Colloquium: February 22, 2007, 3:00 pm, EN6085A

Aircraft Observations of a Coastally Trapped Disturbance off the California Coast

David A. Rahn

Abstract: The summertime marine atmospheric boundary layer off the California coast is normally characterized by northerly winds that are associated with the broad Pacific High. This pattern is disturbed at times by episodes of southerly winds and low clouds adjacent to the coastline, which tends to propagate northward along the coast. These events have been referred to as coastally trapped disturbances (CTD), coastally trapped wind reversals, or southerly surges and have been the focus of research for the past two decades. CTDs occur in a narrow band along the coast that extends approximately 100 km offshore and are characterized by a finger of low stratus or fog that is apparent on visible satellite imagery. CTDs typically move northward with speeds between 5-10 m s^-1 and have a lifespan of several days.

The CTD event of 22-26 June 2006 was explored by the University of Wyoming Raytheon King Air research aircraft to document the physical characteristics of the wind reversal in attempt to infer the forcing mechanisms responsible for the propagation. The various conceptual models are still under debate. Two flights from 23 June are presented that are representative of the CTD during its mature stage. Flight strategies included sawtooth maneuvers to depict the vertical structure of the CTD and isobaric legs to measure the horizontal pressure gradient force (PGF) within the CTD in alongshore and cross-shore directions. Observations showed a thickening of the CTD 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 CTD 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.