NUMERICAL MODELING OF A COASTAL TRAPPED DISTURBANCE - PART I - COMPARISON WITH OBSERVATIONS

The coastal trapped disturbance (CTD) of 15-17 May 1985 represents an example of a strong mesoscale trapped event along the west coast of No rth America with abrupt transitions in many basic meteorological param eters. In this study, a comparison between observations and a numerica l simulation of this event using the Regional Atmospheric Modeling Sys tem (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 a ssociated drops in marine-layer temperature propagates northward along the west coast of North America. Simulated sea level pressure and tem perature fields near the surface match well with observations, especia lly at the synoptic scale. The model realistically simulates mesoscale sea level pressure and 6-h pressure changes during the event. The mod eled hourly time evolution of sea level pressure and the southerly tra nsitions at a series of coastal stations and buoys also agree reasonab ly well with observations. The marine boundary Layer is not well initi alized or very well represented in the model, suggesting that, for thi s particular case, the details of the boundary layer are not crucial i n the evolution of the CTD. It is suggested that the RAMS model can be usefully applied to investigate CTD evolution.