Observed Dynamics of Coastal Flow at Cape Mendocino during Coastal Waves 1996

Abstract

Airborne in situ and remote meteorological measurements from around Cape Mendocino, California, sampled during the Coastal Waves 1996 field program are analyzed for three different days: 7, 12, and 26 June 1996. Two days conformed to typical summertime conditions, with a strong northerly downcoast flow, while the wind on the third day was weaker. On the first 2 days, the flow was supercritical in the sense that the Froude number was larger than unity and these 2 days feature expansion fans in the lee of the cape. On the third day, no such phenomenon was observed. All 3 days had a strong thermal wind caused by the marine inversion sloping down toward the coast. On the first 2 days, the flow aloft was westerly or northerly, so that the thermal wind added to the background flow results in a strong jet. On the third day the flow aloft was southerly, and consequently even with the added thermal wind, the northerly flow in the marine layer was too weak to be supercritical. The main difference between the first 2 days was the fact that 7 June is cloud free, while 12 June had a stratocumulus cover. The 3 days are analyzed as composites, and several scales are identified and described: 1) the large-scale synoptic forcing, determined from the wind aloft and synoptic conditions; 2) blocking by the coastal topography, the thermal wind balance in the near-coast zone, and nondimensional properties; 3) the hydraulic properties in the sub- and supercritical flows as they pass the change in coastline orientation at Cape Mendocino; 4) the impact of the local blocking by the terrain at the cape itself, generating a lee-wave phenomena for the high-wind days, which was the actual cause of the collapse of the marine layer in the lee of the cape; 5) the boundary layer interaction, apparently generating SST anomalies, but with little or no feedback to the wind field. A momentum budget analysis for the two high-wind days show a significant difference between one day (7 June) when mesoscale perturbation dominated the flow and the other day (12 June), when large-scale forcing dominated and the mesoscale perturbation was smaller. The importance of the cloud layer on 12 June is illustrated, using lidar data.