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  • 1. Achtert, P.
    et al.
    Brooks, I. M.
    Brooks, B. J.
    Moat, B. I.
    Prytherch, J.
    Persson, P. O. G.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Measurement of wind profiles by motion-stabilised ship-borne Doppler lidar2015In: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 8, no 11, 4993-5007 p.Article in journal (Refereed)
    Abstract [en]

    Three months of Doppler lidar wind measurements were obtained during the Arctic Cloud Summer Experiment on the icebreakerOden during the summer of 2014. Such ship-borne Doppler measurements require active stabilisation to remove the effects of ship motion. We demonstrate that the combination of a commercial Doppler lidar with a custom-made motion-stabilisation platform enables the retrieval of wind profiles in the Arctic atmospheric boundary layer during both cruising and ice-breaking with statistical uncertainties comparable to land-based measurements. This held true particularly within the atmospheric boundary layer even though the overall aerosol load was very low. Motion stabilisation was successful for high wind speeds in open water and the resulting wave conditions. It allows for the retrieval of vertical winds with a random error below 0.2 m s−1. The comparison of lidar-measured wind and radio soundings gives a mean bias of 0.3 m s−1 (2°) and a mean standard deviation of 1.1 m s−1 (12°) for wind speed (wind direction). The agreement for wind direction degrades with height. The combination of a motion-stabilised platform with a low-maintenance autonomous Doppler lidar has the potential to enable continuous long-term high-resolution ship-based wind profile measurements over the oceans.

  • 2.
    Achtert, Peggy
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lidar Measurements of Polar Stratospheric Clouds in the Arctic2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Polar Stratospheric Clouds (PSCs) play a key role for ozone depletion in the polar stratosphere. Its magnitude depends on the type of PSC and its lifetime and extent. This thesis presents PSC observations conducted with the Esrange lidar and the space-borne CALIPSO lidar.

    PSCs are separated into three types according to their optical properties. The occurrence rate of the different types which are often observed simultaneously as well as their interaction and connection is not well understood. To better understand the processes that govern PSC formation, observations need to be combined with a detailed view of the atmospheric background in which PSCs develop, exist, and are transformed from one type to another.

    This thesis introduces a new channel of the Esrange lidar for temperature profiling at heights below 35 km. The design of this channel and first temperature measurements within PSCs and cirrus clouds are presented. This is an important step since the majority of PSC-related literature extracts temperatures within PSCs from reanalysis data.

    In contrast to ground–based measurements space–borne lidar does not rely on cloud–free conditions. Hence, it provides an unprecedented opportunity for studying the connection between PSCs and the underlying synoptic–scale conditions which manifest as tropospheric clouds. This thesis shows that most of the PSCs observed in the Arctic during winter 2007/08 occurred in connection with tropospheric clouds.

    A combined analysis of ground-based and space-borne lidar observation of PSCs in combination with microphysical modeling can improve our understanding of PSC formation. A first case study of this approach shows how a PSC that was formed by synoptic-scale processes is transformed into another type while passing the Scandinavian mountains.

    Today a variety of classification schemes provides inconsistent information on PSC properties and types. This thesis suggests a unified classification scheme for lidar measurements of PSCs.

  • 3.
    Achtert, Peggy
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Birmili, W.
    Nowak, A.
    Wehner, B.
    Wiedensohler, A.
    Takegawa, N.
    Kondo, Y.
    Miyazaki, Y.
    Hu, M.
    Zhu, T.
    Hygroscopic growth of tropospheric particle number size distributions over the North China Plain2009In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, D00G07- p.Article in journal (Refereed)
    Abstract [en]

    The hygroscopic growth of atmospheric submicrometer particle size distributions (diameter D-p ranging from 22 to 900 nm) was studied at a rural/suburban site in the North China Plain within the framework of the international Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing-2006) research project. The goal was to characterize the regional aerosol in the polluted northeastern plain in China. Size descriptive hygroscopic growth factors (DHGFs) were determined as a function of relative humidity (RH) by relating the particle number size distribution at a dry condition ( 100 nm), the DHGF are substantially higher than in the Aitken particle mode (D-p < 100 nm) as a result of different chemical composition. The size-dependent behavior of the DHGF highlights the relevance of particulate sulfate production over the North China Plain, accomplished by secondary formation from the gas phase and, potentially, liquid phase processes in convective clouds. Furthermore, all results concerning the DHGF show a significant dependency on meteorological air masses. The hygroscopic growth of accumulation mode particles correlates significantly with the PM1-mass fraction of sulfate ions determined by chemical analysis. Finally, this investigation provides a parameterization of the hygroscopic growth of 250-nm particles, which might be useful when predicting visibility and radiative forcing and performing atmospheric aerosol model validations.

  • 4.
    Achtert, Peggy
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson Andersson, M.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Khosrawi, Farahnaz
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the linkage between tropospheric and Polar Stratospheric clouds in the Arctic as observed by space-borne lidar2012In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, no 8, 3791-3798 p.Article in journal (Refereed)
    Abstract [en]

    The type of Polar stratospheric clouds (PSCs) as well as their temporal and spatial extent are important for the occurrence of heterogeneous reactions in the polar stratosphere. The formation of PSCs depends strongly on temperature. However, the mechanisms of the formation of solid PSCs are still poorly understood. Recent satellite studies of Antarctic PSCs have shown that their formation can be associated with deep-tropospheric clouds which have the ability to cool the lower stratosphere radiatively and/or adiabatically. In the present study, lidar measurements aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite were used to investigate whether the formation of Arctic PSCs can be associated with deep-tropospheric clouds as well. Deep-tropospheric cloud systems have a vertical extent of more than 6.5 km with a cloud top height above 7 km altitude. PSCs observed by CALIPSO during the Arctic winter 2007/2008 were classified according to their type (STS, NAT, or ice) and to the kind of underlying tropospheric clouds. Our analysis reveals that 172 out of 211 observed PSCs occurred in connection with tropospheric clouds. 72% of these 172 observed PSCs occurred above deep-tropospheric clouds. We also find that the type of PSC seems to be connected to the characteristics of the underlying tropospheric cloud system. During the Arctic winter 2007/2008 PSCs consisting of ice were mainly observed in connection with deep-tropospheric cloud systems while no ice PSC was detected above cirrus. Furthermore, we find no correlation between the occurrence of PSCs and the top temperature of tropospheric clouds. Thus, our findings suggest that Arctic PSC formation is connected to adiabatice cooling, i.e. dynamic effects rather than radiative cooling.

  • 5.
    Achtert, Peggy
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Khaplanov, Mikhail
    Stockholm University, Faculty of Science, Department of Meteorology .
    Khosrawi, Farahnaz
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology .
    Pure rotational-Raman channels of the Esrange lidar for temperature and particle extinction measurements in the troposphere and lower stratosphere2013In: Atmospheric Measurement Techniques, ISSN 1867-8548, Vol. 6, no 1, 91-98 p.Article in journal (Refereed)
    Abstract [en]

    The Department of Meteorology at Stockholm University operates the Esrange Rayleigh/Raman lidar at Esrange(68° N, 21° E) near the Swedish city of Kiruna. This paper describes the design and first measurements of the newpure rotational-Raman channel of the Esrange lidar. The Esrange lidar uses a pulsed Nd:YAG solid-state laser operating at 532 nm as light source with a repetition rate of 20 Hz and a pulse energy of 350 mJ. The minimum vertical resolution is 150m and the integration time for one profile is 5000 shots. The newly implemented channel allows for measurements of atmospheric temperature at altitudes below 35 km and is currently optimized for temperature measurements between 180 and 200 K. This corresponds to conditions in the lower Arctic stratosphere during winter. In addition to the temperature measurements, the aerosol extinction coefficientand the aerosol backscatter coefficient at 532 nm can be measured in dependently. Our filter-based design minimizes the systematic error in the obtained temperature profile to less than 0.51 K. By combining rotational-Raman measurements (5–35 km height) and the integration technique (30–80 kmheight), the Esrange lidar is now capable of measuring atmospheric temperature profiles from the upper troposphere up to the mesosphere. With the improved setup, the system can be used to validate current lidar-based polar stratospheric cloud classification schemes. The new capability of the instrument measuring temperature and aerosol extinction furthermore enables studies of the thermal structure and variability of the upper troposphere/lower stratosphere. Although several lidars are operated at polar latitudes, there are few instruments that are capable of measuring temperature profiles in the troposphere, stratosphere, and mesosphere, as well as aerosols extinction in the troposphere and lower stratospherewith daylight capability.

