Optical and geometrical characteristics of cirrus clouds over a mid-latitude lidar station

Abstract

Optical and geometrical characteristics of cirrus clouds overThessaloniki, Greece (40.6 degrees N, 22.9 degrees E) have beendetermined from the analysis of lidar and radiosonde measurementsperformed during the period from 2000 to 2006. Cirrus clouds aregenerally observed in a mid-altitude region ranging from 8.6 to 13 km,with mid-cloud temperatures in the range from -65 degrees to -38 degreesC. The cloud thickness generally ranges from 1 to 5 km and 38% of thecases studied have thickness between 2 and 3 km. The retrieval ofoptical depth and lidar ratio of cirrus clouds is performed using threedifferent methods, taking into account multiple scattering effect. Themean optical depth is found to be 0.31 +/- 0.24 and the correspondingmean lidar ratio is 30 +/- 17 sr following the scheme of Klett-Fernaldmethod. Sub-visual, thin and opaque cirrus clouds are observed at 3%,57% and 40% of the measured cases, respectively. A comparison of theresults obtained between the three methods shows good agreement. Themultiple scattering errors of the measured effective extinctioncoefficients range from 20 to 60%, depending on cloud optical depth.The temperature and thickness dependencies on optical properties havealso been studied in detail. A maximum mid-cloud depth of similar to 3.5km is found at temperatures around similar to-47.5 degrees C, whilethere is an indication that optical depth and mean extinctioncoefficient increases with increasing mid-cloud temperature. Acorrelation between optical depth and thickness was also found. However,no clear dependence of the lidar ratio values on the cloud temperatureand thickness was found.