Transcritical flows in the coastal marine atmospheric boundary layer

Numerical solutions of shallow water flow in a variable-width channel are c omputed to model the summertime marine atmospheric boundary layer off the U .S. west coast. Using an idealization of the coastline in the vicinity of P oint Arena, California, as an example, several steady-state base flows are presented that are hydraulically transcritical. These flows are strongly no nlinear, weakly rotational, and are forced by constant pressure gradients a nd damped by nonlinear bottom drag at the sea surface. The transcritical ba se flows are supercritical in the vicinity of bends in the coastline but ar e subcritical to the north (upstream) and to the south (downstream) where t he coastline is straight. Within the supercritical region, orographic bends in the coastline produce expansion fans and compression jumps, the same st ructures found in globally supercritical flows. When the imposed pressure-g radient forcing is increased, the resulting base flow has a supercritical-t o-subcritical transitional jump that is weaker and located farther downstre am, increasing the extent of the supercritical region. Perturbations are ap plied to the transcritical base flows in the south to study the interaction of coastal-trapped disturbances with the base flows. The disturbances prop agate northward in the subcritical region of the base flow but can be halte d after they reach the supercritical region. Very strong nonlinear disturba nces can overcome the supercriticality of the base flow and propagate all t he way up the coast but are severely attenuated in the process. The interac tion of strong coastal-trapped disturbances with the transcritical base how s is accompanied by an eddy-generation process that resembles satellite ima ges of stratus observed during the May 1982 coastal-trapped event off Calif ornia.