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  • 151.
    de Boer, Agatha M.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper.
    Pascual-Ahuir, Estanislao Gavilan
    Stevens, David P.
    Chafik, Léon
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Hutchinson, David K.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper.
    Zhang, Qiong
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi.
    Sime, Louise C.
    Willmott, Andrew J.
    Interconnectivity Between Volume Transports Through Arctic Straits2018Inngår i: Journal of Geophysical Research - Oceans, ISSN 2169-9275, E-ISSN 2169-9291, Vol. 123, nr 12, s. 8714-8729Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Arctic heat and freshwater budgets are highly sensitive to volume transports through the Arctic-Subarctic straits. Here we study the interconnectivity of volume transports through Arctic straits in three models; two coupled global climate models, one with a third-degree horizontal ocean resolution (High Resolution Global Environmental Model version 1.1 [HiGEM1.1]) and one with a twelfth-degree horizontal ocean resolution (Hadley Centre Global Environment Model 3 [HadGEM3]), and one ocean-only model with an idealized polar basin (tenth-degree horizontal resolution). The two global climate models indicate that there is a strong anticorrelation between the Bering Strait throughflow and the transport through the Nordic Seas, a second strong anticorrelation between the transport through the Canadian Arctic Archipelago and the Nordic Seas transport, and a third strong anticorrelation is found between the Fram Strait and the Barents Sea throughflows. We find that part of the strait correlations is due to the strait transports being coincidentally driven by large-scale atmospheric forcing patterns. However, there is also a role for fast wave adjustments of some straits flows to perturbations in other straits since atmospheric forcing of individual strait flows alone cannot lead to near mass balance fortuitously every year. Idealized experiments with an ocean model (Nucleus for European Modelling of the Ocean version 3.6) that investigate such causal strait relations suggest that perturbations in the Bering Strait are compensated preferentially in the Fram Strait due to the narrowness of the western Arctic shelf and the deeper depth of the Fram Strait. Plain Language Summary The Arctic is one of the most fragile places on the Earth, facing double the rate of warming as the rest of the globe. This warming is partly due to melting of sea ice because open water reflects less sunlight than ice. One of the major controls on Arctic sea ice concentration is the heat flowing into the Arctic through its straits. However, due to the harsh conditions in the Arctic, there are limited long-term observations of the currents flowing through these straits. Here we turn to climate models to investigate these Arctic straits flows and in particular focus on how flows into and out of the Arctic balance each other. We find that in some instances specific pairs of strait flows are simultaneously affected by large-scale atmospheric. In other instances, the inflow through one strait flows out through another distant strait because of the way the ocean floor guides the currents. Traditionally, the flows through Arctic straits are studied in relation to local forces such as wind and sea level. Our work suggests value in a more holistic approach; one that also accounts for flow changes in a strait as a response to flow changes in other straits.

  • 152. de Boer, G.
    et al.
    Shupe, M. D.
    Caldwell, P. M.
    Bauer, S. E.
    Persson, O.
    Boyle, J. S.
    Kelley, M.
    Klein, S. A.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Near-surface meteorology during the Arctic Summer Cloud Ocean Study (ASCOS): evaluation of reanalyses and global climate models2014Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, nr 1, s. 427-445Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Atmospheric measurements from the Arctic Summer Cloud Ocean Study (ASCOS) are used to evaluate the performance of three atmospheric reanalyses (European Centre for Medium Range Weather Forecasting (ECMWF)-Interim reanalysis, National Center for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) reanalysis, and NCEP-DOE (Department of Energy) reanalysis) and two global climate models (CAM5 (Community Atmosphere Model 5) and NASA GISS (Goddard Institute for Space Studies) ModelE2) in simulation of the high Arctic environment. Quantities analyzed include near surface meteorological variables such as temperature, pressure, humidity and winds, surface-based estimates of cloud and precipitation properties, the surface energy budget, and lower atmospheric temperature structure. In general, the models perform well in simulating large-scale dynamical quantities such as pressure and winds. Near-surface temperature and lower atmospheric stability, along with surface energy budget terms, are not as well represented due largely to errors in simulation of cloud occurrence, phase and altitude. Additionally, a development version of CAMS, which features improved handling of cloud macro physics, has demonstrated to improve simulation of cloud properties and liquid water amount. The ASCOS period additionally provides an excellent example of the benefits gained by evaluating individual budget terms, rather than simply evaluating the net end product, with large compensating errors between individual surface energy budget terms that result in the best net energy budget.

  • 153. De Geer, Lars-Erik
    et al.
    Persson, Christer
    Rodhe, Henning
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    A Nuclear Jet at Chernobyl Around 21:23:45 UTC on April 25, 19862018Inngår i: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 201, nr 1, s. 11-22Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The nature of two explosions that were witnessed within 3 s at the Chernobyl-4 reactor less than a minute after 21:23:00 UTC on April 25, 1986, have since then been the subject of sprawling interpretations. This paper renders the following hypothesis. The first explosion consisted of thermal neutron mediated nuclear explosions in one or rather a few fuel channels, which caused a jet of debris that reached an altitude of some 2500 to 3000 m. The second explosion would then have been the steam explosion most experts believe was the first one. The solid support for this new scenario rests on two pillars and three pieces of corroborating evidence. The first pillar is that a group at the V. G. Khlopin Radium Institute in then Leningrad on April 29, 1986, detected newly produced, or fresh, xenon fission products at Cherepovets, 370 km north of Moscow and far away from the major track of Chernobyl debris ejected by the steam explosion and subsequent fires. The second pillar is built on state-of-the-art meteorological dispersion calculations, which show that the fresh xenon signature observed at Cherepovets was only possible if the injection altitude of the fresh debris was considerably higher than that of the bulk reactor core releases that turned toward Scandinavia and central Europe. These two strong pieces of evidence are corroborated by what were manifest physical effects of a downward jet in the southeastern part of the reactor, by seismic measurements some 100 km west of the reactor, and by observations of a blue flash above the reactor a few seconds after the first explosion.

  • 154. de Lavergne, C.
    et al.
    Falahat, Saeed
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Madec, G.
    Roquet, F.
    Nycander, Jonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Vic, C.
    Toward global maps of internal tide energy sinks2019Inngår i: Ocean Modelling, ISSN 1463-5003, E-ISSN 1463-5011, Vol. 137, s. 52-75Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Internal tides power much of the observed small-scale turbulence in the ocean interior. To represent mixing induced by this turbulence in ocean climate models, the cascade of internal tide energy to dissipation scales must be understood and mapped. Here, we present a framework for estimating the geography of internal tide energy sinks. The mapping relies on the following ingredients: (i) a global observational climatology of stratification; (ii) maps of the generation of M-2, S-2 and K-1 internal tides decomposed into vertical normal modes; (iii) simplified representations of the dissipation of low-mode internal tides due to wave-wave interactions, scattering by small-scale topography, interaction with critical slopes and shoaling; (iv) Lagrangian tracking of low-mode energy beams through observed stratification, including refraction and reflection. We thus obtain a global map of the column-integrated energy dissipation for each of the four considered dissipative processes, each of the three tidal constituents and each of the first five modes. Modes >= 6 are inferred to dissipate within the local water column at the employed half-degree horizontal resolution. Combining all processes, modes and constituents, we construct a map of the total internal tide energy dissipation, which compares well with observational inferences of internal wave energy dissipation. This result suggests that tides largely shape observed spatial contrasts of dissipation, and that the framework has potential in improving understanding and modelling of ocean mixing. However, sensitivity to poorly constrained parameters and simplifying assumptions entering the parameterized energy sinks calls for additional investigation. The attenuation of low-mode internal tides by wave-wave interactions needs particular attention.

  • 155. de lavergne, C.
    et al.
    Madec, G.
    Roquet, Fabien
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Holmes, R. M.
    McDougall, T. J.
    Abyssal ocean overturning shaped by seafloor distribution2017Inngår i: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 551, nr 7679, s. 181-186Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The abyssal ocean is broadly characterized by northward flow of the densest waters and southward flow of less-dense waters above them. Understanding what controls the strength and structure of these interhemispheric flows-referred to as the abyssal overturning circulation-is key to quantifying the ocean's ability to store carbon and heat on timescales exceeding a century. Here we show that, north of 32 degrees S, the depth distribution of the seafloor compels dense southernorigin waters to flow northward below a depth of about 4 kilometres and to return southward predominantly at depths greater than 2.5 kilometres. Unless ventilated from the north, the overlying mid-depths (1 to 2.5 kilometres deep) host comparatively weak mean meridional flow. Backed by analysis of historical radiocarbon measurements, the findings imply that the geometry of the Pacific, Indian and Atlantic basins places a major external constraint on the overturning structure.

