Aircraft Observations of a Coastally Trapped Wind Reversal off the California Coast

Abstract

The summertime marine atmospheric boundary layer off the California coast is normally characterized by northerly winds associated with the Pacific high. This pattern is occasionally disturbed by episodes of southerly winds and a finger of fog or low stratus adjacent to the coastline extending approximately 100 km offshore. These events propagate northward along the coast with speeds between 5 and 12 m s−1 and have a life span of several days. These occurrences have been referred to as coastally trapped wind reversals (CTWRs), coastally trapped disturbances, or southerly surges. The CTWR event of 22–25 June 2006 was explored by the University of Wyoming King Air research aircraft to document the physical characteristics of the wind reversal in an attempt to infer the forcing mechanisms responsible for the propagation. Two flights from 23 June are presented that are representative of the CTWR during its mature stage. Sawtooth maneuvers depict the CTWR vertical structure, and isobaric legs directly measure the horizontal pressure gradient force (PGF). Observations showed a thickening of the CTWR layer in an alongshore direction to the south. The inversion layer varies throughout the day with the final sawtooth leg depicting clear dynamic destabilization within the inversion layer. A PGF is present at the head of the CTWR that is directed northward. No significant PGF was detected in the cross-shore direction, suggesting that for this case there is little variation in the depth of the marine boundary layer normal to the coast.