluxes, but there was less agreement with other observations (e.g., daytime profiles). In terms of the surface boundary condition, the LES cases driven by either surface temperature or heat fluxes produced similar trends in terms of the daytime profiles and comparisons with data from soundings. However, in reproducing the fluxes and nighttime profiles, the agreement was better with imposed temperature because of its ability to interact dynamically with the air temperature field. Therefore, it is concluded that surface temperature boundary condition is better suited for simulations of temporally evolving ABL flow as in the diurnal evolution of the ABL.

  • 24. Lazeroms, W. M. J.
    et al.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bazile, E.
    Brethouwer, G.
    Wallin, S.
    Johansson, A. V.
    Study of Transitions in the Atmospheric Boundary Layer Using Explicit Algebraic Turbulence Models2016In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 161, no 1, p. 19-47Article in journal (Refereed)
    Abstract [en]

    We test a recently developed engineering turbulence model, a so-called explicit algebraic Reynolds-stress (EARS) model, in the context of the atmospheric boundary layer. First of all, we consider a stable boundary layer used as the well-known first test case from the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study (GABLS1). The model is shown to agree well with data from large-eddy simulations (LES), and this agreement is significantly better than for a standard operational scheme with a prognostic equation for turbulent kinetic energy. Furthermore, we apply the model to a case with a (idealized) diurnal cycle and make a qualitative comparison with a simpler first-order model. Some interesting features of the model are highlighted, pertaining to its stronger foundation on physical principles. In particular, the use of more prognostic equations in the model is shown to give a more realistic dynamical behaviour. This qualitative study is the first step towards a more detailed comparison, for which additional LES data are needed.

  • 25.
    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.
    Brandenburg, Axel
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Colorado, USA.
    Haugen, N. E. L.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden; National Center for Atmospheric Research, USA.
    Eulerian and Lagrangian approaches to multidimensional condensation and collection2017In: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 9, no 2, p. 1116-1137Article in journal (Refereed)
    Abstract [en]

    Turbulence is argued to play a crucial role in cloud droplet growth. The combined problem of turbulence and cloud droplet growth is numerically challenging. Here an Eulerian scheme based on the Smoluchowski equation is compared with two Lagrangian superparticle (or superdroplet) schemes in the presence of condensation and collection. The growth processes are studied either separately or in combination using either two-dimensional turbulence, a steady flow or just gravitational acceleration without gas flow. Good agreement between the different schemes for the time evolution of the size spectra is observed in the presence of gravity or turbulence. The Lagrangian superparticle schemes are found to be superior over the Eulerian one in terms of computational performance. However, it is shown that the use of interpolation schemes such as the cloud-in-cell algorithm is detrimental in connection with superparticle or superdroplet approaches. Furthermore, the use of symmetric over asymmetric collection schemes is shown to reduce the amount of scatter in the results. For the Eulerian scheme, gravitational collection is rather sensitive to the mass bin resolution, but not so in the case with turbulence. Plain Language Summary The bottleneck problem of cloud droplet growth is one of the most challenging problems in cloud physics. Cloud droplet growth is neither dominated by condensation nor gravitational collision in the size range of 15 mu m similar to 40 mu m [1]. Turbulence-generated collection has been thought to be the mechanism to bridge the size gap, i.e., the bottleneck problem. This study compares the Lagrangian and Eulerian schemes in detail to tackle with the turbulence-generated collection.

  • 26.
    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.
    Brandenburg, Axel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, 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
    Condensational and collisional growth of cloud droplets in a turbulent environmentIn: Article in journal (Refereed)
    Abstract [en]

    The effect of turbulence on combined condensational and collisional growth of cloud droplets is investigated using high-resolution direct numerical simulations. The motion of droplets is subjected to both turbulence and gravity. We solve the thermodynamic equations that govern the supersaturation field together with the hydrodynamic equations describing the turbulence. The collision-coalescence process is approximated by a superparticle approach assuming unit collision and coalescence efficiency, i.e., droplet coalesce upon collision. Condensational growth of cloud droplets due to supersaturation fluctuations depends on the Reynolds number, while the collisional growth was previously found to depend on the mean energy dissipation rate. Here we show that the combined processes depend on both Reynolds number and the mean energy dissipation rate. Droplet size distributions broaden either with increasing Reynolds number or mean energy dissipation rate in the range explored here. Even though collisional growth alone is insensitive to Reynolds number, it is indirectly affected by the large scales of turbulence through condensation. This is argued to be due to the fact that condensational growth results in wider droplet-size distributions, which triggers collisional growth. Since turbulence in warm clouds has a relatively small mean energy dissipation rate, but a large Reynolds number, turbulence mainly affects the condensational growth and thus influences the collisional growth indirectly through condensation. Thus, the combined condensational and collisional growth of cloud droplets is mostly dominated by Reynolds number. This work, for the first time, numerically demonstrates that supersaturation fluctuations enhance the collisional growth. It supports the findings from laboratory experiments and the observations that supersaturation fluctuations are important for precipitation.

