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- Title
Supercooled Drizzle Development in Response to Semi-Coherent Vertical Velocity Fluctuations Within an Orographic Layer Cloud.
- Authors
Majewski, Adam; French, Jeffrey R.
- Abstract
Observations of super-cooled liquid water are nearly ubiquitous within wintertime, orographic layer clouds over the intermountain west; however, observations of regions containing super-cooled drizzle drops (SCDDs) are much rarer and the factors controlling SCDD development and location less well understood. As part of the Seeded and Natural Orographic Wintertime clouds - the Idaho Experiment (SNOWIE) goal of improving understanding of natural cloud structure, this study examines the role of fine-scale (sub-kilometer) vertical velocity fluctuations on the microphysical evolution and location of SCDDs within the observed mixed-phase, wintertime orographic clouds from one research flight of SNOWIE. This flight saw SCDDs develop in an elevated, postfrontal layer cloud with cold cloud tops (T < -30 °C) - containing low number concentrations of both ice (Nice < 0.5 L-1) and droplets (Ncld < 30 cm-3). Regions of supercooled drizzle at flight level extended more than a kilometer along the mean wind direction and were first located at and below layers of semi-coherent vertical velocity fluctuations (SCVVFs) embedded within the cloud. The microphysical development of SCDDs in this environment is catalogued using size and mass distributions derived from in-situ probe measurements. Regions corresponding to hydrometeor growth are determined from radar reflectivity profiles retrieved from an airborne W-band cloud radar. Analysis suggests that SCVVF layers (e.g. from K-H waves) are associated with local SCDD development in response to the kinematic perturbation pattern. This drizzle development and subsequent growth by collision-coalescence is inferred from vertical reflectivity enhancements (-20 dBZ/km), with drizzle production confirmed by in-situ measurements within one of these vertical velocity fluctuation layers. The SCDD production and growth occurs embedded within cloud over shallow (km or less) layers before transitioning to drizzle production at cloud top further downwind, indicating that wind shear and resultant vertical velocity fluctuations may be more important for SCDD development than cloud top broadening mechanisms in the orographic (or similarly sheared) cloud environment(s).
- Subjects
OROGRAPHIC clouds; VERTICAL wind shear; STRATOCUMULUS clouds; VELOCITY
- Publication
Atmospheric Chemistry & Physics Discussions, 2019, p1
- ISSN
1680-7367
- Publication type
Article
- DOI
10.5194/acp-2019-717