On the onset of the ice phase in boundary layer Arctic clouds

Abstract

Airborne measurements in slightly supercooled Arctic boundary layer stratocumulus have been carried out in Spitsbergen on 29 May during the ASTAR 2004 campaign. Cloud measurements have been performed in both warm and cold sectors of a cold front passing the observation area. The results show a north-south gradient in freezing properties and thus evidence of significant differences in the cloud microstructure. Ahead of the front line, in the warm sector (cloud top temperature at - 4°C), no ice particles were detected. The cloud formed in clean air conditions (aerosol concentration of 300 cm -3) with subsequent large effective diameter (20-26 μm) and low concentration (50 cm -3) of cloud droplets. Therefore, the collision-coalescence process was effective, favoring the drizzle formation with concentration up to 300 L -1 (D > 50 μm). In the cold sector behind the front, with a lower cloud top temperature (-6°C), ice crystals were observed in the entire cloud layer, and no droplets larger than about 50 μu(drizzle) were detected. The observations confirm high ice particle concentrations (up to 50 L -1) even with rather warm cloud top (-6°C) compared to previous studies in Arctic clouds. The shattering of isolated drops during freezing and the ice splinter production during riming appear to be the most likely processes to explain the observations of high ice concentration in the cold sector. Analysis of back trajectories did not reveal significant differences in the origin of the air masses in the warm and cold sectors that might have contributed to the differentiation of aerosol composition and thus cloud properties. A cloud top temperature colder than -4°C appears to be required for the onset of the ice phase in this slightly supercooled stratiform cloud. Copyright 2009 bv the American Geophysical Union.