Simulations of supercritical flow around points and gapes in a coastal atmosphere

Fully 3D nonlinear model simulations for supercritical flow along locations at the California coast, at Cape Mendocino, and Point Sur,are presented th e model results are objectively and subjectively verified against measureme nts from the Coastal Waves 1996 experiment with good results. They are then analyzed in terms of the flow structure, the impact of the local terrain, the atmospheric forcing on the ocean surface, and the momentum budgets. It, is verified that the how is supercritical (Fr > 1) within a Rossby radius of deformation from the coast and that it can be treated as a reduced-gravi ty, shallow water flow bounded by a sidewall-the coastal mountain barrier A s the supercritical flow impinges on irregularities in the coastline orient ation, expansion fans and hydraulic jumps appear. The modeled Froude number summarizes well the current understanding of the dynamics of these events. In contrast to inviscid, irrotational hydraulic flow, the expansion fans a ppear as curved lines of equal PBL depth and "lens-shaped" maxima in wind s peed residing at the PBL slope. This is a consequence of the realistic trea tment of turbulent friction. Modeled mean PBL vertical winds in the hydraul ic features range +/- similar to 1-2 cm s(-1), while larger vertical winds (+/- similar to 5-10 cm s(-1)) are due to the flow impinging directly on th e mountain barrier. Local terrain features at points or capes perturb the l ocal flow significantly from the idealized case by emitting buoyancy waves. The momentum budget along straight portions of the coast reveals a semigeo strophic balance modified by surface friction. While being geostrophic in t he across-coast direction, the along-coast momentum shows a balance between the pressure gradient force and the turbulent friction. In the expansion f ans, the flow is ageostrophic, and the imbalance is distributed between tur bulent friction and ageostrophic acceleration according to the magnitude of the former. There is also a good correspondence between the magnitude of t he local curl of the surface stress vector and the measured depression in s ea surface temperature (SST) in areas where the latter is large and the alo ng-coast flow is relatively weak, implying that a substantial portion of th e upwelling is driven locally. Supplying the measured SST in the numerical simulations, with a considerable depression along the coast, had only margi nal feedback effects on the character of the how.