Polarization Lidar and Synoptic Analyses of an Unusual Volcanic Aerosol Cloud

Kenneth Sassen Department of Meteorology, University of Utah, Salt Lake City, Utah

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John D. Horel Department of Meteorology, University of Utah, Salt Lake City, Utah

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Abstract

Over an unusually brief three-day period in early August 1989, spectacular twilight effects indicative of a stratospheric volcanic cloud were seen at Salt Lake City, Utah. Concurrent polarization lidar observations detected an aerosol layer at altitudes between 14 and 16 km in the vicinity of the tropopause. Trajectory analyses indicate that the aerosol source was the relatively minor 19 July volcanic eruption of Santiaguito in Guatemala. Materials injected into the lower stratosphere by this eruption were transported initially by tropical easterlies and then by a subtropical jet stream to the locale. The sulfuric acid droplet cloud that formed during transport was affected locally by tropopause folds that promoted stratospheric-tropospheric exchanges. Although lidar depolarization analysis suggests that the ensuing cloud microphysical processes were usually dominated by acid droplet crystallization effects caused by ammonia gas absorption (yielding 0.1–0.2 linear depolarization ratios), there is also evidence for ice crystals at the coldest temperatures (∼−64°C) and for homogeneous droplets. Cloud optical thickness estimates are 0.01–0.02. Mesoscale cloud bands were observed visually near sunset and also occasionally during daylight, another unusual characteristic for volcanic clouds.

Abstract

Over an unusually brief three-day period in early August 1989, spectacular twilight effects indicative of a stratospheric volcanic cloud were seen at Salt Lake City, Utah. Concurrent polarization lidar observations detected an aerosol layer at altitudes between 14 and 16 km in the vicinity of the tropopause. Trajectory analyses indicate that the aerosol source was the relatively minor 19 July volcanic eruption of Santiaguito in Guatemala. Materials injected into the lower stratosphere by this eruption were transported initially by tropical easterlies and then by a subtropical jet stream to the locale. The sulfuric acid droplet cloud that formed during transport was affected locally by tropopause folds that promoted stratospheric-tropospheric exchanges. Although lidar depolarization analysis suggests that the ensuing cloud microphysical processes were usually dominated by acid droplet crystallization effects caused by ammonia gas absorption (yielding 0.1–0.2 linear depolarization ratios), there is also evidence for ice crystals at the coldest temperatures (∼−64°C) and for homogeneous droplets. Cloud optical thickness estimates are 0.01–0.02. Mesoscale cloud bands were observed visually near sunset and also occasionally during daylight, another unusual characteristic for volcanic clouds.

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