Airborne observations of the summertime coastal jet off the California coast

Coast-parallel low-level jets are ubiquitous features of the summertime marine boundary layer off the west coast of the United States. These coastal jets are typically situated near the top of the marine boundary layer with maximum winds often in excess of 25 m s^-1. The jet profile was previously shown to be the result of frictional slowing near the surface and a pronounced thermal wind associated with the sloping inversion at the top of the marine layer. A field study was conducted in early summer of 1997 to study the kinematics and dynamics of the coastal jet off the California coast. The University of Wyoming King Air research aircraft was the primary observation platform. Seven case studies of the coastal jet were conducted during the month of June. Airborne observations showed that the inversion at the top of the marine layer slopes upward to the west in each case. Maximum coastal jet winds were observed near the top of the marine layer. In each case the coastal jet extended over large horizontal distances, often in excess of 100 km from the coastline. In contrast to other observations, wind speeds associated with the coastal jet typically increased to the west during the June 1997 cases. Measurement of the horizontal pressure gradient force revealed the coastal jet to be in a state of near-geostrophic balance.

Previous work has documented the importance of the coastal terrain in enhancing the local wind field. Hydraulic features such as jumps and expansion fans have been noted. During June 1997, little evidence of such accelerations was observed. Dedicated flights to explore the role of the terrain on the wind structure suggested that only minor local topographic enhancements to the wind field were present. The sloping marine layer and associated coastal jet is consistent with a geostrophic adjustment of a thermally-direct circulation arising due to the horizontal temperature contrast between the cool ocean and warm continent. This view emphasizes the large-scale nature of the coastal jet. During June 1997, terrain-induced wind speed variations associated with expansion fans and hydraulic jumps acted only to modulate the primary jet structure.