  • 6.
    Achtert, Peggy
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Khosrawi, Farahnaz
    Stockholm University, Faculty of Science, Department of Meteorology .
    Blum, U.
    Fricke, K. H.
    Investigation of polar stratospheric clouds in January 2008 by means of ground-based and spaceborne lidar measurements and microphysical box model simulations2011In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, D07201- p.Article in journal (Refereed)
    Abstract [en]

    Polar stratospheric clouds (PSCs) play a key role in heterogeneous chemistry and ozone depletion in the lower stratosphere. The type of PSC as well as their temporal and spatial extent are important for the occurrence of heterogeneous reactions and, thus, ozone depletion. In this study a combination of ground-based and spaceborne lidar measurements were used together with microphysical box model simulations along back trajectories to investigate the formation and alteration of Arctic PSCs. The measurements were made by the Rayleigh/Mie/Raman lidar system at Esrange and by the Cloud-Aerosol Lidar with Orthogonal Polarization aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite. Between 20 and 23 January 2008 PSCs composed of liquid particles were observed by CALIPSO between Greenland and the western side of the Scandinavian Mountains. Between 21 and 23 January 2008 the Esrange lidar observed a PSC composed of distinct layers of liquid and solid particles on the eastern side of the mountain range. Microphysical box model simulations along air parcel back trajectories indicate that liquid particles had formed at least 40 h before the observation at Esrange. Furthermore, the model indicates a high HNO(3) uptake into the liquid layer between 10 and 20 h before the observation. The PSC was formed when the air mass was over Greenland. On two occasions during these 20 h, CALIPSO observed PSCs when its measurement tracks crossed the air parcel back trajectory ending at the location of the Esrange lidar. Backscatter ratios calculated from the output of the box model simulation indicate good agreement with the values observed with the Esrange lidar and by CALIPSO. The box model simulations along the back trajectories from Esrange to the CALIPSO ground track and beyond provide us with the unique opportunity to relate ground-based and spaceborne lidar measurements that were not performed at the same spatial location and time. Furthermore, possible differences in the observations from ground and space can be traced to temporal and/or geographically induced changes in particle microphysics within the measured PSCs.

  • 7.
    Achtert, Peggy
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tesche, Matthias
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Assessing lidar-based classification schemes for polar stratospheric clouds based on 16 years of measurements at Esrange, Sweden2014In: Journal of Geophysical Research: Atmospheres, ISSN 2169-8996, Vol. 119, no 3, 1386-1405 p.Article in journal (Refereed)
    Abstract [en]

    Lidar measurements of polar stratospheric clouds (PSCs) are commonly analyzed in classification schemes that apply the backscatter ratio and the particle depolarization ratio. This similarity of input data suggests comparable results of different classification schemes—despite measurements being performed with a variety of mostly custom-made instruments. Based on a time series of 16 years of lidar measurements at Esrange (68°N, 21°E), Sweden, we show that PSC classification differs substantially depending on the applied scheme. The discrepancies result from varying threshold values of lidar-derived parameters used to define certain PSC types. The resulting inconsistencies could impact the understanding of long-term PSC observations documented in the literature. We identify two out of seven considered classification schemes that are most likely to give reliable results and should be used in future lidar-based studies. Using polarized backscatter ratios gives the advantage of increased contrast for observations of weakly backscattering and weakly depolarizing particles. Improved confidence in PSC classification can be achieved by a more comprehensive consideration of the effect of measurement uncertainties. The particle depolarization ratio is the key to a reliable identification of different PSC types. Hence, detailed information on the calibration of the polarization-sensitive measurement channels should be provided to assess the findings of a study. Presently, most PSC measurements with lidar are performed at 532 nm only. The information from additional polarization-sensitive measurements in the near infrared could lead to an improved PSC classification. Coincident lidar-based temperature measurements at PSC level might provide useful information for an assessment of PSC classification.

  • 8.
    Acosta Navarro, Juan C.
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lewinschal, Anna
    Stockholm University, Faculty of Science, Department of Meteorology .
    Varma, Vidya
    Seland, Øyvind
    Gauss, Michael
    Iversen, Trond
    Kirkevåg, Alf
    Riipinen, Ilona
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Hansson, Hans Christen
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Future response of temperature and precipitation to reduced aerosol emissions as compared with increased greenhouse gas concentrations2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 3, 939-954 p.Article in journal (Refereed)
    Abstract [en]

    Experiments with a climate model (NorESM1) were performed to isolate the effects of aerosol particles and greenhouse gases on surface temperature and precipitation in simulations of future climate. The simulations show that by 2025-2049, a reduction of aerosol emissions from fossil fuels following a maximum technically feasible reduction (MFR) scenario could lead to a global and Arctic warming of 0.26 K and 0.84 K, respectively; as compared with a simulation with fixed aerosol emissions at the level of 2005. If fossil fuel emissions of aerosols follow a current legislation emissions (CLE) scenario, the NorESM1 model simulations yield a non-significant change in global and Arctic average surface temperature as compared with aerosol emissions fixed at year 2005. The corresponding greenhouse gas effect following the RCP4.5 emission scenario leads to a global and Arctic warming of 0.35 K and 0.94 K, respectively.

    The model yields a marked annual average northward shift in the inter-tropical convergence zone with decreasing aerosol emissions and subsequent warming of the northern hemisphere. The shift is most pronounced in the MFR scenario but also visible in the CLE scenario. The modeled temperature response to a change in greenhouse gas concentrations is relatively symmetric between the hemispheres and there is no marked shift in the annual average position of the inter-tropical convergence zone. The strong reduction in aerosol emissions in MFR also leads to a net southward cross-hemispheric energy transport anomaly both in the atmosphere and ocean, and enhanced monsoon circulation in Southeast and East Asia causing an increase in precipitation over a large part of this region.

  • 9.
    Acosta Navarro, Juan Camilo
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Smolander, S.
    Struthers, H.
    Zorita, E.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kaplan, J. O.
    Guenther, A.
    Arneth, A.
    Riipinen, Ilona
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Global emissions of terpenoid VOCs from terrestrial vegetation in the last millennium2014In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 119, no 11, 6867-6885 p.Article in journal (Refereed)
    Abstract [en]

    We investigated the millennial variability (1000 A.D.-2000 A.D.) of global biogenic volatile organic compound (BVOC) emissions by using two independent numerical models: The Model of Emissions of Gases and Aerosols from Nature (MEGAN), for isoprene, monoterpene, and sesquiterpene, and Lund-Potsdam-Jena-General Ecosystem Simulator (LPJ-GUESS), for isoprene and monoterpenes. We found the millennial trends of global isoprene emissions to be mostly affected by land cover and atmospheric carbon dioxide changes, whereas monoterpene and sesquiterpene emission trends were dominated by temperature change. Isoprene emissions declined substantially in regions with large and rapid land cover change. In addition, isoprene emission sensitivity to drought proved to have significant short-term global effects. By the end of the past millennium MEGAN isoprene emissions were 634 TgC yr-1 (13% and 19% less than during 1750-1850 and 1000-1200, respectively), and LPJ-GUESS emissions were 323 TgC yr-1(15% and 20% less than during 1750-1850 and 1000-1200, respectively). Monoterpene emissions were 89 TgC yr-1(10% and 6% higher than during 1750-1850 and 1000-1200, respectively) in MEGAN, and 24 TgC yr-1 (2% higher and 5% less than during 1750-1850 and 1000-1200, respectively) in LPJ-GUESS. MEGAN sesquiterpene emissions were 36 TgC yr-1(10% and 4% higher than during 1750-1850 and 1000-1200, respectively). Although both models capture similar emission trends, the magnitude of the emissions are different. This highlights the importance of building better constraints on VOC emissions from terrestrial vegetation.