  • 27.
    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.
    Brandenburg, Axel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden; Global & Climate Dynamics, National Center for Atmospheric Research, USA.
    Haugen, Nils E. L.
    Mehlig, Bernhard
    Rogachevskii, Igor
    Effect of turbulence on collisional growth of cloud droplets2018In: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 75, p. 3469-3487Article in journal (Refereed)
    Abstract [en]

    We investigate the effect of turbulence on the collisional growth of um-sized droplets through high- resolution numerical simulations with well resolved Kolmogorov scales, assuming a collision and coalescence efficiency of unity. The droplet dynamics and collisions are approximated using a superparticle approach. In the absence of gravity, we show that the time evolution of the shape of the droplet-size distribution due to turbulence-induced collisions depends strongly on the turbulent energy-dissipation rate, but only weakly on the Reynolds number. This can be explained through the energy dissipation rate dependence of the mean collision rate described by the Saffman-Turner collision model. Consistent with the Saffman-Turner collision model and its extensions, the collision rate increases as the square root of the energy dissipation rate even when coalescence is invoked. The size distribution exhibits power law behavior with a slope of -3.7 between a maximum at approximately 10 um up to about 40 um. When gravity is invoked, turbulence is found to dominate the time evolution of an initially monodisperse droplet distribution at early times. At later times, however, gravity takes over and dominates the collisional growth. We find that the formation of large droplets is very sensitive to the turbulent energy dissipation rate. This is due to the fact that turbulence enhances the collisional growth between similar sized droplets at the early stage of raindrop formation. The mean collision rate grows exponentially, which is consistent with the theoretical prediction of the continuous collisional growth even when turbulence-generated collisions are invoked. This consistency only reflects the mean effect of turbulence on collisional growth.

  • 28.
    Li, Xiang-Yu
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Mehlig, Bernhard
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brandenburg, Axel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Haugen, Nils
    Fluctuations and growth histories of cloud droplets: super-particle simulations of the collision-coalescence processManuscript (preprint) (Other academic)
    Abstract [en]

    Direct numerical simulations of collisional aggregation in turbulent aerosols are computationally demanding. Many authors therefore use an approximate model of the collision-coalescence process that is computationally more efficient: it relies on representing physical particles in terms of ‘superparticles’. One monitors collisions between superparticles and accounts for collisions between physical particles using a Monte-Carlo algorithm. It has been shown that this algorithm can faithfully represent mean particle growth in turbulent aerosols. Here we investigate how fluctuations are represented in this algorithm. We study particles of different sizes settling under gravity, assuming that the effect of turbulence is simply to mix the particles horizontally. We compute the statistics of growth histories and analyze their fluctuations in terms of the ‘lucky-droplet’ model. We discuss under which circumstances artefacts change the fluctuations of the growth histories, how these can be avoided, and which questions remain to be answered when turbulent fluctuations are explicitly incorporated.

  • 29.
    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
    Cloud-droplet growth due to supersaturation fluctuations in stratiform cloudsIn: Article 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 with droplets being modeled as Lagrangian particles. 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, we numerically confirm that the time evolution of the size distribution depends strongly on the Reynolds number and only weakly on the mean energy dissipation rate. This is shown to be due to the fact that temperature fluctuations and water vapor mixing ratio fluctuations increases 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 updraft velocity is almost zero.

  • 30.
    Lindvall, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    The diurnal temperature range in the CMIP5 models2015In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 44, no 1-2, p. 405-421Article in journal (Refereed)
    Abstract [en]

    This paper analyzes the diurnal temperature range (DTR) over land in simulations of the recent past and in future projections by 20 models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5). The annually averaged DTR is evaluated for the present-day climate using two gridded datasets (HadGHCND and CRU). The DTR varies substantially between different CMIP5 models, particularly in the subtropics, and is generally underestimated. In future projections of the high emission scenario RCP8.5, the models disagree on both the sign and the magnitude of the change in DTR. Still, a majority of the models project a globally averaged reduction in the DTR, with an increase over Europe, a decrease over the Sahara desert and a substantial decrease in DTR at high latitudes in winter. The general DTR reduction is partly linked to the enhancement of the downwelling clear sky longwave radiation due to greenhouse gases. At high latitudes in winter, the decrease in DTR seems to be enforced by an increase in cloudiness, but in most other regions counteracted by decreases in cloud fraction. Changes in the hydrological cycle and in the clear sky shortwave radiation also impact the DTR. The DTR integrates many processes and neither the model differences in the DTR nor in the change in DTR can be attributed to a single parameter. Which variables that impact the model discrepancies vary both regionally and seasonally. However, clouds seem to matter in most regions and seasons and the evaporative fraction is important in summer.

