The relatively shallow marine boundary layer adjacent to steep coastal topography along the California Coast gives rise to a number of mesoscale phenomena, including coastally trapped disturbances (CTD’s), expansion fans, land/sea breezes, low-level jets, and cyclonic eddies. CTD’s occur several times each year during the period from May to early October and are easily identified in satellite imagery due to the distinctive narrow tongue of low clouds and fog propagating to the north along the coast. The ageostrophic southerly flow associated with CTD’s opposes the large-scale north-westerly winds that are a characteristic feature of the flow at this time of year.

In the present study, we investigate a CTD event which occurred on 15-16 June 2000. The event is simulated using the Naval Research Laboratory’s COAMPS model. The model is run in a triply-nested mode with horizontal resolution of 5 km on the innermost nest. Data assimilation is performed for three days prior to the period of interest. The model 12 h forecast for the period 1200 UTC 15 June to 0000 UTC 16 June faithfully reproduces the movement and speed of the fog along the coast. At 0000 UTC 16 June, the forecast indicates that fog extends from Point Reyes (N of San Francisco) to Los Angeles, with the depth of the fog progressively increasing to the south. In the Southern California Bight, the fog lifts, becoming a low stratus layer whose thickness (~250 m) is comparable to the maximum fog depth to the north.

In an effort to understand the evolution and extent of fog and low clouds in this event, several sensitivity studies are performed. In each of several 12 h simulations, the initial conditions are identical to the control. In the first of the sensitivity studies, both latent and sensible surface heat fluxes are removed at all ocean grid points. The results are remarkably similar to the control, indicating that boundary layer moistening due to latent heat flux (tending to enhance cloudiness) and warming due to sensible heat flux (tending to reduce cloudiness) are nearly compensating one another. Simulations are also performed in which latent and sensible heating are separately removed at ocean points. The results are radically different from the control and interpretation of these results in underway.