The sensitivity of supercritical atmospheric boundary-layer flow along a coastal mountain barrier

Abstract

Hypothetical numerical simulations are carried out to investigate the sensitivity of a coastallytrappedsupercritical flow to various forcing. All simulations are compared with a controlsimulation, that is based on a real case from the coastal waves field experiment off the coast ofCalifornia in 1996, the 7 June case. This case features a northerly supercritical flow along theCalifornia coast that triggers an expansion fan at Cape Mendocino (40.4°N, 124.4°W). Theupstream jet then strengthens as the boundary layer becomes more shallow. Other importantfeatures are the terrain at the cape and the SST depression into Shelter Cove in the lee of thecape. All sensitivity simulations reveal an expansion fan with the associated dynamics; thisappears to be a very stable feature. Altering the local terrain changes the local structure of thejet and removes the lee-wave that is responsible for the apparent collapse of the boundary layerin Shelter Cove. A realistic SST, instead of a spatially constant value, has only minor effectsvery locally. Changing the Froude number of the flow by manipulation of the inversion strengthreveals an unexpected feedback. A weaker inversion increases the entrainment and thus deepensthe boundary layer, and vice versa, while increasing the inversion strength. In the expressionfor the Froude number, these 2 effects cancel and the end results is very similar to the controlrun. Even when reducing the background flow speed by 50%, an expansion fan appeared. Thiscase is thus transcritical–subcritical upstream, turning supercritical at the cape. It experiencesthe largest changes in terms of wind speed. Attempts to generate a solid stratocumulus layer(no clouds were present in reality) in the model failed. Within a realistic increase of the initialboundary layer humidity, it was not possible to maintain clouds in these runs. In summary, the expansion fan dynamics appear strong and quite consistent, and it was virtually impossibleto find a set of conditions, based on perturbations to the control case, that did not revealsupercriticality and the expansion fan dynamics.