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  • 151. 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.

  • 152. 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.

  • 153. 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.

  • 154. 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.

  • 155. 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.

  • 156. 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)
  • 157. 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.

  • 158.
    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.

  • 159.
    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)
  • 160.
    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))
  • 161.
    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.

  • 162.
    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.

  • 163.
    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.

  • 164.
    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)
  • 165.
    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.

  • 166.
    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.

  • 167.
    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.

  • 168.
    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.

  • 169. 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.

  • 170.
    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.

  • 171.
    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.

  • 172.
    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.

  • 173. 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.

  • 174.
    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)
  • 175.
    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.

  • 176.
    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.

  • 177.
    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

  • 178.
    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.

  • 179.
    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.

  • 180.
    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.

  • 181.
    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.

  • 182. 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.

  • 183. 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.

  • 184.
    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.

  • 185.
    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)
  • 186. 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)
  • 187.
    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)
  • 188.
    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.

  • 189.
    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.

  • 190.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Bender, Frida A.-M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Charlson, R. J.
    Wood, R.
    The nonlinear relationship between albedo and cloud fraction on near-global, monthly mean scale in observations and in the CMIP5 model ensemble2015Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, nr 21, s. 9571-9578Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the relation between monthly mean albedo and cloud fraction over ocean,60∘S–60∘N. Satellite obser vations indicate that these clouds all fall on the same near-exponential curve,with a monotonic distribution over the ranges of cloud fractions and albedo. Using these observational dataas a reference, we examine the degree to which 26 climate models capture this feature of the near-globalmarine cloud population. Models show a general increase in albedo with increasing cloud fraction, but noneof them display a relation that is as well defined as that characterizing the observations. Models t ypicallydisplay larger albedo variability at a given cloud fraction, larger sensitivity in albedo to changes in cloudfraction, and lower cloud fractions. Several models also show branched distributions, contrasting with thesmooth observational relation. In the models the present-day cloud scenes are more reflective than thepreindustrial, demonstrating the simulated impact of anthropogenic aerosols on planetary albedo.

  • 191. Engström, Anders
    et al.
    Bender, Frida A-M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Karlsson, Johannes
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Improved Representation of Marine Stratocumulus Cloud Shortwave Radiative Properties in the CMIP5 Climate Models2014Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, nr 16, s. 6175-6188Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The radiative properties of subtropical marine stratocumulus clouds are investigated in an ensemble of current-generation global climate models from phase 5 of the Climate Model Intercomparison Project (CMIP5). Using a previously documented method for determining regional mean cloud albedo, the authors find a closer agreement with observations in the CMIP5 models as compared to the previous generation of models (phase 3 of CMIP). The multimodel average indicates regional mean, monthly mean cloud albedos ranging from 0.32 to 0.5 among 26 models and five regions, to be compared with satellite observations that indicate a range from 0.32 to 0.39 for the same five regions. The intermodel spread in cloud fraction gives rise to a spread in albedo. Within models, there is a tendency for large cloud fraction to be related to low cloud albedo and vice versa, a relationship that dampens the intermodel variability in total albedo. The intramodel variability in albedo, for a given cloud fraction, is found to be up to twice as large in magnitude in models as in satellite observations. The reason for this larger variability in models is not settled, but possible contributing factors may be imperfect representation in the models of cloud type distribution or of sensitivity to meteorological variability or aerosols. Changes in aerosol loading are found to be the likely cause of an increase in cloud albedo over time. The radiative effect of such a scene brightening in marine stratocumulus cloud regions, from preindustrial times to present day, is estimated to be up to -1W m(-2) for the global ocean, but there are no observations to verify this number.

  • 192.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ekman, Annica M.L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Impact of meteorological factors on the correlation between aerosol optical depth and cloud fraction2010Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 37, s. L18814-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The aerosol optical depth has in several recent studies been found to correlate with cloud fraction. This study examines the global distribution of the total correlation between aerosol optical depth, cloud fraction and meteorological conditions using satellite observations together with atmospheric re-analysis data from the ECMWF. The results show large regional differences in the correlation between aerosol optical depth and cloud fraction, where a higher correlation is found over remote ocean. The one meteorological variable that correlates significantly with both aerosol optical depth and cloud fraction is the 10-meter wind speed. Constructing the partial correlation between aerosol optical depth and cloud fraction, with the impact from 10-meter wind speed removed, yields a significant difference compared to the total correlation. In several regions the remaining partial correlation is reduced from 0.4 to below 0.1. The results highlight the need to investigate all possible correlations between meteorological variables, cloud properties and aerosols. Citation: Engstrom, A., and A. M. L. Ekman (2010), Impact of meteorological factors on the correlation between aerosol optical depth and cloud fraction

