Deep Orographic Cloud Structure and Composition Derived from Comprehensive Remote Sensing Measurements

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  • 1 Department of Meteorology, University of Utah, Salt Lake City, UT 84112
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Abstract

Coordinated polarization lidar, Ku-band radar and dual-channel microwave radiometer observations of a deep orographic cloud system were collected from a mountain-base site in northwestern Colorado as part of the Colorado Orographic Seeding Experiment (COSE) research effort. The remote sensing observations are presented for three distinct storm stages, corresponding to a pre-frontal altostratus cloud layer, a local orographically-induced cloud development, and a peak in storm activity accompanying the passage of a weak cold front. Supercooled liquid water in the form of thin but often dense liquid layers, and expansive, more weakly mixed-phase cloud regions were usually present even to temperatures approaching −40°C. The liquid water amounts present were often below the detection threshold of the vertically-pointing radiometer measurements, but during one brief interval a liquid water content as high as 0.5 g m−3 may have occurred. The lidar depolarization data also show the presence of a persistent layer of oriented planar ice crystals at the −15°C level, which was responsible for generating aggregates and light snowfall in the downwind mountains. The monitoring of the quantities of the water substance present in the vapor, liquid, and solid phases provides a unique image of the behavior of the storm, and it is concluded that this remote sensor ensemble is well suited for the study of orographic clouds and their potential for modification.

Abstract

Coordinated polarization lidar, Ku-band radar and dual-channel microwave radiometer observations of a deep orographic cloud system were collected from a mountain-base site in northwestern Colorado as part of the Colorado Orographic Seeding Experiment (COSE) research effort. The remote sensing observations are presented for three distinct storm stages, corresponding to a pre-frontal altostratus cloud layer, a local orographically-induced cloud development, and a peak in storm activity accompanying the passage of a weak cold front. Supercooled liquid water in the form of thin but often dense liquid layers, and expansive, more weakly mixed-phase cloud regions were usually present even to temperatures approaching −40°C. The liquid water amounts present were often below the detection threshold of the vertically-pointing radiometer measurements, but during one brief interval a liquid water content as high as 0.5 g m−3 may have occurred. The lidar depolarization data also show the presence of a persistent layer of oriented planar ice crystals at the −15°C level, which was responsible for generating aggregates and light snowfall in the downwind mountains. The monitoring of the quantities of the water substance present in the vapor, liquid, and solid phases provides a unique image of the behavior of the storm, and it is concluded that this remote sensor ensemble is well suited for the study of orographic clouds and their potential for modification.

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