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Arctic clouds - interactions with radiation and thermodynamic structure
Stockholm University, Faculty of Science, Department of Meteorology .
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Clouds play in important role in the climate system through their interaction with radiation. Globally, clouds tend to cool the Earth by reflecting solar radiation and shading the surface. Over the Arctic, clouds tend to have the opposite impact, where they instead warm the surface through the cloud greenhouse effect because the surface is generally quite reflective. The magnitude and overall effect of clouds on the surface varies significantly with the surface, cloud and thermodynamic characteristics and can have large impacts on the energy budget at the surface.

Low-level central-Arctic stratus clouds interact with the thermodynamics in a manner differently than sub-tropical stratus. Observations from several Arctic observatories indicate that these clouds penetrate and persist within stable temperature inversion structures, rather than being limited to the base of the stable layer as observed in the subtropics. It is hypothesized that such interactions with the thermodynamics can impact for example the cloud phase, lifetime, and their relationship with the sub-cloud layer and surface. Analysis indicates both the thermodynamic setting and the cloud properties affect the vertical location of the cloud top relative to inversion base. Hypothetical longwave radiative impacts resulting from liquid water redistributions are identified and discussed.

Clouds primarily influence the energy at the surface via interactions with radiation. Measurements from the central Arctic suggest that the transition of season from melting to freezing was largely determined by the presence, or absence, of liquid-containing clouds and the incumbent cloud longwave warming effect. The components affecting the cloud-radiative forcing are described with relation to the energy budget and the change of season. Additionally, the influence of altering cloud condensation nuclei as a mechanism for limiting cloud liquid water is shown to have strong influences on surface temperature and lower atmospheric stability.

Finally, regional climate models, RCMs, are evaluated against an annual dataset to assess the ability of RCMs to represent cloud and radiation processes in the Arctic. It is shown that both inter-model and model-observation spread are rather significant. Biases in the cloud representations yield distinct biases in the radiative fluxes, and can result in significant local climate variations solely through these parameters.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2010. , p. 46
Keywords [en]
Arctic, stratus, radiation, thermodynamic structure, cloud radiative forcing, seasonal transition
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences
Identifiers
URN: urn:nbn:se:su:diva-43935ISBN: 978-91-7447-176-2 (print)OAI: oai:DiVA.org:su-43935DiVA, id: diva2:359928
Public defence
2010-12-03, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Accepted. Paper 4: Manuscript published in Atmospheric Chemistry and Physics Discussions.Available from: 2010-11-11 Created: 2010-11-01 Last updated: 2022-02-24Bibliographically approved
List of papers
1. Stratiform Cloud—Inversion Characterization During the Arctic Melt Season
Open this publication in new window or tab >>Stratiform Cloud—Inversion Characterization During the Arctic Melt Season
2009 (English)In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 132, no 3, p. 455-474Article in journal (Refereed) Published
Abstract [en]

Data collected during July and August from the Arctic Ocean Experiment 2001illustrated a common occurrence of specific-humidity (q) inversions, where moistureincreases with height, coinciding with temperature inversions in the central Arctic boundarylayer and lower troposphere. Low-level stratiform clouds and their relationship to temperatureinversions are examined using radiosonde data and data from a suite of remote sensinginstrumentation. Two low-level cloud regimes are identified: the canonical case of stratiformclouds, where the cloud tops are capped by the temperature inversion base (CCI—CloudsCapped by Inversion) and clouds where the cloud tops were found well inside the inversion(CII—Clouds Inside Inversion). The latter case was found to occur more than twiceas frequently than the former. The characteristic of the temperature inversion is shown tohave an influence on the cloud regime that was supported. Statistical analyses of the cloudregimes using remote sensing instruments suggest that CCI cases tend to be dominated bysingle-phase liquid cloud droplets; radiative cooling at the cloud top limits the vertical extentof such clouds to the inversion base height. The CII cases, on the other hand, display characteristicsthat can be divided into two situations—(1) clouds that only slightly penetrate thetemperature inversion and exhibit a microphysical signal similar to CCI cases, or (2) cloudsthat extend higher into the inversion and show evidence of a mixed-phase cloud structure.An important interplay between the mixed-phase structure and an increased potential for turbulentmixing across the inversion base appears to support the lifetime of CII cases existingwithin the inversion layer.

Keywords
Arctic Clouds
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:su:diva-32472 (URN)10.1007/s10546-009-9407-1 (DOI)000270129600006 ()
Available from: 2009-12-10 Created: 2009-12-10 Last updated: 2022-02-25Bibliographically approved
2. On the relationship between thermodynamic structure and cloud top, and its climate significance in the Arctic
Open this publication in new window or tab >>On the relationship between thermodynamic structure and cloud top, and its climate significance in the Arctic
(English)Manuscript (preprint) (Other academic)
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:su:diva-44322 (URN)
Available from: 2010-11-05 Created: 2010-11-05 Last updated: 2022-02-24Bibliographically approved
3. A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing
Open this publication in new window or tab >>A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing
Show others...
2011 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 37, no 7-8, p. 1643-1660Article in journal (Refereed) Published
Abstract [en]

