Session 3A.1 A Numerical Modeling Study of the Nocturnal Boundary Layer Inside Arizona's Meteor Crater

Tuesday, 3 August 2010: 3:30 PM
Torrey's Peak I&II (Keystone Resort)
Michael T. Kiefer, Michigan State University, East Lansing, MI; and S. Zhong

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The Advanced Regional Prediction System (ARPS) model is utilized to examine the features of the nocturnal boundary layer as observed within Arizona's Meteor Crater during the METCRAX experiment that took place in October 2006. Four aspects of the observed nocturnal crater atmosphere are investigated: 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 three-layer structure, consisting of a strong surface inversion capped by an isothermal layer, and a secondary inversion at crater top, is the main focus of analysis.

Before examining physical processes in the nocturnal boundary layer, a preliminary step was executed to examine the impact of several model parameters on thermal and wind fields in the crater, and thus determine what combination of parameters most closely replicated observed crater profiles of potential temperature and wind speed/direction. Parameters varied included near surface vertical grid spacing, turbulent length scale formulation, and soil moisture. It is shown that simulated thermal structure exhibits strong sensitivity to vertical grid spacing and soil moisture, with minor sensitivity to turbulent length scale.

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. Analysis of the model thermodynamic and momentum forcing terms provides valuable clues as to the source of the observed aspects of the crater 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. During the ma┬Čjority of the night, cooling of the lower crater atmosphere is dominated by turbulent flux divergence. 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.

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