  • 31.
    Lindvall, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    The impact of changes in parameterizations of surface drag and vertical diffusion on the large- scale circulation in the Community Atmosphere Model (CAM5)Manuscript (preprint) (Other academic)
  • 32.
    Lindvall, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    The impact of changes in parameterizations of surface drag and vertical diffusion on the large-scale circulation in the Community Atmosphere Model (CAM5)2017In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 48, no 11, p. 3741-3758Article in journal (Refereed)
    Abstract [en]

    Simulations with the Community Atmosphere Model version 5 (CAM5) are used to analyze the sensitivity of the large-scale circulation to changes in parameterizations of orographic surface drag and vertical diffusion. Many GCMs and NWP models use enhanced turbulent mixing in stable conditions to improve simulations, while CAM5 cuts off all turbulence at high stabilities and instead employs a strong orographic surface stress parameterization, known as turbulent mountain stress (TMS). TMS completely dominates the surface stress over land and reduces the near-surface wind speeds compared to simulations without TMS. It is found that TMS is generally beneficial for the large-scale circulation as it improves zonal wind speeds, Arctic sea level pressure and zonal anomalies of the 500-hPa stream function, compared to ERA-Interim. It also alleviates atmospheric blocking frequency biases in the Northern Hemisphere. Using a scheme that instead allows for a modest increase of turbulent diffusion at higher stabilities only in the planetary boundary layer (PBL) appears to in some aspects have a similar, although much smaller, beneficial effect as TMS. Enhanced mixing throughout the atmospheric column, however, degrades the CAM5 simulation. Evaluating the simulations in comparison with detailed measurements at two locations reveals that TMS is detrimental for the PBL at the flat grassland ARM Southern Great Plains site, giving too strong wind turning and too deep PBLs. At the Sodankyla forest site, the effect of TMS is smaller due to the larger local vegetation roughness. At both sites, all simulations substantially overestimate the boundary layer ageostrophic flow.

  • 33.
    Lindvall, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hannay, Cecile
    Evaluation of near surface parameters in the two versions of the atmospheric model in cesm1 using flux station observations2013In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 1, p. 26-44Article in journal (Refereed)
    Abstract [en]

    This paper describes the performance of the Community Atmosphere Model (CAM) versions 4 and 5 in simulating near-surface parameters. CAM is the atmospheric component of the Community Earth System Model (CESM). Most of the parameterizations in the two versions are substantially different, and that is also true for the boundary layer scheme: CAM4 employs a nonlocal K-profile scheme, whereas CAM5 uses a turbulent kinetic energy (TKE) scheme. The evaluation focuses on the diurnal cycle and global observational and reanalysis datasets are used together with multiyear observations from 35 flux tower sites, providing high-frequency measurements in a range of different climate zones. It is found that both model versions capture the timing of the diurnal cycle but considerably overestimate the diurnal amplitude of net radiation, temperature, wind, and turbulent heat fluxes. The seasonal temperature range at mid-and high latitudes is also overestimated with too warm summer temperatures and too cold winter temperatures. The diagnosed boundary layer is deeper in CAM5 over ocean in regions with low-level marine clouds as a result of the turbulence generated by cloud-top cooling. Elsewhere, the boundary layer is in general shallower in CAM5. The two model versions differ substantially in their representation of near-surface wind speeds over land. The low-level wind speed in CAM5 is about half as strong as in CAM4, and the difference is even larger in areas where the subgrid-scale terrain is significant. The reason is the turbulent mountain stress parameterization, only applied in CAM5, which acts to increase the surface stress and thereby reduce the wind speed.

  • 34.
    Lundén, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    The inter-annual variability of atmospheric DMS in the high Arctic summerManuscript (preprint) (Other academic)
    Abstract [en]

    Meteorological aspects on the DMS(g) dispersion in the high Arctic atmosphere during the summer season are studied with a regional model that is applied over the pan-Arctic region. Four individual years, 1991, 1996, 2001 and 2007 with variable seasonal ice-melt are studied. The model results show that the Kara, Barents and Greenland Seas are the dominating source regions for DMS(g) found around the North-Pole region in three of the four years. The situation is quite different in 2007 where the concentration mainly originates from the East Siberian Sea. This due to the meteorological conditions as well as the extensive ice-melt in August which increase the source region as well as reduce the distance to the North-Pole. The modeled temporal variability is large in near-surface DMS(g) concentration over the pack-ice region, where the highest concentrations are found in the Atlantic sector. The prescribed seasonal decline is overshadowed by the increase in DMS(g) concentration due to decreased ice extent. Our results show that DMS(g) concentrations are reaching the North-Pole region in well defined plumes originating from different source regions. In 2007 the plumes are more frequent and have high concentration compared to 1996 where only a few plumes reach the area early in the season. The horizontal and vertical distribution of DMS(g) over the pack-ice is determined by the transport associated to individual low-pressure systems entering the pack-ice region. This variability both in time and space illustrates the difficulty to generalize observations taken at specific locations and times over larger areas and time periods.

