Synoptic control over orographic precipitation distributions during the Olympics Mountains Experiment (OLYMPEX)

Abstract

The synoptic controls on orographic precipitation during the Olympics Mountains Experiment (OLYMPEX) are investigated using observations and numerical simulations. Observational precipitation retrievals for six warm-frontal (WF), six warm-sector (WS), and six postfrontal (PF) periods indicate that heavy precipitation occurred in both WF and WS periods, but the latter saw larger orographic enhancements. Such enhancements extended well upstreamof the terrain inWFperiods but were focused over the windward slopes in both PF and WS periods. Quasi-idealized simulations, constrained by OLYMPEX data, reproduce the key synoptic sensitivities of the OLYMPEX precipitation distributions and thus facilitate physical interpretation. These sensitivities are largely explained by three upstream parameters: the large-scale precipitation rate rup, the impinging horizontal moisture flux I, and the low-level static stability. BothWFandWS events exhibit large rup and I, and thus, heavy orographic precipitation, which is greatly enhanced in amplitude and areal extent by the seeder-feeder process. However, the stronger stability of the WFperiods, particularly within the frontal inversion (even when it lies above crest level), causes their precipitation enhancement to weaken and shift upstream. In contrast, the small rup and I, larger static stability, and absence of stratiform feeder clouds in the nominally unsaturated and convective PF events yield much lighter time- and areaaveraged precipitation. Modest enhancements still occur over the windward slopes due to the local development and invigoration of shallow convective showers.