An analysis of POAM II solar occultation observations of polar mesospheric clouds in the southern hemisphere

The second Polar Ozone and Aerosol Measurement (FOAM II) instrument is a space-borne visible/near IR photometer which uses the solar occultation technique to measure vertical profiles of ozone, nitrogen dioxide, and water vapor as well as aerosol extinction and atmospheric temperature in the stratosphere and upper troposphere. Here we report on the detection of polar mesospheric clouds (PMCs) in the high-latitude southern hemisphere by FOAM II during the 1993 and 1994 summer seasons. These measurements are noteworthy because they are the first measurements of PMCs in atmospheric extinction. The POAM II PMC data,set has been analyzed using a simple geometric cloud model. We find that mean cloud altitudes deduced from these data are 82-83 km, consistent with previous ground-based and satellite measurements. In addition, the 0.7 km vertical resolution of FOAM II allows for accurate determination of cloud thickness. For the PMCs detected by FOAM II we find a mean thickness of 2.4 km. The mean peak slant optical depth was determined to be 1.2 x 10(-3) for the 1993 season and 1.8 x 10(-3) for the 1994 season, corresponding to a cloud extinction coefficient of 3.9 x 10(-6) and 6.1 x 10(-6) km(-1), respectively. The multichannel capability of POAM II also makes it possible to study the wavelength dependence of the measured slant optical depth for the clouds with largest extinction. The results of this analysis suggest an upper limit to the modal particle radii for these clouds of approximately 70 nm.