  • 193.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ekman, Annica M.L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Modeled response in radiative properties of isolated shallow convective clouds due to perturbations in meteorological state variables and atmospheric aerosol loadingManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    The aim of the present study is to estimate the range of variability in observational data of meteorological variables permitted to determine a clear and unambiguous signal in cloud fraction and cloud albedo due to changes in aerosol concentrations. Three different cases of tropical shallow convection have been simulated using a two-dimensional cloud resolving model with aerosol-cloud interactions. For each case, 30 different small perturbations were added to the initial meteorological profiles of zonal wind, potential temperature and specific humidity. For each meteorological per- turbation an additional 14 simulations were performed with different boundary layer accumulation mode aerosol concentrations ranging from clean (50 cm-3) to polluted (1350 cm-3) conditions. A total of 5040 simulations were performed to elucidate the impact of aerosols on the simulated cloud fraction and cloud albedo and to compare this to the corresponding impact to the influence of small meteorological perturbations. For the simulated cases, we find that for cloud fraction, the aerosol signal is in general much weaker than the change due to small changes in meteorology. This was especially true for parameters that influence the relative humidity of the environment, i.e. tempera- ture and specific humidity. For cloud albedo, the aerosol signal surpasses that of the relatively small meteorological perturbations. We find up to 40% difference in cloud albedo going from clean to polluted conditions. The corresponding maximum change due to the meteorological perturbations was only 14%. We conclude that for the simulated shallow convective clouds, isolating an aerosol effect on cloud fraction is not possible if using meteorological analysis data containing errors of the same order of magnitude as the imposed meteorological perturbations. However, the meteorological constraints may be sufficient to isolate an aerosol effect on cloud albedo.

  • 194.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ekman, Annica M.L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ström, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    de Reus, Marian
    Wang, Chien
    Observational and modelling evidence of tropical deep convective clouds as a source of mid-tropospheric accumulation mode aerosols2008Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, s. L23813-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    High concentrations (up to 550 cm−3 STP) of aerosols in the accumulation mode (>0.12 μm) were observed by aircraft above 7.5 km altitude in the dynamically active regions of several deep convective clouds during the INDOEX campaign. Using a coupled 3-D aerosol-cloud-resolving model, we find that significant evaporation of hydrometeors due to strong updrafts and exchange with ambient air occurs at the boundaries and within the cloud tower. Assuming that each evaporated hydrometeor release an aerosol, an increase in the aerosol concentration by up to 600 cm−3 STP is found in the model at altitudes between 6 and 10 km. The evaporation and release of aerosols occur as the cloud develops, suggesting that deep convective clouds are important sources of mid-tropospheric aerosols during their active lifetime. This source may significantly impact the vertical distribution as well as long-range transport of aerosols in the free troposphere.

  • 195.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Karlsson, Johannes
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The Importance of Representing Mixed-Phase Clouds for Simulating Distinctive Atmospheric States in the Arctic2014Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, nr 1, s. 265-272Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 196.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Magnusson, Linus
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Estimating trajectory uncertainties due to flow dependent errors in the atmospheric analysis2009Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 9, nr 22, s. 8857-8867Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The uncertainty of a calculated trajectory is dependent on the uncertainty in the atmospheric analysis. Using the Ensemble Transform method (originally adapted for ensemble forecasting) we sample the analysis uncertainty in order to create an ensemble of analyses where a trajectory is started from each perturbed analysis. This method, called the Ensemble analysis method (EA), is compared to the Initial spread method (IS), where the trajectory receptor point is perturbed in the horizontal and vertical direction to create a set of trajectories used to estimate the trajectory uncertainty. The deviation growth is examined for one summer and one winter month and for 15 different geographical locations. We find up to a 40% increase in trajectory deviation in the mid-latitudes using the EA method. A simple model for trajectory deviation growth speed is set up and validated. It is shown that the EA method result in a faster error growth compared to the IS method. In addition, two case studies are examined to qualitatively illustrate how the flow dependent analysis uncertainty can impact the trajectory calculations. We find a more irregular behavior for the EA trajectories compared to the IS trajectories and a significantly increased uncertainty in the trajectory origin. We conclude that by perturbing the analysis in agreement with the analysis uncertainties the error in backward trajectory calculations can be more consistently estimated.

