Cold Air Intrusions into Basins and Valleys

Downslope flows typically develop in undisturbed, clear sky conditions on inclined surfaces, with the flow origin at the top of the slope. Such downslope flows, however, can be significantly modified or, even, overpowered when a continuous cold air source is present at the top of the slope. This can occur, for example, when mesoscale or synoptic scale flows advect cold air over a mountain ridge, a valley or a drainage basin. In this presentation we consider the case at Meteor Crater, AZ, where cold air advected by a mesoscale nocturnal southwesterly katabatic flow on the gently inclined Colorado Plateau is carried over the 30-50 m high rim of a 170 m deep and 1.2-km diameter circular meteorite crater.

A simple buoyancy-based conceptual model is used to predict the penetration depth into the crater of cold air intrusions and their variations with time and angular distance from the katabatic flow direction using lines of temperature data loggers running up the northeast, southeast, south-southwest, southwest and west crater sidewalls. A 40-m-tall tower on the south-southwest rim provides the vertical profiles necessary to observe the dynamics and thermodynamics of the cold air intrusion layer.

Key determinants affecting the characteristics of the cold-air intrusion are expected to include the depth and temperature deficit of the cold air source, the strength of the flow bringing the cold air across the rim, the sidewall slope angle, the ambient atmospheric stability within the crater, the frictional drag of the underlying sidewall surface, the entrainment of ambient air at the top of the intrusion and any terrain-induced flow convergence. These factors are considered, in turn, using a simple katabatic flow model and analyses of October 2013 data from the Second Meteor Crater Experiment (METCRAX II).