EVOLUTION OF THE CLOUDY BOUNDARY-LAYER DURING THE AUTUMNAL FREEZING OF THE BEAUFORT SEA

The Beaufort and Arctic Seas Experiment (BASE) occurred during Septemb er and October 1994, off the Canadian Beaufort coast. In this paper, w e provide a description of the characteristics of the the cloudy bound ary layer that were observed from a research aircraft during BASE and provide an interpretation for the coevolution of the atmospheric bound ary layer and sea ice characteristics during the autumnal freezing of the Beaufort Sea. Observations from a total of 12 research flights are presented. During the course of the experiment the underlying surface made a transition from late summer melt conditions to full-fledged wi nter conditions. All of the observed boundary layer cloud decks are as sociated with stable temperature profiles, with temperature inversions occurring either above or within each of the cloud decks. Unlike obse rvations of summertime boundary layer clouds, no multiple-layered clou ds in the boundary layer were observed. Examination of the cloud phase indicates a transition from predominantly liquid phase clouds at the beginning of the experiment to predominantly crystalline clouds at the end of the experiment. Local interactions between the clouds and unde rlying surface were examined in several case studies, including the ef fects of leads. Using a time series of soundings over a period of 5 da ys in conjunction with the the National Center for Environmental Predi ction analyses, an air mass modification event was examined that began after the intrusion of warm air from the Pacific Ocean. The evolution of the air mass is apparent in decreasing temperatures in the lower a tmosphere and formation of a low-level mixed phase cloud that evolves with time into a deeper cloud layer that is crystalline.