Boundary layer - cloud interaction in the summer Arctic

Low-level clouds play a decisive role for the surface energy budget. For the Arctic these clouds represent a warming factor most of the year. This is partly due to a relatively high surface albedo and large solar zenith angles. These clouds are also often optically thin thus making them sensitive to changes, natural as well as anthropogenic. Moldes have great difficulty describing these clouds in a reasonable fashion; it has been suggested that the large inter-model scatter in climate change scenarios is to large extent due to differences in cloud descriptions in climate models; if this sensitivity is incorrect in the models, climate change scenarios for the Arctic are not reliable.

In this presentation we describe a study from the Arctic, using data collected at the Arctic Summer Cloud Ocean Study. We will demonstarte the effects of low stratiform clouds on the surface energy balance and elaborate on the linkages between the clouds and the surface. Besides sometimes being coupled to the surface by turbulent fluxes, there is also a tight radiative coupling between the clouds and the surface such that the difference between the surface temperature and the sky brightness temperature derived from downwelling longwave radiation appears in distinct regimes. We will discuss the possible implications of this as tied to PBL, cloud microphysics and cloud/radiation interactions.