Effects of Shear, Stability and Valley Characteristics on the Destruction of Temperature Inversions

Abstract

A dry two-dimensional version of the Colorado State Cloud/Mesoscale Model was used to study the morning, inversion destruction cycle in a variety of deep mountain valley configurations. Eleven simulations were run to examine the effects of valley width, surface heating rate, wind shear above the valley, valley orientation, sidewall slope, initial stability and variable surface albedo on the evolution of the daytime boundary layer in the valley. Each was initiated with a stable layer filling the valley to ridgetop with a neutral layer above the ridge. The model was driven at the lower surface by a sinusoidally varying potential temperature flux which approximates the diurnal heating cycle. All simulations show that the initial inversion layer is destroyed by a combination of three processes; a growing surface based neutral layer over the valley floor, the destabilization of the stable air mass by the recirculation of air warmed over the slopes and the descent of the inversion top by the transport of air beneath the stable layer out of the valley in the slope flows. The results show a wide variety of boundary layer behavior typical of that observed in several western Colorado valleys. Most of the model inversions were destroyed 3.5?5 h after sunrise, which is consistent with thermodynamic calculations. Slope effects decrease with increasing valley width and become unimportant when ridgetop width-to-depth ratios exceed 24. Decreasing the surface heating rate influences the rate but not the structure of the boundary layer development. A very weakly heated valley, typical of those with a high surface albedo due to snow, will hold a stable layer until very late in the day. Moderate wind shear and valley orientation have very little effect on the simulated boundary layer evolution. Steeper sidewall slopes and stronger initial stabilities inhibit slope flow development and exhibit less inversion descent. Conversely, lower surface albedos along the valley sidewalls can dramatically increase the magnitude of the stable layer descent.