Polar stratospheric clouds from satellite observational data

Abstract

The spectral aerosol-attenuation coefficients (SAACs) obtained from SAGE III measurements are used to study the physical and integral microphysical characteristics of polar stratospheric clouds (PSCs). Dif- ferent criteria for PSC identification from SAAC measurements are considered and analyzed based on model and field measurements. An intercomparison of them is performed, and the agreement and difference of the results obtained with the use of different criteria are shown. A new criterion is proposed for PSC identification, which is based on the estimate of how close the measured vector of the spectral attenuation coefficient is to a model distribution of the PSC ensemble. On the basis of different criteria, cases of PSCs are isolated from all SAGE III observations (over 30000). All selection criteria lead to a qualitatively and quantitatively similar space-time distribution of the regions of PSC localization. The PSCs observed in the region accessible to SAGE III measurements are localized in the latitudinal zones 65 ° -80 ° in the Northern Hemisphere and 45 ° -60 ° in the Southern Hemisphere during the winter-spring period. In the Northern Hemisphere, PSCs are observed within the longitudinal zone 120 ° W-100 ° E with the maximum frequency of PSC observation in the vicinity of the Greenwich meridian. In the Southern Hemisphere, the region of PSC observation is almost the same in longi- tude but with a certain shift in the maximum frequency of PSC observation to the west. This maximum is observed in the vicinity of 40 ° W, and the region of usual PSC observation is the neighborhood of 60 ° of the maximum's longitude. The physical parameters of PSCs are estimated: the mean heights of the lower and upper boundaries of PSCs are 19.5 and 21.9 km, respectively, and the mean cloud temperature is 191.8 K. The integral microphysical parameters of PSCs are estimated: the total surface of NAT particles S NAT = 0.41 µ m 2 /cm 3 ; the total volume of NAT particles V NAT = 1.1 µ m 3 /cm 3 ; and, for all aerosol and cloud particles together, S is 2.9 ± 1.5 at a standard deviation of 2.7 µ m 2 /cm 3 and V is 2.8 ± 1.5 at a standard deviation of 4.2 µ m 3 /cm 3 . A high frequency of PSC occurrence and high values of S and V in PSCs both for all particles and for NAT particles have been noted in January-February 2005 as compared to the rest of the period of SAGE III measurements for