Modification of Precipitation Processes by Complex Terrain: What Are We Learning from The Olympic Mountains Experiment (OLYMPEX)?

The Olympic Mountains Experiment was a multi-faceted, international, multi-agency field campaign that took place over the Olympic Mountains in the northwest corner of Washington State during the 2015 – 2016 winter season. The goals of OLYMPEX were to provide physical validation of satellite-derived precipitation measurements by the Global Precipitation Measurement (GPM) satellites and to investigate precipitation processes in winter cyclones and how they are modified as the storms encounter a coastal mountain range. 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, supplemental soundings on the Washington coast and on Vancouver Island, multi frequency (S-, Ka/Ku, and X-band) ground-based dual-polarization radars, and three instrumented aircraft (NASA’s DC-8 and ER-2 and the University of North Dakota’s Citation). This presentation summarizes OLYMPEX results from a collaborative group of scientists including analyses of the OLYMPEX datasets, results from numerical modeling, and applications of these data to the improvement of satellite-based precipitation algorithms over complex terrain.

One overarching result that has come to light from these multiple studies is that the most pronounced modification of precipitation processes by complex terrain occurs during environmental conditions found predominantly during the warm sector periods of midlatitude cyclones. These conditions include high melting layer height, strong low-level flow impinging orthogonally on the Olympic Mountain barrier, high values in integrated water vapor transport and near-neutral low-level stability. Under these conditions, moist low-level flow is lifted when it encounters the terrain, and the warm-rain processes of collection and coalescence make significant contributions to the total observed rainfall on the windward slopes. In addition, there is broad lifting of the entire moist column, such that there is enhancement in the ice-layer above the melting level over terrain compared to the same levels over the ocean before the atmosphere encounters terrain. The implications of these findings are activation of ice-layer and mixed phase processes above the melting level contributing to orographic enhancement on the windward slopes and generation of ice particles that are then lofted over the terrain towards the lee side. Model verification studies have shown that most microphysical schemes have difficulty with accurately reproducing the distribution of precipitation on the windward slopes and the lee side, especially during the heavy-rain producing warm sector periods of cyclones. In addition, satellite retrievals significantly underestimate precipitation possible due to the difficulty of observing warm-rain processes since the ground clutter affects satellite retrievals in lowest few kilometers of the atmosphere. Nevertheless, ongoing work is addressing these modeling and satellite retrieval issues as it becomes clear from OLYMPEX that the modification of precipitation processes during the warm sector periods of winter cyclones have a profound effect on the precipitation distribution over coastal mountain ranges.