  • 156. de Lavergne, Casimir
    et al.
    Madec, Gurvan
    Capet, Xavier
    Maze, Guillaume
    Roquet, Fabien
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Getting to the bottom of the ocean2016Inngår i: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 9, nr 12, s. 857-858Artikkel i tidsskrift (Fagfellevurdert)
  • 157. De Marchis, M.
    et al.
    Milici, B.
    Sardina, Gaetano
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Napoli, E.
    Interaction between turbulent structures and particles in roughened channel2016Inngår i: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 78, s. 117-131Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The distribution of inertial particles in turbulent flows is highly non-uniform and is driven by the local dynamics of the turbulent structures of the underlying carrier flow field. In the specific context of dilute particle-laden wall-bounded flows, deposition and resuspension mechanisms are dominated by the interaction between inertial particles and coherent turbulent structures characteristic of the wall region. The macroscopic behavior of these two-phase systems is influenced by particle inertia, which plays a role at the microscale of a single dispersed element. These turbulent structures, which control the turbulent regeneration cycles, are strongly affected by the wall roughness. The effect of the roughness on turbulent transport in dilute suspension has been still poorly investigated. The issue is discussed here by addressing direct numerical simulation (DNS), at friction Reynolds number Re-tau = 180, of a dilute dispersion of heavy particles in a turbulent channel flow, spanning two orders of magnitude of particle inertia. The irregular wall roughness is obtained through the superimposition of four sinusoidal functions of different wavelengths and random amplitudes. We use DNS combined with Lagrangian particle tracking to characterize the effect of inertia on particle preferential accumulation, looking at the effect of roughness on particle distribution, by comparing the statistics computed for fluid and particles of different size and observing differences in terms of distribution patterns and preferential sampling.

  • 158. de Vries, Hylke
    et al.
    Scher, Sebastian
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Haarsma, Rein
    Drijfhout, Sybren
    van Delden, Aarnout
    How Gulf-Stream SST-fronts influence Atlantic winter storms: Results from a downscaling experiment with HARMONIE to the role of modified latent heat fluxes and low-level baroclinicity2019Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 52, nr 9-10, s. 5899-5909Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The strong horizontal gradients in sea surface temperature (SST) of the Atlantic Gulf Stream exert a detectable influence on extratropical cyclones propagating across the region. This is shown in a sensitivity experiment where 24 winter storms taken from ERA-Interim are simulated with HARMONIE at 10-km resolution. Each storm is simulated twice. First, using observed SST (REF). In the second simulation a smoothed SST is offered (SMTH), while lateral and upper-level boundary conditions are unmodified. Each storm pair propagates approximately along the same track, however their intensities (as measured by maximal near-surface wind speed or 850-hPa relative vorticity) differ up to +/- 25%. A 30-member ensemble created for one of the storms shows that on a single-storm level the response is systematic rather than random. To explain the broad response in storm strength, we show that the SST-adjustment modifies two environmental parameters: surface latent heat flux (LHF) and low-level baroclinicity (B). LHF influences storms by modifying diabatic heating and boundary-layer processes such as vertical mixing. The position of each storm's track relative to the SST-front is important. South of the SST-front the smoothing leads to lower SST, reduced LHF and storms with generally weaker maximum near-surface winds. North of the SST-front the increased LHF tend to enhance the winds, but the accompanying changes in baroclinicity are not necessarily favourable. Together these mechanisms explain up to 80% of the variability in the near-surface maximal wind speed change. Because the mechanisms are less effective in explaining more dynamics-oriented indicators like 850 hPa relative vorticity, we hypothesise that part of the wind-speed change is related to adjustment of the boundary-layer processes in response to the LHF and B changes.

  • 159. Decremer, Damien
    et al.
    Chung, Chul E.
    Ekman, Annica M. L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Brandefelt, Jenny
    Which significance test performs the best in climate simulations?2014Inngår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 66, s. 23139-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate change simulated with climate models needs a significance testing to establish the robustness of simulated climate change relative to model internal variability. Student's t-test has been the most popular significance testing technique despite more sophisticated techniques developed to address autocorrelation. We apply Student's t-test and four advanced techniques in establishing the significance of the average over 20 continuous-year simulations, and validate the performance of each technique using much longer (375-1000 yr) model simulations. We find that all the techniques tend to perform better in precipitation than in surface air temperature. A sizable performance gain using some of the advanced techniques is realised in the model Ts output portion with strong positive lag-1 yr autocorrelation (> +/- 0.6), but this gain disappears in precipitation. Furthermore, strong positive lag-1 yr autocorrelation is found to be very uncommon in climate model outputs. Thus, there is no reason to replace Student's t-test by the advanced techniques in most cases.

  • 160.
    Dekker, Evelien
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Bintanja, Richard
    Severijns, Camiel
    Nudging the Arctic Ocean to Quantify Sea Ice Feedbacks2019Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 32, nr 8, s. 2381-2395Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    With Arctic summer sea ice potentially disappearing halfway through this century, the surface albedo and insulating effects of Arctic sea ice will decrease considerably. The ongoing Arctic sea ice retreat also affects the strength of the Planck, lapse rate, cloud, and surface albedo feedbacks together with changes in the heat exchange between the ocean and the atmosphere, but their combined effect on climate sensitivity has not been quantified. This study presents an estimate of all Arctic sea ice related climate feedbacks combined. We use a new method to keep Arctic sea ice at its present-day (PD) distribution under a changing climate in a 50-yr CO2 doubling simulation, using a fully coupled global climate model (EC-Earth, version 2.3). We nudge the Arctic Ocean to the (monthly dependent) year 2000 mean temperature and minimum salinity fields on a mask representing PD sea ice cover. We are able to preserve about 95% of the PD mean March and 77% of the September PD Arctic sea ice extent by applying this method. Using simulations with and without nudging, we estimate the climate response associated with Arctic sea ice changes. The Arctic sea ice feedback globally equals 0.28 +/- 0.15 W m(-2) K-1. The total sea ice feedback thus amplifies the climate response for a doubling of CO2, in line with earlier findings. Our estimate of the Arctic sea ice feedback agrees reasonably well with earlier CMIP5 global climate feedback estimates and shows that the Arctic sea ice exerts a considerable effect on the Arctic and global climate sensitivity.

  • 161. Deng, Junjie
    et al.
    Harff, Jan
    Schimanke, Semjon
    Meier, H. E. Markus
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish Meteorological and Hydrological Institute, Sweden.
    A method for assessing the coastline recession due to the sea level rise by assuming stationary wind-wave climate2015Inngår i: Oceanological and Hydrobiological Studies, ISSN 1730-413X, Vol. 44, nr 3, s. 362-380Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The method introduced in this study for future projection of coastline changes hits the vital need of communicating the potential climate change impact on the coast in the 21th century. A quantitative method called the Dynamic Equilibrium Shore Model (DESM) has been developed to hindcast historical sediment mass budgets and to reconstruct a paleo Digital Elevation Model (DEM). The forward mode of the DESM model relies on paleo-scenarios reconstructed by the DESM model assuming stationary wind-wave climate. A linear relationship between the sea level, coastline changes and sediment budget is formulated and proven by the least square regression method. In addition to its forward prediction of coastline changes, this linear relationship can also estimate the sediment budget by using the information on the coastline and relative sea level changes. Wind climate change is examined based on regional climate model data. Our projections for the end of the 21st century suggest that the wind and wave climates in the southern Baltic Sea may not change compared to present conditions and that the investigated coastline along the Pomeranian Bay may retreat from 10 to 100 m depending on the location and on the sea level rise which was assumed to be in the range of 0.12 to 0.24 m.

