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MS Defense

Department of Atmospheric Science

Tues., Dec. 11, 3:10 pm, EN6085

Supercooled Drizzle Drop Development in a Postfrontal Orographic Layer Cloud Containing Kelvin-Helmholtz Billows

Adam Majewski

University of Wyoming


Observations of supercooled liquid water are nearly ubiquitous in wintertime orographic clouds throughout the intermountain west of the United States. The development of supercooled cloud water in such clouds is also well understood. However, observations of supercooled drizzle drops (SCDDs), supercooled drops of size large enough to have appreciable fall velocities, are rarer and factors controlling their development and location are less-well documented. One of the overarching goals of the Seeded and Natural Orographic Wintertime clouds—the Idaho Experiment (SNOWIE) is to improve understanding of natural cloud structure and key dynamical and microphysical processes governing precipitation formation within mixed-phase, wintertime orographic clouds.

Here a case from SNOWIE is examined, which saw supercooled drizzle drops (SCDDs) develop in a postfrontal layer cloud with cold cloud tops (TCT < -30 °C). This elevated layer cloud—at times decoupled from the orographic cloud beneath—had low background number concentrations of both ice (Nice < 1 L-1) and droplets (Ncld < 30 cm-3) and contained regions of supercooled drizzle at flight level extending more than a kilometer along the mean wind direction, whose vertical location was frequently tied to Kelvin-Helmholtz (K-H) waves forming in critical shear layers. Detailed microphysical development of SCDDs in this environment is presented using size and mass distributions derived from in-situ probe measurements, and regions corresponding to enhanced SCDD production and growth are determined from cumulative frequency by altitude diagrams derived from Wyoming cloud radar profiles.

It is concluded that (1) strongly bimodal droplet spectra from secondary droplet activation within kinetically-limited regions of ascent act to enhance the SCDD development in cloud and (2) that layers of K-H waves characteristically accelerate hydrometeor growth vertically. These conclusions have far reaching implications for the way that model microphysics schemes do or do not use subgrid-scale vertical velocity information, the way that K-H waves are understood to enhance liquid phase microphysics as a function of the kinetics of cloud parcels, and the understood spatial distribution and frequency of hazardous aircraft icing conditions.

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