  • 10.
    Acosta Navarro, Juan Camilo
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Varma, Vidya
    Stockholm University, Faculty of Science, Department of Meteorology .
    Riipinen, Irina
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Seland, O.
    Kirkevag, A.
    Struthers, Hamish
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Linköping University, Sweden.
    Iversen, T.
    Hansson, Hans-Christen
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Amplification of Arctic warming by past air pollution reductions in Europe2016In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 9, no 4, 277-+ p.Article in journal (Refereed)
    Abstract [en]

    The Arctic region is warming considerably faster than the rest of the globe(1), with important consequences for the ecosystems(2) and human exploration of the region(3). However, the reasons behind this Arctic amplification are not entirely clear(4). As a result of measures to enhance air quality, anthropogenic emissions of particulate matter and its precursors have drastically decreased in parts of the Northern Hemisphere over the past three decades(5). Here we present simulations with an Earth system model with comprehensive aerosol physics and chemistry that show that the sulfate aerosol reductions in Europe since 1980 can potentially explain a significant fraction of Arctic warming over that period. Specifically, the Arctic region receives an additional 0.3Wm(-2) of energy, and warms by 0.5 degrees C on annual average in simulations with declining European sulfur emissions in line with historical observations, compared with a model simulation with fixed European emissions at 1980 levels. Arctic warming is amplified mainly in fall and winter, but the warming is initiated in summer by an increase in incoming solar radiation as well as an enhanced poleward oceanic and atmospheric heat transport. The simulated summertime energy surplus reduces sea-ice cover, which leads to a transfer of heat from the Arctic Ocean to the atmosphere. We conclude that air quality regulations in the Northern Hemisphere, the ocean and atmospheric circulation, and Arctic climate are inherently linked.

  • 11. Almroth-Rosell, Elin
    et al.
    Eilola, Kari
    Kuznetsov, Ivan
    Hall, Per O. J.
    Meier, H. E. Markus
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    A new approach to model oxygen dependent benthic phosphate fluxes in the Baltic Sea2015In: Journal of Marine Systems, ISSN 0924-7963, Vol. 144, 127-141 p.Article in journal (Refereed)
    Abstract [en]

    The new approach to model the oxygen dependent phosphate release by implementing formulations of the oxygen penetration depths (OPD) and mineral bound inorganic phosphorus pools to the Swedish Coastal and Ocean Biogeochemical model (SCOBI) is described. The phosphorus dynamics and the oxygen concentrations in the Baltic proper sediment are studied during the period 1980-2008 using SCOBI coupled to the 3D-Rossby Centre Ocean model. Model data are compared to observations from monitoring stations and experiments. The impact from oxygen consumption on the determination of the OPD is found to be largest in the coastal zones where also the largest OPD are found. In the deep water the low oxygen concentrations mainly determine the OPD. Highest modelled release rate of phosphate from the sediment is about 59 x 10(3) t P year(-1) and is found on anoxic sediment at depths between 60-150 m, corresponding to 17% of the Baltic proper total area. The deposition of organic and inorganic phosphorus on sediments with oxic bottom water is larger than the release of phosphorus, about 43 x 10(3) t P year(-1). For anoxic bottoms the release of total phosphorus during the investigated period is larger than the deposition, about 19 x 10(3) t P year(-1). In total the net Baltic proper sediment sink is about 23.7 x 10(3) t P year(-1). The estimated phosphorus sink efficiency of the entire Baltic Sea is on average about 83% during the period.

  • 12. Amyx, K.
    et al.
    Sternovsky, Z.
    Knappmiller, S.
    Robertson, S.
    Horanyi, M.
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology.
    In-situ measurement of smoke particles in the wintertime polar mesosphere between 80 and 85 km altitude2008In: Journal of Atmospheric and Solar-Terrestrial Physics, Vol. 70, 61-70 p.Article in journal (Refereed)
    Abstract [en]

    The MAGIC sounding rocket, launched in January 2005 into the polar mesosphere, carried two detectors for charged aerosol particles. The detectors are graphite patch collectors mounted flush with the skin of the payload and are connected to sensitive electrometers. The measured signal is the net current deposited on the detectors by heavy aerosol particles. The collection of electrons and ions is prevented by magnetic shielding and a small positive bias, respectively. Both instruments detected a layer of heavy aerosol particles between 80 and 85 km with a number density approximately 103 cm−3. Aerodynamic flow simulations imply that the collected particles are larger than 1 nm in radius. The particles are detected as a net positive charge deposited on the graphite collectors. It is suggested that the measured positive polarity is due to the electrification of the smoke particles upon impact on the graphite collectors.

  • 13. Anderson, Bruce T.
    et al.
    Hassanzadeh, Pedram
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Persistent anomalies of the extratropical Northern Hemisphere wintertime circulation as an initiator of El Nino/Southern Oscillation events2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, 10145Article in journal (Refereed)
    Abstract [en]

    Climates across both hemispheres are strongly influenced by tropical Pacific variability associated with the El Nino/Southern Oscillation (ENSO). Conversely, extratropical variability also can affect the tropics. In particular, seasonal-mean alterations of near-surface winds associated with the North Pacific Oscillation (NPO) serve as a significant extratropical forcing agent of ENSO. However, it is still unclear what dynamical processes give rise to year-to-year shifts in these long-lived NPO anomalies. Here we show that intraseasonal variability in boreal winter pressure patterns over the Central North Pacific (CNP) imparts a significant signature upon the seasonal-mean circulations characteristic of the NPO. Further we show that the seasonal-mean signature results in part from year-to-year variations in persistent, quasi-stationary low-pressure intrusions into the subtropics of the CNP, accompanied by the establishment of persistent, quasi-stationary high-pressure anomalies over high latitudes of the CNP. Overall, we find that the frequency of these persistent extratropical anomalies (PEAs) during a given winter serves as a key modulator of intraseasonal variability in extratropical North Pacific circulations and, through their influence on the seasonal-mean circulations in and around the southern lobe of the NPO, the state of the equatorial Pacific 9-12 months later.

  • 14.
    Andersson, Erik
    Stockholm University, Faculty of Science, Department of Meteorology .
    A Variational data assimilation scheme and its use of meteorological observations1999Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Meteorological data assimilation is the process by which a model simulation of the atmosphere is corrected by observations. In operational weather forecasting, this is done in near real-time, to provide accurate initial conditions for numerical forecasts. The accuracy of the forecasts depends crucially on the accuracy of the data assimilation.

    The increase in available computing power and the theoretical advances in data assimilation methods have recently enabled the European Centre for Medium-Range Weather Forecasts (ECMWF) to develop and introduce operationally a new data assimilation scheme (4D-Var), based on variational methods. The four papers of this thesis present recent developments that have exploited some of the most promising possibilities of 4D-Var with respect to observation usage. They have all been incorporated in operational practice at ECMWF.

    Variational data assimilation schemes have several advantages that make the new scheme more flexible than earlier schemes with respect to the use of observations. The introduction of 4D-Var has enabled the effective use of a wider range of meteorological observations than previously possible. This includes, as will be demonstrated in this thesis, both conventional observations and satellite measurements.

    In the first paper we develop a method for the use of satellite radiance observations, and investigate the aspects of 4D-Var that make the scheme particularly well suited for the assimilation of data from a growing variety of space based observing systems. In the second paper we develop a variational quality control method. The third paper investigates the flow-dependence of the 4D-Var response to time series of surface pressure observations in the vicinity of a rapidly developing storm. The final paper presents a diagnostic method for tuning of the 4D-Var system.

  • 15.
    Andersson, Mathias H
    et al.
    Stockholm University, Faculty of Science, Department of Zoology, Department of Animal Ecology.
    Dock-Åkerman, Emily
    Stockholm University, Faculty of Science, Department of Zoology, Department of Animal Ecology.
    Ubral-Hedenberg, Ramona
    Stockholm University, Faculty of Science, Department of Zoology, Department of Animal Ecology.
    Öhman, Marcus C
    Stockholm University, Faculty of Science, Department of Zoology, Department of Animal Ecology.
    Sigray, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Swimming Behavior of Roach (Rutilus rutilus) and Three-spined Stickleback Gasterosteus aculeatus) in Response to Wind Power Noise and Single tone frequencies2007In: Ambio, ISSN 0044-7447, Vol. 36, no 8, 636-638 p.Article in journal (Refereed)
    Abstract [en]

    Human-induced underwater noise is drastically increasing as the result of offshore installations and human activities in the marine environment. Many of these structures and activities produce low-frequency noise that could potentially disturb or have harmful effects on several species of teleost fishes. Within the next decade, thousands of wind turbines will be in use in coastal and offshore waters and there is increasing concern on how they may influence marine life. The aims of this study were to examine how swimming behavior of roach (Rutilus rutilus) and three-spined stickleback (Gasterosteus aculeatus) were influenced by single-frequency sounds and noise generated by an offshore wind turbine, and the function of sound pressure level.

