Observations of Super-cooled Drizzle Production in a Wintertime, Orographic Cloud

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 more rare and the factors controlling SCDD development and location less well understood. This study documents a case in which SCDDs developed in an elevated, postfrontal layer cloud over the mountains of southwest Idaho despite extremely cold cloud tops (T < -30 °C). The analysis focuses on 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.

SCDDs developed in an elevated layer cloud with low number concentrations of both ice (N_ice < 0.5 L^-1) and cloud droplets (N_cld < 30 cm^-3). The SCDD regions extended more than a kilometer along the mean wind direction and first appeared to be located at and below layers of semi-coherent vertical velocity fluctuations 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 hydrometer growth are determined from radar reflectivity profiles retrieved from an airborne W-band cloud radar. Analysis suggests that layers of semi-coherent vertical velocity fluctuations (e.g. from K-H waves) are associated with local SCDD development in response to the kinematic perturbation pattern. This development and subsequent growth by collision-coalescence is inferred from vertical reflectivity enhancements (~ -20 dBZ/km) attributed to hydrometeor collectional growth, with drizzled production confirmed by in situ measurements 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).