Abstract
Abstract. Observations of supercooled liquid water are nearly ubiquitous within wintertime orographic-layer clouds over the Intermountain West; however, observations of regions containing supercooled 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) and its 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 in SNOWIE. For the case examined, SCDDs developed in an elevated, postfrontal-layer cloud with cold cloud tops (T<-30 ∘C) and low number concentrations of both ice (less than 0.5 L−1) and cloud droplets (less than 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 and subsequently below cloud top. 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 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−1), with drizzle production confirmed by in situ measurements within one of these SCVVF 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 act to enhance or speed up SCDD development compared to classic cloud top broadening mechanisms in orographic (or similarly sheared) cloud environment(s).
Highlights
Over the last 40 years, there have been numerous field campaigns either directly or indirectly examining mixed-phase, orographic-layer clouds over the American Intermountain West (Hobbs, 1975; Cooper and Saunders, 1980; Heggli and Reynolds, 1985; Rasmussen et al, 1992; Ikeda et al, 2007; Rosenfeld et al, 2013)
The elevated cellular cloud layer contained both low background number concentrations of ice and cloud droplets and embedded kilometer or longer regions of Supercooled drizzle drops (SCDDs) that formed in a larger pattern of orographic lift
Much of the previous work describing SCDD development in orographic, mixed-phase cloud systems focused on the necessary conditions for development – namely the low cloud droplet and ice number concentrations coupled with condensate supply rates sufficient to support condensational growth to the droplet sizes required for active collision–coalescence (Rauber, 1992; Ikeda et al, 2007)
Summary
Over the last 40 years, there have been numerous field campaigns either directly or indirectly examining mixed-phase, orographic-layer clouds over the American Intermountain West (Hobbs, 1975; Cooper and Saunders, 1980; Heggli and Reynolds, 1985; Rasmussen et al, 1992; Ikeda et al, 2007; Rosenfeld et al, 2013). SLW mass may be distributed entirely across cloud droplets, i.e., those liquid hydrometeors that are relatively small and have not attained appreciable fall speeds, and taken to have diameters less than 50 μm for the purpose of this study. Drizzle drops, with diameters 50 to 500 μm, have appreciable fall velocities (0 to 2 m s−1) relative to cloud droplets and can grow rapidly via collision–coalescence in the presence of cloud droplets (Lamb and Verlinde, 2011). R. French : Supercooled drizzle development from vertical velocity fluctuations such as pneumatic boots (Ashenden et al, 1996). This study aims to catalogue the effect of local, kilometer-scale kinematic perturbation patterns on the development and location of SCDDs for one such mixed-phase cloud system
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