Orographic Precipitation Enhancement as observed during the Olympic Mountains Experiment (OLYMPEX)

The Olympic Mountains Experiment (OLYMPEX), a Global Precipitation Measurement (GPM) Satellite ground validation field project, took place on the Olympic Peninsula in the northwest corner of Washington State during fall 2015 – winter 2016. This unprecedented dataset has proven valuable not only as a means for evaluating the physical basis of the algorithms used to interpret GPM satellite measurements but also for understanding the precipitation processes and their modulation by synoptic conditions and complex terrain. The observational assets included dual-polarization Doppler radars deployed on the coast, in the interior windward slopes of the Olympic Mountains, and on the leeside, an extensive ground network of disdrometers and rain gauges, supplemental soundings and airborne passive microwave radiometers, radar, and in situ microphysics instruments. These data are shedding light on the microphysical and dynamical processes associated with precipitation production as extratropical cyclones pass over the windward, high terrain and leeward sides of the Olympic Mountains.

Statistics of the dropsize distributions (DSDs) obtained during stratiform rain periods derived from disdrometers placed on the windward slopes of the Olympic Mountains and at a variety of elevations show that the DSDs fall into distinct regimes. Each regime is associated with specific environmental conditions related to the low-level environmental flow impinging on the mountain barrier. The most frequently observed DSD regime contains modest concentrations of both small and large drops and is associated with a climatologically average synoptic environment. In this regime, orographic enhancement is modest and predominately at higher elevations. The heaviest rain DSD regime is associated with large concentrations of drops that range from very small to medium-large sizes. The melting level height is high (greater than 2.5 km), the integrated water vapor transport from the ocean to the mountains is high (greater than 500 kg m^-1 s^-1) and the low-level flow is strong (greater than 20 m s^-1) and from the west-southwest. The static stability is close to moist neutral, allowing for unimpeded lift of the low-level flow over the topographic barrier. This produces the largest rain rates at low-mid elevation locations near the front of the topographic barrier and the greatest orographic enhancement. In the heavy rain regime at these low-mid elevation locations, warm-rain processes of condensation, collision and coalescence produce a multitude of small-medium drops that strongly contribute to the observed orographic enhancement with up to half the observed rain rate originating from these smaller drops.