On the linkage between tropospheric and Polar Stratospheric clouds in the Arctic as observed by space-borne lidar

by P. Achtert, M. Karlsson Andersson, F. Khosrawi, J. Gumbel
Atmospheric Chemistry and Physics ()
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The type of Polar stratospheric clouds (PSCs) as well as their temporal and spatial extent are important for the occurrence of heterogeneous reactions in the polar strato-sphere. The formation of PSCs depends strongly on temper-ature. However, the mechanisms of the formation of solid PSCs are still poorly understood. Recent satellite studies of Antarctic PSCs have shown that their formation can be as-sociated with deep-tropospheric clouds which have the abil-ity to cool the lower stratosphere radiatively and/or adiabat-ically. In the present study, lidar measurements aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Obser-vation (CALIPSO) satellite were used to investigate whether the formation of Arctic PSCs can be associated with deep-tropospheric clouds as well. Deep-tropospheric cloud sys-tems have a vertical extent of more than 6.5 km with a cloud top height above 7 km altitude. PSCs observed by CALIPSO during the Arctic winter 2007/2008 were classified accord-ing to their type (STS, NAT, or ice) and to the kind of un-derlying tropospheric clouds. Our analysis reveals that 172 out of 211 observed PSCs occurred in connection with tropo-spheric clouds. 72 % of these 172 observed PSCs occurred above deep-tropospheric clouds. We also find that the type of PSC seems to be connected to the characteristics of the un-derlying tropospheric cloud system. During the Arctic winter 2007/2008 PSCs consisting of ice were mainly observed in connection with deep-tropospheric cloud systems while no ice PSC was detected above cirrus. Furthermore, we find no correlation between the occurrence of PSCs and the top tem-perature of tropospheric clouds. Thus, our findings suggest that Arctic PSC formation is connected to adiabatice cooling, i.e. dynamic effects rather than radiative cooling.

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