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Colloquium

Department of Atmospheric Science

Tues., Nov. 15, 3:10 pm, EN6085

Freezing Drizzle and Ice Multiplication in an Orographic Cloud

Dr. Jeff French

Department of Atmospheric Science

University of Wyoming

Abstract

Super-cooled clouds that produce freezing drizzle pose a serious threat to aviation due to the potential for severe icing. Here we examine the details of such a case that was composed of two distinct cloud layers, a lower cloud between -1 and -4 °C and an upper cloud with tops around -12 °C. Both clouds had very low droplet concentrations, well less than 100 cm-3, which is quite unusual for clouds over the continental US. The clouds formed as part of an atmospheric river event that impacted the western and northwestern US. Because the origin of the air was maritime in nature and decoupled from the underlying surface, it likely contributed to the observed low cloud droplet concentrations.
Much of the presentation focuses on measurements provided from in situ probes mounted on the UW King Air research aircraft. In particular we examine cloud droplet spectra, size distribution and phase of larger hydrometeors, and bulk measures of water content. A profile from the base of the lowest cloud to near the top of the upper cloud indicates that the majority of precipitation mass is liquid, although the relatively rare large ice crystal was detected. Interestingly, as the precipitation falls into the top of the lower cloud, significantly more ice is produced and the precipitation mass becomes more equally split between liquid and ice. We conjecture that the initial production of ice in the upper cloud occurs through primary nucleation, likely near the cloud top. As these very few ice crystals grow through vapor deposition and eventually fall into the lower cloud, they interact with cloud droplets producing more ice through secondary processes (presumably Hallet-Mossop).

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