West Coast winter cyclogenesis

During outbreaks of arctic air over western North America, an inverted trough is often observed immediately along the coast of British Columbia and the U.S. Pacific Northwest. Occasionally, coastal cyclogenesis can take place within this vorticity-rich coastal environment; this synoptic scenario can occasionally bring heavy snow to the inland sections of western Washington and Oregon. These cyclones develop in an environment of substantial baroclinicity, in the presence of large surface heat and moisture flux, and ahead of a shortwave trough embedded within northerly or northwesterly flow aloft. Topography appears to play an important role in setting up the precursor environment for these cyclonic systems, and land-sea contrasts contribute to baroclinicity in the trough. The objective of this research is to identify the relative importance of land-sea contrasts, topography and lee troughing, and diabatic heating in the marine boundary layer to the cyclogenesis.

A representative northwesterly flow cyclone event from 7-9 January 1980 is investigated using the Weather Research and Forecasting (WRF) model. Model simulations using NCEP/NCAR reanalysis data for initial and lateral boundary conditions were run with the objective of isolating topographic, baroclinic, and diabatic processes to the cyclone development in this event. The WRF runs are used to provide a physically consistent, high-resolution data set with which to analyze the event, isolate the contributions from terrain and land-sea contrasts, and perform physics experiments that allow isolation of diabatic processes in the lower-tropospheric development.

Initial model simulations are able to capture the formation of the inverted coastal trough and cyclogenesis event only in simulations with terrain included. More challenging is the ability to initialize the model with proper representation of the upper-level precursor disturbance that triggers the event. Experiments will be presented in which (i) the land-sea contrast is removed so as to make the entire domain land, (ii) the terrain is made flat everywhere but the coastline is retained, and (iii) the terrain is flat and there is no coastline. A potential vorticity diagnosis, and comparisons between full-physics and adiabatic simulations allow determination of the significance of diabatic processes in the event.