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Discussion papers
https://doi.org/10.5194/acp-2019-717
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-717
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 07 Oct 2019

Submitted as: research article | 07 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Supercooled Drizzle Development in Response to Semi-Coherent Vertical Velocity Fluctuations Within an Orographic Layer Cloud

Adam Majewski and Jeffrey R. French Adam Majewski and Jeffrey R. French
  • Department of Atmospheric Science, University of Wyoming, Laramie, 82070, USA

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).

Adam Majewski and Jeffrey R. French
Interactive discussion
Status: open (until 02 Dec 2019)
Status: open (until 02 Dec 2019)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Adam Majewski and Jeffrey R. French
Data sets

UW King Air Hydrometeor Size Spectra from SNOWIE J. R. French https://doi.org/10.5065/D6GT5KXK

UW King Air Wyoming Cloud Radar Data from SNOWIE S. Haimov https://doi.org/10.15786/M2CD4J

UW King Air Flight Level Data from SNOWIE J. R. French https://doi.org/10.15786/M2MW9F

Adam Majewski and Jeffrey R. French
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Latest update: 20 Oct 2019
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Short summary
The study reports formation of super-cooled drizzle drops in response to repeating kilometer-wide up- and down- drafts within a mixed-phase, mountain layer cloud containing very little ice despite cold cloud top temperatures (T ~ −30 °C). The discrete, embedded hydrometeor growth layers and downwind transition to drizzle production at cloud top indicates the relative importance of kinematic mechanisms in determining the location of precipitation development in cloud.
The study reports formation of super-cooled drizzle drops in response to repeating...
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