The Summertime Low-Level Jet and Marine Boundary Layer Structure along the California Coast

Abstract

This paper examines the strong, summertime northerly low-level jet (LLJ) that frequently exists along the California coast. The persistent synoptic-scale pressure distribution (North Pacific high to the west, thermal low to the east) and baroclinity created by the juxtaposition of the heated continent and the cool marine layer produce the mean structure of this LLJ. Strong diurnal thermal forcing, coupled with topographic influences on the flow, modulate the jet structure, position, and intensity. A mesoscale model is used to examine many of the complex facets of the LLJ flow dynamics. Several sensitivity studies, in addition to a control experiment, aid in this investigation. Principal findings of this study include the following. The pronounced east?west slope of the marine planetary boundary layer (MPBL) is not due primarily to colder SST values along the coast. Dynamically forced low-level coastal divergence, coupled with synoptic-scale divergence, appears to be dominant in determining MPBL inversion slope and profoundly impacts the coastal stratus distribution. Maximum baroclinity occurs in midafternoon, whereas the LLJ maximum occurs in the evening. An analytical treatment of the dynamics shows that diurnal variation of the jet-level baroclinity, coupled with inertial and friction effects, explain this jet timing. In a no-terrain simulation, the jet is broader, somewhat weaker, and tilts more to the west than in our control case. A deeper boundary layer occurs over the location of the Central Valley of California in the no-terrain simulation than in the more realistic control run. Consequently, a delay in time of maximum baroclinity aloft occurs in 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 capping the MPBL, lowers and moves toward the coast during the day and lifts and moves farther away from the coast at night. Meso-?-scale structure of the LLJ along the coast is forced by the topography of points and capes. Thee mesoscale model simulation has supercritical flow, showing 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 lee side of Cape Mendocino. The LLJ maxima in the lee of points and capes produce local maxima in surface stress. The position of these wind stress maxima correlate well with the location of cold pools observed in the SST, implying that locally enhanced, wind-forced upwelling plays a major role in the creation of such cold SST patches.