The Nocturnal Evolution of Atmospheric Structure in a Basin as a Larger-Scale Katabatic Flow Is Lifted over Its Rim

AbstractThe successive stages of nocturnal atmospheric structure inside a small isolated basin are investigated when a katabatically driven flow on an adjacent tilted plain advects cold air over the basin rim. Data came from Arizona’s Meteor Crater during intensive observing period 4 of the Second Meteor Crater Experiment (METCRAX II) when a mesoscale flow above the plain was superimposed on the katabatic flow leading to a flow acceleration and then deceleration over the course of the night. Following an overflow-initiation phase, the basin atmosphere over the upwind inner sidewall progressed through three stages as the katabatic flow accelerated: 1) a cold-air-intrusion phase in which the overflowing cold air accelerated down the upwind inner sidewall, 2) a bifurcation phase in which the katabatic stable layer lifted over the rim included both a nonnegatively buoyant upper layer that flowed horizontally over the basin and a negatively buoyant lower layer (the cold-air intrusion) that continued on the slope below to create a hydraulic jump at the foot of the sidewall, and 3) a final warm-air-intrusion phase in which shear instability in the upper overflowing layer produced a lee wave that brought warm air from the elevated residual layer downward into the basin. Strong winds during the third phase penetrated to the basin floor, stirring the preexisting, intensely stable, cold pool. Later in the night a wind direction change aloft decelerated the katabatic wind and the atmosphere progressed back through the bifurcation and cold-air-intrusion phases. A conceptual diagram illustrates the first four evolutionary phases.