Super-cooled Large Drop Production in Orographic Wintertime Clouds Observed in Idaho

The presence of super-cooled liquid water (SLW) in wintertime clouds is a prerequisite criterion for a cloud to be targeted for cloud seeding to enhance snowfall and it poses a serious threat to aviation due to the potential for aircraft icing. Some wintertime clouds produce super-cooled drizzle-size drops, which poses a unique icing hazard for aviation, yet the mechanisms by which these clouds produce super-cooled large drops (SLD) is less well understood. It is often assumed that, if ice is present in a mixed-phase cloud, the SLW will be depleted by ice-phase processes and the cloud would not be capable of producing SLD. Yet, some clouds exhibit mixed-phase conditions and still contain SLD.

Clouds with SLD were frequently observed during the Seeded and Natural Orographic Wintertime clouds: the Idaho Experiment (SNOWIE) field campaign, which was conducted between January–March 2017 near Boise, Idaho. SNOWIE aims to investigate natural orographic precipitation processes, with a focus on ice initiation, snow growth, and the impacts of orography on the development of supercooled liquid water, as well as to evaluate the impact of cloud seeding on orographic clouds. This paper will present a suite of observations, including W-band cloud radar, radiometer, sounding, and in situ aircraft measurements from a few cases with SLD to investigate the processes that led to the production of SLD.