STRUCTURE OF THE LOWER ATMOSPHERE OVER THE NORTHERN CALIFORNIA COAST DURING WINTER

The structure of the lower atmosphere over the northern California coa stal ocean upwelling area was studied during the Shelf Mixed Layer Exp eriment in the winter of 1989. Surface data were collected at seven au tomated coastal stations and six buoys. Boundary layer soundings were made using balloons at the coast and a research aircraft over the ocea n. The aircraft was also used to map the low-level (30 m) mean and flu x fields over the 80 km x 120 km shelf area. The wintertime coastal we ather conditions were more variable than in summer and were observed t o fit into three categories: strong northerly (downcoast) winds, stron g southerly (upcoast) winds, and weak winds. The variability was cause d by the passage of wintertime cyclones interspersed with periods of s mall pressure gradients. The strong wind cases had small diurnal varia tions, whereas the diurnal variations were large for the weak wind cas e. The vertical structure of the coastal boundary layer was more unifo rm compared to that in summer, with weak or nonexistent temperature in versions. Winds below 600 m were not correlated with those above 1.5 k m except during strong alongshore winds. The presence of a coastal mou ntain ridge suppresses low-level cross-shore Bow. The horizontal struc ture over the ocean shelf measured by the low-level aircraft tracks sh owed 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. T his implies positive Ekman pumping of the shelf waters in this area re gardless of wind direction.