A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing
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Snow surface and sea-ice energy budgets were measured near 87.5°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 −7°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.
KeywordsArctic Sea ice Surface energy budget Cloud radiative forcing Shortwave radiation Longwave radiation
This work is part of ASCOS (the Arctic Summer Cloud Ocean Study) and was funded by the Swedish Science Research Council and the DAMOCLES European Union 6th Framework Program Integrated Research Project. ASCOS was made possible by funding from the Knut and Alice Wallenberg Foundation. IMB and CEB were also funded by the UK Natural Environment Research Council; CEB was additionally partially supported by the UK Met Office that also provided real-time weather forecasting for ASCOS. POGP and MDS were also funded by the National Science Foundation. MN also received funding through the NorClim project, financed in the Norklima program of the Norwegian Research Council; Sebastian Gerland is acknowledged for dual-responsibility of data with MN. The Swedish Polar Research Secretariat (SPRS) provided access to the icebreaker Oden and logistical support. Chief Scientists were Caroline Leck and Michael Tjernström. We are grateful to the SPRS logistical staff and to Oden’s Captain Mattias Peterson and his crew. ASCOS is an IPY project under the AICIA-IPY umbrella and an endorsed SOLAS project.
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