Precipitation Processes in Cyclones Passing over a Coastal Mountain Range: Recent Results from The Olympic Mountains Experiment (OLYMPEX)

The Olympic Mountains Experiment (OLYMPEX) was a multi-faceted, international, multi-agency field campaign that took place over the Olympic Mountains in the Pacific Northwest during the fall 2015 and continued through the winter 2016. The goals of OLYMPEX were to provide physical validation and verification of satellite-derived precipitation measurements by the Global Precipitation Measurement (GPM) satellites and to document the precipitation processes in land-falling wintertime cyclones as they approach land and are modified by complex terrain. The data assets of OLYMPEX covered both the windward and lee sides of the Olympic Mountains including an array of rain gauges and disdrometers placed at a variety of elevations, multi-frequency ground-based dual-polarization radars (NASA’s S-band NPOL and Ka/Ku-band D3R, NSF's X-band DOW, and Environment Canada's X-band), and three aircraft (NASA's DC-8 and ER-2 and the University of North Dakota’s Citation). This presentation summarizes a wide variety of results from these OLYMPEX datasets that document the nature orographic enhancement of precipitation as storms pass over a coastal mountain range. These results include:

• Data from the rain gauges and disdrometers illustrate that the drop size distributions (DSDs) and rainfall enhancement patterns during stratiform raining periods exhibit considerable variability. During heavy rain periods, the enhancement is strongest on the lower slopes of the Olympic Mountains and DSDs are marked by large concentrations of small to medium drops and varying concentrations of large drops.
• Kelvin-Helmholtz billows were documented by ground-based radar in several baroclinic systems. KH waves centered in the melting layer produced the most notable signatures in dual-polarization variables, suggesting that the KH billows promote both riming and aggregation.
• Down-valley flow in the Quinault Valley on the windward side of the Olympic Mountains was frequently observed during OLYMPEX. When the down-valley flow was deep and forced by large-scale pressure patterns, heavier rain was found near the coast and lighter precipitation was located in the interior, presumably forced by incoming air forced over the lower, stable layer. This scenario had a lesser difference in precipitation between the coast and the interior and forward slopes of the Olympic Mountains than the usual enhancement pattern during onshore moist flow.
• Secondary maxima in dual-polarization signatures observed above the melting level indicate an enhancement of ice-based microphysical processes as systems move over the terrain. A variety of particle types were observed in these elevated enhanced signatures including a layer of aggregating bullet rosettes in widespread stratiform and rimed particles during periods of generating cells. While an effect on windward precipitation was not obvious in the surface instruments, downstream implications on precipitation at the high terrain and leeward side will be explored.