Sea Breezes Shallow and Deep on the California Coast

Abstract

Analyses of Doppler lidar data reveal sea breezes occurring on two different depth and time scales at Monterey Bay, California, on a day with offshore gradient flow indicated before sunrise and after sunset. The lidar data used in this study consist of vertical cross sections and profiles of the westerly, onshore wind component u. In the morning after 0900 PST a shallow sea breeze formed, which reached a depth of 300 m by noon. Starting in early afternoon a deeper sea-breeze layer formed in the lowest kilometer, and by late afternoon the shallow sea breeze blended into the deeper sea breeze and was no longer evident. Maximum speeds of 6 m s?1 in the shallow sea breeze occurred at the surface, whereas those in the deep sea breeze (also 6 m s?1) were about 300 m above the surface. It is hypothesized that the shallow sea breeze is a local phenomenon responding to a more local temperature contrast between the sea and the region between the ocean and the mountain ranges. The deeper sea breeze, on the other hand, is seen as a more regional circulation, driven by the larger-scale contrast between the atmosphere over the ocean and that over the hot interior valleys of California, or perhaps even a larger continental scale. The lidar observations also included the evening transition, which began as a very shallow land breeze observed only by surface observing stations. In the deep sea-breeze layer between 250 m and 1 km AGL, the flow returned to offshore gradient flow simultaneously through the entire layer 2?4 h after sunset. The sea breeze was thus seen as a daytime interruption of the basic gradient offshore flow.