OLYMPEX: A Grand Field Campaign to Study Pacific Storms Passing over a West Coast Mountain Range

The Olympic Mountains Experiment (OLYMPEX) was a NASA-led multi-agency international field campaign to study storms producing rain and snow in the Olympic Mountains of Washington State. The project had the dual-purpose of: 1) collecting data to validate and improve algorithms for determining precipitation amounts from the Global Precipitation Mission (GPM) core satellite, which has Ku and Ka band radars and passive microwave sensors in channels ranging from 10 to 183 GHz, and 2) elucidating the physical processes in oceanic frontal systems passing over mountain ranges. Such storms produce huge rainfall amounts on the windward slopes of the Olympic Range. The rainfall producing capacity of the storm is greatly enhanced over the windward slopes by a combination of dynamical and microphysical processes. The most intensive phase of observations was from 10 November to 22 December 2015. Three aircraft flew coordinated missions. The NASA ER-2 and DC8 aircraft flew at high altitudes making remote sensing measurements similar to those obtained on the GPM core satellite. The University of North Dakota Citation made in situ microphysical measurements. The flights of the three aircraft were carefully coordinated; making measurements over a network of ground instruments located along the southwest-northeast oriented Quinault Valley on the windward side of the mountains and at Hurricane Ridge on the leeside of the range. The high-level aircraft mostly flew along lines over the Quinault Valley and Hurricane Ridge. In one storm the three aircraft flew directly under the GPM satellite overpass for direct comparison with the satellite observations. The NPOL S-band, D3R Ka/Ku-band, and the DOW X-band radar collected data in RHI sectors over the Quinault Valley, and an Environment Canada X-band on Vancouver Island conducted RHI sectors over Hurricane Ridge. The ground instruments collected extensive data on particle-size distributions and other microphysical characteristics of the rain and snow at a variety of altitudes. All radars collected a full suite of dual-polarization and Doppler radial velocity data. The precipitation on the Olympic Peninsula was greater than normal, and all observation systems worked nearly flawlessly. During the intensive field phase, approximately 13 storms passed over. Supplementary soundings at the coastal location of the NPOL and D3R radars were obtained at ~3 hour intervals during these storms in addition to all the other observations. The coordinated data collection by aircraft, radars, and ground instruments form a voluminous set of data on Pacific Northwest storms. Preliminary analysis indicates that one of the primary modes of orographic enhancement was low-level moist flow rising over the lower windward slopes and producing many very small drops. Ice-phase processes producing larger particles also varied in relation to shear-enhanced turbulence over topography.