Vertical Motions in Orographic Cloud Systems Retrieved from W-Band Radar over the Idaho Mountains during SNOWIE: Controls on Supercooled Liquid Water Content and Cloud Droplet Number Concentrations

This paper examines the controls on supercooled liquid water content (SLWC) and droplet number concentrations (N_t,_CDP) over the Salmon River Mountains of Idaho. Composites of retrieved vertical motion fields (w) from the W-band Wyoming Cloud Radar were created along fixed flight tracks flown parallel to mean midlevel flow. 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. Transient vertical motions over the terrain varied in time and space so averaging vertical motions over the cross sections from a large number of flight legs effectively removed the contribution of transient updrafts while retaining fixed vertical circulations tied to the terrain. Localized terrain ridges were associated with fixed orographic wave couplets that had updraft and downdraft magnitudes on the order of 0.3-0.5 m s^-1. 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. SLWC occurrence and magnitudes, and N_t,CDP associated with fixed updrafts were found to be normally distributed about ridgelines when SLW was present. The largest vertical air parcel displacements occurred near the terrain. Together with the high mixing ratios at lower altitudes, the trajectories implied that SLW production was most likely close to the terrain slope associated with fixed waves. 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 will be discussed.