  • 162. Devasthale, A.
    et al.
    Sedlar, J.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Characteristics of water-vapour inversions observed over the Arctic by Atmospheric Infrared Sounder (AIRS) and radiosondes2011Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, nr 18, s. 9813-9823Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An accurate characterization of the vertical structure of the Arctic atmosphere is useful in climate change and attribution studies as well as for the climate modelling community to improve projections of future climate over this highly sensitive region. Here, we investigate one of the dominant features of the vertical structure of the Arctic atmosphere, i.e. water-vapour inversions, using eight years of Atmospheric Infrared Sounder data (2002-2010) and radiosounding profiles released from the two Arctic locations (North Slope of Alaska at Barrow and during SHEBA). We quantify the characteristics of clear-sky water vapour inversions in terms of their frequency of occurrence, strength and height covering the entire Arctic for the first time. We found that the frequency of occurrence of water-vapour inversions is highest during winter and lowest during summer. The inversion strength is, however, higher during summer. The observed peaks in the median inversion-layer heights are higher during the winter half of the year, at around 850 hPa over most of the Arctic Ocean, Siberia and the Canadian Archipelago, while being around 925 hPa during most of the summer half of the year over the Arctic Ocean. The radiosounding profiles agree with the frequency, location and strength of water-vapour inversions in the Pacific sector of the Arctic. In addition, the radiosoundings indicate that multiple inversions are the norm with relatively few cases without inversions. The amount of precipitable water within the water-vapour inversion structures is estimated and we find a distinct, two-mode contribution to the total column precipitable water. These results suggest that water-vapour inversions are a significant source to the column thermodynamics, especially during the colder winter and spring seasons. We argue that these inversions are a robust metric to test the reproducibility of thermodynamics within climate models. An accurate statistical representation of water-vapour inversions in models would mean that the large-scale coupling of moisture transport, precipitation, temperature and water-vapour vertical structure and radiation are essentially captured well in such models.

  • 163. Devasthale, A.
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Caian, M.
    Thomas, M. A.
    Kahn, B. H.
    Fetzer, E. J.
    Influence of the arctic oscillation on the vertical distribution of clouds as observed by the a train constellation of satellites2012Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, nr 21, s. 10535-10544Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The main purpose of this study is to investigate the influence of the Arctic Oscillation (AO), the dominant mode of natural variability over the northerly high latitudes, on the spatial (horizontal and vertical) distribution of clouds in the Arctic. To that end, we use a suite of sensors on-board NASA's A-Train satellites that provide accurate observations of the distribution of clouds along with information on atmospheric thermodynamics. Data from three independent sensors are used (AQUA-AIRS, CALIOP-CALIPSO and CPR-CloudSat) covering two time periods (winter half years, November through March, of 2002-2011 and 2006-2011, respectively) along with data from the ERA-Interim reanalysis. We show that the zonal vertical distribution of cloud fraction anomalies averaged over 67-82 degrees N to a first approximation follows a dipole structure (referred to as Greenland cloud dipole anomaly, GCDA), such that during the positive phase of the AO, positive and negative cloud anomalies are observed eastwards and westward of Greenland respectively, while the opposite is true for the negative phase of AO. By investigating the concurrent meteorological conditions (temperature, humidity and winds), we show that differences in the meridional energy and moisture transport during the positive and negative phases of the AO and the associated thermodynamics are responsible for the conditions that are conducive for the formation of this dipole structure. All three satellite sensors broadly observe this large-scale GCDA despite differences in their sensitivities, spatio-temporal and vertical resolutions, and the available lengths of data records, indicating the robustness of the results. The present study also provides a compelling case to carry out process-based evaluation of global and regional climate models.

  • 164. Devasthale, Abhay
    et al.
    Sedlar, Joseph
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Kahn, Brian H.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Fetzer, Eric J.
    Tian, Baijun
    Teixeira, Joao
    Pagano, Thomas S.
    A DECADE OF SPACEBORNE OBSERVATIONS OF THE ARCTIC ATMOSPHERE: Novel Insights from NASA's AIRS Instrument2016Inngår i: Bulletin of The American Meteorological Society - (BAMS), ISSN 0003-0007, E-ISSN 1520-0477, Vol. 97, nr 11, s. 2163-2176Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Arctic sea ice is declining rapidly and its annual ice extent minima reached record lows twice during the last decade. Large environmental and socioeconomic implications related to sea ice reduction in a warming world necessitate realistic simulations of the Arctic climate system, not least to formulate relevant environmental policies on an international scale. However, despite considerable progress in the last few decades, future climate projections from numerical models still exhibit the largest uncertainties over the polar regions. The lack of sufficient observations of essential climate variables is partly to blame for the poor representation of key atmospheric processes, and their coupling to the surface, in climate models. Observations from the hyper spectral Atmospheric Infrared Sounder (AIRS) instrument on board National Aeronautics and Space Administration (NASA)'s Aqua satellite are contributing toward improved understanding of the vertical structure of the atmosphere over the poles since 2002, including the lower troposphere. This part of the atmosphere is especially important in the Arctic, as it directly impacts sea ice and its short-term variability. Although in situ measurements provide invaluable ground truth, they are spatially and temporally inhomogeneous and sporadic over the Arctic. A growing number of studies are exploiting AIRS data to investigate the thermodynamic structure of the Arctic atmosphere, with applications ranging from understanding processes to deriving climatologies; all of which are also useful to test and improve parameterizations in climate models. As the AIRS data record now extends more than a decade, a select few of many such noteworthy applications of AIRS data over this challenging and rapidly changing landscape are highlighted here.

  • 165. Devasthale, Abhay
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ali, Omar
    The vertical distribution of thin features over the Arctic analysed from CALIPSO observations: Part 2: Aerosols2011Inngår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 63, nr 1, s. 86-95Artikkel i tidsskrift (Fagfellevurdert)
  • 166. Devasthale, Abhay
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Karlsson, Karl-Goran
    Thomas, Manu Anna
    Jones, Colin
    Sedlar, Joseph
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Omar, Ali H.
    The vertical distribution of thin features over the Arctic analysed from CALIPSO observations: Part 1: Optically thin clouds2011Inngår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 63, nr 1, s. 77-85Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Clouds play a crucial role in the Arctic climate system. Therefore, it is essential to accurately and reliably quantify and understand cloud properties over the Arctic. It is also important to monitor and attribute changes in Arctic clouds. Here, we exploit the capability of the CALIPSO-CALIOP instrument and provide comprehensive statistics of tropospheric thin clouds, otherwise extremely difficult to monitor from passive satellite sensors. We use 4 yr of data (June 2006-May 2010) over the circumpolar Arctic, here defined as 67-82 degrees N, and characterize probability density functions of cloud base and top heights, geometrical thickness and zonal distribution of such cloud layers, separately for water and ice phases, and discuss seasonal variability of these properties. When computed for the entire study area, probability density functions of cloud base and top heights and geometrical thickness peak at 200-400, 1000-2000 and 400-800 m, respectively, for thin water clouds, while for ice clouds they peak at 6-8, 7-9 and 400-1000 m, respectively. In general, liquid clouds were often identified below 2 km during all seasons, whereas ice clouds were sensed throughout the majority of the upper troposphere and also, but to a smaller extent, below 2 km for all seasons.

  • 167.
    Dey, Dipanjan
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Döös, Kristofer
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The coupled ocean–atmosphere hydrologic cycle2019Inngår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 71, nr 1, s. 1-11Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The freshwater cycle has in the present study been traced as one integrated process in the coupled ocean–atmosphere system for both present and possible future climates simulated with an Earth-System Model. A method based on water-mass conservation was used in order to calculate mass fluxes of water from regions of evaporation to regions of precipitation. These fluxes include not only advection of moisture by the winds but also the vertical water-mass transport due to precipitation forming hence a mass-conserved 3D water-mass transport field. Six atmospheric hydrological cells were revealed, which cross the sea surface, where they join the oceanic overturning circulation. These atmospheric water cells can be interpreted as an extension of the oceanic overturning circulation, since the otherwise open ocean streamlines at the surface continue into the atmosphere due to evaporation and back into the ocean due to precipitation. Although these atmospheric water cells are related to the usual air cells, they are only half part of the coupled water cells and located differently. The future-climate scenario shows that the mid- and high-latitude atmospheric water-mass cells will transport more moisture towards the poles as well as increase of the northward cross-Equatorial atmospheric water-mass transport.

