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William T. Sommers

Abstract

Limited-area Fine Mesh (LFM) forecasts were interpolated and compared with surface observations in the southern California mountains and on-site soundings.

Descriptive case studies show that during sea-breeze type of flow, when synoptic-scale forcing is weak, surface flow in mountain canyons can be uncoupled from free atmosphere flow and totally driven by local diurnal forcing mechanisms, while ridgeline sites conform to free atmosphere conditions. During Santa Ana flow, when synoptic-scale forcing is vigorous, surface flow in canyons and on ridges shows only kinematic adjustment of the synoptic-scale flow.

Root-mean-square difference comparisons of interpolated 12 and 24 h LFM 850, 700 and 500 mb forecasts of height, temperature, dew point and wind with on-site sounding data are presented.

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William T. Sommers

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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William T. Sommers

Abstract

Surface weather conditions in the southern California mountains during Santa Ana occurrences are related to LFM forecast variables. The strong downslope winds and low relative humidities during Santa Ana conditions pose a serious fire problem. Synoptic conditions leading to Santa Anas should incorporate subsidence and flow perpendicular to the local mountains, at mid-tropospheric levels, on the windward side of the mountains.

Twelve hour LFM 500 mb forecasts of height, wind and omega, in relation to Santa Ana wind occurrence, were examined for 16 cases. A case study illustrates the typical synoptic-scale transition to subsiding, northerly flow that leads to Santa Ana occurrence. A graphic summary shows how the relative strength of the 16 cases was related to LFM v wind component and omega forecasts.

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