  • 16.
    Andersson, Mathias H.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology, Animal Ecology.
    Sigray, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Persson, Leif K. G.
    FOI.
    Wind farm noise influence on the audibility of fishManuscript (preprint) (Other academic)
  • 17.
    Andesson, Mathias H.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology, Animal Ecology.
    Sigray, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Particle motion measured at an operational wind turbine in relation to hearing sensitivity in fishManuscript (preprint) (Other academic)
  • 18. Angevine, Wayne
    et al.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zagar, Mark
    Stockholm University, Faculty of Science, Department of Meteorology .
    Modeling of the coastal boundary layer and pollutant transport in New England2006In: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 45, 137-154 p.Article in journal (Refereed)
  • 19. Asmus, H.
    et al.
    Robertson, S.
    Dickson, S.
    Friedrich, M.
    Megner, Linda
    Stockholm University, Faculty of Science, Department of Meteorology .
    Charge balance for the mesosphere with meteoric dust particles2015In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, Vol. 127, 137-149 p.Article in journal (Refereed)
    Abstract [en]

    An aerosol particle charging model initially developed for noctilucent cloud ice particles has been extended in several steps in order to better explain the data for charged meteoric smoke particles (MSPs) obtained by the nighttime and daytime CHAMPS rockets launched from Andoya, Norway, in October 2011. Addition of photodetachment to the model shows that this process reduces the number density of positively charged MSPs as well as the number density of negatively charged MSPs as a consequence of the photodetached electrons neutralizing the positively charged MSPs. In addition, the model shows that the ionization rate can be deduced from the electron number density and the electron-ion recombination rate only at the highest altitudes (those with ionization rates above 20 cm(-3) s(-1)) as a consequence of recombination on the MSPs being dominant at lower altitudes. The differences between the daytime and the nighttime rocket data suggest a photodetachment rate between 0.1 and 0.01 s(-1). A further extension of the model to include the formation of negative ions and their destruction helps explain the ledge seen in the number density of the lightest negatively charged particles. The MSP number densities that are the inputs to the charging model are taken from the CARMA/CHEM2D model. The CHAMPS data are more consistent with number densities generated with an assumed input flux from ablation of 4 t d(-1) than with 44 t d(-1) assumed previously.

  • 20.
    Axelsson, Per
    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 .
    An ensemble of arctic simulations of the AOE-2001 field experiment2011In: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 2, 146-170 p.Article in journal (Refereed)
  • 21. Baars, Holger
    et al.
    Kanitz, Thomas
    Engelmann, Ronny
    Althausen, Dietrich
    Heese, Birgit
    Komppula, Mika
    Preissler, Jana
    Tesche, Matthias
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Stockholm University, Faculty of Science, Department of Meteorology .
    Ansmann, Albert
    Wandinger, Ulla
    Lim, Jae-Hyun
    Ahn, Joon Young
    Stachlewska, Iwona S.
    Amiridis, Vassilis
    Marinou, Eleni
    Seifert, Patric
    Hofer, Julian
    Skupin, Annett
    Schneider, Florian
    Bohlmann, Stephanie
    Foth, Andreas
    Bley, Sebastian
    Pfuller, Anne
    Giannakaki, Eleni
    Lihavainen, Heikki
    Viisanen, Yrjo
    Hooda, Rakesh Kumar
    Pereira, Sergio Nepomuceno
    Bortoli, Daniele
    Wagner, Frank
    Mattis, Ina
    Janicka, Lucja
    Markowicz, Krzysztof M.
    Achtert, Peggy
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Stockholm University, Faculty of Science, Department of Meteorology .
    Artaxo, Paulo
    Pauliquevis, Theotonio
    Souza, Rodrigo A. F.
    Sharma, Ved Prakesh
    van Zyl, Pieter Gideon
    Beukes, Johan Paul
    Sun, Junying
    Rohwer, Erich G.
    Deng, Ruru
    Mamouri, Rodanthi-Elisavet
    Zamorano, Felix
    An overview of the first decade of Polly(NET): an emerging network of automated Raman-polarization lidars for continuous aerosol profiling2016In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, no 8, 5111-5137 p.Article in journal (Refereed)
    Abstract [en]

    A global vertically resolved aerosol data set covering more than 10 years of observations at more than 20 measurement sites distributed from 63 degrees N to 52 degrees S and 72 degrees W to 124 degrees E has been achieved within the Raman and polarization lidar network Polly(NET). This network consists of portable, remote-controlled multiwavelength-polarization-Raman lidars (Polly) for automated and continuous 24/7 observations of clouds and aerosols. Polly(NET) is an independent, voluntary, and scientific network. All Polly lidars feature a standardized instrument design with different capabilities ranging from single wavelength to multiwavelength systems, and now apply unified calibration, quality control, and data analysis. The observations are processed in near-real time without manual intervention, and are presented online at polly.tropos.de. The paper gives an overview of the observations on four continents and two research vessels obtained with eight Polly systems. The specific aerosol types at these locations (mineral dust, smoke, dust-smoke and other dusty mixtures, urban haze, and volcanic ash) are identified by their Angstrom exponent, lidar ratio, and depolarization ratio. The vertical aerosol distribution at the Polly(NET) locations is discussed on the basis of more than 55 000 automatically retrieved 30 min particle backscatter coefficient profiles at 532 nm as this operating wavelength is available for all Polly lidar systems. A seasonal analysis of measurements at selected sites revealed typical and extraordinary aerosol conditions as well as seasonal differences. These studies show the potential of Polly(NET) to support the establishment of a global aerosol climatology that covers the entire troposphere.

  • 22. Bahrami, Fariba
    et al.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology.
    Existence of energy minimizing vortices attached to a flat-top seamount2007In: Nonlinear Analysis: Real World Applications, ISSN 1468-1218, Vol. 8, 288-294 p.Article in journal (Refereed)
    Abstract [en]

    The existence of an energy minimizer relative to a class of

    rearrangements of a given function is proved. The minimizers are stationary and stable solutions of the two-dimensional barotropic vorticity equation, governing the evolution of geophysical flow over a surface of variable height. The theorem proved implies the existence of a family of stable anticyclonic vortices with cyclonic potential vorticity over a seamount, and a corresponding family of cyclonic vortices with anticyclonic potential vorticity over a localized depression. The seamount is described by a characteristic function (corresponding to a flat top) with arbitrary shape.

  • 23.
    Bahrami, Fariba
    et al.
    University of Tabriz, Dept. of Mathematics.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Alikhani, Robab
    University of Tabriz, Dept. of Mathematics.
    Existence of energy maximizing vortices in a three-dimensionalquasigeostrophic shear flow with bounded height2010In: Nonlinear Analysis, ISSN 1468-1218, Vol. 11, no 3, 1589-1599 p.Article in journal (Refereed)
    Abstract [en]

    The existence of an energy maximizer relative to a class of rearrangements of agiven function is proved. The maximizers are stationary and stable solutions of thequasigeostrophic equation, which governs the time evolution of large-scale threedimensionalgeophysical flow in a vertically bounded domain. The background flow isunidirectional, with linear horizontal shear. The theorem proved implies the existence of afamily of stationary and stable vortices that rotate in the same direction as the backgroundshear. It extends an earlier theorem by Burton and Nycander, which is valid for a verticallyunbounded domain.

  • 24. Bahrami, Fariba
    et al.
    Taghvafard, Hadi
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Mohammadi, Abbasali
    Stability investigation for steady solutions of the barotropic vorticity equation in R-22013In: Communications in nonlinear science & numerical simulation, ISSN 1007-5704, Vol. 18, no 3, 541-546 p.Article in journal (Refereed)
    Abstract [en]

    In this paper, we investigate the p-norm stability for vortices of geophysical flows over a surface of variable height that are strict maximizers of the kinetic energy relative to all isovortical flows. In this note, stability means nonlinear stability in the p-norm on the space of vorticity.