  • 168.
    Dey, Dipanjan
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Indian Institute of Technology Bhubaneswar, India.
    Sil, Sourav
    Jana, Sudip
    Pramanik, Saikat
    Pandey, P. C.
    An assessment of TropFlux and NCEP air-sea fluxes on ROMS simulations over the Bay of Bengal region2017Inngår i: Dynamics of atmospheres and oceans (Print), ISSN 0377-0265, E-ISSN 1872-6879, Vol. 80, s. 47-61Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This study presents an assessment of the TropFlux and the National Centers for Environmental Prediction (NCEP) reanalysis air-sea fluxes in simulating the surface and subsurface oceanic parameters over the Bay of Bengal (BoB) region during 2002-2014 using the Regional Ocean Modelling System (ROMS). The assessment has been made by comparing the simulated fields with in-situ and satellite observations. The simulated surface and subsurface temperatures in the TropFlux forced experiment (TropFlux-E) show better agreement with the Research Moored Array for African-Asian-Australian Monsoon Analysis (RAMA) and Argo observations than the NCEP forced experiment (NCEP-E). The BoB domain averaged sea surface temperature (SST) simulated in the NCEP-E is consistently cooler than the satellite SST, with a root mean square error (RMSE) of 0.79 C. Moreover, NCEP-E shows a limitation in simulating the observed seasonal cycle of the SST due to substantial underestimation of the pre-monsoon SST peak. These limitations are mostly due to the lower values of the NCEP net hedt flux. The seasonal and interannual variations of SST in the TropFlux-E are better comparable to the observations with correlations and skills more than 0.80 and 0.90 respectively. However, SST is overestimated during summer monsoon periods mainly due to higher net heat flux. The superiority of TropFlux forcing over the NCEP reanalysis can also.be seen when simulating the interannual variabilities of the magnitude and vertical extent of Wyrtki jets at two equatorial RAMA buoy locations. The jet is weaker in the NCEP-E relative to the TropFlux-E and observations. The simulated sea surface height anomalies (SSHA) from both the experiments are able to capture the regions of positive and negative SSHA with respect to satellite-derived altimeter data with better performance in the TropFlux-E. The speed of the westward propagating Rossby wave along 18 N in the TropFlux-E is found to be about 4.7 cm/s, which is close to the theoretical phase speed of Rossby waves.

  • 169.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engqvist, Anders
    Assessment of water exchange between a discharge region and the open sea: A comparison of different methodological concepts2007Inngår i: Estuarine coastal and shelf science, ISSN 0272-7714, Vol. 74, nr 4, s. 709-721Artikkel i tidsskrift (Fagfellevurdert)
  • 170.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engqvist, Anders
    Book review of Lagrangian Analysis and Prediction of Coastal and Ocean Dynamics2008Inngår i: Marin Geophysical Researches, Vol. 29, nr 2, s. 159-Artikkel, omtale (Annet (populærvitenskap, debatt, mm))
  • 171.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Jönsson, Bror
    Kjellsson, Joakim
    Evaluation of oceanic and atmospheric trajectory schemes in the TRACMASS trajectory model v6.02017Inngår i: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 10, nr 4, s. 1733-1749Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Three different trajectory schemes for oceanic and atmospheric general circulation models are compared in two different experiments. The theories of the trajectory schemes are presented showing the differential equations they solve and why they are mass conserving. One scheme assumes that the velocity fields are stationary for set intervals of time between saved model outputs and solves the trajectory path from a differential equation only as a function of space, i.e. stepwise stationary. The second scheme is a special case of the stepwise-stationary scheme, where velocities are assumed constant between general circulation model (GCM) outputs; it uses hence a fixed GCM time step. The third scheme uses a continuous linear interpolation of the fields in time and solves the trajectory path from a differential equation as a function of both space and time, i.e. a time-dependent scheme. The trajectory schemes are tested offline, i.e. using the already integrated and stored velocity fields from a GCM. The first comparison of the schemes uses trajectories calculated using the velocity fields from a high-resolution ocean general circulation model in the Agulhas region. The second comparison uses trajectories calculated using the wind fields from an atmospheric reanalysis. The study shows that using the time-dependent scheme over the stepwise-stationary scheme greatly improves accuracy with only a small increase in computational time. It is also found that with decreasing time steps the stepwise-stationary scheme becomes increasingly more accurate but at increased computational cost. The time-dependent scheme is therefore preferred over the stepwise-stationary scheme. However, when averaging over large ensembles of trajectories, the two schemes are comparable, as intrinsic variability dominates over numerical errors. The fixed GCM time step scheme is found to be less accurate than the stepwisestationary scheme, even when considering averages over large ensembles.

  • 172.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Kjellsson, Joakim
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Jönsson, Bror Fredrik
    Department of Geosciences, Princeton University.
    TRACMASS—A Lagrangian Trajectory Model2013Inngår i: Preventive Methods for Coastal Protection: Towards the Use of Ocean Dynamics for Pollution Control / [ed] Soomere, Tarmo; Quak, Ewald, Springer, 2013, s. 225-249Kapittel i bok, del av antologi (Fagfellevurdert)
    Abstract [en]

    A detailed description of the Lagrangian trajectory model TRACMASS is presented. The theory behind the original scheme for steady state velocities is derived for rectangular and curvilinear grids with different vertical coordinates for the oceanic and atmospheric circulation models. Two different ways to integrate the trajectories in time in TRACMASS are presented. These different time schemes are compared by simulating inertial oscillations, which show that both schemes are sufficiently accurate not to deviate from the analytical solution.The TRACMASS are exact solutions to differential equations and can hence be integrated both forward and backward with unique solutions. Two low-order trajectory subgrid parameterizations, which are available in TRACMASS, are explained. They both enable an increase of the Lagrangian dispersion, but are, however, too simple to simulate some of the Lagrangian properties that are desirable. The mass conservation properties of TRACMASS are shown to make it possible to follow the water or air masses both forward and backward in time, which also opens up for all sorts of calculations of water/air mass exchanges as well as Lagrangian stream functions.

  • 173.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Kjellsson, Joakim
    Zika, Jan
    Laliberte, Frederic
    Brodeau, Laurent
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Aldama Campino, Aitor
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The Coupled Ocean-Atmosphere Hydrothermohaline Circulation2017Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, nr 2, s. 631-647Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The thermohaline circulation of the ocean is compared to the hydrothermal circulation of the atmosphere. The oceanic thermohaline circulation is expressed in potential temperature-absolute salinity space and comprises a tropical cell, a conveyor belt cell, and a polar cell, whereas the atmospheric hydrothermal circulation is expressed in potential temperature-specific humidity space and unifies the tropical Hadley and Walker cells as well as the midlatitude eddies into a single, global circulation. The oceanic thermohaline streamfunction makes it possible to analyze and quantify the entire World Ocean conversion rate between cold-warm and fresh-saline waters in one single representation. Its atmospheric analog, the hydrothermal streamfunction, instead captures the conversion rate between cold-warm and dry-humid air in one single representation. It is shown that the ocean thermohaline and the atmospheric hydrothermal cells are connected by the exchange of heat and freshwater through the sea surface. The two circulations are compared on the same diagramby scaling the axes such that the latent heat energy required to move an air parcel on the moisture axis is equivalent to that needed to move a water parcel on the salinity axis. Such a comparison leads the authors to propose that the Clausius-Clapeyron relationship guides both the moist branch of the atmospheric hydrothermal circulation and the warming branches of the tropical and conveyor belt cells of the oceanic thermohaline circulation.

