Aircraft Measurements and Numerical Simulations of an Expansion Fan off the California Coast

Abstract

Mountains along the California coastline play a critical role in the dynamics of marine atmospheric boundary layer (MBL) airflow in the vicinity of the shoreline. Large changes in the MBL topology have been known to occur downwind of points and capes along the western coast of the United States. Large spatial gradients in wind and temperature become established that can cause anomalous electromagnetic wave propagation. Detailed airborne measurements using the University of Wyoming King Air were conducted to study the adjustment of the MBL to the Point Arguello and Point Conception headlands. Pronounced thinning of the MBL consistent with an expansion fan occurred to the south of Point Conception on 13 June 2012. A sharp cloud edge was collocated with the near collapse of the MBL. D-value cross sections derived from differential GPS altitude measurements allow assessment of the vertical profile of the horizontal pressure gradient force and hence thermal wind forcing in response to the near collapse of the MBL. The Weather Research and Forecasting Model was run with a 1-km grid spacing to examine the atmospheric adjustment around Point Conception during this period. Results from the simulations including the vertical cross sections of the horizontal pressure gradient force were consistent with the aircraft observations. Model results suggest that divergence occurs as the flow rounds Point Conception, characteristic of an expansion fan. Wind speeds in the MBL increase coincident with the decrease in MBL thickness, and subsiding flow associated with the near collapse of the MBL is responsible for the sharp cloud edge.