Observation and Analysis of Coastally Trapped Wind Reversals

During the warm season (April-September), the California coast is under the influence of persistent northwesterly flow. Periodically, this flow is replaced by a narrow band of southerly winds along the coast. The transition to southerly flow is often accompanied by a rise in sea-level pressure, lower temperatures, coastal stratus, and fog. The mesoscale disturbance responsible for this southerly transition has become known as a coastally trapped wind reversal (CTWR). While it is clear that these mesoscale disturbances are forced by the interaction of the coastal topography with the synoptic-scale flow, the exact mechanisms for their development and their governing dynamics remain the subject of much debate. The present study examines three cases from 1996 that appear to have the characteristics of a CTWR. Each case is analyzed to determine the associated synoptic-scale forcing and the respective mesoscale structure. The observed synoptic-scale forcing is compared to the results of a climafological study conducted by Mass and Bond (1996). Results from a modeling study by Skamarock et al. (1998) are used to create a conceptual model for comparison with the observed development and mesoscale% structure of each event. Results of this study show that only two of the cases can be classified as a CTWR. The study also shows that variability exists in the synoptic-scale forcing associated with the initiation of a CTWR. Two mechanisms for the development of the offshore mesoscale low, and ultimately the CTWR, have been identified. The variability of the mesoscale structure in each CTWR is also documented