AN INVESTIGATION OF THE CATALINA EDDY

The Catalina eddy event of 21 July 1992 is simulated using a mesoscale data assimilation system featuring an optimum interpolation analysis, incremental update, and second-order closure physics. The results are contrasted with other recent modeling studies of the Catalina eddy. G enesis of the eddy occurs when changes on the synoptic scale lead to a n intensification of the east-west pressure gradient near the coast, r esulting in enhanced northwesterly flow along the coast and over the m ountains east of Point Conception. Lee troughing results in an alongsh ore pressure gradient at the coast with higher pressure to the south. Topographically trapped, ageostrophic southerly flow is then initiated . The combination of southerly flow along the coast with strong northw esterly flow to the west results in formation of a cyclonic eddy in th e bight. The zone of southerly flow is. characterized by a deep, cool, cloud-topped boundary layer that can considerably alter coastal weath er and impact activities involved with aviation, air quality, fire wea ther, and microwave refractivity. While other recent modeling studies have failed to properly represent boundary layer structure, the data a ssimilation system used in the present study reproduces these features . Results show that the model forecast eddy is in relatively good agre ement with surface wind observations. The data assimilation system, wh ich consists of the analysis-initialization scheme and the forecast mo del, retains much of the mesoscale structure of the forecast, while ad justing the position of the eddy to better fit the observations. Withi n the zone of southerly flow, rapid deepening of the boundary layer is accompanied by the formation of stratus clouds. Through the use of se nsitivity studies, the authors demonstrate that the deepening of the b oundary layer results from convergence and upward motion forced by the topographic barrier along the coast and that the interaction between clouds and radiation plays a significant role.