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  • 1. Birch, C. E.
    et al.
    Brooks, I. M.
    Tjernstrom, M.
    Shupe, M. D.
    Mauritsen, T.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Lock, A. P.
    Earnshaw, P.
    Persson, P. O. G.
    Milton, S. F.
    Leck, C.
    Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies2012Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, nr 7, s. 3419-3435Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Observations made during late summer in the central Arctic Ocean, as part of the Arctic Summer Cloud Ocean Study (ASCOS), are used to evaluate cloud and vertical temperature structure in the Met Office Unified Model (MetUM). The observation period can be split into 5 regimes; the first two regimes had a large number of frontal systems, which were associated with deep cloud. During the remainder of the campaign a layer of low-level cloud occurred, typical of central Arctic summer conditions, along with two periods of greatly reduced cloud cover. The short-range operational NWP forecasts could not accurately reproduce the observed variations in near-surface temperature. A major source of this error was found to be the temperature-dependant surface albedo parameterisation scheme. The model reproduced the low-level cloud layer, though it was too thin, too shallow, and in a boundary-layer that was too frequently well-mixed. The model was also unable to reproduce the observed periods of reduced cloud cover, which were associated with very low cloud condensation nuclei (CCN) concentrations (< 1 cm(-3)). As with most global NWP models, the MetUM does not have a prognostic aerosol/cloud scheme but uses a constant CCN concentration of 100 cm(-3) over all marine environments. It is therefore unable to represent the low CCN number concentrations and the rapid variations in concentration frequently observed in the central Arctic during late summer. Experiments with a single-column model configuration of the MetUM show that reducing model CCN number concentrations to observed values reduces the amount of cloud, increases the near-surface stability, and improves the representation of both the surface radiation fluxes and the surface temperature. The model is shown to be sensitive to CCN only when number concentrations are less than 10-20 cm(-3).

  • 2. Brooks, Ian M.
    et al.
    Tjernstrom, Michael
    Persson, P. Ola G.
    Shupe, Matthew D.
    Atkinson, Rebecca A.
    Canut, Guylaine
    Birch, Cathryn E.
    Mauritsen, Thorsten
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Brooks, Barbara J.
    The Turbulent Structure of the Arctic Summer Boundary Layer During The Arctic Summer Cloud-Ocean Study2017Inngår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 122, nr 18, s. 9685-9704Artikkel i tidsskrift (Fagfellevurdert)
  • 3.
    Devasthale, Abhay
    et al.
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Kahn, Brian H.
    Tjernstrom, Michael
    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, ISSN 0003-0007, Vol. 97, nr 11, s. 2163-2176Artikkel i tidsskrift (Fagfellevurdert)
  • 4.
    Devasthale, Abhay
    et al.
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Koenigk, Torben
    SMHI, Forskningsavdelningen, Klimatforskning - Rossby Centre.
    Fetzer, E. J.
    The thermodynamic state of the Arctic atmosphere observed by AIRS: comparisons during the record minimum sea ice extents of 2007 and 20122013Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, nr 15, s. 7441-7450Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The record sea ice minimum (SIM) extents observed during the summers of 2007 and 2012 in the Arctic are stark evidence of accelerated sea ice loss during the last decade. Improving our understanding of the Arctic atmosphere and accurate quantification of its characteristics becomes ever more crucial, not least to improve predictions of such extreme events in the future. In this context, the Atmospheric Infrared Sounder (AIRS) instrument onboard NASA's Aqua satellite provides crucial insights due to its ability to provide 3-D information on atmospheric thermodynamics. Here, we facilitate comparisons in the evolution of the thermodynamic state of the Arctic atmosphere during these two SIM events using a decade-long AIRS observational record (2003-2012). It is shown that the meteorological conditions during 2012 were not extreme, but three factors of preconditioning from winter through early summer played an important role in accelerating sea ice melt. First, the marginal sea ice zones along the central Eurasian and North Atlantic sectors remained warm throughout winter and early spring in 2012 preventing thicker ice build-up. Second, the circulation pattern favoured efficient sea ice transport out of the Arctic in the Atlantic sector during late spring and early summer in 2012 compared to 2007. Third, additional warming over the Canadian archipelago and southeast Beaufort Sea from May onward further contributed to accelerated sea ice melt. All these factors may have lead the already thin and declining sea ice cover to pass below the previous sea ice extent minimum of 2007. In sharp contrast to 2007, negative surface temperature anomalies and increased cloudiness were observed over the East Siberian and Chukchi seas in the summer of 2012. The results suggest that satellite-based monitoring of atmospheric preconditioning could be a critical source of information in predicting extreme sea ice melting events in the Arctic.