Snow surface and sea-ice energy budgets were measured near 87.5A degrees N during the Arctic Summer Cloud Ocean Study (ASCOS), from August to early September 2008. Surface temperature indicated four distinct temperature regimes, characterized by varying cloud, thermodynamic and solar properties. An initial warm, melt-season regime was interrupted by a 3-day cold regime where temperatures dropped from near zero to -7A degrees C. Subsequently mean energy budget residuals remained small and near zero for 1 week until once again temperatures dropped rapidly and the energy budget residuals became negative. Energy budget transitions were dominated by the net radiative fluxes, largely controlled by the cloudiness. Variable heat, moisture and cloud distributions were associated with changing air-masses. Surface cloud radiative forcing, the net radiative effect of clouds on the surface relative to clear skies, is estimated. Shortwave cloud forcing ranged between -50 W m(-2) and zero and varied significantly with surface albedo, solar zenith angle and cloud liquid water. Longwave cloud forcing was larger and generally ranged between 65 and 85 W m(-2), except when the cloud fraction was tenuous or contained little liquid water; thus the net effect of the clouds was to warm the surface. Both cold periods occurred under tenuous, or altogether absent, low-level clouds containing little liquid water, effectively reducing the cloud greenhouse effect. Freeze-up progression was enhanced by a combination of increasing solar zenith angles and surface albedo, while inhibited by a large, positive surface cloud forcing until a new air-mass with considerably less cloudiness advected over the experiment area.

National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:su:diva-44323 (URN)10.1007/s00382-010-0937-5 (DOI)000295522600022 ()
Note
authorCount :10Available from: 2010-11-05 Created: 2010-11-05 Last updated: 2022-02-24Bibliographically approved
4. Aerosols indirectly warm the Arctic
Open this publication in new window or tab >>Aerosols indirectly warm the Arctic
Show others...
2010 (English)Manuscript (preprint) (Other academic)
Abstract [en]

On average, airborne aerosol particles cool the Earth's surface directly by absorbing and scattering sunlight and indirectly by influencing cloud reflectivity, life time, thickness or extent. Here we show that over the central Arctic Ocean, where there is frequently a lack of aerosol particles upon which clouds may form, a small increase in aerosol loading may enhance cloudiness thereby likely causing a climatologically significant warming at the ice-covered Arctic surface. Under these low concentration conditions cloud droplets grow to drizzle sizes and fall, even in the absence of collisions and coalescence, thereby diminishing cloud water. Evidence from a case study suggests that interactions between aerosol, clouds and precipitation could be responsible for attaining the observed low aerosol concentrations.

National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:su:diva-44321 (URN)10.5194/acpd-10-16775-2010 (DOI)
Note
This manuscript is published in Atmospheric Chemistry and Physics Discussions, where it is currently under review for publication in Atmospheric Chemistry and Physics.Available from: 2010-11-05 Created: 2010-11-05 Last updated: 2022-02-24Bibliographically approved
5. How well do regional climate models reproduce radiation and clouds in the Arctic?: An evolution of ARCMIP simulations
Open this publication in new window or tab >>How well do regional climate models reproduce radiation and clouds in the Arctic?: An evolution of ARCMIP simulations
2008 (English)In: Journal of Applied Meteorology and Climatology, ISSN 1558-8424, E-ISSN 1558-8432, Vol. 47, no 9, p. 2405-2422Article in journal (Refereed) Published
Abstract [en]

Downwelling radiation in six regional models from the Arctic Regional Climate Model Intercomparison (ARCMIP) project is systematically biased negative in comparison with observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment, although the correlations with observations are relatively good. In this paper, links between model errors and the representation of clouds in these models are investigated. Although some modeled cloud properties, such as the cloud water paths, are reasonable in a climatological sense, the temporal correlation of model cloud properties with observations is poor. The vertical distribution of cloud water is distinctly different among the different models; some common features also appear. Most models underestimate the presence of high clouds, and, although the observed preference for low clouds in the Arctic is present in most of the models, the modeled low clouds are too thin and are displaced downward. Practically all models show a preference to locate the lowest cloud base at the lowest model grid point. In some models this happens also to be where the observations show the highest occurrence of the lowest cloud base; it is not possible to determine if this result is just a coincidence. Different factors contribute to model surface radiation errors. For longwave radiation in summer, a negative bias is present both for cloudy and clear conditions, and intermodel differences are smaller when clouds are present. There is a clear relationship between errors in cloud-base temperature and radiation errors. In winter, in contrast, clear-sky cases are modeled reasonably well, but cloudy cases show a very large intermodel scatter with a significant bias in all models. This bias likely results from a complete failure in all of the models to retain liquid water in cold winter clouds. All models overestimate the cloud attenuation of summer solar radiation for thin and intermediate clouds, and some models maintain this behavior also for thick clouds.

National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:su:diva-14592 (URN)10.1175/2008JAMC1845.1 (DOI)000259317400009 ()
Available from: 2009-01-13 Created: 2009-01-13 Last updated: 2022-02-25Bibliographically approved

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