83rd Annual

Monday, 10 February 2003: 1:45 PM
Investigation of 23 Feb 1998 California extratropical storm using COAMPSTM
Brian J. Gaudet, NRL, Monterey, CA; and J. M. Schmidt
Poster PDF (1000.7 kB)
During the period 2-3 Feb 1998 an intense low pressure region migrated to a region offshore of northern California. A front passed ashore to the south of the low over central and southern California. In advance of the front, strong low-level prefrontal winds (to 25 m s-1) and intense rains caused considerable damage; after frontal passage winds and rains continued to be intense, though of a different nature than previously.

The storm was simulated using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) developed at the Naval Research Laboratory. The atmospheric component is nonhydrostatic and compressible, and possesses both liquid and ice phase bulk microphysics. The model was run using four nested grids, possessing grid spacings of 81 km, 27 km, 9 km, and 3 km, respectively.

The model was able to reproduce the general features of the storm well, although the prefrontal low-level winds and precipitation were simulated better than the postfrontal features and the timing of the frontal passage. The influences on the prefrontal winds and precipitation were found to be a complex interaction of topographic forcing, oceanic fluxes, and moist convective thermodynamics. Specifically, the thermodynamic forcing of the frontal moist convection was instrumental to the further intensification of the thermal and momentum gradient along the front. A low-level jet in advance of the front seemed to be explainable by thermal wind balance and advection over the ocean, but was greatly intensified by the Coast Ranges to produce intense winds in the Central Valley. The sensitivity of these features to the abnormally high sea surface temperatures will be presented. The effect of a modification in the bulk microphysics, including the addition of a graupel category, will also be discussed.

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