  • 174.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Meier, H E Markus
    Döscher, Ralf
    The Baltic haline conveyor belt or the overturning circulation and mixing in the Baltic.2004Inngår i: Ambio, ISSN 0044-7447, Vol. 33, nr 4-5, s. 261-6Artikkel i tidsskrift (Fagfellevurdert)
  • 175.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nilsson, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Analysis of the Meridional Energy Transport by Atmospheric Overturning Circulations2011Inngår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, nr 8, s. 1806-1820Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The atmospheric meridional overturning circulation is computed using the interim European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-Interim) data. Meridional mass transport streamfunctions are calculated not only using pressure as a vertical coordinate but also using temperature, specific humidity, and geopotential height as generalized vertical coordinates. Moreover, mass transport streamfunctions are calculated using the latent, the dry static, or the moist static energy as generalized vertical coordinates. The total meridional energy transport can be obtained by integrating these streamfunctions vertically over their entire energy range. The time-averaged mass transport streamfunctions are also decomposed into mean-flow and eddy-induced components. The meridional mass transport streamfunctions with temperature and specific humidity as independent variables yield a two-cell structure with a tropical Hadley-like cell and a pronounced extratropical Ferrel-like cell, which carries warm and moist air poleward. These Ferrel-like cells are much stronger than the Eulerian zonal-mean Ferrel cell, a feature that can be understood by considering the residual circulation related to specific humidity or temperature. Regardless of the generalized vertical coordinate, the present meridional mass transport streamfunctions yield essentially a two-layer structure with one poleward and one equatorward branch. The strongest meridional overturning in the midlatitudes is obtained when the specific humidity or the moist static energy is used as the vertical coordinate, indicating that the specific humidity is the variable that best distinguishes between poleward- and equatorward-moving air in the lower troposphere.

  • 176.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nilsson, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nycander, Jonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Brodeau, Laurent
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ballarotta, Maxime
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The world ocean thermohaline circulation2012Inngår i: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 42, nr 9, s. 1445-1460Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A new global streamfunction is presented and denoted the thermohaline streamfunction. This is defined as the volume transport in terms of temperature and salinity (hence no spatial variables). The streamfunction is used to analyze and quantify the entire World Ocean conversion rate between cold/warm and fresh/saline waters. It captures two main cells of the global thermohaline circulation, one corresponding to the conveyor belt and one corresponding to the shallow tropical circulation. The definition of a thermohaline streamfunction also enables a new method of estimating the turnover time as well as the heat and freshwater transports of the conveyor belt. The overturning time of the conveyor belt is estimated to be between 1000 and 2000 yr, depending on the choice of stream layer. The heat and freshwater transports of these two large thermohaline cells have been calculated by integrating the thermohaline streamfunction over the salinity or temperature, yielding a maximum heat transport of the conveyor belt of 1.2 PW over the 34.2-PSU salinity surface and a freshwater transport of 0.8 Sv (1 Sv = 10(6) m(3) s(-1)) over the 9 degrees C isotherm. This is a measure of the net interocean exchange of heat between the Atlantic and Indo-Pacific due to the thermohaline circulation.

  • 177.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nycander, Jonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Coward, Andrew C.
    Lagrangian decomposition of the Deacon Cell2008Inngår i: Journal of Geophysical Research: Oceans, Vol. 113, s. C07028-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The meridional overturning cells in the Southern Ocean are decomposed by Lagrangian tracing using velocity and density fields simulated with an ocean general circulation model. Particular emphasis is given to the Deacon Cell. The flow is divided into four major components: 1) water circling around Antarctica in the Antarctic Circumpolar Current (ACC), 2) water leaving the ACC towards the north into the three world oceans, 3) water coming from the north and joining the ACC, mainly consisting of North Atlantic Deep Water (NADW) and 4) inter-ocean exchange between the three world oceans without circling around Antarctica. The Deacon Cell has an amplitude of 20 Sv, of which 6 Sv can be explained by the the east-west tilt of the ACC, 5 Sv by the east-west tilt of the sub-tropical gyre, and the remaining 9 Sv by the differences of the slope and depth of the southward transport of NADW and its return flow as less dense water. The diabatic or cross-isopycnal Deacon Cell is only 2 Sv.

  • 178.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Rupolo, Volfango
    Brodeau, Laurent
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Dispersion of surface drifters and model-simulated trajectories2011Inngår i: Ocean Modelling, ISSN 1463-5003, E-ISSN 1463-5011, Vol. 39, nr 3-4, s. 301-310Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    From a data set encompassing the years 1990-2008 pairs of surface drifters with maximum initial separations of 5, 10 and 25 km have been identified. Model trajectories have been calculated using the same initial positions and times as the selected pairs of surface drifters. The model trajectories are based on the TRACMASS trajectory code and driven by the ocean general circulation model NEMO. The trajectories are calculated off-line, i.e. with the stored velocity fields from the circulation model. The sensitivity of the trajectory simulations to the frequency of the stored velocity fields was tested for periods of 3 and 6 h as well as 5 days. The relative dispersion of the surface-drifter and model trajectories has been compared, where the latter was found to be too low compared to the relative dispersion of the drifters. Two low-order trajectory sub-grid parameterisations were tested and successfully tuned so that the total amplitude of the relative dispersion of the model trajectories is similar to that associated with the drifter trajectories. These parameterisations are, however, too simple for a correct simulation of Lagrangian properties such as the correlation time scales and the variance of the eddy kinetic energy. The importance of model-grid resolution is quantified by comparing the relative dispersion from an eddy-permitting and a coarse-resolution model, respectively. The dispersion rate is halved with the coarse grid. The consequences of the two-dimensionality of the trajectories is evaluated by comparing the results obtained with the 2D and the Lagrangian 3D trajectories. This shows that the relative dispersion is 15% stronger when the trajectories are freely advected with the 3D velocity field.

  • 179. Eberhart, Martin
    et al.
    Löhle, Stefan
    Strelnikov, Boris
    Hedin, Jonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Khaplanov, Mikhail
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Fasoulas, Stefanos
    Gumbel, Jörg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Lübken, Franz-Josef
    Rapp, Markus
    Atomic oxygen number densities in the mesosphere-lower thermosphere region measured by solid electrolyte sensors on WADIS-22019Inngår i: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 12, nr 4, s. 2445-2461Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 180. Efrati, S.
    et al.
    Lehahn, Y.
    Rahav, E.
    Kress, N.
    Herut, B.
    Gertman, I.
    Goldman, R.
    Ozer, T.
    Lazar, M.
    Heifetz, Eyal
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Intrusion of coastal waters into the pelagic eastern Mediterranean: in situ and satellite-based characterization2013Inngår i: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 10, nr 5, s. 3349-3357Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A combined dataset of near-real-time multi-satellite observations and in situ measurements from a high-resolution survey is used for characterizing physical-biogeochemical properties of a patch stretching from the coast to the open sea in the Levantine Basin (LB) of the eastern Mediterranean (EM). Spatial analysis of the combined dataset indicates that the patch is a semi-enclosed system, bounded within the mixed layer and separated from ambient waters by transport barriers induced by horizontal stirring. As such, the patch is characterized by physical-biogeochemical properties that significantly differ from those of the waters surrounding it, with lower salinity and higher temperatures, concentrations of silicic acid and chlorophyll a, and abundance of Synechococcus and picoeukaryote cells. Based on estimates of patch dimensions (similar to 40 km width and similar to 25m depth) and propagation speed (similar to 0.09ms(-1)), the volume flux associated with the patch is found to be on the order of 0.1 Sv. Our observations suggest that horizontal stirring by surface currents is likely to have an important impact on the ultra-oligotrophic Levantine Basin ecosystem, through (1) transport of nutrients and coastally derived material, and (2) formation of local, dynamically isolated niches. In addition, this work provides a satellite-based framework for planning and executing high-resolution sampling strategies in the interface between the coast and the open sea.

  • 181.
    Ehard, Benedikt
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Achtert, Peggy
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Dörnbrack, Andreas
    Gisinger, Sonja
    Gumbel, Jörg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Khaplanov, Mikhail
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Rapp, Markus
    Wagner, Johannes
    Combination of Lidar and Model Data for Studying Deep Gravity Wave Propagation2016Inngår i: Monthly Weather Review, ISSN 0027-0644, E-ISSN 1520-0493, Vol. 144, nr 1, s. 77-98Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The paper presents a feasible method to complement ground-based middle atmospheric Rayleigh lidar temperature observations with numerical simulations in the lower stratosphere and troposphere to study gravity waves. Validated mesoscale numerical simulations are utilized to complement the temperature below 30-km altitude. For this purpose, high-temporal-resolution output of the numerical results was interpolated on the position of the lidar in the lee of the Scandinavian mountain range. Two wintertime cases of orographically induced gravity waves are analyzed. Wave parameters are derived using a wavelet analysis of the combined dataset throughout the entire altitude range from the troposphere to the mesosphere. Although similar in the tropospheric forcings, both cases differ in vertical propagation. The combined dataset reveals stratospheric wave breaking for one case, whereas the mountain waves in the other case could propagate up to about 40-km altitude. The lidar observations reveal an interaction of the vertically propagating gravity waves with the stratopause, leading to a stratopause descent in both cases.

