Forcing of the Summertime Low-Level Jet along the California Coast

Coast-parallel low-level jets are commonplace in the offshore environment along the west coast of the United States during summer. The jet often has wind speeds in excess of 30 m s−1 and is typically situated near the top of the marine boundary layer. A field study was conducted in early summer of 1997 to study the kinematics and dynamics of the low-level jet off the California coast. The University of Wyoming King Air research aircraft was the primary observation platform. Measurement of the horizontal pressure gradient force was fundamental to understanding the dynamics of the jet. By flying at constant pressure, the height of an isobaric surface could be determined by the radar altimeter. The slope of a constant pressure surface is proportional to the pressure gradient force and hence provides an estimate of the geostrophic wind. Data are presented for two episodes of the low-level jet. In both cases wind speed maxima extending in excess of 100 km from the coast were observed. In contrast to previous observational studies, little evidence of hydraulic effects near the coastal margin was found. Measurements of the horizontal pressure gradient force within the marine boundary layer showed that the coastal jet is in a state of near-geostrophic balance. The observed vertical shear of the geostrophic wind components matched direct measurements of the thermal wind and confirms the importance of the sloping marine boundary layer in forcing the jet as proposed previously. It is offered that the large-scale structure of sloping marine layer and its attendant low-level jet is consistent with the geostrophic adjustment of thermally direct circulation forced by the horizontal temperature contrast between land and ocean.