LIDAR OBSERVATION OF SPHERICAL-PARTICLES IN A -65-DEGREES COLD CIRRUSOBSERVED ABOVE SODANKYLA (FINLAND) DURING SESAME

The absence of LIDAR depolarization in a polar cirrus observed above S odankyla (Finland) showed the presence of spherical particles at -65 d egrees C. The presence of pure liquid water is excluded, since homogen eous freezing should occur at or above -40 degrees C. The cirrus; laye r was detected at the tropopause, during the horizontal and vertical a dvection above northern Scandinavia of warm and wet oceanic air. Two a lternative explanations are suggested, the first one involving the pre sence of large, deliquescent tropospheric CN, freezing at very low tem perature, and the second one involving the presence of metastable, sph erical ice-particles produced in the Fast adiabatic cooling of the air masses. The second hypothesis is less reasonable because the absence o f depolarized LIDAR signal implies a strict cylindrical symmetry in th e particles, a symmetry that is difficult to maintain during the freez ing process. (C) 1998 Elsevier Science Ltd. All rights reserved.