  • 182.
    Ehard, Benedikt
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Achtert, Peggy
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Gumbel, Jörg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Long-term lidar observations of wintertime gravity wave activity over northern Sweden2014Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 32, nr 11, s. 1395-1405Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper presents an analysis of gravity wave activity over northern Sweden as deduced from 18 years of wintertime lidar measurements at Esrange (68ºN, 21ºE). Gravity wave potential energy density (GWPED) was used to characterize the strength of gravity waves in the altitude regions 30–40km and 40–50 km. The obtained values ex- ceed previous observations reported in the literature. This is suggested to be due to Esrange’s location downwind of the Scandinavian mountain range and due to differences in the various methods that are currently used to retrieve gravity wave parameters. The analysis method restricted the identification of the dominating vertical wavelengths to a range from 2 to 13 km. No preference was found for any wavelength in this window. Monthly mean values of GW- PED show that most of the gravity waves’ energy dissipates well below the stratopause and that higher altitude regions show only small dissipation rates of GWPED. Our analy- sis does not reproduce the previously reported negative trend in gravity wave activity over Esrange. The observed inter-annual variability of GWPED is connected to the occurrence of stratospheric warmings with generally lower wintertime mean GWPED during years with major stratospheric warmings. A bimodal GWPED occurrence frequency indicates that gravitywave activity at Esrange is affected by both ubiq- uitous wave sources and orographic forcing.

  • 183.
    Eilola, K.
    et al.
    Swedish Meteorol & Hydrol Inst, S-60176 Norrköping, Sweden.
    Mårtensson, Sebastian
    Swedish Meteorol & Hydrol Inst, S-60176 Norrköping, Sweden.
    Meier, Markus
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish Meteorol & Hydrol Inst, S-60176 Norrköping, Sweden.
    Modeling the impact of reduced sea ice cover in future climate on the Baltic Sea biogeochemistry2013Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, nr 1, s. 149-154Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In a warming future climate, the sea ice cover is expected to decrease, with very likely large consequences for the marine ecosystem. We investigated the impact of future sea ice retreat on the Baltic Sea biogeochemistry at the end of the century, using an ensemble of regionalized global climate simulations. We found that the spring bloom will start by up to one month earlier and winds and wave-induced resuspension will increase, causing an increased transport of nutrients from the productive coastal zone into the deeper areas. The internal nutrient fluxes do not necessarily increase because they also depend on oxygen and temperature conditions of the bottom water. Winter mixing increases in areas having reduced ice cover and in areas having reduced stratification due to increased freshwater supply. The reduced sea ice cover therefore partly counteracts eutrophication because increased vertical mixing improves oxygen conditions in lower layers.

  • 184. Eilola, Kari
    et al.
    Almroth-Rosell, Elin
    Meier, H. E. Markus
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish Meteorological and Hydrological Institute, Sweden.
    Impact of saltwater inflows on phosphorus cycling and eutrophication in the Baltic Sea: a 3D model study2014Inngår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 66, s. 23985-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The impact of dense saltwater inflows on the phosphorus dynamics in the Baltic Sea is studied from tracer experiments with a three-dimensional physical model. Model simulations showed that the coasts of the North West Gotland Basin and the Gulf of Finland, the Estonian coast in the East Gotland Basin are regions where tracers from below the halocline are primarily lifted up above the halocline. After 1 yr tracers are accumulated at the surface along the Swedish east coast and at the western and southern sides of Gotland. Elevated concentrations are also found east and southeast of Gotland, in the northern Bornholm Basin and in the central parts of the East Gotland Basin. The annual supplies of phosphorus from the deeper waters to the productive surface layers are estimated to be of the same order of magnitude as the waterborne inputs of phosphorus to the entire Baltic Sea. The model results suggest that regionally the impact of these nutrients may be quite large, and the largest regional increases in surface concentrations are found after large inflows. However, the overall direct impact of major Baltic inflows on the annual uplift of nutrients from below the halocline to the surface waters is small because vertical transports are comparably large also during periods without major inflows. Our model results suggest that phosphorus released from the sediments between 60 and 100 m depth in the East Gotland Basin contributes to the eutrophication, especially in the coastal regions of the eastern Baltic Proper.

  • 185.
    Ekman, Annica
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engström, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Wang, C.
    The effect of aerosol composition and concentration on the development and anvil properties of a continental deep convective cloud2007Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, Vol. 133, nr 627, s. 1439-1452Artikkel i tidsskrift (Fagfellevurdert)
  • 186.
    Ekman, Annica
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engström, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ström, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    de Reus, Marian
    Max Planck Institute for Chemistry, Mainz, Germany.
    Williams, Jonathan
    Max Planck Institute for Chemistry, Mainz, Germany.
    Andreae, Meinrat
    Max Planck Institute for Chemistry, Mainz, Germany.
    Do organics contribute to small particle formation in the Amazonian upper troposphere?2008Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, nr L17810, s. 5-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    3-D cloud-resolving model simulations including explicit aerosol physics and chemistry are compared with observations of upper tropospheric (12 km) aerosol size distributions over the Amazon Basin. The model underestimates the aerosol number concentration for all modes, especially the nucleation mode (d < 18 nm). We show that a boundary layer SO2 mixing ratio of approximately 5 ppb would be needed in order to reproduce the high nucleation mode number concentrations observed. This high SO2 mixing ratio is very unlikely for the pristine Amazon Basin at this time of the year. Hence, it is suggested that vapours other than H2SO4 participate in the formation and growth of small aerosols. Using activation nucleation theory together with a small (0.4–10%) secondary organic aerosol mass yield, we show that isoprene has the potential of substantially increasing the number of small particles formed as well as reducing the underestimate for the larger aerosol modes.

  • 187.
    Ekman, Annica
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engström, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ström, Johan
    Institutionen för tillämpad miljövetenskap (ITM).
    de Reus, Marian
    Williams, Jonathan
    Andreae, Meinrat O.
    Do organics contribute to new particle formation in the Amazonian upper troposphere?2008Inngår i: Geophysical Research Letters, Vol. 35, s. L17810-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    3-D cloud-resolving model simulations including explicit aerosol physics and chemistry are compared with observations of upper tropospheric (12 km) aerosol size distributions over the Amazon Basin. The model underestimates the aerosol number concentration for all modes, especially the nucleation mode (d< 18nm). We show that a boundary layer SO2 mixing ratio of approximately 5 ppb would be needed in order to reproduce the high nucleation mode number concentrations observed. This high SO2 mixing ratio is very unlikely for the pristine Amazon Basin at this time of the year. Hence, it is suggested that vapours other than H2SO4 participate in the formation and growth of small aerosols. Using activation nucleation theory together with a small (0.4-10%) secondary organic aerosol mass yield, we show that isoprene has the potential of substantially increasing the number of small particles formed as well as reducing the underestimate for the larger aerosol modes.