     

  • 35.
    Lundén, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Influence of meteorological processes on the spatial and temporal variability of atmospheric dimethyl sulfide in the high Arctic summer2007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no D13308Article in journal (Refereed)
    Abstract [en]

    Near-surface observations of gas phase dimethyl sulfide, DMS(g), over the central Arctic Ocean display large temporal variability. By using a three-dimensional numerical model, the atmospheric part of COAMPS2.0®, we show that meteorological processes such as transport and mixing cause variability in DMS(g) of the same order as in the observations. The observations used in this study were taken on board the icebreaker Oden that cruised the high Arctic during the following three expeditions: the International Arctic Ocean Expedition 1991, the Arctic Ocean Expedition 1996, and the Arctic Ocean Experiment 2001. Calculation of air-sea flux and photochemical decay of DMS(g) was added to COAMPS2.0®. A 10-day period in August 2001 was modeled. The time development of observed DMS(g) is captured by the model, correlation coefficient 0.76, in spite of a simplified treatment of DMS processes. Also, the model results clearly show that DMS(g) is advected over the pack ice in plumes originating from different source areas around the pack ice.

  • 36.
    Lundén, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wisthaler, Armin
    Institut fur Ionenphysik and Angewandte Physik, Universität Innsbruck.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hansel, Armin
    Institut fur Ionenphysik and Angewandte Physik, Universität Innsbruck.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    The vertical distribution of atmospheric DMS in the high Arctic summer2010In: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 62, no 3, p. 160-171Article in journal (Refereed)
    Abstract [en]

    The vertical structure of gas-phase dimethyl sulfide, DMS(g), in the high Arctic atmosphere is investigated during a summer season. The model results show that the near-surface DMS(g) concentration over open ocean is very variable both in time and space, depending on the local atmospheric conditions. Profiles over ocean have typically highest concentration near the surface and decrease exponentially with height. Over the pack-ice, the concentrations are typically lower and the vertical structure changes as the air is advected northward. Modeled DMS(g) maxima above the local boundary layer were present in about 3\% of the profiles found over the pack-ice. These maxima were found in association to frontal zones. Our results also show that DMS(g) can be mixed downward by turbulence into the local boundary layer and act as a local near--surface DMS(g) source over the pack-ice and may hence influence the growth of cloud condensation nuclei and cloud formation in the boundary layer. Profile observations are presented in support to the model results. They show that significant DMS(g) concentrations exist in the Arctic atmosphere at altitudes not to be expected when only considering vertical mixing in the boundary layer.

     

  • 37.
    Mauritsen, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Observations of stably stratified shear-driven atmospheric turbulence at low and high Richardson numbers2007In: Journal of the Atmospheric Sciences, ISSN 0022-4928, Vol. 64, no 2, p. 645-655Article in journal (Refereed)
    Abstract [en]

    Stably stratified shear-driven turbulence is analyzed using the gradient Richardson number, Ri, as the stability parameter. The method overcomes the statistical problems associated with the widely used Monin–Obukhov stability parameter. The results of the Ri-based scaling confirm the presence of three regimes: the weakly and the very stable regimes and the transition in between them. In the weakly stable regime, fluxes scale in proportion with variance, while in the very stable regime, stress and scalar fluxes behave differently. At large Ri, the velocity field becomes highly anisotropic and the turbulent potential energy becomes approximately equal to half of the turbulent kinetic energy. It appears that even in the strongly stable regime, beyond what is known as the critical gradient Richardson number, turbulent motions are present.

  • 38.
    Mauritsen, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Grisogono, Branko
    Department of Geophysics, Faculty of Science, Zagreb, Croatia.
    Wave flow simulations over Arctic leads2005In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 117, no 2, p. 259-273Article in journal (Refereed)
    Abstract [en]

    We investigate the flow over Arctic leads using a mesoscale numerical model, typical of both summer and winter, under idealised conditions. We find that Arctic leads may be the source of standing atmospheric internal gravity waves during both seasons. The summertime wave may be compared with the wave generated by a small ridge, though with the phase reversed. The mechanism for exciting the wave is found to be the internal boundary layer developing due to horizontal variations in surface temperature and roughness length. During the more exploratory wintertime simulations, with substantial temperature difference between the lead and the ice surface, we find that secondary circulations and intermittent wave-breaking may occur. The effects of the lead appear far downstream.

