Vertical Structure and Microphysical Characteristics of Precipitation on the High Terrain and Lee Side of the Olympic Mountains As Seen in OLYMPEX

As mid-latitude cyclones pass over coastal mountain ranges, the processes producing their clouds and precipitation are modified when they encounter complex terrain, leading to a maximum in precipitation fallout on the windward slopes and a minimum on the lee side. Few observational studies have focused on the microphysical processes contributing to the de-enhancement of precipitation over the high terrain and lee side of high coastal mountain ranges. Lower precipitation totals often make these “rain shadow” regions more vulnerable to wildfires, droughts and water shortages than the wetter windward sides. The 2015-16 Olympic Mountains Experiment (OLYMPEX) collected unprecedented observations over the coastal Olympic Mountains including frequent soundings and dual-polarization Doppler radar on both the windward and lee sides, multi-frequency airborne radar over the entire mountain range, and ground-based particle size observations at the coast, windward slopes, and high elevation Hurricane Ridge site. This study emphasizes the airborne component obtained by the NASA DC-8 aircraft, the only OLYMPEX platform to observe both upwind and downwind of the mountain range with the same instrument package. Thirteen DC-8 missions are divided into five sub-regions: ocean, coast, windward, high terrain, and lee side. Nine missions observed predominantly stratiform precipitation within the prefrontal, warm, and/or frontal storm sectors and four missions observed predominantly cellular convection within the postfrontal sector. As a result, the DC-8 observations show the variability in precipitation amount and composition across a coastal mountain range.

Contoured Frequency by Altitude Diagrams (CFADs) of Ku- and Ka-band reflectivity distributions from the NASA Airborne Third-Generation Precipitation Radar (APR-3) reveal that precipitating cloud structures depend on both the environment determined by the synoptic-scale storm sector and the spatial location relative to the mountain range. On the windward side, enhancement of reflectivity is observed through the entire depth of the cloud beginning upstream of the mountain range at the coast and continuing over the high terrain. On the lee side, a decrease in reflectivity compared to the windward side is observed mainly below 3 km elevation. Significant precipitation over the high terrain often occurred in the postfrontal sector and was associated with high concentrations of small ice particles at the Hurricane Ridge ground site. These new insights on the variability in precipitation amount and composition across a coastal mountain range will advance ongoing efforts to measure precipitation from space and parameterize microphysical processes in numerical models.