Vertical Motions in Orographic Cloud Systems over the Payette River Basin. Part IV: Controls on Supercooled Liquid Water Content and Cloud Droplet Number Concentrations

This paper examines the controls on supercooled liquid water content (SLWC) and drop number concentrations (N_t,CDP) over the Payette River basin during the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE) campaign. During SNOWIE, 27.4% of 1-Hz in situ cloud droplet probe samples were in an environment containing supercooled liquid water (SLW). The interquartile range of SLWC, when present, was found to be 0.02–0.18 g m^-3 and 13.3–37.2 cm^-3 for N_t,CDP, with the most extreme values reaching 0.40–1.75 g m^-3 and 150–320 cm^-3 in isolated regions of convection and strong shear-induced turbulence. SLWC and N_t,CDP distributions are shown to be directly related to cloud-top temperature and ice particle concentrations, consistent with past research over other mountain ranges. Two classes of vertical motions were analyzed as potential controls on SLWC and N_t,CDP, the first forced by the orography and fixed in space relative to the topography (stationary waves) and the second transient, triggered by vertical shear and instability within passing synoptic-scale cyclones. SLWC occurrence and magnitudes, and N_t,CDP associated with fixed updrafts were found to be normally distributed about ridgelines when SLW was present. SLW was more likely to form at low altitudes near the terrain slope associated with fixed waves due to higher mixing ratios and larger vertical air parcel displacements at low altitudes. When considering transient updrafts, SLWC and N_t,CDP appear more uniformly distributed over the flight track with little discernable terrain dependence as a result of time and spatially varying updrafts associated with passing weather systems. The implications for cloud seeding over the basin are discussed.