Extinction coefficient (1 μm) properties of high‐altitude clouds from solar occultation measurements (1985–1990): Evidence of volcanic aerosol effect

Abstract

The properties of the l‐μm volume extinction coefficient of two geographically different high‐altitude cloud systems have been examined for the posteruption period (1985–1990) of the April 1982 El Chichón volcanic event with emphasis on the effect of volcanic aerosols on clouds. These two high‐altitude cloud systems are the tropical clouds in the tropopause region observed by the Stratospheric Aerosol and Gas Experiment (SAGE) II and the polar stratospheric clouds (PSCs) sighted by the Stratospheric Aerosol Measurement (SAM) II. The results indicate that volcanic aerosols alter the frequency distributions of these high‐altitude clouds in such a manner that the occurrence of clouds having high extinction coefficients (6×10−3 – 2×10−2 km−1) is suppressed, while that of clouds having low extinction coefficients (2×10−3 – 6×10−2 km−1) is enhanced. This influence of the volcanic aerosols appears to be opposite to the increase in the extinction coefficient of optically thick clouds observed by the Earth Radiation Budget Experiment (ERBE) during the initial posteruption period of the June 1991 Pinatubo eruption. A plausible explanation of this difference, based on the Mie theory, is presented. The Mie calculation indicates that there are two possible types of response of cloud extinction coefficient to changes in aerosol concentration depending on the primary effective radius (re) of cloud systems observed by the instrument. These two types of response are separated by the cloud particle effective radius of about 0.8 μm. When re is smaller than 0.8 μm, the cloud extinction coefficient decreases in response to increases of aerosol concentration, and when re is greater than 0.8 μm, the opposite happens. As a consequence, the effective radius of most, if not all, of the high‐altitude clouds, measured by the SAGE series of satellite instruments must be less than about 0.8 μm. This mean cloud particle size implied by the satellite extinction‐coefficient data at a single wavelength (1 μm) is further substantiated by the particle size analysis based on cloud extinction coefficient at two wavelengths (0.525 and 1.02 μm) obtained by the SAGE II observations. Most of the radiation measured by ERBE is reflected by cloud systems comprised of particles having effective radii much greater than 1 μm. A reduction in the effective radius of these clouds due to volcanic aerosols is expected to increase their extinction‐coefficient values, opposite the effect observed by SAGE II and SAM II. This work further illustrates the capability of the solar occultation satellite sensor to provide particulate extinction‐coefficient measurements important to the study of the aerosol‐cloud interactions. Finally, the June 1991 Mount Pinatubo major eruption put 3 times more material into the stratosphere than that of the 1982 El Chichón volcanic event. It is important to examine the variations of the extinction coefficient of these two high‐altitude cloud systems for the posteruption years of the Pinatubo volcanic event for further evidence of the impact of volcanic aerosols on high‐altitude clouds.