Name
Michael Tjernström
Professor at MISU

Abstract

Michael Tjernström, professor MISU.
Michael Tjernström, professor MISU.

Arctic climate is ultimately determined by a balance between heat transported into the area meridionally, and heat lost at the top of the atmosphere over the same area, by radiation. Since the net radiation loss is mainly due to small-scale processes, parameterized in models, and meridional heat fluxes are due to larger-scale dynamics, directly resolved by models, the two processes has usually been studied separately.

In a week-long episode in summer 2014, during the Arctic Clouds in Summer Experiment (ACSE), warm air from Siberia flowed northward over the East-Siberian Sea, out over melting sea-ice. As the ~25 °C warm air adjusted to the melting sea-ice surface, a strong surface inversion developed and dense fog formed. This vertical structure is contrary to the well-mixed cloud-capped boundary layer observed over central Arctic sea ice in summer, and resulted in positive surface net longwave radiation while the turbulent sensible heat flux was downward. Although solar radiation was attenuated by the fog, the net effect was additional energy to the surface, by ~10-30 Wm-2.

This led us to hypothesize a zone along the ice edge where the surface will receive enhanced energy when warm air flows northward in over melting ice. To test this hypothesis, we analyzed the entire ACSE field campaign. All temperature profiles over sea ice were categorized into cases with or without surface inversions; inversions were further divided in two, based on their humidity profiles. When projecting other observations onto these classes, surface inversion where moisture increases with height systematically added an extra 10-20 Wm-2 to the surface energy budget, indicating that meridional heat flux must be considered together with small-scale processes caused by air-mass transformation.

 

Time and Place
Tuesday 12 September 2017, 11.15
Rossbysalen C609, Arrhenius Laboratory, 6th floor

(This event has taken place.)