A Mass Flux Model of Nocturnal Cold-Air Intrusions into a Closed Basin

Abstract

bservations made during the Meteor Crater Experiment (METCRAX) field campaign revealed unexpected nighttime cooling characteristics in Arizona?s Meteor Crater. Unlike in other natural closed basins, a near-isothermal temperature profile regularly develops over most of the crater depth, with only a shallow stable layer near the crater floor. A conceptual model proposed by Whiteman et al. attributes the near-isothermal stratification to the intrusion, and subsequent detrainment, of near-surface air from outside the crater into the crater atmosphere. To quantify and test the hypothesis, a mass flux model of the intrusion process is developed. It is found that the observed temperature profile can be reproduced, providing confirmation of the conceptual model. The near-isothermal stratification can be explained as a result of progressively cooler air entering the crater and detraining into the atmosphere, combined with the finite time of ascent in the compensating rising motion. The strength of detrainment largely determines the characteristics of the cooling process. With weak detrainment, most of the cooling arises from an adiabatic rising motion (?filling-up? mode). Stronger detrainment leads to reduced rising motion and enhanced cooling at upper levels in the crater (?destabilization? mode). Of interest is that the detrainment also reduces the total cooling, which, for a given intrusion mass flux, is determined by the temperature difference between the intruding air and the crater atmosphere at rim height.