The West Coast Thermal Trough: Climatology, Evolution and Sensitivity to Terrain and Surface Fluxes

Abstract

Although the West Coast thermal trough (WCTT) is the most important mesoscale feature over the U.S. West Coast during the warm season, its initiation, evolution, and structure are not well understood. Originating in the southwest U.S., this inverted trough can extend northward into Oregon, Washington, and British Columbia, with large impacts on temperature, wind, humidity, and air quality. Using NCEP's North American Regional Reanalysis (NARR), annual and diurnal climatologies of WCTTs reaching the northwest U.S. were constructed. For the entire year, WCTTs are most frequent along the coast near the California/Oregon border, with weaker maxima west of the Cascade and coastal mountains. Over the coastal region, they occur most often during autumn, while east of the Cascade Mountains, the highest frequency is during summer. There is strong diurnal variability in WCTT frequency during the summer, with little diurnal variation in winter. Though compositing revealed important seasonal differences in WCTT evolution, some common features emerged. An upper-level ridge moves over the northwest U.S. and associated high pressure builds in the lower troposphere over southwest Canada, resulting in the development of near-surface easterlies and downslope flow over the western slopes of major terrain barriers of the region. Simultaneously, the WCTT extends northward from California into the Pacific Northwest. As the synoptic configuration changes, the WCTT either moves eastward and merges with the larger thermal low over the Great Basin region, which is most common in summer, or recedes back into California and dissipates, as often happens in winter Another important question is the relative importance of terrain forcing, advection, and surface heating on its formation and evolution. To investigate this, the 13-16 May 2007 WCTT event was examined using observations and simulations from the Weather Research and Forecasting (WRF) model. An analysis of the thermodynamic energy equation for these simulations was completed, as well as sensitivity experiments in which terrain or surface fluxes were removed or modified. For the May 2007 event, vertical advection of potential temperature is the primary driver of local warming and WCTT formation west of the Cascades. The downslope flow that drives this warming is forced by easterly flow associated with high pressure over British Columbia. When the terrain is removed from the model, the WCTT does not form, and high pressure builds over the northwest U.S. When the WCTT forms on the east side of the Cascades, diabatic heating dominates over the other terms in the thermodynamic energy equation, with warm advection playing a small role. If surface heat fluxes are turned off in the model, an area of low pressure remains east of the Cascades, though it is substantially attenuated.