  • 25. Bahrami, Fariba
    et al.
    Zarroug, Moundheur
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lundberg, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Analysis of a thermosyphon using a Mandelstam condition2016In: Proceedings of the Institution of Civil Engineers: Engineering and Computational Mechanics, ISSN 1755-0777, E-ISSN 1755-0785, Vol. 169, no 1, 29-39 p., 1500011Article in journal (Refereed)
    Abstract [en]

    The characteristics of a thermally forced connected-vessel thermosyphon operating in an oscillatory mode have been determined using analytical techniques, the outcome of which is compared with results obtained by numerical integration of the governing equations. From a previous investigation it was known that adequate phase-plane representations of the limit cycles associated with oscillations could be obtained if the vessel-volume ratio was sufficiently small. This study aims at demonstrating how this constraint on the vessel volumes can be relaxed by prescribing a Mandelstam condition, that is, by postulating that the total heat content of the system remains conserved during the rapid phases of the oscillation. It was concluded that incorporating this Mandelstam condition in the analysis had the highly beneficial consequence that good analytical results could be obtained for much larger values of the vessel-volume ratio than those previously permitted.

  • 26.
    Ballarotta, Maxime
    Stockholm University, Faculty of Science, Department of Meteorology .
    The thermohaline circulation during the Last Glacial Maximum and in the Present-Day climate2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The thermohaline circulation (THC) corresponds to the large time- and spatial-scales ocean circulation associated with the transport of heat and salt, and is known to be an important factor controlling the climate variability. The large scales involved in the THC make it difficult to observe, and therefore the synergy of numerical models and climate proxy reconstructions is particularly relevant to study the characteristics of this circulation in the present and past climates.

    In this doctoral thesis, the THC during the Last Glacial Maximum (LGM) and the Present-Day (PD) is explored using a state-of-the-art Ocean General Circulation Model in its high- and low-resolution regimes. By comparing the LGM model outputs with the paleo-proxy reconstructions, it is shown that the high-resolution simulation improves the representation of the sea surface tem- peratures in the regions where the current structures appear to be complex, i.e., the western boundary currents (Agulhas, Kuroshio, Gulf Stream) and the Antarctic Circumpolar Current, although statistical comparisons with paleo- proxy reconstructions are not significantly improved on a global scale.

    The THC involves a superposition of processes acting at widely different spatial and temporal scales, from the geostrophic large-scale and slowly-varying flow to the mesoscale turbulent eddies and at even smaller-scale, the mixing generated by the internal wave field. Not all these processes can be properly resolved in numerical models, and thus need to be parameterized. Analyzing the THC in an eddy-permitting numerical model, it was found that the temporal scales required for diagnosing the Southern Ocean circulation should not exceed 1 month and the spatial scales needed to be taken into account must be smaller than 1°. Important changes in the nature and intensity of the THC were observed between the LGM and PD simulations. An estimation of the turnover times (i.e., the time it takes for the water parcel to make and entire loop on the Conveyor Belt) revealed that the LGM THC could be more vigorous than under the PD conditions. As a result, the ocean transports of heat and freshwater, the oceanic uptake of CO2, the ventilation of the deep ocean and the reorganization of the passive and active tracers (e.g., temperature, salinity, greenhouse gases, nutrients) can be altered in these different regimes.

  • 27.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brandefelt, Jenny
    Lundberg, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    A Last Glacial Maximum world-ocean simulation at eddy-permitting resolution – Part 1: Experimental design and basic evaluation2013In: Climate of the Past Discussions, ISSN 1814-9340, E-ISSN 1814-9359, Vol. 9, 297-328 p.Article in journal (Refereed)
    Abstract [en]

    Most state-of-the-art climate models include a coarsely resolved oceanic compo- nent, which has difficulties in capturing detailed dynamics, and therefore eddy- permitting/eddy-resolving simulations have been developed to reproduce the observed World Ocean. In this study, an eddy-permitting numerical experiment is conducted to simulate the global ocean state for a period of the Last Glacial Maximum (LGM, ∼26500 to 19000yr ago) and to investigate the improvements due to taking into account these higher spatial scales. The ocean general circulation model is forced by a 49-yr sample of LGM atmospheric fields constructed from a quasi-equilibrated climate-model simulation. The initial state and the bottom boundary condition conform to the Paleoclimate Modelling Intercomparison Project (PMIP) recommendations. Be- fore evaluating the model efficiency in representing the paleo-proxy reconstruction of the surface state, the LGM experiment is in this first part of the investigation, compared with a present-day eddy-permitting hindcast simulation as well as with the available PMIP results. It is shown that the LGM eddy-permitting simulation is consistent with the quasi-equilibrated climate-model simulation, but large discrepancies are found with the PMIP model analyses, probably due to the different equilibration states. The strongest meridional gradients of the sea-surface temperature are located near 40° N and S, this due to particularly large North-Atlantic and Southern-Ocean sea-ice covers. These also modify the locations of the convection sites (where deep-water forms) and most of the LGM Conveyor Belt circulation consequently takes place in a thinner layer than today. Despite some discrepancies with other LGM simulations, a glacial state is captured and the eddy-permitting simulation undertaken here yielded a useful set of data for comparisons with paleo-proxy reconstructions. 

  • 28.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Drijfhout, Sybren
    Kuhlbrodt, Till
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    The residual circulation of the Southern Ocean: Which spatio-temporal scales are needed?2013In: Ocean Modelling, ISSN 1463-5003, Vol. 64, 46-55 p.Article in journal (Refereed)
    Abstract [en]

    The Southern Ocean circulation consists of a complicated mixture of processes and phenomena that arise at different time and spatial scales which need to be parametrized in the state-of-the-art climate models. The temporal and spatial scales that give rise to the present-day residual mean circulation are here inves- tigated by calculating the Meridional Overturning Circulation (MOC) in density coordinates from an eddy-permitting global model. The region sensitive to the temporal decomposition is located between 38°S and 63°S, associated with the eddy-induced transport. The ‘‘Bolus’’ component of the residual circu- lation corresponds to the eddy-induced transport. It is dominated by timescales between 1 month and 1 year. The temporal behavior of the transient eddies is examined in splitting the ‘‘Bolus’’ component into a ‘‘Seasonal’’, an ‘‘Eddy’’ and an ‘‘Inter-monthly’’ component, respectively representing the correlation between density and velocity fluctuations due to the average seasonal cycle, due to mesoscale eddies and due to large-scale motion on timescales longer than one month that is not due to the seasonal cycle. The ‘‘Seasonal’’ bolus cell is important at all latitudes near the surface. The ‘‘Eddy’’ bolus cell is dominant in the thermocline between 50°S and 35°S and over the whole ocean depth at the latitude of the Drake Passage. The ‘‘Inter-monthly’’ bolus cell is important in all density classes and is maximal in the Brazil– Malvinas Confluence and the Agulhas Return Current. The spatial decomposition indicates that a large part of the Eulerian mean circulation is recovered for spatial scales larger than 11.25°, implying that small-scale meanders in the Antarctic Circumpolar Current (ACC), near the Subantarctic and Polar Fronts, and near the Subtropical Front are important in the compensation of the Eulerian mean flow. 

  • 29.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lundberg, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brandefelt, Jenny
    A Last Glacial Maximum World-Ocean simulation at eddy-permitting resolution – Part 2: Confronting the paleo-proxy data2013In: Climate of the Past Discussions, ISSN 1814-9340, E-ISSN 1814-9359, Vol. 9, 329-350 p.Article in journal (Refereed)
    Abstract [en]

    Previous investigations concerning the design of an eddy-permitting LGM oceanic sim- ulation are here extended with focus on whether this type of simulation is capable of improving the numerical results with regard to the available paleo-proxy reconstructions. Consequently, an eddy-permitting and two coarse-grid simulations of the same LGM period are confronted with a dataset from the Multiproxy Approach for the Recon- struction of the Glacial Ocean Sea Surface Temperatures (MARGO SSTs) and a num- ber of sea-ice reconstructions. From a statistical analysis it was found that the eddy- permitting simulation does not significantly improve the SST representation with regard to the paleo-reconstructions. The western boundary currents are better resolved in the high-resolution experiment than in the coarse simulations, but, although these more detailed SST structures yield a locally improved consistency between modelled pre- dictions and proxies, they do not contribute significantly to the global statistical score. As in the majority of the PMIP2 simulations, the modelled sea-ice conditions are still inconsistent with the paleo-reconstructions, probably due to the choice of the model equilibrium. 

