The dynamics of northwest summer winds over the Santa Barbara Channel

A mesoscale model is used to examine the dynamics of northwest flow over th e Santa Barbara Channel region. Three cases are considered, each characteri zed by typical summertime synoptic conditions, but with differences in pres sure gradient strength and marine boundary layer depth (MBL). The first cas e examines a relatively deep MBL and strong pressure gradient. Case 2 is ch aracterized by a more shallow MBL and weaker pressure gradient, and case 3 represents a transition from a deep MBL to shallow conditions. In all cases , simulated surface winds show reasonable agreement with observations over most of the model domain, with the exception of regions near abrupt terrain changes. Results from the model indicate that the flow with a deep MBL (similar to 4 00 m) and strong pressure gradient (case 1) is supercritical, causing regio ns of acceleration and expansion in the lee of Point Conception. When the M BL is shallow (similar to 150 m) (case 2), a transcritical flow scenario ex ists with subcritical flow upstream from Point Conception and a supercritic al flow region over the Santa Barbara Channel and downstream from the Chann el Islands. Flow over the channel is strongly affected by diurnal heating i n shallow MBL cases, reversing direction in step with a land breeze circula tion induced by nighttime cooling. The land breeze forces an internal wave disturbance that propagates westward across the channel, eliminating the su percritical flow region in the lee of Point Conception. Conditions with a d eep MBL (similar to 400 m) produce less variability in the surface winds, e xcept for the region sheltered by the Santa Ynez Mountains. An expansion fa n is still evident in this case, but it is produced by the interaction of t he flow with higher terrain north and east of the channel. The low hills on Point Conception and the Channel Islands do not have a large blocking effe ct on the surface flow when the MBL is deep. Analysis of the momentum budget supports the conclusion that the boundary l ayer behaves like a transcritical hydraulic flow when the MBL is shallow. E xcept for the open ocean region, the Coriolis term is minor in comparison w ith the pressure and advection terms. Diurnal heating effects are evident i n the nearshore pressure term, which varies from offshore during the late e vening to onshore in the afternoon. These effects are most significant when the MBL is shallow and can augment the hydraulically forced pressure patte rn, causing a stronger expansion fan in the late afternoon and a collapse o f the expansion fan during the early morning.