Diurnal evolution of temperature inversion structure in Arizona's Meteor Crater

A comprehensive boundary layer research program called METCRAX (the METeor CRAter EXperiment) was conducted with research support from NSF, and field support from NCAR in October 2006 in the Barringer or Arizona Meteor Crater. This bowl-shaped crater, 40 km east of Flagstaff, is the best preserved meteor crater on Earth, and was formed by a meteorite impact 50,000 years ago. The impact produced a crater that is 1.2 km in diameter and 165 m in depth, with a rim that rises 40-60 m above the surrounding Colorado Plateau.

Experiments were focused on the evolution of the boundary layer inside the crater using a continuously operating east-west line of 9-m-tall flux towers instrumented at 4 or 5 levels and running E-W across the crater floor and up the sidewalls. In addition 56 temperature data loggers were operated continuously on E-W and N-S lines intersecting at the crater floor center and running up the sidewalls, across the rim and out onto the surrounding plain. A radar boundary layer wind profiler with a radio acoustic sounding system (RASS) and a SODAR with RASS were operated continuously outside the crater.

During Intensive Observational Periods (IOPs) with low background winds and clear skies, three tethersondes were operated simultaneously inside the crater on an E-W line, and rawinsondes were launched at 3-h intervals outside the crater.

IOP observations showed that the time of initiation of the stable boundary layer (SBL) and its asymmetric formation were governed by shadows cast into the crater by the surrounding rim topography. The SBL temperature structure built up in an unusual way that has not been seen previously in other small basins. A sharp temperature inversion formed in the lowest 30 m or so of the crater, but this was surmounted by a near-isothermal layer that extended all the way to the ridgetop, where it was capped by an elevated inversion that connected the cold air in the crater to the still-warm air above the crater. The near-isothermal layer appears to have been produced by mixing of the air in the crater with colder air that is advected over the southwest rim by a larger-scale drainage flow. This interpretation is supported by measurements of colder temperatures on the southwest rim of the crater than on northeast rim. The breakup of the nocturnal temperature inversion occurred in the several hours following sunrise by the upward growth of a convective boundary layer (CBL) from the basin floor and the sinking of the top of the nocturnal inversion in the upper elevations of the basin. Unequal insolation on the sidewalls caused asymmetries in CBL development. Differences in temperature of up to 4°C on the opposing sidewalls occurred when one sidewall was in sunlight and the other was in shadow. Computations of the rate of heat loss from the crater atmospheric volume as normalized by the horizontal area at the crater's rim show that the overall rate of heat loss in the early evening initially reaches 30-40 W m-2, but then decreases during the rest of the night until sunrise.

Diurnal temperature structure evolution in the Meteor Crater will be compared to temperature structure evolution in two similar-size basins in the eastern Alps and in the Bear River Mountains of Utah.