A numerical study of the boundary layer dynamics inside Arizona's Meteor Crater

Fine-scale numerical simulations using the Advanced Regional Prediction System (ARPS) were performed to help explain and understand the observed boundary layer dynamics inside the Meteor Crater, a small, closed basin in northern Arizona. Model results are compared to data obtained during the METCRAX field campaign that took place near Winslow, Ari¬zona during October 2006. Several aspects of the observed nocturnal crater atmosphere are investigated including a quasi-steady state three-layer thermal structure, horizontal homogeneity of the crater atmosphere, asymmetric inflow, and the tendency for katabatic flow to spill over rather than descend into the crater. The study also examined the interactions of crater boundary layer with the ambient atmosphere by simulating cases with substantially different ambient atmospheric conditions.

Despite limitations of the modeling strategy, including the relatively coarse terrain dataset (x ~ 100 m) and incomplete model radiation physics, the numerical simulations were able to reproduce the salient features of the nocturnal boundary layer under different conditions. Analysis of the model thermodynamic and momentum forcing terms provides valuable clues as to the source of the observed aspects of the crater boundary layer and its interactions with the ambient atmosphere. It is found that steady cooling of the sloping terrain west of the crater leads to gradual intensification of a downslope flow through a combination of negative buoyancy and the horizontal pressure gradient force. An increase in katabatic winds directed toward the crater subsequently triggers development of gravity waves immediately above the crater. Cold air advection associated with gravity wave activity immediately above the crater is found to be critical to destabilization of the boundary layer and thus the creation of the isothermal layer. Cooling of the lower crater atmosphere is dominated by turbulent flux divergence during the majority of the overnight period. Model results suggest that the dominant factor in the development of horizontal homogeneity is the cessation of drainage flow into the crater, with the development of gravity waves being the key element in diverting regional downslope flow away from the crater slopes. The results also illustrate how the crater converts kinetic energy from the ambient mean flow to turbulent kinetic energy inside the crater.