Numerical Modeling of a Coastal Trapped Disturbance. Part I: Comparison with Observations

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.