THE AIRFLOW OVER MOUNTAINS. A review of the state of current knowledge

Abstract

AbstractA brief survey is first given of the observational evidence regarding special airflow effects in the neighbourhood of mountains, as provided by the visual evidence of clouds, the experiences of glider pilots and effects noted by the pilots of powered aircraft. This serves to indicate the phenomena requiring explanation. The review of theoretical studies is confined mainly to the work of Queney and Scorer. Both these authors apply the perturbation method to laminar, isentropic and inviscid flow over an ideal ridge. Queney deals with a uniform airstream (constant velocity and constant stability) and derives streamlines showing wave motions, both vertical and horizontal. Queney's waves are dependent partly on the stability and partly on geostrophic forces, and the order of the wavelength is several hundred km. Scorer, in his treatment, deals with smaller‐scale phenomena, so enabling geostrophic forces to be neglected, and provides for variations of stability and wind shear in the vertical. He finds that realistic wave effects, of the type observed in nature are predicted by the theory provided the variation of stability and wind shear with height is in accord with certain criteria. Past experimental work with models in wind tunnels is briefly considered in the light of what can be deduced from theoretical treatments regarding the conditions for dynamical similarity. More recent work by Long, using a tank containing several layers of immiscible liquid to simulate a density gradient is referred to briefly; this technique appears to have promise. A comprehensive field study by Forchtgott, who wed teams of gliders, is summarized. From his observations, Forchtgott has built up a theory of the flow patterns which occur in the lee of mountains. The main value of Forchtgott's work lies in the practical support it provides for some of the results deduced by perturbation theory and in throwing light on the nature of non‐laminar flow patterns which cannot be dealt with by existing theoretical treatments. In a concluding section, aviation applications of existing knowledge on this subject are referred to briefly. The criteria have some practical value for forecasting purposes.