Numerical Modeling of a Coastal Trapped Disturbance. Part II: Structure and Dynamics

Abstract

A detailed analysis of a simulation of a coastal trapped disturbance (CTD) using the Colorado State University Regional Atmospheric Modeling System (RAMS) is presented. The CTD considered (15?18 May 1985) represents an example of a strong mesoscale trapped event with abrupt gravity current?like transitions in many meteorological parameters, and which was closely tied to the synoptic forcing. Propagation of this event along the west coast of North America occurred from initiation in the Southern California Bight?Baja California coastal region to the northern tip of Vancouver Island, and the event appeared to have no difficulty in negotiating significant bends or gaps in the coastal mountains unlike some other events that have ceased or stalled near Cape Mendocino, Point Arena, and the mouth of the Columbia River. It is found that warm offshore flow ahead of the CTD, and cool onshore flow in the Southern California Bight?northern Baja California coastal region, both driven by the westward tracking of a synoptic low, are very important for initiation, and subsequent propagation, of the model CTD, similar to observations. Convergence of the initial onshore cool flow in the south combined with warm offshore flow in the north lead to a northward-directed pressure gradient and, hence, a southerly wind transition. The adjustment timescale of the onshore flow to form the southerlies of the CTD is found to be consistent with expectations from theory. During the propagating stage of the event, the pressure gradient and Coriolis terms were found to be most important for the meridional wind tendency, with advection and diffusion making smaller contributions. Consistent with semigeostrophic theory for CTD, the length scale in the across-mountain direction of the model CTD is much less than the along-mountain scale. Although the model transitions in winds, pressure, and temperature are not as sharp as observed (attributed to the lack of boundary layer structure in the NCEP fields used for model initialization), there is some signature of the gravity current nature of the event. Decay of the event occurred when the favorable synoptic forcing related to the synoptic low moved to the northwest. There appeared to be no evidence of any topographic influence on the decay or termination of this particular event, unlike for several other cases. Taken together, this and the previous validation study suggest that RAMS can be usefully employed to better understand the nature of at least these strong CTD events.