Boundary Layer Updrafts Driven by Airflow over Heated Terrain

This study presents linear and nonlinear scalings for boundary layer ascent forced by airflow over heated terrain and compares them to results from corresponding high-resolution numerical simulations. Close agreement between theory and simulation is found over most of the parameter space considered, including variations in background winds, boundary layer stability, mountain height, and diabatic heating rate. As expected, the linear and nonlinear scalings perform best for linear and nonlinear flows, respectively. For a convective boundary layer, the scalings accurately predict vertical motion for all flows considered, including those that extend well into the nonlinear regime. Thus, these scalings may ultimately help to improve the parameterization of subgrid orographic ascent in large-scale models. The vertical velocity scalings are less accurate for mechanically blocked flows in stable boundary layers, for which a simple vertical displacement scaling is superior. Although the scalings do not treat interactions between mechanical and thermal flow responses, these interactions are generally weak except in blocked flows with strong surface heating. Numerical simulations of such cases suggest that a hydrostatically induced pressure decrease in the lee associated with the diabatic surface heating drives stronger flow reversal within the wake and leeside convergence downwind of it, both of which produce strong surface-based updrafts. Thus, nonlinear interactions between mechanical and thermal flow responses may significantly enhance the likelihood of convection initiation over heated mountains.