Diurnal Winds in the Himalayan Kali Gandaki Valley. Part III: Remotely Piloted Aircraft Soundings

Abstract

In 1998 a field campaign has been conducted in the north?south-oriented Kali Gandaki valley in Nepal to explore the structure of its extreme valley wind system. Piloted ballon (pibal) observations were made to map the strong upvalley winds as well as the weak nocturnal flows (Part I). The stratification of the valley atmosphere was not explored. In Part II of this multipart paper, numerical simulations are presented that successfully simulate most of the wind observations. Moreover, the model results suggest that the vigorous upvalley winds can be seen as supercritical flow induced by contractions of the valley. Here, the results of a further campaign are reported where remotely piloted airplanes were used to obtain vertical profiles of temperature and humidity up to heights of ?2000 m above the ground. Such profiles are needed for an understanding of the flow dynamics in the valley and for a validation of the model results. This technique is novel in some respects and turned out to be highly reliable even under extreme conditions. In addition four automatic stations were installed along the valley's axis. Winds were observed via pibal ascents. These data complement the wind data of 1998 so that the diurnal wind system of the Kali Gandaki valley is now documented reasonably well. It is found that the fully developed upvalley flow is confined to a turbulent layer that tends to be neutrally stratified throughout the domain of observations. The stratification above this layer is stable. A capping inversion is encountered occasionally. This finding excludes explanations of the strong winds in terms of hydraulic theories that rely on the presence of strong inversions. Pairs of simultaneous ascents separated by 5?10 km along the valley axis reveal a remarkable variability induced by the topography and, perhaps, by an instability of the flow. The analysis of the surface data as well as that of the soundings shows that the flow above the neutral layer affects the surface pressure distribution and, therefore, the acceleration of the extreme upvalley winds.