Nocturnal Cold-Air Intrusions into a Closed Basin: Observational Evidence and Conceptual Model

Observations are analyzed to explain an unusual feature of the nighttime atmospheric structure inside Arizona's idealized, basin-shaped Meteor Crater. The upper 75%-80% of the crater's atmosphere, which overlies an intense surface-based inversion on the crater's floor, maintains a near-isothermal lapse rate during the entire night, even while continuing to cool. Evidence is presented to show that this near-isothermal layer is produced by cold-air intrusions that come over the crater's rim. The intrusions are driven by a regional-scale drainage flow that develops over the surrounding inclined Colorado Plateau. Cold air from the drainage flow builds up on the upwind side of the crater and splits around the crater at low levels. A shallow layer of cold air, however, spills over the 30-60-m-high rim and descends partway down the crater's upwind inner sidewall until reaching its buoyancy equilibrium level. Detrainment of cold air during its katabatic descent and compensatory rising motions in the crater atmosphere destabilize the basin atmosphere, producing the observed near-isothermal lapse rate. A conceptual model of this phenomenon is presented.