Atmospheric Science: Thermal Inversion at the Barringer Meteorite Crater

10/01/2009

Atmospheric thermal inversion occurs when cold air is trapped beneath warmer air. Barringer Meteorite Crater, with its simple bowl-shaped topography, is an ideal experiment site for investigating this phenomenon.

If you live in a western city that is surrounded by mountains (for example, Phoenix, Salt Lake City, and Boise), the effects of atmospheric thermal inversion may be familiar. Within those basins, cold air is often trapped beneath warmer air. This turns off atmospheric convection near the ground. The air grows still and pollutants begin to build up beneath the ceiling of warm air, causing murky conditions that vex visibility and cause respiratory problems.

Although the basic physics of the phenomenon are known, the details that may affect (or mitigate) the severity of thermal inversions are still being investigated. Atmospheric scientists use a variety of tools: in situ measurements in cities with thermal inversions, small scale experimental models of air flow in basins, and theoretical calculations of air movements. Unfortunately, the different scales of past work are often difficult to link together, so scientists began looking for an experimental site of intermediate size.

Barringer Meteorite Crater, with its simple bowl-shaped topography, became the favored experiment site. A large integrative team from several universities, in coordination with the National Science Foundation and the National Center for Atmospheric Research, deployed a large number of sensors in and around the crater. The leaders of the team were David Whiteman, Andreas Muschinski, Sharon Zhong, and David Fritts. The team measured the effects of solar radiation as the sun heated different portions of the crater walls between dawn and dusk, and consequences of the overnight loss of solar radiation. The flow of air around the crater, over its rim, and down its slopes was monitored to determine if the flow was steady, unstable, or episodically disrupted.

This meteorological field campaign showed that thermal inversions occur in Barringer Meteorite Crater, creating a stable boundary layer about 30 meters deep in the crater interior. The inversion often begins in the late afternoon, remains overnight, and is then destroyed two to three hours after sunrise. Surprisingly, the basal layer of air is covered with an isothermal layer of air that separated the cold air on the crater floor from the overlying warmer air of the atmospheric inversion. The team is investigating the origin of this atmospheric structure and its implications for thermal inversions elsewhere.

Although the impetus for this study was terrestrial, we have used these results to explore conditions on Mars where rovers have detected thermal inversions within impact craters. Based on the measurements made at Barringer Meteorite Crater, the daily temperature variations should be greater on the crater floors of Mars, than on their rims. The average temperature on a crater floor on Mars will also be less than that on the rim. We can also infer that temperature inversions on Mars may be greater now than in the geologic past, when lakes may have mitigated the magnitude of any thermal inversions. Interestingly, the types of down-slope air flow detected at Barringer Meteorite Crater can enhanced the ablation of volatile material, like water, on Mars, and, thus, may be an important factor in the potential habitability of impact craters on Mars.