  • 39.
    Mauritsen, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zilitinkevich, Sergej S.
    University of Helsinki. Division of Atmospheric Sciences, Department of Physical Sciences.
    Esau, Igor
    Nansen Environmental and Remote Sensing Center, Bergen.
    Enger, Leif
    Uppsala University. Department of Earth Sciences, Meteorology.
    Grisogono, Branko
    Department of Geophysics, Faculty of Science, University of Zagreb, Zagreb, Croatia.
    A total turbulent energy closure model for neutrally and stably stratified atmospheric boundary layers2007In: Journal of the Atmospheric Sciences, ISSN 0022-4928, Vol. 64, no 11, p. 4113-4136Article in journal (Refereed)
    Abstract [en]

    This paper presents a turbulence closure for neutral and stratified atmospheric conditions. The closure is based on the concept of the total turbulent energy. The total turbulent energy is the sum of the turbulent kinetic energy and turbulent potential energy, which is proportional to the potential temperature variance. The closure uses recent observational findings to take into account the mean flow stability. These observations indicate that turbulent transfer of heat and momentum behaves differently under very stable stratification. Whereas the turbulent heat flux tends toward zero beyond a certain stability limit, the turbulent stress stays finite. The suggested scheme avoids the problem of self-correlation. The latter is an improvement over the widely used Monin–Obukhov-based closures. Numerous large-eddy simulations, including a wide range of neutral and stably stratified cases, are used to estimate likely values of two free constants. In a benchmark case the new turbulence closure performs indistinguishably from independent large-eddy simulations.

  • 40.
    Mortin, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, Rune G.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Evaluation of pan-Arctic melt-freeze onset in CMIP5 climate models and reanalyses using surface observations2014In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 42, no 7-8, p. 2239-2257Article in journal (Refereed)
    Abstract [en]

    The seasonal melt-freeze transitions are fun- damental features of the Arctic climate system. The representation of the pan-Arctic melt and freeze onset (north of 60°N) is assessed in two reanalyses and eleven CMIP5 global circulation models (GCMs). The seasonal melt-freeze transitions are retrieved from surface air temperature (SAT) across the land and sea-ice domains and evaluated against surface observations. While monthly averages of SAT are reasonably well represented in models, large model-observation and model–model disparities of timing of melt and freeze onset are evident. The evaluation against surface observations reveals that the ERA-Interim reanalysis performs the best, closely followed by some of the climate models. GCMs and reanalyses capture the seasonal melt-freeze transitions better in the central Arctic than in the marginal seas and across the land areas. The GCMs project that during the 21st century, the summer length—the period between melt and freeze onset—will increase over land by about 1 month at all latitudes, and over sea ice by 1 and 3 months at low and high latitudes, respectively. This larger summer-length increase over sea ice at pro- gressively higher latitudes is related to a retreat of summer sea ice during the 21st century, since open water freezes roughly 40 days later than ice-covered ocean. As a consequence, by the year 2100, the freeze onset is projected to be initiated within roughly 10 days across the whole Arctic Ocean, whereas this transition varies by about 80 days today.

  • 41.
    Mortin, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Howell, Stephen E. L.
    Climate Research Division, Environment Canada.
    Derksen, Chris
    Climate Research Division, Environment Canada.
    Wang, Libo
    Climate Research Division, Environment Canada.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, Rune G.
    Stockholm University, Faculty of Science, Department of Meteorology .
    OSCAT as a successor to QuikSCAT: a comparison over Arctic sea ice with emphasis on the seasonal melt-freeze transitionsIn: Annals of Glaciology, ISSN 0260-3055, E-ISSN 1727-5644Article in journal (Refereed)
    Abstract [en]

    It is important to continuously monitor the seasonal melt-freeze transitions because of their influence on the Arctic climate system. The Ku-band scatterometer QuikSCAT was widely used to retrieve the seasonal transitions before its antenna failed in November 2009. In this study, we show that OSCAT, a Ku-band scatterometer launched in September 2009, can serve as a successor to QuikSCAT over Arctic sea ice. This is done by (1) comparing backscatter measurements acquired in overlapping time periods, and by (2) retrieving the seasonal melt-freeze transitions over Arctic sea ice from resolution-enhanced QuikSCAT and OSCAT data and comparing the results, also with other datasets. The main difference between the instruments, in terms of backscatter measurements, is the incidence angle in which backscatter is acquired, which yields backscatter discrepancies of 1-2 dB. This discrepancy is significantly smaller than the response of both sensors to surface melting and freezing processes. An edge-detection algorithm is employed that retrieves seasonal transitions from QuikSCAT (1999-2009) and OSCAT (2011-2013) at a 4.45-km spatial resolution. A comparison with transitions retrieved from the C-band scatterometer ASCAT (2009-2013) and from passive microwave radiometers (1999-2013) indicates that the transitions from scatterometers are largely consistent across all regions in the Arctic sea-ice domain.

