Session 14.6 Radar Observations of Orographic Precipitation Over Coastal Northern California

Thursday, 24 June 2004: 2:45 PM
Curtis N. James, Embry-Riddle Aeronautical University, Prescott, AZ; and R. A. Houze

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This study investigates orographic precipitation in flood-prone coastal Northern California over a 2.5-year period. Heavy rainfall events during the period were associated with strong southwesterly flow, high relative humidity, and conditional instability below the 800-mb level. In addition, general clockwise turning of the wind with height throughout the lower troposphere indicated the prevalence of warm advection. Reflectivity measurements from an operational Doppler radar located near the coast were generally strongest and most frequent over broad windward slopes of the terrain, with local maxima over small-scale terrain peaks. Rainfall enhancement was also observed upwind from the barrier within a distance of about one Rossby radius for unsaturated air, and more pronounced enhancement was observed within the shorter moist Rossby radius of the barrier. Observations therefore suggest that combined orographic influences from upslope enhancement, seeder-feeder effects, small-scale convective triggering, and upstream enhancement all contributed to the formation of heavy precipitation in the region.

Key dynamic and thermodynamic variables of the impinging low-level flow (900-800 mb) strongly modulated the observed precipitation amounts. The low-level wind direction characterized the synoptic and thermodynamic setting of the flow and controlled the upslope component of the wind relative to the terrain. In general, south-southwesterly flow produced the heaviest rainfall over the region. Greater wind speeds were responsible for increased rainfall over the windward slopes and downwind from the barrier summit. The relative humidity and flow speed of the mid-level flow (between 700 and 500 mb) were also positively correlated with heavy precipitation, indicating the presence of strong synoptic forcing and mid-level cloud during heavy precipitation.

Precipitation upwind from the barrier was mainly controlled by the low-level static stability. Upstream rainfall was heavier when the thermodynamic profile of the impinging flow was absolutely stable. Strong low-level stability was generally accompanied by weaker low-level synoptic-scale flow, which flow further decelerated as it approached the terrain. The radar data also reveal a diurnal precipitation pattern, with heaviest accumulation observed in the early to late morning hours, which could be at least partially attributed to variations in static stability.

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