Response of the Summer Marine Layer Flow to an Extreme California Coastal Bend

• Published in: Monthly weather review 2008-08, Vol.136 (8), p.2894-2922
• Main Authors: DORMAN, Clive E, KORACIN, Darko
• Format: Article
• Language: English
• Subjects: Acceleration; Air pollution; Coasts; Compression; Diurnal variations; Earth, ocean, space; Exact sciences and technology; External geophysics; Flow; Fog; Froude number; High speed; Hydraulics; Islands; Maximum winds; Meteorology; Mountains; Offshore; Outdoor air quality; Pollution control; Potential temperature; Sea surface; Sea surface temperature; Strong winds; Summer; Surface temperature; Surface wind; Synoptic conditions; Temperature differences; Temperature gradients; Temperature patterns; Temperature structure; Topography; Weather; Wind; Wind effects; Wind speed; Wind structure; Winds
• ISSN: 0027-0644; 1520-0493
• DOI: 10.1175/2007MWR2336.1

A summer wind speed maximum extending more than 200 km occurs over water around Point Conception, California, the most extreme bend along the U.S. West Coast. The following several causes were investigated for this wind speed maximum: 1) synoptic conditions, 2) marine layer hydraulic flow effects, 3) diurnal variations, 4) mountain leeside downslope flow, 5) sea surface temperature structure, and 6) island influence. Synoptic conditions set the general wind speed around Point Conception, and these winds are classified as strong, moderate, or weak. The strong wind condition extends about Point Conception, reaching well offshore toward the southwest, and the highest speeds are within 20 km to the south. Moderate wind cases do not extend as far offshore, and they have a moderate maximum wind speed that occurs over a smaller area in the western mouth of the Santa Barbara Channel. The weak wind speed case consists of light and variable winds about Point Conception. Each category occurs about one-third of the time. Atmospheric marine layer hydraulic dynamics dominate the situation after the synoptic condition is set. This includes an expansion fan on the south side of the point and a compression bulge on the north side. The expansion fan significantly increases the wind speeds over a large area that extends to the southwest, south, and east of Point Conception, and the maximum wind speed is increased for the strong and moderate synoptic cases as well. The horizontal sea surface temperature pattern contributes to the sea surface wind maximum through the Froude number, which links the potential temperature difference between the sea surface temperature and the capping inversion temperature with marine layer acceleration in an expansion fan. A greater potential temperature difference across the top of the marine layer also causes more energy to be trapped in the marine layer, instead of escaping upward. The thermally driven flow resulting from differential heating over land in the greater Los Angeles, California, coastal and elevated area to the east is not directly related to the wind speed maximum, either in the Santa Barbara Channel or in the open ocean extending farther offshore. The effects of the thermally driven flow extend only to the east of the Santa Barbara Channel. The downslope flow on the south side of the Santa Ynez Mountains that is generated by winds crossing the Santa Ynez Mountain ridge contributes neither to the high-speed wind maximum in the Sa