Analysis of lidar observations of Arctic polar stratospheric clouds during January 1989

We present analyses of lidar backscatter and depolarization ratios for polar stratospheric clouds (PSCs) observed during the 1989 Airborne Arctic Stratospheric Experiment. The backscatter and depolarization ratios are available at one visible and one infrared wavelength. Water ice PSCs were identified at low ambient temperatures based upon their relatively large back-scattering and depolarization ratios. The remaining clouds fall into four major categories. First, we observe a class of clouds that are not depolarizing at either of the two wavelengths. These clouds are identified as Type Ib PSCs, which are assumed to be composed of ternary solutions of H2SO4/HNO3/H2O. Type Ib clouds were never dominant, though on some dates they accounted for 25 to 40% of the observations. We find from the wavelength dependence of the backscattering by these clouds that their size distributions must be very narrow. Other optical observations of these clouds should consider the possible impact of these narrow size distributions on their data analysis. These clouds have a relatively large total particulate mass that is comparable to the known gas phase reservoir of nitric acid. The number density of Type Ib particles is similar to the concentrations of the ambient sulfate aerosols. The second category of clouds is highly depolarizing at both lidar wavelengths, but has relatively low backscattering ratios. We identify these clouds as Type 1a PSCs (assumed to be nitric acid tri- or dihydrate) which form only on a small subset by number, approximately 1% or less, of the ambient sulfate aerosols. These clouds were the first to be observed, and were especially common on the first few flights. They accounted for more than 70% of the observations on three flights. Type Ia particles are near 1 μm or larger in radius. The third type of cloud is depolarizing at visible wavelengths, but not at near infrared wavelengths. These clouds were seen during portions of nearly every flight and comprised 10 to 25% of all of the observations. These clouds, which we refer to as Type 1c, are composed of small, solid particles. These clouds contain a relatively large mass of material, comparable to the gas phase reservoir of nitric acid. Type 1c particles are a few tenths of a micrometer in radius and have a concentration that is similar to that of the ambient sulfate aerosols. The final class of clouds has no depolarization at the lidar's visible wavelength but has significant depolarization at the infrared wavelength. Such particles were seen on many of the flights and sometimes accounted for as much as 30% of all the observations. We interpret these clouds as being mixtures of Type 1a and 1b PSCs. Although some polar stratospheric clouds have fairly homogeneous properties over very large spatial scales, many have variable properties at relatively small scales. Thus the various types of particles are often observed within a single cloud. Homogeneous clouds composed only of solid particles seem inconsistent with a wave cloud origin. However, clouds in which several types of particles occur together may represent situations in which mountain waves have triggered solid particle formation. Alternatively, clouds composed of different types of particles may be in the process of freezing. We observed in regions upwind of the coldest temperatures that the air often did not contain clouds but may have had large supersaturations with respect to NAT. In contrast, regions downwind of the coldest temperatures often contained large solid particles in air that may have been close to NAT supersaturation. The correlation between PSC types and their respective temperature histories suggest that, given enough time, liquid particles will convert to solids.