Dual-Polarization Radar Observations of Precipitation Processes in Complex Terrain during OLYMPEX

The Olympic Mountains Experiment (OLYMPEX), a GPM ground validation project occurring fall 2015 – winter 2016, provided a unique opportunity to investigate precipitation processes as wintertime mid-latitude cyclones encounter complex terrain. The extensive instrument assets deployed during the field campaign upstream and across a range of elevations of the Olympic Mountains allowed for documentation of orographic enhancement of precipitation in a variety of synoptic events.  As part of this observational network, ground-based dual-polarization Doppler radars operated nearly continuously on the coast, windward interior valley, and leeside of the mountains. These data provided unprecedented detail to illuminate the role of terrain on microphysical and dynamical processes associated with this precipitation enhancement.  In particular, NASA’s S-band, NPOL, radar operating from the coast collected high-resolution vertical cross sections through systems over the ocean and inland toward the Quinault Valley. An X-band Doppler on Wheels (DOW) radar, located at Lake Quinault beneath the NPOL beam, documented necessary details for understanding processes occurring in the valley and over the adjacent lower windward slopes.

The relative roles of warm-rain and ice-based microphysical processes are explored through these radar observations in a range of environmental conditions dictated by frontal sector and storm type. Dual-polarization radar-inferred hydrometeors are compared to in situ aircraft data from the University of North Dakota Citation, which flew spirals through the radar RHI sectors, to determine dominant microphysical processes. These hydrometeor characteristics are placed within the context of flow patterns revealed by radar radial velocities. Flow impinging on the Olympic Peninsula was found to lift upstream of the furthest ridge and was often undercut by low-level, down-valley flow; a persistent pattern observed by the DOW located in the valley. At times, signatures of Kelvin-Helmholtz waves within this shear zone were observed in the radar radial velocity and spectral width data; their possible role in influencing microphysical processes in their vicinity will be discussed. The height of the 0°C level in relation to this overall shear zone will be explored to determine the dominant processes responsible for the observed patterns and variability of orographic precipitation enhancement during OLYMPEX.