| Description |
Differential solar irradiation on opposing mountain sidewalls produces local temperature gradients. Flows across the valley or basin develop due to the ensuing horizontal pressure gradients, which are directed from the less irradiated and colder sidewall toward the more irradiated and warmer sidewall. These thermal flows are investigated for the small and almost circular basin of Arizona's Meteor Crater using observations and numerical simulations. Observations from the Meteor Crater show a pronounced cross-basin flow in the center of the crater basin under undisturbed conditions, which develops as an easterly flow in the morning when the sun is to the east and the west sidewall is more strongly irradiated, and which then shifts to a southerly direction around noon and eventually to a westerly direction in the evening. The direction of the cross-basin flow agrees with the direction of the crossbasin temperature and pressure gradients as the sun moves across the sky during the day. Large-eddy simulations for an idealized, rotationally symmetric basin produce a cross-basin circulation with a three-layer structure in the morning, that is, a nearsurface southeasterly cross-basin flow topped by an opposing, northwesterly return flow and a secondary southeasterly flow near or above the top of the basin. Based on an analysis of the horizontal momentum and the thermodynamic balance equations, a different formation mechanism is identified for each layer, with each of the formation mechanisms being related to asymmetric irradiation. Additional simulations are run with a prescribed surface heat flux, which produces a spatially constant heat-flux gradient, and with varying background wind speeds and directions for different basin sizes. Results indicate that persistent cross-basin flows develop only in basins that are smaller than 5 km. Background winds induce a secondary circulation near the top of the basin, which interacts with the thermally driven circulation. The resulting wind field depends on the direction of the background winds with respect to the prescribed heat-flux gradient and on the stratification of the basin atmosphere. |
