Basin temperature inversions and their relationship to ambient atmospheric conditions

Observations from a recent meteorological experiment in the Meteor Crater, a small, near circular, enclosed basin, in northern Arizona of the United States, are used to investigate the status of the atmosphere in the basin as indicated by temperature inversion and its relationship to ambient atmospheric conditions. Surface winds over the surrounding plain of the basin was identified as an important factor in determining whether the basin atmosphere would be coupled to, or decoupled from, its ambient environment. A mean nighttime surface wind of 5 m s-1 over the plain is found to be the threshold value below which the basin would be completely decoupled from air above and a strong temperature inversion would form. Because the stable nocturnal boundary layer acts to prevent large-scale winds aloft from penetrating down to the surface of the surrounding plain, large-scale winds may not be a reliable aid to the forecast of whether or not a strong temperature inversion would form in the basin. Instead, local or regional thermally driven flows are likely to be more important. The status of the basin atmosphere is also directly related to ambient stability in the nocturnal boundary layer just above the surface of the plain, with a sub-critical Richardson number enabling the coupling of the basin atmosphere to the ambient environment and vice versa. Longwave radiation loss is found to be a poor indicator for whether the basin would be decoupled and an inversion would form, but for the nights when an inversion does form, the strength of the inversion is positively correlated to the amount of the net longwave radiation at night except over the basin floor where the cooling is also affected by cold air drainage from the sidewalls.