  • 5.
    Devasthale, Abhay
    et al.
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Tjernstrom, M.
    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.

  • 6.
    Devasthale, Abhay
    et al.
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Tjernstrom, Michael
    Karlsson, Karl-Göran
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Thomas, Manu Anna
    SMHI, Forskningsavdelningen, Luftmiljö.
    Jones, Colin
    SMHI, Forskningsavdelningen, Klimatforskning - Rossby Centre.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Omar, Ali H.
    The vertical distribution of thin features over the Arctic analysed from CALIPSO observations2011Inngår 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.

  • 7.
    Karlsson, Karl-Göran
    et al.
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Anttila, Kati
    Trentmann, Jorg
    Stengel, Martin
    Meirink, Jan Fokke
    Devasthale, Abhay
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Hanschmann, Timo
    Kothe, Steffen
    Jaaskelainen, Emmihenna
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Benas, Nikos
    van Zadelhoff, Gerd-Jan
    Schlundt, Cornelia
    Stein, Diana
    Finkensieper, Stefan
    Håkansson, Nina
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Hollmann, Rainer
    CLARA-A2: the second edition of the CM SAF cloud and radiation data record from 34 years of global AVHRR data2017Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, nr 9, s. 5809-5828Artikkel i tidsskrift (Fagfellevurdert)
  • 8.
    Karlsson, Karl-Göran
    et al.
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Riihela, A.
    Mueller, R.
    Meirink, J. F.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Stengel, M.
    Lockhoff, M.
    Trentmann, J.
    Kaspar, F.
    Hollmann, R.
    Wolters, E.
    CLARA-A1: a cloud, albedo, and radiation dataset from 28 yr of global AVHRR data2013Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, nr 10, s. 5351-5367Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A new satellite-derived climate dataset - denoted CLARA-A1 ("The CM SAF cLoud, Albedo and RAdiation dataset from AVHRR data") - is described. The dataset covers the 28 yr period from 1982 until 2009 and consists of cloud, surface albedo, and radiation budget products derived from the AVHRR (Advanced Very High Resolution Radiometer) sensor carried by polar-orbiting operational meteorological satellites. Its content, anticipated accuracies, limitations, and potential applications are described. The dataset is produced by the EUMETSAT Climate Monitoring Satellite Application Facility (CM SAF) project. The dataset has its strengths in the long duration, its foundation upon a homogenized AVHRR radiance data record, and in some unique features, e. g. the availability of 28 yr of summer surface albedo and cloudiness parameters over the polar regions. Quality characteristics are also well investigated and particularly useful results can be found over the tropics, mid to high latitudes and over nearly all oceanic areas. Being the first CM SAF dataset of its kind, an intensive evaluation of the quality of the datasets was performed and major findings with regard to merits and shortcomings of the datasets are reported. However, the CM SAF's long-term commitment to perform two additional reprocessing events within the time frame 2013-2018 will allow proper handling of limitations as well as upgrading the dataset with new features (e. g. uncertainty estimates) and extension of the temporal coverage.

  • 9. Kupiszewski, P.
    et al.
    Leck, C.
    Tjernstrom, M.
    Sjogren, S.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Graus, M.
    Mueller, M.
    Brooks, B.
    Swietlicki, E.
    Norris, S.
    Hansel, A.
    Vertical profiling of aerosol particles and trace gases over the central Arctic Ocean during summer2013Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, nr 24, s. 12405-12431Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Unique measurements of vertical size-resolved aerosol particle concentrations, trace gas concentrations and meteorological data were obtained during the Arctic Summer Cloud Ocean Study (ASCOS, www.ascos.se), an International Polar Year project aimed at establishing the processes responsible for formation and evolution of low-level clouds over the high Arctic summer pack ice. The experiment was conducted from on board the Swedish icebreaker Oden, and provided both ship-and helicopter-based measurements. This study focuses on the vertical helicopter profiles and onboard measurements obtained during a three-week period when Oden was anchored to a drifting ice floe, and sheds light on the characteristics of Arctic aerosol particles and their distribution throughout the lower atmosphere. Distinct differences in aerosol particle characteristics within defined atmospheric layers are identified. Within the lowermost couple hundred metres, transport from the marginal ice zone (MIZ), condensational growth and cloud processing develop the aerosol population. During two of the four representative periods defined in this study, such influence is shown. At altitudes above about 1 km, long-range transport occurs frequently. However, only infrequently does large-scale subsidence descend such air masses to become entrained into the mixed layer in the high Arctic, and there-fore long-range transport plumes are unlikely to directly influence low-level stratiform cloud formation. Nonetheless, such plumes can influence the radiative balance of the planetary boundary layer (PBL) by influencing formation and evolution of higher clouds, as well as through precipitation transport of particles downwards. New particle formation was occasionally observed, particularly in the near-surface layer. We hypothesize that the origin of these ultrafine particles could be in biological processes, both primary and secondary, within the open leads between the pack ice and/or along the MIZ. In general, local sources, in combination with upstream boundary-layer transport of precursor gases from the MIZ, are considered to constitute the origin of cloud condensation nuclei (CCN) particles and thus be of importance for the formation of interior Arctic low-level clouds during summer, and subsequently, through cloud influences, for the melting and freezing of sea ice.

