The Olympic Mountains Experiment (OLYMPEX): An Opportunity to Explore Terrain-Influenced Precipitation Processes in Mid-Latitude Cyclones

During fall 2015–winter 2016, extensive instrument assets were deployed upstream and across a range of elevations of the Olympic Mountains. This Olympic Mountains Experiment (OLYMPEX), a GPM validation project, provided a unique opportunity to document orographic enhancement of precipitation as wintertime mid-latitude cyclones encounter complex terrain. As part of the observational network, ground-based dual-polarization Doppler radars operated nearly continuously on the coast (NASA’s NPOL S-band), windward interior valley (CSWR’s DOW6 X-band), and on the leeside of the mountains (Environment Canada’s X-band), providing unprecedented detail on microphysical and dynamical processes associated with precipitation enhancement through high-resolution vertical cross sections.

The degree of enhancement and relative roles of warm-rain and ice-based microphysical processes are presented using these radar observations in a range of environmental conditions dictated by frontal sector and storm type. In situ aircraft data from the University of North Dakota Citation, which flew spirals through the radar RHI sectors, provides a means to validate radar-inferred hydrometeor characteristics, while the radars provide a broader context for which to place aircraft observations, including probe images and measurements of liquid water content. 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. Periods when Kelvin-Helmholtz wave signatures were detected in this shear zone highlight a possible influence on microphysical processes. Additionally, an example of generating cells aloft showcases production of liquid water above the freezing level, contributing to riming and variations in the brightband below. These interesting process results from the OLYMPEX dataset will be further explored and presented.