  • 30.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Falahat, Saeed
    Stockholm University, Faculty of Science, Department of Meteorology .
    The vigorous large-scale ocean circulations during the Last Glacial MaximumManuscript (preprint) (Other academic)
    Abstract [en]

    The representation of the ocean thermohaline circulation (THC) under glacial and interglacial climate conditions is investigated using a new global thermohaline stream function. Consequently, the interglacial and glacial THCs are compared from two experiments based on an ocean general circulation model forced at the surface by conditions representing the present-day and the period of the Last Glacial Maximum (LGM, ≈ 21kyr ago). It is shown  that the LGM THC is amplified by the salinity/density contrast between the Atlantic and the Pacific basins, as well as in the abyss due to larger salinity gradients. Even though the circuit along the Conveyor Belt loop is not drastically changed, the water mass transformations can regionally differ between the two periods. Additionally, the LGM Conveyor Belt Cell is more  isolated from the abyss and its turnover time is between two and three times shorter than in the present-day simulation, suggesting vigorous large-scale circulation. 

  • 31.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Falahat, Saeed
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the glacial and interglacial thermohaline circulation and the associated transports of heat and freshwater2014In: Ocean Science, ISSN 1812-0784, E-ISSN 1812-0792, Vol. 10, no 6, 907-921 p.Article in journal (Refereed)
    Abstract [en]

    The thermohaline circulation (THC) and the oceanic heat and freshwater transports are essential for understanding the global climate system. Streamfunctions are widely used in oceanography to represent the THC and estimate the transport of heat and freshwater. In the present study, the regional and global changes of the THC, the transports of heat and freshwater and the timescale of the circulation between the Last Glacial Maximum (LGM, ≈ 21 kyr ago) and the present-day climate are explored using an Ocean General Circulation Model and streamfunctions projected in various coordinate systems. We found that the LGM tropical circulation is about 10% stronger than under modern conditions due to stronger wind stress. Consequently, the maximum tropical transport of heat is about 20% larger during the LGM. In the North Atlantic basin, the large sea-ice extent during the LGM constrains the Gulf Stream to propagate in a more zonal direction, reducing the transport of heat towards high latitudes by almost 50% and reorganising the freshwater transport. The strength of the Atlantic Meridional Overturning Circulation depends strongly on the coordinate system. It varies between 9 and 16 Sv during the LGM, and between 12 to 19 Sv for the present day. Similar to paleo-proxy reconstructions, a large intrusion of saline Antarctic Bottom Water takes place into the Northern Hemisphere basins and squeezes most of the Conveyor Belt circulation into a shallower part of the ocean. These different haline regimes between the glacial and interglacial period are illustrated by the streamfunctions in latitude–salinity coordinates and thermohaline coordinates. From these diagnostics, we found that the LGM Conveyor Belt circulation is driven by an enhanced salinity contrast between the Atlantic and the Pacific basin. The LGM abyssal circulation lifts and makes the Conveyor Belt cell deviate from the abyssal region, resulting in a ventilated upper layer above a deep stagnant layer, and an Atlantic circulation more isolated from the Pacific. An estimate of the timescale of the circulation reveals a sluggish abyssal circulation during the LGM, and a Conveyor Belt circulation that is more vigorous due to the combination of a stronger wind stress and a shortened circulation route.

  • 32.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Laurent, Brodeau
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jenny, Brandefelt
    Lundberg, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Last Glacial Maximum world ocean simulations at eddy-permitting and coarse resolutions: do eddies contribute to a better consistency between models and palaeoproxies?2013In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 9, no 6, 2669-2686 p.Article in journal (Refereed)
    Abstract [en]

    Most state-of-the-art climate models include a coarsely resolved oceanic component, which hardly captures detailed dynamics, whereas eddy-permitting and eddy-resolving simulations are developed to reproduce the observed ocean. In this study, an eddy-permitting and a coarse resolution numerical experiment are conducted to simulate the global ocean state for the period of the Last Glacial Maximum (LGM, ~26 500 to 19 000 yr ago) and to investigate the improvements due to taking into account the smaller spatial scales. The ocean state from each simulation is confronted with a data set from the Multiproxy Approach for the Reconstruction of the Glacial Ocean (MARGO) sea surface temperatures (SSTs), some reconstructions of the palaeo-circulations and a number of sea-ice reconstructions. The western boundary currents and the Southern Ocean dynamics are better resolved in the high-resolution experiment than in the coarse simulation, but, although these more detailed SST structures yield a locally improved consistency between model predictions and proxies, they do not contribute sig- nificantly to the global statistical score. The SSTs in the tropical coastal upwelling zones are also not significantly improved by the eddy-permitting regime. The models perform in the mid-latitudes but as in the majority of the Paleo- climate Modelling Intercomparison Project simulations, the modelled sea-ice conditions are inconsistent with the palaeo-reconstructions. The effects of observation locations on the comparison between observed and simulated SST suggest that more sediment cores may be required to draw reliable conclusions about the improvements introduced by the high resolution model for reproducing the global SSTs. One has to be careful with the interpretation of the deep ocean state which has not reached statistical equilibrium in our simula-tions. However, the results indicate that the meridional overturning circulations are different between the two regimes, suggesting that the model parametrizations might also play a key role for simulating past climate states. 

  • 33. 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, 57-72 p.Article 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.

  • 34. Bari, D. Demirhan
    et al.
    Gabriel, A.
    Kornich, H.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Peters, D. W. H.
    The effect of zonal asymmetries in the Brewer-Dobson circulation on ozone and water vapor distributions in the northern middle atmosphere2013In: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, ISSN 2169-897X, Vol. 118, no 9, 3447-3466 p.Article in journal (Refereed)
    Abstract [en]

    The longitudinal variations in the time-mean transport by the Brewer-Dobson circulation are studied using a three-dimensional (3-D) residual circulation approach to analyze the effects on zonal asymmetries in stratospheric ozone (O-3) and middle atmospheric water vapor (H2O). For January, the monthly mean residual winds, including both the Eulerian flow and the eddy-induced time-mean flow, were derived from general circulation model simulations with interactive chemistry (HAMMONIA), reanalysis (ERA-Interim), and satellite data (Aura/MLS). Extending the picture of the zonal mean two-dimensional Brewer-Dobson circulation, we find a 3-D circulation structure in relation to the zonal wave one in the middle atmosphere, including northward and downward residual winds over northern Europe/Asia with the downwelling directed toward the center of the polar vortex over northern Siberia, as well as southward and upward residual winds over the northern Pacific/Aleutians, and a pronounced cross-polar transport from Asia to North America in the middle stratosphere. The residual advection of O-3 and H2O shows that the observed wave one patterns in O-3 and H2O are produced by the zonal asymmetries in the residual mass transport in which Eulerian and eddy time-mean transports are largely counteracting. In comparison to observations, the model underestimates the effects of planetary waves but overestimates those of transient waves in configuring the stationary waves in O-3 and H2O. Overall, the 3-D residual circulation approach provides a useful diagnostic for understanding regional differences in middle atmospheric trace gas distributions and for validating general circulation models with interactive chemistry.

  • 35. Baron, Phillippe
    et al.
    Murtagh, Donal
    Urban, J
    Sagawa, S
    Ochiai, H
    Koernich, H
    Khosrawi, Farahnaz
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kikuchi, K
    Mizobuchi, S
    Sagi, K
    Kasai, Y
    Yasui, M
    Observation of horizontal winds in the middle-atmosphere between 30° S and 55° N during the northern winter 2009–20102012In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, 6049-6064 p.Article in journal (Refereed)
    Abstract [en]

    Although the links between stratospheric dynamics, climate and weather have been demonstrated, direct observations of stratospheric winds are lacking. We report observations of winds between 8 and 0.01 hPa (~35–80 km) from October 2009 to April 2010 by the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the International Space Station. The altitude range covers the region between 35–60 km where previous space-borne wind instruments show a lack of sensitivity. Both zonal and meridional wind components were obtained, though not simultaneously, in the latitude range from 30° S to 55° N and with a single profile precision of 7–9 m s−1 between 8 and 0.6 hPa and better than 20 m s−1 at altitudes above. The vertical resolution is 5–7 km except in the upper part of the retrieval range (10 km at 0.01 hPa). In the region between 1–0.05 hPa, a mean difference <2 m s−1 is found between SMILES profiles retrieved from different spectroscopic lines and instrumental settings. Good agreement (mean difference of ~2 m s−1) is also found with the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis in most of the stratosphere except for the zonal winds over the equator (mean difference of 5–10 m s−1). In the mesosphere, SMILES and ECMWF zonal winds exhibit large differences (> 20 m s−1), especially in the tropics. We illustrate our results by showing daily and monthly zonal wind variations, namely the semi-annual oscillation in the tropics and reversals of the flow direction between 50° N–55° N during sudden stratospheric warmings in the stratosphere. The daily comparison with ECMWF winds reveals that in the beginning of February, a significantly stronger zonal westward flow is measured in the tropics at 2 hPa compared to the flow computed in the analysis (difference of ~20 m s−1). The results show that the comparison between SMILES and ECMWF winds is not only relevant for the quality assessment of the new SMILES winds but it also provides insights on the quality of the ECMWF winds themselves. Although the instrument was not specifically designed for measuring winds, the results demonstrate that space-borne sub-mm wave radiometers have the potential to provide good quality data for improving the stratospheric winds in atmospheric models.

