A Mixed-Layer Simulation of Daytime Boundary-Layer Variations within the Los Angeles Basin

Abstract

The ability of an idealized mixed-layer model to predict mesoscale variations in boundary-layer (BL) properties is tested by simulating a case-study day in the Los Angeles Basin, a day typical of their summertime fair weather conditions. Large horizontal and temporal BL variations result from the strong land-sea thermal discontinuity, which primarily drives the flow, and the complex terrain with large terrain height differences, which primarily channels the flow. Rather than specifying an ?undisturbed level? in the model, the parameterized interaction between the free atmosphere and the BL is derived from gravity wave theory and a representative horizontal length scale determined by the size of the basin. Predicted BL depths, velocities, and temperatures all show good agreement with observed values. Known convergence zones are predicted qualitatively; addition of an ad hoc detrainment increases model quantitative accuracy in those regions by allowing some of the converging air to exit the BL. However, a stringent test of model accuracy?comparison of predicted and observed terms of the equations of motion?reveals that the accuracy of the predicted mean values is partly a consequence of compensating errors. Despite its simplicity, the model adequately predicts horizontal and temporal BL variations when the forcing occurs in the BL, indicating that under those conditions the mixed-layer model contains the physical mechanisms primarily responsible for the BL variations.