Orographically Modified Ice-Phase Precipitation Processes during the Olympic Mountains Experiment (OLYMPEX).

Over mountainous terrain, windward enhancement of stratiform precipitation results from a combination of warm-rain and ice-phase processes. In this study, ice-phase precipitation processes are investigated within frontal systems during the Olympic Mountains Experiment (OLYMPEX). An enhanced layer of radar reflectivity (ZH) above the melting level bright band (i.e., a secondary ZH maximum) is observed over both the windward slopes of the Olympic Mountains and the upstream ocean, with a higher frequency of occurrence and higher ZH values over the windward slopes indicating an orographic enhancement of ice-phase precipitation processes. Aircraft-based in situ observations are evaluated for the 1–2 and 3 December 2015 orographically enhanced precipitation events. Above the secondary ZH maximum, the hydrometeors are primarily horizontally oriented dendritic and branched crystals. Within the secondary ZH maximum, there are high concentrations of large (>~2-mm diameter) dendrites, plates, and aggregates thereof, with a significant degree of riming. In both events, aggregation and riming appear to be enhanced within a turbulent layer near sheared flow at the top of a low-level jet impinging on the terrain and forced to rise above the melting level. Based on windward ground sites at low, mid-, and high elevations, secondary ZH maxima periods during all of OLYMPEX are associated with increased rain rates and larger mass-weighted mean drop diameters compared to periods without a secondary ZH maximum. This result suggests that precipitation originating from secondary ZH maxima layers may contribute to enhanced windward precipitation accumulations through the formation of large, dense particles that accelerate fallout. Significance Statement: Precipitation processes are modified within winter storms passing over the Olympic Mountains, often resulting in increased rain and snow on the windward slopes. This study evaluates precipitation characteristics and inferred growth processes related to radar reflectivity maxima within ice layers of clouds, providing insights into ice-phase contributions to windward precipitation. Ground- and aircraft-based measurements indicate that rapid ice growth may occur when branched and platelike crystals are aggregated within turbulent regions that are prevalent over the windward slopes. Large aggregate particles gain mass by collection of supercooled liquid water, which may increase precipitation fallout. While ground measurements suggest that ice-phase growth contributes to windward accumulations, some uncertainty about the processes that occur between the ice layer and the ground remain. [ABSTRACT FROM AUTHOR]