An Observational and Modeling Study of the Interaction of Low-Level Southwesterly Flow with the Olympic Mountains during COAST IOP 4

This paper presents an observational and modeling study of the three-dimensional flow and precipitation around the Olympic Mountains of northwest Washington State for a case of relatively steady south-southwesterly flow. This study utilized data from COAST IOP 4 (the fourth intensive observing period during the Coastal Observation and Simulation with Topography field experiment) that took place during 9–10 December 1993. One of the most important data sources was the NOAA P-3 aircraft, which provided flight-level data, reflectivity, and Doppler winds as it flew around the Olympics. Features that were mapped successfully with the NOAA P-3 included flow splitting around the windward pressure ridge, strong southeasterlies (to approximately 25 m s−1) northeast of the Olympics caused by downgradient acceleration into the lee trough, the rapid transition from strong southeasterlies to lighter easterlies (5–10 m s−1) in the Strait of Juan de Fuca, strong downslope flow and a hydraulic jump-like transition over the lee slope, the Olympic rain shadow, and the enhancement of precipitation on the windward (southern) slopes of the Olympics. This event was simulated at resolutions down to 3 km using the nonhydrostatic version of the Penn State-NCAR mesoscale model (MMS). Overall, the model realistically simulated the observed features. The model simulation indicated significant asymmetry in the pressure field around the Olympics (the lee troughing was 5–7 mb deeper than the windward ridging). This asymmetry remained when the Olympics were replaced by a symmetrical bell-shaped barrier. A simulation without latent heating shows that this asymmetry was not the result of latent heating reducing the strength of the windward ridging. Instead, in an environment with high stability near crest level and negative vertical wind shear above the barrier, a mountain wave forced strong downslope flow on the lee side and the resulting pressure asymmetry. Momentum diagnostics along the western Washington coast indicate an approximate geostrophic balance normal to the coast and an antitriptic balance parallel to the coast. To further demonstrate the three-dimensional flow around the Olympics, a series of trajectories launched upstream of the barrier are shown.