Diurnal winds in the Himalayan Kali Gandaki Valley. Part I: Observations

The diurnal wind system of the Kali Gandaki Valley in Nepal was explored in September and October 1998 in a held campaign using pilot balloons as the main observational tool. This valley connects the Plateau of Tibet with the Indian plains. The river crosses the Himalayas forming the deepest valley on Earth. intense upvalley winds blow up this valley during the day. Observ ations were made along the river at various spots selected between the exit point from the Himalayas and the source close to the Plateau of Tibet. The strongest upvalley winds were found between Marpha and Chuksang with typic al speeds of 15-20 m s(-1). The upvalley wind sets in first at the ground b ut an upvalley wind layer of 1000-2000-m depth forms quickly after the onse t. This deep inflow layer persists up to Lo Manthang, a town located a few kilometers south of the Plateau. of Tibet. Deceleration in the late afterno on and evening also appears to commence near the ground. Weak drainage flow forms late in the night. The causes of these phenomena are discussed.