  • 42.
    Mortin, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Howell, Stephen E. L.
    Climate Research Division, Environment Canada.
    Wang, Libo
    Climate Research Division, Environment Canada.
    Derksen, Chris
    Climate Research Division, Environment Canada.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, Rune G.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Schrøder, Thomas M.
    California Institute of Technology, USA.
    Extending the QuikSCAT record of seasonal melt–freeze transitions over Arctic sea ice using ASCAT2014In: Remote Sensing of Environment, ISSN 0034-4257, E-ISSN 1879-0704, Vol. 141, no 5, p. 214-230Article in journal (Refereed)
    Abstract [en]

    The seasonal melt–freeze transitions are important to continuously monitor over Arctic sea ice in order to better understand Arctic climate variability. The Ku-band scatterometer QuikSCAT (13.4 GHz), widely used to retrieve pan-Arctic seasonal transitions, discontinued its decadal long record in 2009. In this study, we show that the C-band scatterometer ASCAT (5.3 GHz), in orbit since 2006 and with an anticipated lifetime through 2021, can be used to extend the QuikSCAT record of seasonal melt–freeze transitions. This is done by (1) comparing back- scatter measurements over multiyear and first-year ice, and by (2) retrieving seasonal transitions from resolution-enhanced ASCAT and QuikSCAT measurements and comparing the results with independent datasets. Despite operating in different frequencies, ASCAT and QuikSCAT respond similarly to surface transitions. However, QuikSCAT measurements respond slightly stronger to the early melt of first-year ice, making it less sensitive to sea-ice dynamics. To retrieve the transitions, we employed an improved edge-detector algorithm, which was iterated and constrained using sea-ice concentration data, efficiently alleviating unreasonable outliers. This gives melt–freeze transitions over all Arctic sea ice north of 60°N at a 4.45 km resolution during 1999–2009 and 2009–2012 for QuikSCAT and ASCAT, respectively. Using the sensor overlap period, we show that the retrieved transitions retrieved from the different instruments are largely consistent across all regions in the Arctic sea-ice domain, indicating a robust consistency.

  • 43.
    Mortin, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, Rune G.
    Kapsch, Marie-Luise
    Stroeve, Julienne C.
    Boisvert, Linette N.
    Melt onset over Arctic sea ice controlled by atmospheric moisture transport2016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 12, p. 6636-6642Article in journal (Refereed)
    Abstract [en]

    The timing of melt onset affects the surface energy uptake throughout the melt season. Yet the processes triggering melt and causing its large interannual variability are not well understood. Here we show that melt onset over Arctic sea ice is initiated by positive anomalies of water vapor, clouds, and air temperatures that increase the downwelling longwave radiation (LWD) to the surface. The earlier melt onset occurs; the stronger are these anomalies. Downwelling shortwave radiation (SWD) is smaller than usual at melt onset, indicating that melt is not triggered by SWD. When melt occurs early, an anomalously opaque atmosphere with positive LWD anomalies preconditions the surface for weeks preceding melt. In contrast, when melt begins late, clearer than usual conditions are evident prior to melt. Hence, atmospheric processes are imperative for melt onset. It is also found that spring LWD increased during recent decades, consistent with trends toward an earlier melt onset.

  • 44.
    Pausata, Francesco S. R.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lindvall, Jenny
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Climate effects of a hypothetical regional nuclear war: Sensitivity to emission duration and particle composition2016In: Earth's Future, ISSN 1384-5160, E-ISSN 2328-4277, Vol. 4, no 11, p. 498-511Article in journal (Refereed)
    Abstract [en]

    Here, we use a coupled atmospheric-ocean-aerosol model to investigate the plume development and climate effects of the smoke generated by fires following a regional nuclear war between emerging third-world nuclear powers. We simulate a standard scenario where 5 Tg of black carbon (BC) is emitted over 1 day in the upper troposphere-lower stratosphere. However, it is likely that the emissions from the fires ignited by bomb detonations include a substantial amount of particulate organic matter (POM) and that they last more than 1 day. We therefore test the sensitivity of the aerosol plume and climate system to the BC/POM ratio (1:3, 1:9) and to the emission length (1 day, 1 week, 1 month). We find that in general, an emission length of 1 month substantially reduces the cooling compared to the 1-day case, whereas taking into account POM emissions notably increases the cooling and the reduction of precipitation associated with the nuclear war during the first year following the detonation. Accounting for POM emissions increases the particle size in the short-emission-length scenarios (1 day/1week), reducing the residence time of the injected particle. While the initial cooling is more intense when including POM emission, the long-lasting effects, while still large, may be less extreme compared to the BC-only case. Our study highlights that the emission altitude reached by the plume is sensitive to both the particle type emitted by the fires and the emission duration. Consequently, the climate effects of a nuclear war are strongly dependent on these parameters.

  • 45. Pithan, Felix
    et al.
    Ackerman, Andrew
    Angevine, Wayne M.
    Hartung, Kerstin
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ickes, Luisa
    Kelley, Maxwell
    Medeiros, Brian
    Sandu, Irina
    Steeneveld, Gert-Jan
    Sterk, H. A. M.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Vaillancourt, Paul A.
    Zadra, Ayrton
    Select strengths and biases of models in representing the Arctic winter boundary layer over sea ice: the Larcform 1 single column model intercomparison2016In: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 8, no 3, p. 1345-1357Article in journal (Refereed)
    Abstract [en]

