A case study of the interaction of the summertime coastal jet with the California topography

Coast-parallel low-level jets are commonplace in the marine boundary layer off the west coast of the United States during summer. A field study was co nducted in early summer of 1997 to document the forcing of boundary layer w inds in the near-coastal environment off California. On 8 June 1997 the Wyo ming King Air collected data along a 350-km stretch of coastal margin from Cape Mendocino to San Francisco in order to examine the interaction between the coastal topography and the low-level jet. During the course of the fli ght, 32 soundings were conducted. The maximum speed of the coastal jet was found near the top of the marine boundary layer at altitudes from 200 to 60 0 m. Analysis of the data revealed a westward increase in the height of the marine boundary layer and maximum jet wind speeds. Strongest jet winds wer e observed southwest of Cape Mendocino with a maximum speed of 28 m s(-1). The coastal jet was characterized by a broad horizontal extent. Wind maxima were found at distances approximately 30 km to more than 100 km offshore. Hydraulic features such as jumps and expansion fans have previously been ob served downwind of coastal capes and points along the California coast. The flow upwind of Cape Mendocino and Point Arena was found to be supercritica l, but the King Air data showed that accelerations associated with possible expansion fan phenomena were minimal. It is proposed that the sloping inve rsion at the top of the marine boundary layer and attendant coastal jet are fundamentally the result of a geostrophic adjustment process arising becau se of the horizontal temperature contrast between the cool ocean and warm c ontinent. This view emphasizes that the coastal jet is a ubiquitous, large- scale feature of the summertime coastal environment. Terrain-induced wind s peed variations associated with expansion fans and hydraulic jumps only mod ulate the primary jet structure.