The vertical and horizontal structure of the atmospheric boundary layer near the northern California coast was investigated during spring upwelling conditions as part of the 1981 Coastal Ocean Dynamics Experiment. Two daytime aircraft flights were devised to measure mean and turbulent quantities for 25-km tracks along and across the continental shelf from 30- to 1200-m altitude. The Pacific high offshore and an inland thermal low characterized the synoptic situations, and equatorward winds resulted, which produced intense ocean upwelling. The wind profile was found to be jet shaped: maximum wind speeds were 25 m s-1 and 16 m s-1 at 400 and 100 m, respectively, in the two flights. Above and below the heights of the jet speed maxima, the wind speed decreased rapidly. Sharp density inversions existed at the levels of the peak wind speeds; in one flight the inversion also sloped down toward the coast. Turbulence was effectively confined below the inversions. Strong, zonal temperature gradients above the inversion between the hot land and cool marine air caused a thermal wind for which the wind speed decreased with height. Below the inversion, turbulent stress divergence was large and effective in transferring the horizontal momentum of the wind to the sea, slowing the wind. There was also a small thermal wind below the inversion but opposite to the one above owing to the well-mixed air temperature following the sea surface temperature, which was cold at the coast and relatively warmer offshore owing to the upwelling. These effects explain the jet-shaped wind profile. The mean momentum equations show that an acceleration toward the coast can balance the pressure gradient, Coriolis, and turbulent stress divergence forces, and there were some observations to support this. Complete balance was not obtained for the turbulent kinetic energy budget: inferred dissipation rates were too large. ¿American Geophysical Union 1987