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Structure of the Lower Atmosphere over the Northern California Coast during Winter

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  • 1 Department of Geological Sciences, San Diego State University, San Diego, California
  • | 2 Department of Mechanical and Aerospace Engineering, University of California at Irvine, Irvine, California
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Abstract

The structure of the lower atmosphere over the northern California coastal ocean upwelling area was studied during the Shelf Mixed Layer Experiment in the winter of 1989. Surface data were collected at seven automated coastal stations and six buoys. Boundary layer soundings were made using balloons at the coast and a research aircraft over the ocean. The aircraft was also used to map the low-level (30 m) mean and flux fields over the 80 km × 120 km shelf area.

The wintertime coastal weather conditions were more variable than in summer and were observed to fit into three categories: strong northerly (downcoast) winds, strong southerly (upcoast) winds, and weak winds. The variability was caused by the passage of wintertime cyclones interspersed with periods of small pressure gradients. The strong wind cases had small diurnal variations, whereas the diurnal variations were large for the weak wind case.

The vertical structure of the coastal boundary layer was more uniform compared to that in summer, with weak or nonexistent temperature inversions. Winds below 600 m were not correlated with those above 1.5 km except during strong alongshore winds. The presence of a coastal mountain ridge suppresses low-level cross-shore flow. The horizontal structure over the ocean shelf measured by the low-level aircraft tracks showed an area of large positive wind stress curl [over 1 Pa (100 km)−1] west of Point Arena for both directions of the strong wind cases. This implies positive Ekman pumping of the shelf waters in this area regardless of wind direction.

Abstract

The structure of the lower atmosphere over the northern California coastal ocean upwelling area was studied during the Shelf Mixed Layer Experiment in the winter of 1989. Surface data were collected at seven automated coastal stations and six buoys. Boundary layer soundings were made using balloons at the coast and a research aircraft over the ocean. The aircraft was also used to map the low-level (30 m) mean and flux fields over the 80 km × 120 km shelf area.

The wintertime coastal weather conditions were more variable than in summer and were observed to fit into three categories: strong northerly (downcoast) winds, strong southerly (upcoast) winds, and weak winds. The variability was caused by the passage of wintertime cyclones interspersed with periods of small pressure gradients. The strong wind cases had small diurnal variations, whereas the diurnal variations were large for the weak wind case.

The vertical structure of the coastal boundary layer was more uniform compared to that in summer, with weak or nonexistent temperature inversions. Winds below 600 m were not correlated with those above 1.5 km except during strong alongshore winds. The presence of a coastal mountain ridge suppresses low-level cross-shore flow. The horizontal structure over the ocean shelf measured by the low-level aircraft tracks showed an area of large positive wind stress curl [over 1 Pa (100 km)−1] west of Point Arena for both directions of the strong wind cases. This implies positive Ekman pumping of the shelf waters in this area regardless of wind direction.

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