THE SUMMERTIME LOW-LEVEL JET AND MARINE BOUNDARY-LAYER STRUCTURE ALONG THE CALIFORNIA COAST

This paper examines the strong, summertime northerly low-level jet (LL J) that frequently exists along the California coast. The persistent s ynoptic-scale pressure distribution(North Pacific high to the west, th ermal low to the east) and baroclinity created by the juxtaposition of the heated continent and the cool marine layer produce the mean struc ture of this LLJ. Strong diurnal thermal forcing, coupled with topogra phic influences on the flow, modulate the jet structure, position, and intensity. A mesoscale model is used to examine many of the complex f acets of the LLJ flow dynamics. Several sensitivity studies, in additi on to a control experiment, aid in this investigation. Principal findi ngs of this study include the following. The pronounced east-west slop e of the marine planetary boundary layer (MPBL) is not due primarily t o colder SST values along the coast. Dynamically forced low-level coas tal divergence, coupled with synoptic-scale divergence, appears to be dominant in determining MPBL inversion slope and profoundly impacts th e coastal stratus distribution. Maximum baroclinity occurs in midafter noon, whereas the LLJ maximum occurs in the evening. An analytical tre atment of the dynamics shows that diurnal variation of the jet-level b aroclinity, coupled with inertial and friction effects, explain this j et timing. In a no-terrain simulation, the jet is broader, somewhat we aker, and tilts more to the west than in our control case. A deeper bo undary layer occurs over the location of the Central Valley of Califor nia in the no-terrain simulation than in the more realistic control ru n. Consequently, a delay in time of maximum baroclinity aloft occurs i n the no-terrain case, and the LLJ maximum occurs later as compared to the control. The core of the jet, which resides in the inversion capp ing the MPBL, lowers and moves toward the coast during the day and lif ts and moves farther away from the coast at night. Meso-beta-scale str ucture of the LLJ along the coast is forced by the topography of point s and capes. The mesoscale model simulation has supercritical Bow, sho wing expansion fan characteristics, in the MPBL around Cape Mendocino. Model results are consistent with mountain wave theory in that a near -surface wind speed maximum and pressure minimum are modeled on the le e side of Cape Mendocino. The LLJ maxima in the lee of points and cape s produce local maxima in surface stress. The position of these wind s tress maxima correlate well with the location of cold pools observed i n the SST, implying that locally enhanced, wind-forced upwelling plays a major role in the creation of such cold SST patches.