Waves on a Marine Inversion Undergoing Mountain Leeside Wind Shear

William T. Sommers Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Berkeley, CA 94701

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Abstract

Inland penetration of a shallow layer of marine air is a common occurrence along the coast of southern California. The marine air generally is confined to the coastal basin by surrounding mountains and a capping inversion. Air above the inversion is drier and warmer than the marine air below, resulting in different burning conditions for forest fires in mountain areas above and below the inversion. During periods of Santa Ana winds, synoptic-scale forcing generates strong offshore flow that is very dry, and that confines the marine air to lower elevations nearer to the coast, or offshore. Critical fire conditions are almost synonymous with Santa Ana conditions. When a shallow marine air layer coexists with Santa Ana flow, erratic and dangerous fire behavior occurs along mountain slopes in the vicinity of the air mass interface.

During the period 13–17 November 1977, two fires were burning under Santa Ana conditions in the Santa Monica Mountains, one to the east and the other to the west of Malibu. As a part of a larger Santa Ana field experiment, we observed local variations of the marine inversion (using a monostatic acoustic echo sounder), and surface conditions above and below the inversion, in the vicinity of Malibu. The local observations were supplemented by mesoscale surface observations and soundings as well as synoptic-scale data. During a period when synoptic-scale conditions were little varied, mesoscale soundings revealed destabilization of flow as it crossed the mountains from the desert to the ocean. High-frequency waves and lower frequency oscillations on the marine inversion were related to variations in surface winds above and below the inversion. Inversion oscillations of lower frequency were related to variations in surface wind direction above the inversion, and combined with local topography to force variations in the lower speed surface winds below the inversion. Instances where drier, hotter, higher speed flow reached sea level were directly correlated with inversion surfacing. Inversion wave conditions were consistent with stability and shear values from a nearby rawinsonde sounding.

Abstract

Inland penetration of a shallow layer of marine air is a common occurrence along the coast of southern California. The marine air generally is confined to the coastal basin by surrounding mountains and a capping inversion. Air above the inversion is drier and warmer than the marine air below, resulting in different burning conditions for forest fires in mountain areas above and below the inversion. During periods of Santa Ana winds, synoptic-scale forcing generates strong offshore flow that is very dry, and that confines the marine air to lower elevations nearer to the coast, or offshore. Critical fire conditions are almost synonymous with Santa Ana conditions. When a shallow marine air layer coexists with Santa Ana flow, erratic and dangerous fire behavior occurs along mountain slopes in the vicinity of the air mass interface.

During the period 13–17 November 1977, two fires were burning under Santa Ana conditions in the Santa Monica Mountains, one to the east and the other to the west of Malibu. As a part of a larger Santa Ana field experiment, we observed local variations of the marine inversion (using a monostatic acoustic echo sounder), and surface conditions above and below the inversion, in the vicinity of Malibu. The local observations were supplemented by mesoscale surface observations and soundings as well as synoptic-scale data. During a period when synoptic-scale conditions were little varied, mesoscale soundings revealed destabilization of flow as it crossed the mountains from the desert to the ocean. High-frequency waves and lower frequency oscillations on the marine inversion were related to variations in surface winds above and below the inversion. Inversion oscillations of lower frequency were related to variations in surface wind direction above the inversion, and combined with local topography to force variations in the lower speed surface winds below the inversion. Instances where drier, hotter, higher speed flow reached sea level were directly correlated with inversion surfacing. Inversion wave conditions were consistent with stability and shear values from a nearby rawinsonde sounding.

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