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

Michael Tjernström

doi:10.3402/tellusa.v51i5.14499

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

Tellus

Volume 51 (1999): Issue 5

By: Michael Tjernström

Open Access

|Jan 1999

Abstract

Hypothetical numerical simulations are carried out to investigate the sensitivity of a coastally trapped supercritical flow to various forcing. All simulations are compared with a control simulation, that is based on a real case from the coastal waves field experiment off the coast of California in 1996, the 7 June case. This case features a northerly supercritical flow along the California coast that triggers an expansion fan at Cape Mendocino (40.4°N, 124.4°W). The upstream jet then strengthens as the boundary layer becomes more shallow. Other important features are the terrain at the cape and the SST depression into Shelter Cove in the lee of the cape. All sensitivity simulations reveal an expansion fan with the associated dynamics; this appears to be a very stable feature. Altering the local terrain changes the local structure of the jet and removes the lee-wave that is responsible for the apparent collapse of the boundary layer in Shelter Cove. A realistic SST, instead of a spatially constant value, has only minor effects very locally. Changing the Froude number of the flow by manipulation of the inversion strength reveals an unexpected feedback. A weaker inversion increases the entrainment and thus deepens the boundary layer, and vice versa, while increasing the inversion strength. In the expression for the Froude number, these 2 effects cancel and the end results is very similar to the control run. Even when reducing the background flow speed by 50%, an expansion fan appeared. This case is thus transcritical–subcritical upstream, turning supercritical at the cape. It experiences the 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 initial boundary 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 impossible to find a set of conditions, based on perturbations to the control case, that did not reveal supercriticality and the expansion fan dynamics.

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