Colloquium: 26 Aug 2008, EN6085A, 3:00 pm

Abstract:

      During the warm season coastally-trapped wind reversals (CTWRs) occur periodically in the lowest several hundred meters of the marine boundary layer west of California and disrupt the northerly flow associated with the Pacific High. CTWRs are characterized by southerly winds and a finger of fog or low stratus adjacent to the coastline extending approximately 100 km offshore, which propagates northward along the coast between 5-12 m s-1 and has a lifespan of several days. Several proposed conceptual models for CTWRs exist which include a Kelvin wave, a density current, a hybrid Kelvin wave-bore, a mesoscale response to synoptic forcing, and a topographic Rossby wave. While much modeling work that is often idealized has been conducted, observational data has been sparse. A CTWR was observed off the California coast from 22-25 June 2006 that originated near Point Conception and propagated northward to Cape Mendocino. The University of Wyoming King Air research aircraft documented the primary characteristics of the wind reversal. Numerical simulations using the Weather Research and Forecast modeling system were conducted to provide a broader picture of the CTWR structure and evolution and are compared the observations.
      It is demonstrated that the mature CTWR for this case is best described as a weakly trapped density current. In the cross-shore direction the PGF and MBL were essentially flat and only slight cross-shore wind was detected, suggesting little variation normal to the coast. Alongshore wind and PGF within the CTWR are directed to the north. Propagation occurs in response to a density gradient that is established due to the synoptic preconditioning, which decreases MBL depth, flattens the low-level pressure gradient, warms the coastal area, and removes the cloud cover, and cloud-top nocturnal longwave radiative cooling of the CTWR stratus layer which deepens and cools the MBL. Acceleration is seen in the motion of the CTWR in response to these two processes and is enhanced during nighttime hours when cloud forcing is at a maximum, producing antitriptic flow.