Boundary Layer Evolution within a Canyonland Basin. Part II: Numerical Simulations of Nocturnal Flows and Heat Budgets

Abstract

A mesoscale model is used to simulate the nocturnal evolution of the wind and temperature fields within a small, elliptical basin located in western Colorado that has a drainage area of about 84 km2. The numerical results are compared to observed profiles of wind and potential temperature. The thermal forcing of the basin wind system and the sources of air that support the local circulations are determined. Individual terms of the basin atmospheric heat budget are also calculated from the model results. The model is able to reproduce key features of the observed potential temperature profiles over the basin floor and winds exiting the basin through the narrow canyon that drains the basin. Complex circulations are produced within the basin atmosphere as a result of the convergence of drainage flows from the basin sidewalls. The strength of the sidewall drainage flow varies around the basin and is a function of the source area above the basin, the local topography, and the ambient winds. Flows on the basin floor are affected primarly by the drainage winds from the northern part of the basin. The near-surface sidewall drainage flows converge within the southern portion of the basin, producing a counterclockwise eddy during most of the evening. Evaluation of the individual terms of the atmospheric heat budget show that the forcing due to advection and turbulent diffusion is significantly larger above the sidewalls than over the basin floor; therefore, measurements made over the basin floor would not be representative of the basin as a whole. The cooling in the center of the basin results from the local radiative flux divergence and the advection of cold air from the sidewalls, and the cooling above the basin sidewalls is due primarily to turbulent sensible heat flux divergence. A high rate of atmospheric cooling occurs within the basin throughout the evening, although the strongest cooling occurs in the early evening hours. Sensitivity tests show that the thermal structure, circulations, and rate of cooling can be significantly affected by ambient wind direction and, to a lesser extent, vegetation coverage.