  • 197.
    Engström, Erik J.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Characterization of soot in air and rain over southern Asia2009Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Filter-based optical measurements of light absorbing particulate matter at awavelength of about 550 nm, here referred to as soot, in air and rainwaterhave been performed during the period from 1st June 2005 to 31th May 2009at Godavari in Nepal, Sinhagad in India and Hanimaadhoo in The Maldives.A method for determination of water-insoluble light absorbing matter inrainwater has been developed. Analysis of environmental samples has beensuccessfully performed with the described method on samples collected atHanimaadhoo and Godavari. At Hanimaadhoo the average soot concentrationin rainwater was 48 mgl-1 and at Godavari 86 mgl-1.In order to reduce systematic errors at optical determination of soot due tothe light scattering of non-absorbing particles co-deposited on the filter, suchas inorganic salts and mineral dust, an additional sensor recording backscatteredlight was used. Two alternative protocols of corrections (optical andchemical) were applied to the samples. Simultaneous measurements of sootand inorganic ions in aerosol and precipitation at Hanimaadhoo during theperiod May 2005 to February 2007 made it possible to calculate the washoutratio (WR) of these components as a measure of how efficiently they are scavengedby precipitation. During the monsoon season the WR for soot was similarto that of sulphate and other fine mode aerosol components, indicating thatsoot containing particles in these situations were efficient as cloud condensationnuclei. During the polluted winter days, on the other hand, the WR forsoot was 3 times smaller than that of sulphate, showing that the soot containingparticles had retained a hydrophobic character even after a travel time ofseveral days.The Indian monsoon circulation with its two annual phases in combinationwith the location of the main combustion source areas dominated the observedpatterns of soot at the observatories in India and Maldives. Godavari in Nepalwas however mainly influenced by combustion sources all year around concealingpossible variability related to the monsoon circulation. At Hanimaadhoo,peak values in the soot concentration occurred during the winter season(December to April) when air was transported from the polluted Indian subcontinentout over the Indian Ocean. At least a factor of ten lower values wererecorded in air that had spent more than 10-days over the Indian Ocean duringthe monsoon season (July to September).

     

  • 198.
    Engström, Erik J.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Reducing uncertainties associated with filter-based optical measurements of soot aerosol particles with chemical information.Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Of the many identified and potential effects ofatmospheric aerosol particles on climate, those of soot particlesare the most uncertain, in that analytical techniques concerningsoot are far from satisfactory. One concern whenapplying filter-based optical measurements of soot is thatthey suffer from systematic errors due to the light scatteringof non-absorbing particles co-deposited on the filter, suchas inorganic salts and mineral dust. In addition to an opticalcorrection of the non-absorbing material this study providesa protocol for correction of light scattering based onthe chemical quantification of the material, which is a novelty.A newly designed Particle Soot Absorption Photometerwas constructed to measure light transmission on particleaccumulating filters, which includes an additional sensorrecording backscattered light. The choice of polycarbonatemembrane filters avoided high chemical blank values and reducederrors associated with length of the light path throughthe filter.Two protocols of corrections were applied to aerosol samplescollected at the Maldives Climate Observatory Hanimaadhooduring episodes with either continentally influencedair from the Indian/Arabian subcontinents (wintermonsoon) or pristine air from the Southern Indian Ocean(summer monsoon). The two ways of correction (optical andchemical) lowered the particle light absorption of soot by 63 to 61 %, respectively, for data from the Arabian Sea sourcedgroup, resulting in median soot absorption coefficients of 4.2 and 3.5 Mm-1. Corresponding values for the South IndianOcean data were 69 and 97 % (0.38 and 0.02 Mm-1). A comparison with other studies in the area indicated anoverestimation of their soot levels, by up to two orders ofmagnitude. This raises the necessity for chemical correctionprotocols on optical filter-based determinations of soot, before even the sign on the radiative forcing based on their effectscan be assessed.

  • 199.
    Engström, Erik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Determination of water-insoluble light absorbing matter in rainwater using polycarbonate membrane lters and photometricdetection.Artikkel i tidsskrift (Fagfellevurdert)
  • 200.
    Engström, Erik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Seasonal variability in light absorption particulate matter or soot in air at three stations in the South-Asian region situated in Nepal, India and MaldivesManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    Filter-based optical measurements of light absorbing particulate matter at awavelength of about 550 nm, referred to as soot, in air have been performed during theperiod from 1st June 2005 to 31th May 2009 at Godavari in Nepal, Sinhagad in India andHanimaadhoo in the Maldives. In order to reduce systematic errors due to the lightscattering of non-absorbing particles co-deposited on the filter, such as inorganic saltsand mineral dust, an additional sensor recording backscattered light was implemented.Two protocols of corrections (optical and chemical) were applied to the samplescollected at the observatories. The Indian monsoon circulation with its two annualphases in combination with the location of the combustion sources and their contributionrelative other non-anthropogenic sources dominated the observed patterns of soot at theobservatories in India and Maldives. The observatory in Nepal was however mainlyinfluenced by combustion sources all year around concealing possible variability relatedto the monsoon circulation. At the receptor observatory in the Maldives, peak values inthe soot absorption coefficient occurred during the winter season (December to April)when air was transported from the polluted Indian subcontinent out over the IndianOcean. A close to two orders of magnitude lower values were recorded in air that hadspent more than 10-days over the Indian Ocean during the monsoon season (July toSeptember), suggested to be dominated by particulate matter from remote marinebiogenic sources.

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