Synoptic Control over Orographic Precipitation Distributions During OLYMPEX

During the OLYMPEX field campaign in Washington State in winter 2015-16, intensive precipitation and upper-air measurements were obtained within frontal systems traversing the Olympic mountain range. In this study, an analysis and interpretation of the observed precipitation distributions, as a function of synoptic conditions, is undertaken. The synoptic conditions are categorized as warm-frontal (ahead of a surface warm front), warm sector (between the surface warm and cold fronts), and post-frontal (behind the surface cold front). Six events of each type are selected, for which observed precipitation distributions are retrieved using a combination of operational S-band radars and a relatively dense regional rain-gauge network. Not surprisingly, far greater orographic precipitation amounts are observed during warm-frontal and warm-sector events than during post-frontal events. The warm-sector and warm-frontal events differ in that former exhibits much greater orographic enhancement, with a maximum shifting closer to the ridge crest. While some of these differences follow directly from basic analysis of the upstream flow properties, others are less straightforward. To enhance the physical interpretation, quasi-idealized simulations with the WRF model are conducted. The simulations use the real Olympics terrain, idealized soundings based on the upper-air observations, and a large-scale lifting parameterization to produce realistic background precipitation rates (an element often missing from idealized orographic precipitation simulations). Simulations of warm-frontal, warm sector, and post-frontal conditions reproduce the key differences in precipitation amounts and distributions between these events. The simulations are interpreted to gain insight into the dynamical and microphysical mechanisms that underlie the observed precipitation distributions.