Boundary layer evolution within a canyonland Basin. Part I: Mass, heat, and moisture budgets from observations

Abstract

Individual terms of the mass, heat, and moisture budget equations are evaluated for an atmospheric control volume in Colorado's Sinbad Basin using tethered balloon and surface energy budget data obtained during a 16.5-h period on 15-16 July 1988. The basin was chosen for its simple topography and arid climate, which simplified the evaluation of some of the budget terms. The paper documents the many assumptions that are required to evaluate the mass, heat, and moisture budget equations in confined terrain using small datasets. The nighttime outflow of air from the basin produced a compensatory mean sinking motion of 0.026 m s-1 at the top of the basin control volume and brought warm air into the top of the basin atmosphere. In contrast to previous reports for well-drained valleys, a high rate of atmospheric cooling continued in this basin throughout the entire night. The cooling is attributed primarily to turbulent sensible heat flux divergence and, to a lesser extent, radiative flux divergence. Measured sensible heat fluxes on the basin floor were small, suggesting that downward turbulent sensible heat fluxes must be relatively stronger in the downslope flows that develop above the sidewalls. A general means of characterizing and comparing the energetics of basin and valley atmospheres is developed from the heat budget equation and is illustrated using data from other valleys and basins. An accompanying paper takes a complementary approach of evaluating the heat budget terms using a dynamic model.