  • 10. Leung, W. -YH.
    et al.
    Savre, J.
    Bender, F. A. -M
    Komppula, M.
    Portin, H.
    Romakkaniemi, S.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Noone, K.
    Ekman, A. M. L.
    Sensitivity of a continental night-time stratocumulus-topped boundary layer to varying environmental conditions2016Inngår i: QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, ISSN 0035-9009, Vol. 142, nr 700, s. 2911-2924Artikkel i tidsskrift (Fagfellevurdert)
  • 11. Loewe, Katharina
    et al.
    Ekman, Annica M. L.
    Paukert, Marco
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Tjernstrom, Michael
    Hoose, Corinna
    Modelling micro- and macrophysical contributors to the dissipation of an Arctic mixed-phase cloud during the Arctic Summer Cloud Ocean Study (ASCOS)2017Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, nr 11, s. 6693-6704Artikkel i tidsskrift (Fagfellevurdert)
  • 12.
    Sedlar, Joseph
    et al.
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Devasthale, Abhay
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Clear-sky thermodynamic and radiative anomalies over a sea ice sensitive region of the Arctic2012Inngår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 117, artikkel-id D19111Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Monthly clear-sky anomalies of atmospheric temperature and water vapor over the East Siberian and Laptev Sea regions of the Arctic for 2003-2010 are examined here. This region experiences significant interannual variations in sea ice concentration and is also where ice loss was most apparent in the record year 2007. Clear-sky thermodynamic profiles come from the Atmospheric Infrared Sounder (AIRS) sensor onboard the Aqua satellite. Associated longwave (LW) and shortwave (SW) radiation-flux anomalies are estimated through radiative transfer modeling. Anomalies of temperature (+/- 10 K) and water vapor (+/- 1 g kg(-1)) often positively covary, resulting in distinct signatures in the clear-sky downwelling LW (LWD) anomalies, occasionally larger than +/- 10 W m(-2) around the 2003-2010 climatology. Estimates of mean greenhouse anomalies indicate a shift from negative to positive anomalies midway through the 8-year record. Sensitivity tests suggest that temperature anomalies are the strongest contributor to both LWD and greenhouse anomalies, relative to water-vapor anomalies; monthly averaging of column precipitable water yields relatively small anomalies (order 1 mm) that produce a linear response in greenhouse anomalies. Finally the clear-sky contribution to 2007 monthly ice thickness is estimated. Anomalous clear-sky radiation retards the total 2007 ice thickness by 0.3 m (15-30% of ice-thickness climatology), and anomalous LW radiation is most important for preconditioning the ice during the months prior to, and after, the summer melt season. A highly sensitive interaction between cloud fraction, surface albedo and LWD anomalies is found, and we develop a metric for determining clear-sky anomalous ice melt potential.

  • 13.
    Sedlar, Joseph
    et al.
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Shupe, Matthew D.
    Tjernstrom, Michael
    On the Relationship between Thermodynamic Structure and Cloud Top, and Its Climate Significance in the Arctic2012Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 25, nr 7, s. 2374-2393Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Cloud and thermodynamic characteristics from three Arctic observation sites are investigated to understand the collocation between low-level clouds and temperature inversions. A regime where cloud top was 100-200 m above the inversion base [cloud inside inversion (CII)] was frequently observed at central Arctic Ocean sites, while observations from Barrow, Alaska, indicate that cloud tops were more frequently constrained to inversion base height [cloud capped by inversion (CCI)]. Cloud base and top heights were lower, and temperature inversions were also stronger and deeper, during CII cases. Both cloud regimes were often decoupled from the surface except for CCI over Barrow. In-cloud lapse rates differ and suggest increased cloud-mixing potential for CII cases. Specific humidity inversions were collocated with temperature inversions for more than 60% of the CCI and more than 85% of the CII regimes. Horizontal advection of heat and moisture is hypothesized as an important process controlling thermodynamic structure and efficiency of cloud-generated motions. The portion of CII clouds above the inversion contains cloud radar signatures consistent with cloud droplets. The authors test the longwave radiative impact of cloud liquid above the inversion through hypothetical liquid water distributions. Optically thin CII clouds alter the effective cloud emission temperature and can lead to an increase in surface flux on the order of 1.5 W m(-2) relative to the same cloud but whose top does not extend above the inversion base. The top of atmosphere impact is even larger, increasing outgoing longwave radiation up to 10 W m(-2). These results suggest a potentially significant longwave radiative forcing via simple liquid redistributions for a distinctly dominant cloud regime over sea ice.