    Weather and climate models struggle to represent lower tropospheric temperature and moisture profiles and surface fluxes in Arctic winter, partly because they lack or misrepresent physical processes that are specific to high latitudes. Observations have revealed two preferred states of the Arctic winter boundary layer. In the cloudy state, cloud liquid water limits surface radiative cooling, and temperature inversions are weak and elevated. In the radiatively clear state, strong surface radiative cooling leads to the build-up of surface-based temperature inversions. Many large-scale models lack the cloudy state, and some substantially underestimate inversion strength in the clear state. Here, the transformation from a moist to a cold dry air mass is modeled using an idealized Lagrangian perspective. The trajectory includes both boundary layer states, and the single-column experiment is the first Lagrangian Arctic air formation experiment (Larcform 1) organized within GEWEX GASS (Global atmospheric system studies). The intercomparison reproduces the typical biases of large-scale models: some models lack the cloudy state of the boundary layer due to the representation of mixed-phase microphysics or to the interaction between micro- and macrophysics. In some models, high emissivities of ice clouds or the lack of an insulating snow layer prevent the build-up of surface-based inversions in the radiatively clear state. Models substantially disagree on the amount of cloud liquid water in the cloudy state and on turbulent heat fluxes under clear skies. Observations of air mass transformations including both boundary layer states would allow for a tighter constraint of model behavior.

  • 46.
    Ranjha, Raza
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Semedo, Alvaro
    Global distribution and seasonal variability of coastal low-level jets derived from ERA-Interim reanalysis2013In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 65, p. 20412-Article in journal (Refereed)
    Abstract [en]

    A low-level wind maximum is often found over the oceans near many coasts around the world. These Coastal Low-Level Jets (CLLJs) play an important role in the coastal weather and have significant impacts on regional climate and ecology as well as on a number of human activities. The presence of CLLJs is related to various local circumstances such as land-sea temperature contrasts, upwelling, coastal terrain, orientation of the coast, etc., but also to the large-scale atmospheric dynamics. This makes studies of CLLJs not only interesting but also challenging.

    In the present study, based on ERA-Interim reanalysis data, the global distribution, spatio-temporal structure, and the seasonal variability of CLLJs are documented. Seasonal data from 1980 to 2011 are used to identify areas where CLLJs are frequently found in the lowest 2 km, following criteria based on the vertical profiles of wind speed and temperature. The results are analyzed to highlight the fundamental aspects and distinctive features of the CLLJs across the globe, including their occurrence rate, jet height, wind-speed maximum and horizontal extent.

    Global maps of CLLJs are constructed for the summer and winter seasons. The west coasts of North America, the Iberian Peninsula, north-western Africa and the south-eastern coast of the Arabian Peninsula make up the northern-hemispheric CLLJ regions, while the west coasts of South America, Australia and southern Africa comprise the south-hemispheric equivalents. The existence and characteristics of CLLJs along the southern coast of Oman and the western coast of the Iberian Peninsula regions are also discussed, not fully envisaged before in the context of CLLJs. The highest occurrence of CLLJs is found during the summer in both hemispheres, and the coast of Oman has the globally highest CLLJ frequency, with also the highest maximum wind speeds. The most commonly found CLLJ has a maximum wind speed between 9 and 15 m s-1, and occurs at heights between 500 and 700 m a.s.l.

  • 47.
    Ranjha, Raza
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Lund University, Sweden.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Semedo, Alvaro
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Cardoso, Rita M.
    Structure and Variability of the Oman Coastal Low-Level Jet2015In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 67, article id 25285Article in journal (Refereed)
    Abstract [en]

    During the boreal summer strong southwesterly, coast parallel low-level winds prevail off the coast of Oman and over the Arabian Sea. The offshore large-scale structure and variability of these coastal winds has been a topic of numerous studies because of their strong connection to the south-Asian summer monsoon. However, low-level coastal jets along the coast of Oman have not been studied in detail, especially on a mesoscale level, despite their globally high frequency of occurrence. In the current study, regional atmospheric modeling has been utilized in an attempt to resolve the mesoscale structure, spatial variability and temporal characteristics of the Oman coastal low-level jet (CLLJ). The limited area model COAMPS® has been used at 6-km resolution for a five month period from May through September, during 2009. The model output compares favorably with the seasonal climatology for the coast of Oman.

    In this study, analysis of high resolution model fields reveals the mesoscale structure of the Oman CLLJ, clearly distinguishing it from the large-scale southwesterly monsoon circulation farther offshore over the Arabian Sea. The Oman CLLJ is closer to the coast and spread northeastwards along the coast of Oman. Although the synoptic forcing that preconditions the Oman CLLJ is different from other areas of CLLJ occurrences, nevertheless, the jet is still driven primarily by the coastal baroclinicity due to sharp land-sea thermal contrasts. Within the study period, July exhibits the highest CLLJ frequency with ~80% occurrences and also manifests highest winds around 27 m s-1.

