Transcritical flows in the coastal marine atmospheric boundary layer

Numerical solutions of shallow water flow in a variable-width channel are computed to model the summertime marine atmospheric boundary layer off the U.S. west coast. Using an idealization of the coastline in the vicinity of Point Arena, California, as an example, several steady-state base flows are presented that are hydraulically transcritical. These flows are strongly nonlinear, weakly rotational, and are forced by constant pressure gradients and damped by nonlinear bottom drag at the sea surface. The transcritical base flows are supercritical in the vicinity of bends in the coastline but are subcritical to the north (upstream) and to the south (downstream) where the coastline is straight. Within the supercritical region, orographic bends in the coastline produce expansion fans and compression jumps, the same structures found in globally supercritical flows. When the imposed pressure-gradient forcing is increased, the resulting base flow has a supercritical-to-subcritical transitional jump that is weaker and located farther downstream, increasing the extent of the supercritical region. Perturbations are applied to the transcritical base flows in the south to study the interaction of coastal-trapped disturbances with the base flows. The disturbances propagate northward in the subcritical region of the base flow but can be halted after they reach the supercritical region. Very strong nonlinear disturbances can overcome the supercriticality of the base flow and propagate all the way up the coast but are severely attenuated in the process. The interaction of strong coastal-trapped disturbances with the transcritical base hows is accompanied by an eddy-generation process that resembles satellite images of stratus observed during the May 1982 coastal-trapped event off California.