OLYMPEX: A Unique and Comprehensive Dataset for Understanding Precipitation Processes in Complex Terrain

Building upon experience in mid-latitude winter experiments over mountainous terrain (e.g., COAST, MAP and IMPROVE II), Prof. Houze led the Olympic Mountains Experiment (OLYMPEX) during fall 2015 - winter 2016, which sought to deepen our understanding of the processes controlling orographic precipitation enhancement. Located adjacent to the Pacific Ocean and within an active storm track, the Olympic Mountains are an ideal location to examine these processes as mid-latitude storms move inland from the ocean and encounter complex terrain. Strong emphasis was placed on isolating important microphysical processes through targeted sampling of clouds embedded within a variety of extratropical cyclone environments. Taking advantage of technological advancements since prior campaigns, OLYMPEX combined a diverse set of instruments, from ground instrumentation to airborne platforms, to obtain a holistic view of these processes from the ocean to the windward and leeside of the mountains.

This presentation highlights initial results from OLYMPEX, emphasizing the important contributions by Prof. Houze to the understanding of precipitation enhancement in mountainous regions. The ground-based instrument network highlighted varying degrees of enhancement, often occurring over the lower windward peaks. As in prior studies, the background synoptic forcing during OLYMPEX exerted substantial control over the dominant microphysical processes leading to enhancement through the corresponding thermodynamic and kinematic environments. Comparison of ground-based radar data with the surface measurements of particle size distributions yields insight into the processes responsible for enhancement within varying synoptic regimes. In “warm” events with an elevated 0° C level occurring above the first ridges, precipitation enhancement occurs through increased number of small- to medium-sized drops. In comparatively “cold” events, more numerous medium- to large-sized drops are observed as ice-based processes begin to dominate. A strong, moist jet impinging on the coast was found to lift upstream of the mountains, indicating an orographic influence far from the first windward ridge. A notable shear layer generated by either the synoptic flow field or orographically-driven down-valley flow is suspected to influence the microphysical processes leading to enhancement, as was suggested in previous studies. The presence of a Doppler on Wheels (DOW) radar in the Quinault Valley provided unprecedented, necessary detail on this nearly persistent shallow down-valley flow layer. Dual-polarization capabilities of the DOW and other ground-based radars, combined with in-situ microphysical measurements from the University of North Dakota Citation, provide details on the microphysical structures relative to this shear layer. The presence or absence of upward motion (resulting from either forced uplift or turbulence due to a shear layer) relative to the height of the 0° C level likely contributes to observations of supercooled water and the resulting hydrometeor habit. Another consequence of the shear layer is the occasional appearance of low-level Kelvin-Helmholtz waves. As a prominent signature in the radar data, the resulting impact of these turbulent waves on surface precipitation is under investigation. Finally, airborne radar data from NASA’s DC-8 provided measurements in regions inaccessible by the ground-based instruments, including high terrain and inland valleys. Consistent with surface measurements, the maximum precipitation enhancement occurs over the initial slopes and not over the highest elevations. The mobility of the airborne radar complements the stationary ground radars with continuous measurements from the windward slopes to the lee, showing that the leeside precipitation structure exhibits strong spatial and temporal variations.