Supercooled Drizzle Drop Development In a Postfrontal Orographic Layer Cloud in Response to Semi-Coherent Vertical Velocity Fluctuations
Abstract
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 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 < 0.5 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 semi-coherent (0.5-3 km spatial scale) vertical velocity fluctuations associated with wind shear layers or weak static instability. 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 contoured frequency by altitude diagrams derived from Wyoming cloud radar profiles.The observations suggest that layers of semi-coherent vertical velocity fluctuations (e.g. from K-H waves) may enhance collision-coalescence growth vertically, with larger vertical growth rate (dD/dt) compared to drizzling cloud without this kinematic pattern. This has far reaching implications for the way that model microphysics schemes do or do not use subgrid-scale vertical velocity information and in turn the impact on the modeled spatial distribution and frequency of hazardous aircraft icing conditions.
- Publication:
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Masters Thesis
- Pub Date:
- 2019
- Bibcode:
- 2019MsT.........17M
- Keywords:
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- Atmospheric sciences;Hydrologic sciences;Meteorology