LIDAR OBSERVATIONS OF POLAR STRATOSPHERIC CLOUDS ABOVE THE ESRANGE AND ALOMAR IN NORTHERN SCANDINAVIA: STATISTICS AND SIMULTANEOUS OBSERVATION

摘要

Polar stratospheric clouds play a key role in polar ozone depletion and have been thus subject of intense research since several years. In particular groundbased lidar instruments in polar regions are very well suited to monitor PSCs on a long-term basis. The Bonn University lidar at the Esrange, the ALOMAR RMR lidar, and the ALOMAR O_3 lidar can observe polar stratospheric clouds on different wavelengths and polarisations. Since January 1997 the U. Bonn lidar at the Esrange had performed measurement campaigns of 4 - 6 weeks each winter. This extensive database shows that PSCs at the Esrange are predominantly caused by leewave-induced modulation of the temperature profile. In contrast, PSCs at Alomar exist mainly due to synoptic cold temperatures. Since the last winters have been dominated by major stratospheric warmings, PSC detections at Alomar and at the Esrange were rare. The last winter 2004/05, however, showed a stable, cold vortex leading to synoptic PSC existence. Cloud-free skies on both sides of the Scandinavian mountains - which is a very unusual feature during polar winter - on 5 January 2005 allowed observation of a polar stratospheric cloud simultaneously by all three lidar systems on different wavelengths and polarisations. De-polarisation measurements revealed that this cloud consisted mostly of STS and NAT. The particle size was estimated from the multi-colour measurements to be about 300 nm with a narrow distribution width of σ ≈ 1.02. The synoptic temperatures provided by ECMWF were cold enough for PSC existence. Backward trajectory calculations indicate that the observed clouds existed for at least 24 hours before observation.