  • 48.
    Ranjha, Raza
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Semedo, Alvaro
    Modelling coastal low-level wind-jets: does horizontal resolution matter?2016In: Meteorology and atmospheric physics (Print), ISSN 0177-7971, E-ISSN 1436-5065, Vol. 128, no 2, p. 263-278Article in journal (Refereed)
    Abstract [en]

    Atmospheric flows in coastal regions are impacted by land-sea temperature contrasts, complex terrain, shape of the coastline, among many things. Along the west coast of central North America, winds in the boundary layer are mainly from north or northwest, roughly parallel to the coastline. Frequently, the coastal low-level wind field is characterized by a sharp wind maximum along the coast in the lowest kilometer. This feature, commonly referred to as a Coastal Low-Level Jet (CLLJ), has significant impact on the climatology of the coastal region, influencing aviation, shipping, fishing and a number of other human activities along the littoral zone, and better understanding and forecasting of this is vital.

    The purpose of this study is to elucidate the issue of proper resolution in a mesoscale numerical model to describe the physics of a CLLJ, and its impact on the upper ocean. The COAMPS® model is used for a summer event to determine the realism of the model results compared to observations, from an area of supercritical flow adjustment between Pt. Sur and Pt. Conception, California. Simulations at different model horizontal resolutions, from 54 km to 2 km are performed. While the model produces realistic results with increasing details at higher resolution, the results do not converge even at a resolution of only few kilometers and an objective analysis of model errors do not show an increased skill with increasing resolution. New methods may thus have to be developed to evaluate models at very high resolution. Based on all available information, a compromise resolution appears to be at least 6 km.

  • 49.
    Savre, Julien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Technical note: Introduction to MIMICA, a large-eddy simulation solver for cloudy planetary boundary layers2014In: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 6, no 3, p. 630-649Article in journal (Refereed)
    Abstract [en]

    In large-eddy simulation (LES), large-scale turbulent structures are explicitly resolved on the numerical grid while the dissipative turbulent eddies, typically smaller than the grid size, must be modeled. Because in the atmospheric boundary layer a large disparity of turbulent scales exists (about 9 orders of magnitude separate the largest and smallest scales), LES is considered as an essential modeling approach to capture the physics and dynamics of boundary layer clouds. A new LES solver developed at Stockholm University is presented here for the first time. The model solves for nonhydrostatic anelastic equations using high-order low-dissipative numerical schemes for the advection of scalars and momentum. A two-moment bulk microphysics scheme is implemented representing five types of hydrometeors including ice crystals and snow. The LES is evaluated based on simulations of two well-documented stratiform cloud events that were previously used for LES intercomparisons. In the first one, a marine drizzling stratocumulus observed during DYCOMS-II, the model is shown to predict bulk cloud microphysical and dynamical properties within the range of the intercomparison model results. In the second case, based on a monolayer Arctic mixed-phase cloud observed during ISDAC, we found that when using fast-falling crystals, ice quickly precipitates out of the cloud without significant growth, resulting in very low ice water paths. The simulated clouds are also found to be very sensitive to the prescribed ice crystal number concentration: multiplying the ice concentration by a factor 2.5 results in rapid cloud dissipation in the most extreme case. Overall, these results are found to be consistent with former studies of Arctic mixed-phase clouds as well as in situ measurements. More specifically, when the ice number concentration and parameterized ice habit are constrained by measurements, simulated microphysical properties such as the ice water path and ice crystal size distribution are found to agree well with observations.

  • 50.
    Savre, Julien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Large-eddy simulations of an Arctic mixed-phase stratiform cloud observed during ISDAC: sensitivity to moisture aloft, surface fluxes and large-scale forcing2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 689, p. 1177-1190Article in journal (Refereed)
    Abstract [en]

    Large-eddy simulation (LES) is used to examine the complex interactions between cloud properties and boundary-layer structure in Arctic low-level mixed-phase clouds using idealised conditions based on the Indirect and Semi-Direct Aerosol Campaign (ISDAC, April 2008). The persistence of steady mixed-phase conditions depends mostly on a balance between ice vertical redistribution and ice growth by vapour deposition in such a way that ice crystals cannot accumulate within the cloud layer to consume the available liquid water. An external source of water vapour is necessary to balance the net sink of total water in the cloud layer. Two main local sources of moisture are present: the initial moist surface layer and the free troposphere. In the studied case, the surface layer is found to be the dominant source of vapour to the cloud, the temperature inversion preventing significant entrainment from above. In most of the cases, the simulated boundary layer becomes rapidly well-mixed despite the stabilising effect of ice sublimation and latent cooling close to the surface. The minor effect of near-surface latent cooling on stability is connected to the initially moist surface layer limiting ice sublimation. Water vapour supply in the sub-cloud layer, resulting from entrainment of moisture from aloft, reduces ice sublimation above the surface layer and contributes to the maintenance of some degree of boundary-layer decoupling. In contrast, moisture surface fluxes reduce sublimation in the surface layer and accelerate cloud-surface coupling. Overall, the persistence of cloud-surface decoupling remains mostly driven by large-scale heat and moisture advection.

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