Synoptic and Orographic Control of Observed Drop Size Distribution Regimes During the OLYMPEX Field Campaign

A dense network of rain gauges and disdrometers deployed in the 2015-16 OLYMPEX field campaign showed in unprecedented detail the drop size distributions (DSDs) associated with orographic precipitation enhancement in frontal systems passing over a mountain range. Considerably higher precipitation totals were recorded inland, with a gradient of increasing rainfall from the coast to the windward facing slopes. A rain shadow was ubiquitous on the lee side. Normalized DSDs from the disdrometer network reveal several coherent DSD modes as well as profound differences between coastal and interior sites. These modes are often persistent for time periods ranging from an hour up to a half day or longer. Shifts in DSD modes are closely related to transitions in larger scale synoptic regimes. Low-level shear and stability also directly influence microphysical processes near mountain barriers leading to localized variations in DSDs between instruments spaced 5-10 km apart.

Combining observations for the full wet season reveals that DSD modes have the following characteristics: 1) Coastal DSDs are dominated by stratiform precipitation with broad distributions of larger drops formed by melting. 2) Windward inland sites observed an additional DSD mode consisting of large concentrations of small (< 1 mm) drops, likely caused by the rapid fallout of droplets produced by condensation and collision/coalescence. 3) Windward high elevation sites observed both additional small drops as well as an increase in the size and concentration of larger drops (> 2 mm). The larger drops are likely generated by enhanced riming and aggregation from shear-induced turbulence. This study evaluates the relative importance of warm and cold orographic enhancement processes, how they manifest in DSDs, and how they are controlled by the synoptic situation (warm frontal, warm sector, cold frontal, and postfrontal).