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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 upstream of the terrain in WF periods 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.

This figure shows the results of a new, two-step variational precipitation retrieval from Purnell and Kirshbaum (Monthly Weather Review, 2018). The retrieval was performed on 18 precipitation events during the Olympics Experiment (OLYMPEX) field campaign in Washington State (2015-16). Six events were analyzed for three different synoptic categories: (a) warm-frontal (WF), (b) warm-sector (WS), and (c) postfrontal (PF) precipitation. Composites of each category show that the heaviest precipitation occurred during the WS events, with the largest accumulations over the southwestern (and, typically, windward) slopes. The WF events saw less precipitation over the higher terrain and more over the upstream ocean, and the PF events were characterized by much lighter precipitation focused over the lower-windward slopes.

These sensitivities are largely explained by three upstream parameters: the large-scale precipitation rate , the impinging horizontal moisture flux I, and the low-level static stability. Both WF and WS events exhibit large  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 WF periods, 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  and I, larger static stability, and absence of stratiform feeder clouds in the nominally unsaturated and convective PF events yield much lighter time- and area-averaged precipitation. Modest enhancements still occur over the windward slopes due to the local development and invigoration of shallow convective showers.

Reference: Purnell, D.J. and D.J. Kirshbaum, 2018: Synoptic Control over Orographic Precipitation Distributions during the Olympics Mountains Experiment (OLYMPEX). Mon. Wea. Rev., 146, 1023–1044, 

Area of Research: Weather

Associate Professor and Chair, , research focus is on improving the conceptual understanding and prediction of mesoscale processes, namely cumulus convection and topographically forced/modified flows. Through ongoing advancements in computer power, these processes are becoming better resolved in numerical weather prediction (NWP) and climate models. However, lingering deficiencies in the numerical representation and conceptual understanding of these processes continue to limit the accuracy of both NWP and climate forecasts. Improvements in these areas are required for reliable guidance on short- and long-term environmental hazards.

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