  • 188.
    Ekman, Annica M. L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Do sophisticated parameterizations of aerosol-cloud interactions in CMIP5 models improve the representation of recent observed temperature trends?2014Inngår i: Journal of Geophysical Research: Atmospheres, ISSN 2169-897X, Vol. 119, nr 2, s. 817-832Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Model output from the Coupled Model Intercomparison Project phase 5 (CMIP5) archive was compared with the observed latitudinal distribution of surface temperature trends between the years 1965 and 2004. By comparing model simulations that only consider changes in greenhouse gas forcing (GHG) with simulations that also consider the time evolution of anthropogenic aerosol emissions (GHGAERO), the influence of aerosol forcing on modeled surface temperature trends, and the dependence of the forcing on the model representation of aerosols and aerosol indirect effects, was evaluated. One group of models include sophisticated parameterizations of aerosol activation into cloud droplets; viz., the cloud droplet number concentration (CDNC) is a function of the modeled supersaturation as well as the aerosol concentration. In these models, the temperature trend bias was reduced in GHGAERO compared to GHG in more regions than in the other models. The ratio between high- and low-latitude warming also improved compared to observations. In a second group of models, the CDNC is diagnosed using an empirical relationship between the CDNC and the aerosol concentration. In this group, the temperature trend bias was reduced in more regions than in the model group where no aerosol indirect effects are considered. No clear difference could be found between models that include an explicit aerosol module and the ones that utilize prescribed aerosol. There was also no clear difference between models that include aerosol effects on the precipitation formation rate and the ones that do not. The results indicate that the best representation of recent observed surface temperature trends is obtained if the modeled CDNC is a function of both the aerosol concentration and the supersaturation. Key Points <list list-type=bulleted> <list-item id=jgrd51052-li-0001>CMIP5 GCMs disagree on late 20th century zonal average aerosol forcing <list-item id=jgrd51052-li-0002>Including aerosol indirect effects reduces the zonal mean temperature bias <list-item id=jgrd51052-li-0003>A more sophisticated parameterization of droplet activation is beneficial

  • 189.
    Ekman, Annica M. L.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engström, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Söderberg, Anders
    Impact of Two-Way Aerosol-Cloud Interaction and Changes in Aerosol Size Distribution on Simulated Aerosol-Induced Deep Convective Cloud Sensitivity2011Inngår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, nr 4, s. 685-698Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Recent cloud-resolving model studies of single (isolated) deep convective clouds have shown contradicting results regarding the response of the deep convection to changes in the aerosol concentration. In the present study, a cloud-resolving model including explicit aerosol physics and chemistry is used to examine how the complexity of the aerosol model, the size of the aerosols, and the aerosol activation parameterization influence the aerosol-induced deep convective cloud sensitivity. Six sensitivity series are conducted. A significant difference in the aerosol-induced deep convective cloud sensitivity is found when using different complexities of the aerosol model and different aerosol activation parameterizations. In particular, graupel impaction scavenging of aerosols appears to be a crucial process because it efficiently may limit the number of cloud condensation nuclei (CCN) at a critical stage of cloud development and thereby dampen the convection. For the simulated case, a 100% increase in aerosol concentration results in a difference in average updraft between the various sensitivity series that is as large as the average updraft increase itself. The change in graupel and rain formation also differs significantly. The sign of the change in precipitation is not always directly proportional to the change in updraft velocity and several of the sensitivity series display a decrease of the rain amount with increasing updraft velocity. This result illustrates the need to account for changes in evaporation processes and subsequent cooling when assessing aerosol effects on deep convective strength. The model simulations also show that an increased number of aerosols in the Aitken mode (here defined by 23 <= d <= 100.0 nm) results in a larger impact on the convective strength compared to an increased number of aerosols in the accumulation mode (here defined by 100 <= d <= 900.0 nm). When accumulation mode aerosols are activated and grow at the beginning of the cloud cycle, the supersaturation near the cloud base is lowered, which to some extent limits further aerosol activation. The simulations indicate a need to better understand and represent the two-way interaction between aerosols and clouds when studying aerosol-induced deep convective cloud sensitivity.

  • 190.
    Ekman, Annica M. L.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Hermann, Markus
    Gross, Peter
    Heintzenberg, Jost
    Kim, Dongchul
    Wang, Chien
    Sub-micrometer aerosol particles in the upper troposphere/lowermost stratosphere as measured by CARIBIC and modeled using the MIT-CAM3 global climate model2012Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, s. D11202-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this study, we compare modeled (MIT-CAM3) and observed (CARIBIC) sub-micrometer nucleation (N4-12, 4 <= d <= 12 nm) and Aitken mode (N-12, d > 12 nm) particle number concentrations in the upper troposphere and lowermost stratosphere (UT/LMS). Modeled and observed global median N4-12 and N-12 agree fairly well (within a factor of two) indicating that the relatively simplified binary H2SO4-H2O nucleation parameterization applied in the model produces reasonable results in the UT/LMS. However, a comparison of the spatiotemporal distribution of sub-micrometer particles displays a number of discrepancies between MIT-CAM3 and CARIBIC data: N4-12 is underestimated by the model in the tropics and overestimated in the extra-topics. N-12 is in general overestimated by the model, in particular in the tropics and during summer months. The modeled seasonal variability of N4-12 is in poor agreement with CARIBIC data whereas it agrees rather well for N-12. Modeled particle frequency distributions are in general narrower than the observed ones. The model biases indicate an insufficient diffusive mixing in MIT-CAM3 and a too large vertical transport of carbonaceous aerosols. The overestimated transport is most likely caused by the constant supersaturation threshold applied in the model for the activation of particles into cloud droplets. The annually constant SO2 emissions in the model may also partly explain the poor representation of the N4-12 seasonal cycle. Comparing the MIT-CAM3 with CARIBIC data, it is also clear that care has to be taken regarding the representativeness of the measurement data and the time frequency of the model output.

  • 191.
    Ekman, Annica M. L.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Lewinschal, Anna
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Struthers, Hamish
    Can an influence of changing aerosol emissions be detected in thepattern of surface temperature change between 1970 and 2000?Manuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    The general circulation model CAM-Oslo was used to examine the influence of varyingaerosol and greenhouse gas emissions on the pattern of surface temperature change betweenthe years 1970 and 2000, and whether the temperature response over different regions wasgoverned by local (due to changes in energy fluxes) or far-field (due to changes in large-scale circulation) processes. Circulation changes, originating from precipitation anomaliesmainly over the west/central Pacific and off the east coast of North America, influenced asubstantial part of the northern hemisphere temperature change pattern in CAM-Oslo, inparticular over southern North America, but also over Europe and Asia. The result highlightsthe importance of better understanding zonally asymmetric precipitation changes due todifferent forcing agents. A local response in surface temperature due to net surface radiativeflux (RF) anomalies could also be detected over Europe and Asia, where the differencein all-sky net surface RF was mainly driven by aerosol- or circulation-induced changes inliquid water path and cloud cover. A local anthropogenic aerosol effect on the cloud dropletsize and subsequent short-wave (SW) RF was found over Europe and Asia, but only whenexcluding a change in the greenhouse gas concentration. For clear skies, the SW RF patternwas well-correlated with the aerosol optical depth anomalies. However, this correlation wasat least partly governed by relative humidity fluctuations. Overall, the greenhouse andaerosol effects on surface temperature were in the simulations found to be non-linear with asignificant feedback on the aerosol population from a warming climate.

  • 192.
    Ekström, Sanna
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nozière, Barbara
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Hansson, Hans-Christen
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    The Cloud Condensation Nuclei (CCN) properties of 2-methyltetrols and C3–C6 polyols from osmolality and surface tension measurements (Discussion paper)2008Inngår i: Atmospheric Chemistry and Physics Discussion, ISSN 1680-7367, Vol. 8, nr 5, s. 17237-17256Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A significant fraction of the organic material in aerosols is made of highly soluble compounds such as sugars (mono- and polysaccharides) and polyols, including the 2-methyltetrols, methylerythritol and methyltreitol. The high solubility of these compounds has brought the question of their potentially high CCN efficiency. For the 2-methyltetrols, this would have important implications for cloud formation at global scale because they are thought to be produced by the atmospheric oxidation of isoprene. To investigate this question, the complete Köhler curves for C3–C6 polyols and the 2-methyltetrols have been determined experimentally from osmolality and surface tension measurements. Contrary to what expected, none of these compounds displayed a critical supersaturation lower than those of inorganic salts or organic acids. Their Raoult terms show that this limited CCN efficiency is due to their absence of dissociation in water, this in spite of slight surface-tension effects for the 2-methyltetrols. Thus, compounds such as sugars and polyols would not contribute more to cloud formation in the atmosphere than any other organic compounds studied so far. In particular, the presence of 2-methyltetrols in aerosols would not particularly enhance cloud formation in the atmosphere, contrary to what has been suggested.

  • 193. El-Hames, A. S.
    et al.
    Hannachi, Abdelwaheb
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Al-Ahmadi, M.
    Al-Amri, N.
    Groundwater Quality Zonation Assessment using GIS, EOFs and Hierarchical Clustering2013Inngår i: Water resources management, ISSN 0920-4741, E-ISSN 1573-1650, Vol. 27, nr 7, s. 2465-2481Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Three methods are utilized in this paper to assist in the groundwater clustering, in an arid region aquifer, into similar zones according to its quality. A multiple regression is first applied in order to assess the importance of the different chemical constituents in the amount of total dissolved salt, which shows the dominance of chlorine and sodium. A multivariate analysis based on empirical orthogonal functions and hierarchical clustering (EOFs) is applied to assist in water quality clustering in the studied aquifer. The clustering has produced five distinguished categories of groundwater quality, which agree well with World Health Organisation criteria and limits for water usage. Based on these categories, spatial distribution maps of groundwater quality are produced by Kriging and GIS software.

