An Investigation of the Catalina Eddy

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Abstract
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. Genesis of the eddy occurs when changes on the synoptic scale lead to an intensification of the east–west pressure gradient near the coast, resulting in enhanced northwesterly flow along the coast and over the mountains east of Point Conception. Lee troughing results in an alongshore 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 northwesterly flow to the west results in formation of a cyclonic eddy in the bight. The zone of southerly flow is characterized by a deep, cool, cloud-topped boundary layer that can considerably alter coastal weather and impact activities involved with aviation, air quality, fire weather, and microwave refractivity. While other recent modeling studies have failed to properly represent boundary layer structure, the data assimilation system used in the present study reproduces these features. Results show that the model forecast eddy is in relatively good agreement with surface wind observations. The data assimilation system, which consists of the analysis–initialization scheme and the forecast model, retains much of the mesoscale structure of the forecast, while adjusting the position of the eddy to better fit the observations. Within the zone of southerly flow, rapid deepening of the boundary layer is accompanied by the formation of stratus clouds. Through the use of sensitivity studies, the authors demonstrate that the deepening of the boundary 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.

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