  • 14.
    Sedlar, Joseph
    et al.
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Tjernstrom, Michael
    Clouds, warm air, and a climate cooling signal over the summer Arctic2017Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, nr 2, s. 1095-1103Artikkel i tidsskrift (Fagfellevurdert)
  • 15. Shupe, M. D.
    et al.
    Persson, P. O. G.
    Brooks, I. M.
    Tjernstrom, M.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Mauritsen, T.
    Sjogren, S.
    Leck, C.
    Cloud and boundary layer interactions over the Arctic sea ice in late summer2013Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, nr 18, s. 9379-9399Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Observations from the Arctic Summer Cloud Ocean Study (ASCOS), in the central Arctic sea-ice pack in late summer 2008, provide a detailed view of cloud-atmosphere-surface interactions and vertical mixing processes over the sea-ice environment. Measurements from a suite of ground-based remote sensors, near-surface meteorological and aerosol instruments, and profiles from radiosondes and a helicopter are combined to characterize a week-long period dominated by low-level, mixed-phase, stratocumulus clouds. Detailed case studies and statistical analyses are used to develop a conceptual model for the cloud and atmosphere structure and their interactions in this environment. Clouds were persistent during the period of study, having qualities that suggest they were sustained through a combination of advective influences and in-cloud processes, with little contribution from the surface. Radiative cooling near cloud top produced buoyancy-driven, turbulent eddies that contributed to cloud formation and created a cloud-driven mixed layer. The depth of this mixed layer was related to the amount of turbulence and condensed cloud water. Coupling of this cloud-driven mixed layer to the surface boundary layer was primarily determined by proximity. For 75% of the period of study, the primary stratocumulus cloud-driven mixed layer was decoupled from the surface and typically at a warmer potential temperature. Since the near-surface temperature was constrained by the ocean-ice mixture, warm temperatures aloft suggest that these air masses had not significantly interacted with the sea-ice surface. Instead, back-trajectory analyses suggest that these warm air masses advected into the central Arctic Basin from lower latitudes. Moisture and aerosol particles likely accompanied these air masses, providing necessary support for cloud formation. On the occasions when cloud-surface coupling did occur, back trajectories indicated that these air masses advected at low levels, while mixing processes kept the mixed layer in equilibrium with the near-surface environment. Rather than contributing buoyancy forcing for the mixed-layer dynamics, the surface instead simply appeared to respond to the mixed-layer processes aloft. Clouds in these cases often contained slightly higher condensed water amounts, potentially due to additional moisture sources from below.

  • 16. Tjernstrom, M.
    et al.
    Birch, C. E.
    Brooks, I. M.
    Shupe, M. D.
    Persson, P. O. G.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Mauritsen, T.
    Leck, C.
    Paatero, J.
    Szczodrak, M.
    Wheeler, C. R.
    Meteorological conditions in the central Arctic summer during the Arctic Summer Cloud Ocean Study (ASCOS)2012Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, nr 15, s. 6863-6889Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Understanding the rapidly changing climate in the Arctic is limited by a lack of understanding of underlying strong feedback mechanisms that are specific to the Arctic. Progress in this field can only be obtained by process-level observations; this is the motivation for intensive ice-breaker-based campaigns such as the Arctic Summer Cloud-Ocean Study (ASCOS), described here. However, detailed field observations also have to be put in the context of the larger-scale meteorology, and short field campaigns have to be analysed within the context of the underlying climate state and temporal anomalies from this. To aid in the analysis of other parameters or processes observed during this campaign, this paper provides an overview of the synoptic-scale meteorology and its climatic anomaly during the ASCOS field deployment. It also provides a statistical analysis of key features during the campaign, such as key meteorological variables, the vertical structure of the lower troposphere and clouds, and energy fluxes at the surface. In order to assess the representativity of the ASCOS results, we also compare these features to similar observations obtained during three earlier summer experiments in the Arctic Ocean: the AOE-96, SHEBA and AOE-2001 expeditions. We find that these expeditions share many key features of the summertime lower troposphere. Taking ASCOS and the previous expeditions together, a common picture emerges with a large amount of low-level cloud in a well-mixed shallow boundary layer, capped by a weak to moderately strong inversion where moisture, and sometimes also cloud top, penetrate into the lower parts of the inversion. Much of the boundary-layer mixing is due to cloud-top cooling and subsequent buoyant overturning of the cloud. The cloud layer may, or may not, be connected with surface processes depending on the depths of the cloud and surface-based boundary layers and on the relative strengths of surface-shear and cloud-generated turbulence. The latter also implies a connection between the cloud layer and the free troposphere through entrainment at cloud top.