  • 36.
    Behrenfeldt, Ulrika
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Meteorology.
    Ström, Johan
    Department of Applied Environmental Science (ITM).
    Stohl, Andreas
    Chemical properties of Arctic aerosol particles collected at the Zeppelin station during the aerosol transition period in May and June of 20042008In: Tellus. Series B: Chemical and Physical Meteorology, Vol. 60, no 3, 405-415 p.Article in journal (Refereed)
  • 37. Belova, E.
    et al.
    Kirkwood, S.
    Latteck, R.
    Zecha, M.
    Pinedo, H.
    Hedin, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology .
    Multi-radar observations of polar mesosphere summer echoes during the PHOCUS campaign on 20-22 July 20112014In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, Vol. 118, 199-205 p.Article in journal (Refereed)
    Abstract [en]

    During the PHOCUS rocket campaign, on 20-22 July 2011, the observations of polar mesosphere summer echoes (PMSE) were made by three mesosphere-stratosphere-troposphere radars, operating at about 50 MHz. One radar, ESRAD is located at Esrange in Sweden, where the rocket was launched, two other radars, MAARSY and MORRO, are located 250 km north-west and 200 km north of the ESRAD, respectively, on the other side of the Scandinavian mountain ridge. We compared PMSE as measured by these three radars in terms of their strength, spectral width and wave modulation. Time-altitude maps of PMSE strength look very similar for all three radars. Cross-correlations with maximum values 0.5-0.6 were found between the signal powers over the three days of observations for each pair of radars. By using cross-spectrum analysis of PMSE signals, we show that some waves with periods of a few hours were observed by all three radars. Unlike the strengths, simultaneous values of PMSE spectral width, which is related to turbulence, sometimes differ significantly between the radars. For interpretation of the results we suggested that large-scale fields of neutral temperature, ice particles and electron density, which are more or less uniform over 150-250 km horizontal extent were 'modulated' by waves and smaller patches of turbulence.

  • 38.
    Bender, Frida
    Stockholm University, Faculty of Science, Department of Meteorology.
    A note on the effect of GCM tuning on climate sensitivity2008In: Environmental Research Letters, ISSN 1748-9318 , Vol. 3, no 1, 014001- p.Article in journal (Refereed)
  • 39.
    Bender, Frida
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Rodhe, Henning
    Stockholm University, Faculty of Science, Department of Meteorology.
    Charlson, Robert
    Univeristy of Washington, Seattle.
    Ekman, Annica
    Stockholm University, Faculty of Science, Department of Meteorology.
    Loeb, Norman
    Hampton University, Hampton.
    22 views of the global albedo - comparison between 20 GCMs and two satellites2006In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, Vol. 58, no 3, 320-330 p.Article in journal (Refereed)
  • 40. Bender, Frida A. -M.
    et al.
    Charlson, Robert J.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leahy, Louise V.
    Quantification of Monthly Mean Regional-Scale Albedo of Marine Stratiform Clouds in Satellite Observations and GCMs2011In: Journal of Applied Meteorology and Climatology, ISSN 1558-8424, E-ISSN 1558-8432, Vol. 50, no 10, 2139-2148 p.Article in journal (Refereed)
    Abstract [en]

    Planetary albedo the reflectivity for solar radiation is of singular importance in determining the amount of solar energy taken in by the Earth-atmosphere system. Modeling albedo, and specifically cloud albedo, correctly is crucial for realistic climate simulations. A method is presented herein by which regional cloud albedo can be quantified from the relation between total albedo and cloud fraction, which in observations is found to be approximately linear on a monthly mean scale. This analysis is based primarily on the combination of cloud fraction data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and albedo data from the Clouds and the Earth's Radiant Energy System (CERES), but the results presented are also supported by the combination of cloud fraction and proxy albedo data from satelliteborne lidar [Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CA LIPSO)]. These data are measured and derived completely independently from the CERES-MODIS data. Applied to low-level marine stratiform clouds in three regions (off the coasts of South America, Africa, and North America), the analysis reveals regionally uniform monthly mean cloud albedos, indicating that the variation in cloud shortwave radiative properties is small on this scale. A coherent picture of low effective cloud albedo emerges, in the range from 0.35 to 0.42, on the basis of data from CERES and MODIS. In its simplicity, the method presented appears to be useful as a diagnostic tool and as a constraint on climate models. To demonstrate this, the same method is applied to cloud fraction and albedo output from several current-generation climate models [from the Coupled Model Intercomparison Project, phase 3 (CMIP3), archive]. Although the multimodel mean cloud albedo estimates agree to within 20% with the satellite-based estimates for the three focus regions, model-based estimates of cloud albedo are found to display much larger variability than do the observations, within individual models as well as between models.

  • 41.
    Bender, Frida A. -M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Engström, Anders
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wood, Robert
    Charlson, Robert J.
    Evaluation of Hemispheric Asymmetries in Marine Cloud Radiative Properties2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 11, 4131-4147 p.Article in journal (Refereed)
    Abstract [en]

    The hemispheric symmetry of albedo and its contributing factors in satellite observations and global climate models is evaluated. The analysis is performed on the annual mean time scale, on which a bimodality in the joint distribution of albedo and cloud fraction is evident, resulting from tropical and subtropical clouds and midlatitude clouds, respectively. Hemispheric albedo symmetry is not found in individual ocean-only latitude bands; comparing the Northern and Southern Hemisphere (NH and SH), regional mean albedo is higher in the NH tropics and lower in the NH subtropics and midlatitudes than in the SH counterparts. This follows the hemispheric asymmetry of cloud fraction. In midlatitudes and tropics the hemispheric asymmetry in cloud albedo also contributes to the asymmetry in total albedo, whereas in the subtropics the cloud albedo is more hemispherically symmetric. According to the observations, cloud contributions to compensation for higher clear-sky albedo in the NH come primarily from cloud albedo in midlatitudes and cloud amount in the subtropics. Current-generation climate models diverge in their representation of these relationships, but common features of the model-data comparison include weaker-than-observed asymmetry in cloud fraction and cloud albedo in the tropics, weaker or reversed cloud fraction asymmetry in the subtropics, and agreement with observed cloud albedo asymmetry in the midlatitudes. Models on average reproduce the NH-SH asymmetry in total albedo over the 60 degrees S-60 degrees N ocean but show higher occurrence of brighter clouds in the SH compared to observations. The albedo bias in both hemispheres is reinforced by overestimated clear-sky albedo in the models.

  • 42.
    Bender, Frida A-M
    Stockholm University, Faculty of Science, Department of Meteorology.
    Earth's albedo in a changing climate2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The albedo is a key parameter in the radiative budget of the Earth and a primary determinant of the planetary temperature and is therefore also central to questions regarding climate stability, climate change and climate sensitivity. Climate models and satellite observations are essential for studying the albedo, and the parameters determining it, on large spatial and temporal scales. Although climate models are able to capture the large-scale characteristics of the albedo, a bias is found between modelled and observed global albedo estimates, and on a regional scale particular problematic regions can be identified.

    Cloud parameters, that are of great importance for determining the albedo, vary widely among models, but lack of observations makes constraining models, and even evaluating models, difficult. The freedom of variability for cloud parameters can be used to make models agree with observations of the better constrained radiative budget. It is shown that tuning a model to different radiative budget estimates by altering cloud parameters can influence the climate sensitivity of the model, but the effect seen is small, compared to the range of climate sensitivities estimated by different models.

    Despite their different parameterizations of clouds, aerosols etc., models do have fundamental features in common, which can further the understanding of the real climate system. For instance it is found that sensitivity to volcanic forcing is related to climate sensitivity in an ensemble of models. If this relation is valid for the real climate as well, observations of the volcanic sensitivity can help restrict the climate sensitivity.

