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The Arctic Atmosphere: Interactions between clouds, boundary-layer turbulence and large-scale circulation
Stockholm University, Faculty of Science, Department of Meteorology .
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Arctic climate is changing fast, but weather forecast and climate models have serious deficiencies in representing the Arctic atmosphere, because of the special conditions that occur in this region. The cold ice surface and the advection of warm air aloft from the south result in a semi-continuous presence of a temperature inversion, known as the “Arctic inversion”, which is governed by interacting large-scale and local processes, such as surface fluxes and cloud formation. In this thesis these poorly understood interactions are investigated using observations from field campaigns on the Swedish icebreaker Oden: The Arctic Summer Cloud Ocean Study (ASCOS) in 2008 and the Arctic Clouds in Summer Experiment (ACSE) in 2014. Two numerical models are also used to explore these data: the IFS global weather forecast model from the European Center for Medium-range Weather Forecasts and the MIMICA LES from Stockholm University.

Arctic clouds can persist for a long time, days to weeks, and are usually mixed-phase; a difficult to model mixture of super-cooled cloud droplets and ice crystals. Their persistence has been attributed to several mechanisms, such as large-scale advection, surface evaporation and microphysical processes. ASCOS observations indicate that these clouds are most frequently decoupled from the surface; hence, surface evaporation plays a minor role. The determining factor for cloud-surface decoupling is the altitude of the clouds. Turbulent mixing is generated in the cloud layer, forced by cloud-top radiative cooling, but with a high cloud this cannot penetrate down to the surface mixed layer, which is forced primarily by mechanical turbulence. A special category of clouds is also found: optically thin liquid-only clouds with stable stratification, hence insignificant in-cloud mixing, which occur in low-aerosol conditions. IFS model fails to reproduce the cloud-surface decoupling observed during ASCOS. A new prognostic cloud physics scheme in IFS improves simulation of mixed-phase clouds, but does not improve the warm bias in the model, mostly because IFS fails to disperse low surface-warming clouds when observations indicate cloud-free conditions.

With increasing summer open-water areas in a warming Arctic, there is a growing interest in processes related to the ice marginal zones and the summer-to-autumn seasonal transition. ACSE included measurements over both open-water and sea-ice surfaces, during melt and early freeze. The seasonal transition was abrupt, not gradual as would have been expected if it was primarily driven by the gradual changes in net solar radiation. After the transition, the ocean surface remained warmer than the atmosphere, enhancing surface cooling and facilitating sea-ice formation. Observations in melt season showed distinct differences in atmospheric structure between the two surface types; during freeze-up these largely disappear. In summer, large-scale advection of warm and moist air over melting sea ice had large impacts on atmospheric stability and the surface. This is explored with an LES; results indicate that while vertical structure of the lowest atmosphere is primarily sensitive to heat advection, cloud formation, which is of great importance to the surface energy budget, is primarily sensitive to moisture advection.

Place, publisher, year, edition, pages
Stockholm, Sweden: Department of Meteorology, Stockholm University , 2016. , 49 p.
Keyword [en]
Arctic, mixed-phase clouds, thermodynamic structure, Arctic inversion, cloud-surface interactions, seasonal transition, IFS model, LES
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-134525ISBN: 978-91-7649-559-9ISBN: 978-91-7649-560-5OAI: oai:DiVA.org:su-134525DiVA: diva2:1033927
Public defence
2016-11-30, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2016-11-07 Created: 2016-10-10 Last updated: 2016-10-31Bibliographically approved
List of papers
1. The thermodynamic structure of summer Arctic stratocumulus and the dynamic coupling to the surface
Open this publication in new window or tab >>The thermodynamic structure of summer Arctic stratocumulus and the dynamic coupling to the surface
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2014 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, no 22, 12573-12592 p.Article in journal (Refereed) Published
Abstract [en]

The vertical structure of Arctic low-level clouds and Arctic boundary layer is studied, using observations from ASCOS (Arctic Summer Cloud Ocean Study), in the central Arctic, in late summer 2008. Two general types of cloud structures are examined: the "neutrally stratified" and "stably stratified" clouds. Neutrally stratified are mixed-phase clouds where radiative-cooling near cloud top produces turbulence that generates a cloud-driven mixed layer. When this layer mixes with the surface-generated turbulence, the cloud layer is coupled to the surface, whereas when such an interaction does not occur, it remains decoupled; the latter state is most frequently observed. The decoupled clouds are usually higher compared to the coupled; differences in thickness or cloud water properties between the two cases are however not found. The surface fluxes are also very similar for both states. The decoupled clouds exhibit a bimodal thermodynamic structure, depending on the depth of the sub-cloud mixed layer (SCML): clouds with shallower SCMLs are disconnected from the surface by weak inversions, whereas those that lay over a deeper SCML are associated with stronger inversions at the decoupling height. Neutrally stratified clouds generally precipitate; the evaporation/sublimation of precipitation often enhances the decoupling state. Finally, stably stratified clouds are usually lower, geometrically and optically thinner, non-precipitating liquid-water clouds, not containing enough liquid to drive efficient mixing through cloud-top cooling.

