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Enhancement of precipitation processes aloft over complex terrain is documented using reflectivity data from an S-band scanning radar (NPOL) that was deployed on the west coast of Washington State during the Olympic Mountains Experiment (OLYMPEX). From November 2015 through mid-January 2016, NPOL obtained high-resolution data within sectors over the ocean and over the windward slopes of the Olympic Mountains. Contoured Frequency by Altitude Diagrams of radar reflectivity highlight a higher frequency of occurrence of larger reflectivities for all heights between 2 and 8 km over land compared to ocean, with the largest difference in the 4- to 6-km range indicating a robust signature of enhancement aloft over the windward slopes. This enhancement pattern is found to some degree under all environmental conditions considered but is especially pronounced during periods of high vapor transport, high melting level height, southwest low-level winds, and neutral stability. These conditions are generally associated with warm sectors of midlatitude cyclones and atmospheric rivers. Past studies have postulated that a secondary enhancement in reflectivity aloft was an intrinsic part of atmospheric river type systems. However, these results show that further significant enhancement of this signature occurs as deep moist-neutral, high water vapor content flow is lifted when it encounters a mountain range. Reflectivity data from the dual-precipitation radar aboard the Global Precipitation Measurement satellite also documents this reflectivity increase aloft over the Olympic Mountains compared to the adjacent ocean, showing the potential for Global Precipitation Measurement to provide reliable estimates of precipitation structure over remote mountainous regions.

Plain Language Summary When frontal cyclones pass over a mountain range, modification of flow often results in precipitation enhancement on the windward slopes. Most studies of this phenomenon rely on the precipitation patterns deduced from surface networks of rain gauges or through numerical modeling. Little attention has been given to orographic modification of the precipitation processes occurring in the middle and upper layers of clouds. This paper uses high-resolution vertical cross sections of radar reflectivity data taken during the Olympic Mountains Experiment. Measurements made over the ocean are compared to those over the windward slopes of the Olympic Mountains and a clear signature of enhancement aloft over the windward slopes is found at all heights from 2 to 8 km strongest between 4 and 6 km. This enhancement of reflectivity aloft is especially pronounced under environmental conditions of large water vapor transport, high melting level, strong onshore-directed low-level winds and neutral stability. Those environmental conditions are commonly found in warm sectors of midlatitude cyclones and in atmospheric river-type events. This same signature of higher reflectivity aloft over windward slopes is found from satellite-derived radar reflectivity measurements implying that satellites have the potential to provide reliable estimates of precipitation structure over remote mountainous regions.