Isothermalcy in a basin atmosphere produced by nocturnal cold air intrusions

Previous experiments that have studied nighttime temperature structure evolution in small basins have found that the depth and strength of inversions increase during the night. Our recent study of temperature inversions in Arizona's Meteor Crater, however, has found an unusual temperature structure evolution that has not been previously seen. There, a 30-m-deep, intense inversion forms in the early evening on the crater floor. This is capped by a deep near-isothermal layer that extends to the crater's rim 170 m above the crater floor. The crater continues to cool during the night while maintaining the near-isothermal temperature structure. The surprising feature of the temperature profile evolution is the persistence of the deep isothermal layer. This paper investigates the processes that produce and maintain this unusual temperature structure.

The near-isothermal layer is produced by a southwesterly regional-scale drainage flow that affects the crater region. The drainage flow on the adjacent plain spills over the rim of the crater, which is only 30-50 meters above the plain. The intruding cold air layer descends the sidewall and during this descent, turbulent detrainment and horizontal mixing of the cold air affects the bulk stability in the crater atmosphere. The detrainment of cold air is greatest at the upper elevations of the crater, continuously destabilizing the crater atmosphere. The cold air intrusion occurs on the upwind inner sidewall of the crater and the cold air penetrates into the crater to depths that depend on the temperature deficit of the air coming over the rim, the inflow volume flux, and the existing temperature structure of the bulk atmosphere inside the crater. The inflows are rarely able to penetrate all the way to the crater floor because of the strong stability in the pre-existing, shallow, intense inversion there.

The evidence for our conceptual model of crater/background flow interactions, including the mesoscale flow modification as it encounters the crater, the resulting shallow inflows of cold air over the rim, and the effects of these inflows on the structure of the nocturnal inversion in the crater will be discussed in the presentation.