A Numerical Study of Inversion-Layer Breakup and the Effects of Topographic Shading in Idealized Valleys

Abstract

Numerical simulations of inversion-layer breakup in idealized steep valleys are performed using the Advanced Regional Prediction System (ARPS) to investigate the effects of valley width and depth, and topographic shade. Simulations of the diurnal pattern of slope winds under weak synoptic conditions are presented in a valley of depth 500 m and floor width 1200 m. Typical up- and downslope wind circulations are reproduced, and their influence on the stability in the valley is analyzed and characterized using the classifications of Whiteman. A systematic investigation of the inversion-layer characteristics in a set of 24 valleys of varying depth and width is conducted. For the narrow-valley cases, the depth and lifetime of the stable layer increase as the depth of the valley increases. For wide valleys, however, the stable-layer depth and lifetime converge toward a single value regardless of the valley depth. An original subroutine accounting for topographic shading is introduced and its effects on both the slope winds and the inversion breakup process are discussed. Results from tests in idealized valleys indicate that topographic shading can delay inversion-layer breakup and, therefore, should be included, when appropriate, in numerical simulations of flow over complex terrain.