National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-110375 (URN)10.5194/acp-14-12573-2014 (DOI)000348536700012 ()
Available from: 2014-12-11 Created: 2014-12-11 Last updated: 2016-10-10Bibliographically approved
2. Late Summer Arctic clouds in the ECMWF forecast model: an evaluation of cloud parameterization scheme
Open this publication in new window or tab >>Late Summer Arctic clouds in the ECMWF forecast model: an evaluation of cloud parameterization scheme
2016 (English)In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 694, 387-400 p.Article in journal (Refereed) Published
Abstract [en]

Mixed-phase clouds are an integral part of the Arctic climate system, for precipitation and for their interactions with radiation and thermodynamics. Mixed-phase processes are often poorly represented in global models and many use an empirically based diagnostic partition between the liquid and ice phases that is dependent solely on temperature. However, increasingly more complex microphysical parametrizations are being implemented allowing a more physical representation of mixed-phase clouds.

This study uses in situ observations from the Arctic Summer Cloud Ocean Study (ASCOS) field campaign in the central Arctic to assess the impact of a change from a diagnostic to a prognostic parametrization of mixed-phase clouds and increased vertical resolution in the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS). The newer cloud scheme improves the representation of the vertical structure of mixed-phase clouds, with supercooled liquid water at cloud top and ice precipitating below, improved further with higher vertical resolution. Increased supercooled liquid water and decreased ice content are both in closer agreement with observations. However, these changes do not result in any substantial improvement in surface radiation, and a warm and moist bias in the lowest part of the atmosphere remains. Both schemes also fail to capture the transitions from overcast to cloud-free conditions. Moreover, whereas the observed cloud layer is frequently decoupled from the surface, the modelled clouds remain coupled to the surface most of the time. The changes implemented to the cloud scheme are an important step forward in improving the representation of Arctic clouds, but improvements in other aspects such as boundary-layer turbulence, cloud radiative properties, sensitivity to low aerosol concentrations and representation of the sea-ice surface may also need to be addressed.

Keyword
IFS model, cloud scheme, evaluation, mixed-phase clouds, Arctic, cloud–surface interactions
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-124362 (URN)10.1002/qj.2658 (DOI)000369978300031 ()
Available from: 2015-12-18 Created: 2015-12-18 Last updated: 2016-10-10Bibliographically approved
3. Atmospheric conditions during the Arctic Clouds in Summer Experiment (ACSE): Contrasting open-water and sea-ice surfaces during melt and freeze-up seasons
Open this publication in new window or tab >>Atmospheric conditions during the Arctic Clouds in Summer Experiment (ACSE): Contrasting open-water and sea-ice surfaces during melt and freeze-up seasons
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2016 (English)In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442Article in journal (Refereed) Epub ahead of print
Abstract [en]

The Arctic Clouds in Summer Experiment (ACSE) was conducted during summer and early autumn 2014, providing a detailed view of the seasonal transition from ice melt into freeze-up. Measurements were taken over both ice-free and ice-covered surfaces, near the ice edge, offering insight to the role of the surface state in shaping the atmospheric conditions. The initiation of the autumn freeze-up was related to a change in air mass, rather than to changes in solar radiation alone; the lower atmosphere cooled abruptly leading to a surface heat loss. During melt season, strong surface inversions persisted over the ice, while elevated inversions were more frequent over open water. These differences disappeared during autumn freeze-up, when elevated inversions persisted over both ice-free and ice-covered conditions. These results are in contrast to previous studies that found a well-mixed boundary layer persisting in summer and an increased frequency of surface-based inversions in autumn, suggesting that our knowledge derived from measurements taken within the pan-Arctic area and on the central ice-pack does not necessarily apply closer to the ice-edge. This study offers an insight to the atmospheric processes that occur during a crucial period of the year; understanding and accurately modeling these processes is essential for the improvement of ice-extent predictions and future Arctic climate projections.

National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-134342 (URN)10.1175/JCLI-D-16-0211.1 (DOI)
Available from: 2016-10-05 Created: 2016-10-05 Last updated: 2016-10-10
4. Large Eddy Simulation of warm-air advection and air-mass transformation in the summer Arctic
Open this publication in new window or tab >>Large Eddy Simulation of warm-air advection and air-mass transformation in the summer Arctic
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(English)Manuscript (preprint) (Other academic)
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-134347 (URN)
Available from: 2016-10-05 Created: 2016-10-05 Last updated: 2016-10-10Bibliographically approved

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