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Shucai Guan, Peter L. Jackson, and Chris J. C. Reason

Abstract

The coastal trapped disturbance (CTD) of 15–17 May 1985 represents an example of a strong mesoscale trapped event along the west coast of North America with abrupt transitions in many basic meteorological parameters. In this study, a comparison between observations and a numerical simulation of this event using the Regional Atmospheric Modeling System (RAMS) is presented. The model is shown to realistically reproduce CTD characteristics such as the coastal transition from northerly to southerly flow, as a mesoscale coastal ridge of higher pressure with associated drops in marine-layer temperature propagates northward along the west coast of North America. Simulated sea level pressure and temperature fields near the surface match well with observations, especially at the synoptic scale. The model realistically simulates mesoscale sea level pressure and 6-h pressure changes during the event. The modeled hourly time evolution of sea level pressure and the southerly transitions at a series of coastal stations and buoys also agree reasonably well with observations. The marine boundary layer is not well initialized or very well represented in the model, suggesting that, for this particular case, the details of the boundary layer are not crucial in the evolution of the CTD. It is suggested that the RAMS model can be usefully applied to investigate CTD evolution.

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Peter L. Jackson, Chris J. C. Reason, and Shucai Guan

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.

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