Vertical Motions in Orographic Cloud Systems over the Payette River Basin: Controls on Liquid Water Content, Drop Number Concentrations, and Drop Size Distributions

Two classes of vertical motions, fixed (associated with vertically propagating gravity waves tied to flow over topography) and transient (associated primarily with vertical wind shear and conditional instability within passing weather systems), were diagnosed over the Payette River basin of Idaho during the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE).

To assess the relationship fixed updrafts had on the distribution of LWC over the terrain, 1 Hz CDP LWC samples were binned every 0.01 degrees longitude (~800 m) along fixed flight tracks flown during the campaign. Composites of vertical motion (w) from all flight legs along fixed tracks A and B were also constructed. Along these legs, transient vertical motions vary in time and space, so averaging the vertical motion fields over cross sections from a large number of flight legs effectively removes the vertical motion contribution of transient updrafts while retaining fixed vertical circulations tied to the terrain.

This work will build off past work by analyzing the controls fixed and transient updrafts had on liquid water content, drop number concentrations and drop size distribution. Five key controls will be explored: an inverse relationship between cloud top temperature and LWC magnitude/drop concentrations, higher magnitudes of LWC/drop concentrations over the high terrain, low LWC/drop concentrations when background ice concentrations were >15 L^‑1, drop concentrations < 50 cm^‑3 when airmass origins were at higher altitudes (2-6 km) over the eastern Pacific Ocean, and drop size distribution broadening near cloud top.

LWC was more common when fixed waves were present but more widespread over the terrain (uniform) when taking into account transient vertical motions. SLW was found equally in both updrafts and downdrafts over the Payette River Basin (normally distributed about 0 m s^-1) and significantly driven by spillover effects associated with fixed vertical motions and pockets of SLW associated with transient vertical motions over the terrain.