A Study of the Forcing of the 22–25 June 2006 Coastally Trapped Wind Reversal Based on Numerical Simulations and Aircraft Observations

Abstract

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 that typically occurs during summer. South winds and coastal fog or low stratus accompany the CTWR, which propagates northward along the coast. A CTWR was observed off the California coast during late June 2006 that originated in the California Bight and propagated northward to Cape Mendocino during the subsequent 2-day period. This CTWR event was explored by the University of Wyoming King Air research aircraft to document the primary characteristics of the wind reversal. Numerical simulations of the CTWR event using the Weather Research and Forecast modeling system were conducted to compare with observations and to provide a broader picture of the CTWR structure and evolution. An analysis of the forcing mechanisms responsible for the June 2006 CTWR event is presented. It is demonstrated that the mature CTWR for this case is a density current propagating northward along the coast in response to the density gradient found to the north of the CTWR with maximum speed during the nighttime hours. Establishment of the density contrast is largely a result of cloud-top longwave radiative cooling of the stratus that accompanies the CTWR, which serves to cool and deepen the boundary layer during the night. Density contrast between the cloudy CTWR air and the ambient environment is enhanced by the persistent offshore flow to the north of the CTWR with attendant warming and a flattening of the horizontal pressure gradient in the marine layer.