  • 194. Enell, Carl-Fredrik
    et al.
    Hedin, Jonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Stegman, Jacek
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Witt, Georg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Friedrich, Martin
    Singer, Werner
    Baumgarten, Gerd
    Kaifler, Bernd
    Hoppe, Ulf-Peter
    Gustavsson, Björn
    Brandström, Urban
    Khaplanov, Mikhail
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Kero, Antti
    Ulich, Thomas
    Turunen, Esa
    The Hotel Payload 2 campaign: Overview of NO, O and electron density measurements in the upper mesosphere and lower thermosphere2011Inngår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 73, nr 14-15, s. 2228-2236Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The ALOMAR eARI Hotel Payload 2 (HotPay 2) rocket campaign took place at Andoya Rocket Range, Norway, in January 2008. The rocket was launched on January 31, 2008 at 19:14 UT, when auroral activity appeared after a long geomagnetically quiet period. In this paper we present an overview of the HotPay 2 measurements of upper mesospheric and lower thermospheric (UMLT) electron, atomic oxygen (O) and nitric oxide (NO) densities. [O] and [NO] were retrieved from a set of three photometers, Night-Time Emissions from the Mesosphere and Ionosphere (NEMI). Faraday rotation receivers on the rocket and the EISCAT UHF incoherent scatter radar provided simultaneous electron density profiles, whereas the ALOMAR Na lidar and meteor radar measured the temperature profile and wind. The aurora was also observed with ground-based imagers. The retrieved oxygen number density profile has a maximum at 89 km, some 10 km lower than expected from earlier measurements and modelled profiles based on climatological averages (such as the MSIS model), and the retrieved NO densities are also lower than the expected. Satellite measurements indicate that subsidence over the winter pole controlled the densities. Quantitative chemistry model results based on climatological average atmospheric density and temperature profiles were, therefore, not in good agreement with the measured profiles. The Hotel Payload 2 measurements thus confirm the importance of downward transport from the thermosphere into the winter polar vortex.

  • 195.
    Eneroth, Kristina
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Holmen, Kim
    Berg, Torunn
    Schmidbauer, Norbert
    Solberg, Sverre
    Springtime depletion of tropospheric ozone, gaseous elemental mercury and non-methane hydrocarbons in the European Arctic, and its relation to atmospheric transport2007Inngår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 41, nr 38, s. 8511-8526Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using a trajectory climatology for the period 1992-2001 we have examined how seasonal changes in transport cause changes in the concentrations of tropospheric ozone (O-3), gaseous elemental mercury (GEM) and non-methane hydrocarbons (NMHCs) observed at the Mt. Zeppelin station, Ny-angstrom lesund (78.9 degrees N, 11.9 degrees E). During April-June O-3 depletion events were frequently observed in connection with air transport across the Arctic Basin. The O-3 loss was most pronounced in air masses advected close to the surface. This result supports the idea that the O-3 depletion reactions take place in the lowermost part of the atmosphere in the central Arctic Basin. A strong positive correlation between springtime O-3 depletion events and the oxidation of GEM to divalent mercury was found. During air mass advection from Siberia, the Barents Sea and the Norwegian Sea the strongest correlation was observed during April-May, whereas air masses originating from the Canadian Arctic and the central Arctic areas showed the highest O-3-GEM correlation in May-June. We suggest that this 1-month lag could either be due to the position of the marginal ice zone or temperature differences between the northwestern and northeastern air masses. In connection with springtime O-3 depletion events low concentrations of some NMHCs, especially ethane and ethyne, were observed, indicating that both bromine (ethyne oxidant) and chlorine radicals (ethane oxidant) are present in the Arctic atmosphere during spring. In winter, negative correlations between O-3 and NMHCs were found in connection with air transport from Europe and Siberia, which we interpret as O-3 destruction taking place in industrially contaminated plumes.

  • 196.
    Engardt, Magnuz
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Climate change and carbon dioxide fluxes in the high latitude northern hemisphere1997Doktoravhandling, med artikler (Annet vitenskapelig)
  • 197. Enger, Leif
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Estimating the effects on the regional precipitation climate in a semiarid region caused by an artificial lake using a mesoscale model1991Inngår i: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 30, s. 227-250Artikkel i tidsskrift (Fagfellevurdert)
  • 198.
    Engqvist, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Systemekologiska institutionen.
    Döös, Kristofer
    Meteorologiska institutionen (MISU).
    Andrejev, O
    Modeling water exchange and contaminant transport through a Baltic coastal region2006Inngår i: Ambio, ISSN 0044-7447, Vol. 35, nr 8, s. 435-447Artikkel i tidsskrift (Fagfellevurdert)
  • 199.
    Engström, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Aerosol-cloud interaction from an observational and modeling perspective2011Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Clouds may respond strongly to changes in the atmospheric aerosol population, and the response of clouds to an increased global aerosol burden could to some extent mask the warming caused by enhanced greenhouse gas concentrations. However, estimates of the impact of aerosols on cloud properties are associated with large uncertainties, both because of difficulties representing the aerosol-cloud interaction within models, and because of problems of unequivocally isolating the effect of aerosols on cloud properties in observational data. This thesis focuses in part on underlying meteorological factors that significantly correlate with both aerosol and cloud properties, and on how sensitive clouds are to small variations in meteorological conditions. It was found that meteorological covariations must be taken into account when estimating the strength of the relationship between aerosols and cloud properties. By studying the response of shallow convective clouds to perturbations in meteorological conditions and aerosol concentration, it was further concluded that variations in meteorological conditions can enhance or mask the relationship between aerosols and cloud properties, making it difficult to isolate the aerosol signature from small meteorological differences. Additionally, the impact of deep convective clouds on the redistribution of aerosols within a cloud life cycle is examined. It was found that mid-tropospheric aerosols can have a substantial source in evaporating cloud droplets within deep convection. Lastly, this thesis focuses on the implications of meteorological analysis uncertainties, in part related to the difficulties of constraining meteorological variability in observational data of clouds and aerosols, but mainly the impact of analysis errors on atmospheric trajectory calculations. A method is presented to consistently estimate the uncertainty in trajectory calculations. It was concluded that the spatial and temporal trajectory error can be substantially underestimated if the analysis error is not taken into account.

  • 200.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Bender, Frida A. -M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Charlson, Robert J.
    Wood, Robert
    Geographically coherent patterns of albedo enhancement and suppression associated with aerosol sources and sinks2015Inngår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 67, s. 1-9, artikkel-id 26442Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Earth's albedo is the primary determinant of the amount of energy absorbed by the Earth-atmosphere system. It is a function of the fractional cloud cover and the cloudy-and clear-sky albedos, and thereby of the aerosol loading of the atmosphere. Here, we introduce a method by which we can examine the spatial distribution of the albedo variability that is independent of variations in the two dominant factors of albedo: cloud fraction and liquid water path (LWP). The analysis is based on data simultaneously retrieved from the CERES and MODIS instruments carried on board the Aqua satellite. We analysed the daily overpass data between July 2002 and June 2014 and showed that perturbations in albedo, accounting for variations induced by cloud fraction and LWP, display a coherent geographical pattern. Positive deviations occur in proximity to known anthropogenic aerosol sources, and negative deviations coincide with areas of intense precipitation, acting as aerosol sinks. A simple multiplication of the observed positive perturbations in albedo with the solar flux of 340Wm(-2) yields a magnitude of that effect of several watts per square meter locally. While the location and scale of the geographical pattern might suggest an anthropogenic contribution to the positive albedo perturbations, it is imperative to first carefully examine all other possible causal factors behind the perturbations. Finally, although we have not attempted a full calculation of detection limits, the analysis is capable of sensing very small changes in average albedo of the order of 0.003 out of a total albedo of the order of 0.3. Hence, the applied method might find utilisation in a variety of situations where there is a need to quantify small perturbations of a dependent variable in noisy global data sets.

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