Diabatic modification of airflow over a mesoscale orographic ridge: A model study of the coupled response

A theoretical model is used to study the combined influence upon mesoscale atmospheric flow of orography and the diabatic effects of non‐precipitating clouds. In the model a CISK‐like representation is adopted for non‐precipitating convective clouds. The resulting coupled response is examined in the limit of small amplitude, steady, hydrostatic perturbations of a flow incident upon a ridge (or a train of ridges) in a non‐rotating environment. It is shown that the combined effect of the two forcing processes can be significantly different from that of orography acting alone. In particular in certain situations an enhanced (indeed resonant) response can occur with strong winds on the lee slope and a concomitant large surface pressure drag. Physical considerations based upon the CISK instability concept indicate that for the present model the converse flow response with reduced or even negative values of the pressure drag is usually (but not always) an inadmissible flow configuration. These results help to clarify the disparate results obtained in earlier studies, and, unlike those studies, suggest that diabatic effects might on occasions have a major role in inducing strong surface lee‐side winds.