  • 17. Tjernstrom, M.
    et al.
    Leck, C.
    Birch, C. E.
    Bottenheim, J. W.
    Brooks, B. J.
    Brooks, I. M.
    Backlin, L.
    Chang, Y. -W
    de Leeuw, G.
    Di Liberto, L.
    de la Rosa, S.
    Granath, E.
    Graus, M.
    Hansel, A.
    Heintzenberg, J.
    Held, A.
    Hind, A.
    Johnston, P.
    Knulst, J.
    Martin, M.
    Matrai, P. A.
    Mauritsen, T.
    Mueller, M.
    Norris, S. J.
    Orellana, M. V.
    Orsini, D. A.
    Paatero, J.
    Persson, P. O. G.
    Gao, Q.
    Rauschenberg, C.
    Ristovski, Z.
    Sedlar, Joseph
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Shupe, M. D.
    Sierau, B.
    Sirevaag, A.
    Sjogren, S.
    Stetzer, O.
    Swietlicki, E.
    Szczodrak, M.
    Vaattovaara, P.
    Wahlberg, N.
    Westberg, M.
    Wheeler, C. R.
    The Arctic Summer Cloud Ocean Study (ASCOS): overview and experimental design2014Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, nr 6, s. 2823-2869Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The climate in the Arctic is changing faster than anywhere else on earth. Poorly understood feedback processes relating to Arctic clouds and aerosol-cloud interactions contribute to a poor understanding of the present changes in the Arctic climate system, and also to a large spread in projections of future climate in the Arctic. The problem is exacerbated by the paucity of research-quality observations in the central Arctic. Improved formulations in climate models require such observations, which can only come from measurements in situ in this difficult-to-reach region with logistically demanding environmental conditions. The Arctic Summer Cloud Ocean Study (ASCOS) was the most extensive central Arctic Ocean expedition with an atmospheric focus during the International Polar Year (IPY) 2007-2008. ASCOS focused on the study of the formation and life cycle of low-level Arctic clouds. ASCOS departed from Longyearbyen on Svalbard on 2 August and returned on 9 September 2008. In transit into and out of the pack ice, four short research stations were undertaken in the Fram Strait: two in open water and two in the marginal ice zone. After traversing the pack ice northward, an ice camp was set up on 12 August at 87 degrees 21' N, 01 degrees 29' W and remained in operation through 1 September, drifting with the ice. During this time, extensive measurements were taken of atmospheric gas and particle chemistry and physics, mesoscale and boundary-layer meteorology, marine biology and chemistry, and upper ocean physics. ASCOS provides a unique interdisciplinary data set for development and testing of new hypotheses on cloud processes, their interactions with the sea ice and ocean and associated physical, chemical, and biological processes and interactions. For example, the first-ever quantitative observation of bubbles in Arctic leads, combined with the unique discovery of marine organic material, polymer gels with an origin in the ocean, inside cloud droplets suggests the possibility of primary marine organically derived cloud condensation nuclei in Arctic stratocumulus clouds. Direct observations of surface fluxes of aerosols could, however, not explain observed variability in aerosol concentrations, and the balance between local and remote aerosols sources remains open. Lack of cloud condensation nuclei (CCN) was at times a controlling factor in low-level cloud formation, and hence for the impact of clouds on the surface energy budget. ASCOS provided detailed measurements of the surface energy balance from late summer melt into the initial autumn freeze-up, and documented the effects of clouds and storms on the surface energy balance during this transition. In addition to such process-level studies, the unique, independent ASCOS data set can and is being used for validation of satellite retrievals, operational models, and reanalysis data sets.

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