    The range of climate sensitivity estimates in models can largely be attributed to variations in cloud response to forcing. It is found that in models with high climate sensitivity changes in cloud cover and cloud reflectivity enhance a positive radiative forcing due to increased CO2 concentrations, feeding back on the warming and in models with low climate sensitivity, cloud response counteracts the positive radiative forcing and warming induced by the same forcing.

    As a consequence the total albedo response to increased CO2 forcing is found to be stronger (more negative) in high sensitivity models and vice versa.

    Cloud albedo and its variation between different cloud regimes, is important in this regard, yet not well known. A method based on the relation between cloud fraction and albedo is presented, giving a way to estimate regional cloud albedo, primarily for homogeneous cloud regimes, but possibly also extended to a global scale.

     

  • 43.
    Bender, Frida
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Charlson, Robert
    University of Washington, Seattle.
    Ekman, Annica
    Stockholm University, Faculty of Science, Department of Meteorology.
    Planetary and cloud albedo in present and strongly forced climateManuscript (Other academic)
  • 44.
    Bender, Frida
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica
    Stockholm University, Faculty of Science, Department of Meteorology .
    Rodhe, Henning
    Stockholm University, Faculty of Science, Department of Meteorology .
    Response to the eruption of Mount Pinatubo in relation to climate sensitivity in the CMIP3 models2010In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 35, no 5, 875-886 p.Article in journal (Refereed)
    Abstract [en]

    The radiative flux perturbations and subsequent temperature responses in relation to the eruption of Mount Pinatubo in 1991 are studied in the ten general circulation models incorporated in the Coupled Model Intercomparison Project, phase 3 (CMIP3), that include a parameterization of volcanic aerosol. Models and observations show decreases in global mean temperature of up to 0.5 K, in response to radiative perturbations of up to 10 W m−2, averaged over the tropics. The time scale representing the delay between radiative perturbation and temperature response is determined by the slow ocean response, and is estimated to be centered around 4 months in the models. Although the magniude of the temperature response to a volcanic eruption has previously been used as an indicator of equilibrium climate sensitivity in models, we find these two quantities to be only weakly correlated. This may partly be due to the fact that the size of the volcano-induced radiative perturbation varies among the models. It is found that the magnitude of the modelled radiative perturbation increases with decreasing climate sensitivity, with the exception of one outlying model. Therefore, we scale the temperature perturbation by the radiative perturbation in each model, and use the ratio between the integrated temperature perturbation and the integrated radiative perturbation as a measure of sensitivity to volcanic forcing. This ratio is found to be well correlated with the model climate sensitivity, more sensitive models having a larger ratio. Further, if this correspondence between “volcanic sensitivity” and sensitivity to CO2 forcing is a feature not only among the models, but also of the real climate system, the alleged linear relation can be used to estimate the real climate sensitivity. The observational value of the ratio signifying volcanic sensitivity is hereby estimated to correspond to an equilibrium climate sensitivity, i.e. equilibrium temperature increase due to a doubling of the CO2 concentration, between 1.7 and 4.1 K. Several sources of uncertainty reside in the method applied, and it is pointed out that additional model output, related to ocean heat storage and radiative forcing, could refine the analysis, as could reduced uncertainty in the observational record, of temperature as well as forcing.

  • 45.
    Bender, Frida
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Engström, Anders
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson, Johannes
    Stockholm University, Faculty of Science, Department of Meteorology .
    Factors controlling cloud albedo in marine subtropical stratocumulus regions in climate models and satellite observations2016In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, no 10, 3559-3587 p.Article in journal (Refereed)
    Abstract [en]

    This study focuses on the radiative properties of five subtropical marine stratocumulus cloud regions, on monthly mean scale. Through examination of the relation between total albedo and cloud fraction, and its variability and relation to other parameters, some of the factors controlling the reflectivity, or albedo, of the clouds in these regions are investigated. It is found that the main part of the variability in albedo at a given cloud fraction can be related to temporal, rather than spatial variability, indicating spatial homogeneity in cloud radiative properties in the studied regions. This is seen most clearly in satellite observations, but also in an ensemble of climate models. Further comparison between satellite data and output from climate models shows that there is good agreement with respect to the role of liquid water path, the parameter that can be assumed to be the primary source of variability in cloud reflectivity for a given cloud fraction. On the other hand, the influence of aerosol loading on cloud albedo differs between models and observations. The cloud-albedo effect, or cloud brightening caused by aerosol through its coupling to cloud droplet number concentration and droplet size, is found not to dominate in the satellite observations on monthly mean scale, as it appears to do on this scale in the climate models. The disagreement between models and observations is particularly strong in regions with frequent occurrence of absorbing aerosols above clouds, where satellite data contrary to the climate models indicate a scene darkening with increasing aerosol loading.

  • 46.
    Bender, Frida
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Rodhe, Henning
    Stockholm University, Faculty of Science, Department of Meteorology.
    Al Gore är inte forskare2008In: Svenska Dagbladet, no 2008-10-25Article in journal (Other (popular science, discussion, etc.))
  • 47.
    Bender, Frida
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Rodhe, Henning
    Stockholm University, Faculty of Science, Department of Meteorology.
    Vem ska man tro på om klimatet?2008In: Svenska Dagbladet, no 2008-10-17Article in journal (Other (popular science, discussion, etc.))
  • 48. Bengtsson, Lennart
    et al.
    Claesson, Stefan
    Rodhe, Henning
    Stockholm University, Faculty of Science, Department of Meteorology .
    Öquist, Gunnar
    Extrema oväder hänger inte ihop med ett varmare klimat2009In: Dagens Nyheter, ISSN 1101-2447, no 16 decemberArticle in journal (Other (popular science, discussion, etc.))
  • 49.
    Bengtsson, Lisa
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the Convective-Scale Predictability of the Atmosphere2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    A well-represented description of convection in weather and climate models is essential since convective clouds strongly influence the climate system. Convective processes interact with radiation, redistribute sensible and latent heat and momentum, and impact hydrological processes through precipitation. Depending on the models’ horizontal resolution, the representation of convection may look very different. However, the convective scales not resolved by the model are traditionally parameterized by an ensemble of non-interacting convective plumes within some area of uniform forcing, representing the “large scale”. A bulk representation of the mass-flux associated with the individual plumes in the defined area provide the statistical effect of moist convection on the atmosphere. Studying the characteristics of the ECMWF ensemble prediction system it is found that the control forecast of the ensemble system is not variable enough in order to yield a sufficient spread using an initial perturbation technique alone. Such insufficient variability may be addressed in the parameterizations of, for instance, cumulus convection where the sub-grid variability in space and time is traditionally neglected. Furthermore, horizontal transport due to gravity waves can act to organize deep convection into larger scale structures which can contribute to an upscale energy cascade. However, horizontal advection and numerical diffusion are the only ways through which adjacent model grid-boxes interact in the models. The impact of flow dependent horizontal diffusion on resolved deep convection is studied, and the organization of convective clusters is found very sensitive to the method of imposing horizontal diffusion. However, using numerical diffusion in order to represent lateral effects is undesirable. To address the above issues, a scheme using cellular automata in order to introduce lateral communication, memory and a stochastic representation of the statistical effects of cumulus convection is implemented in two numerical weather models. The behaviour of the scheme is studied in cases of organized convective squall-lines, and initial model runs show promising improvements.

  • 50.
    Bengtsson, Lisa K.
    et al.
    Sveriges meteorologiska och hydrologiska institut (SMHI), Norrköping.
    Magnusson, Linus
    Stockholm University, Faculty of Science, Department of Meteorology .
    Källén, Erland
    Stockholm University, Faculty of Science, Department of Meteorology .
    Independent Estimations of the Asymptotic Variability in an Ensemble Forecast System2008In: Monthly Weather Review, ISSN 0027-0644, E-ISSN 1520-0493, Vol. 136, no 11, 4105-4112 p.Article in journal (Refereed)
    Abstract [en]

    One desirable property within an ensemble forecast system is to have a one-to-one ratio between the root-mean-square error (rmse) of the ensemble mean and the standard deviation of the ensemble (spread). The ensemble spread and forecast error within the ECMWF ensemble prediction system has been extrapolated beyond 10 forecast days using a simple model for error growth. The behavior of the ensemble spread and the rmse at the time of the deterministic predictability are compared with derived relations of rmse at the infinite forecast length and the characteristic variability of the atmosphere in the limit of deterministic predictability. Utilizing this methodology suggests that the forecast model and the atmosphere do not have the same variability, which raises the question of how to obtain a perfect ensemble.

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