Diurnal Winds in the Himalayan Kali Gandaki Valley. Part II: Modeling

Abstract

The Penn State–NCAR mesoscale model MM5 is used to simulate and better understand the wind observations in the Kali Gandaki Valley reported in the first part of this paper. The Kali Gandaki River originates in Nepal near Tibet, flows southward through the Mustang Basin, crosses the Himalayas in a gorge, and descends to the lowlands of Nepal. Extremely strong diurnal upvalley flow in the gorge and the basin alternates with rather weak drainage flow in the night. As proposed in Part I, the Mustang Basin and the Tibetan Plateau can be considered as an elevated heat source driving the upvalley flow during the day. However, the extreme strength of the diurnal upvalley winds and the order-of-magnitude asymmetry between day and night cannot be explained with a simple plateau circulation theory. The model is successful in simulating almost all aspects of the observations. The simulations strongly suggest that the observed acceleration of the upvalley winds near the entrance to the Mustang Basin is linked to a supercritical-like flow pattern. Gravity waves induced by the ridges protruding into the valley appear to contribute to this flow structure. Humidity is found to be essential for simulating the strength of the observed day–night asymmetry because of its impact on the boundary layer structure above the Himalayan foothills, especially due to the evaporation of rain. In addition, advection of relatively stable air from the foreland into the basin is important for the formation of the gravity waves and also explains part of the asymmetry. The Plateau of Tibet appears to have a small but positive impact on the flow speeds in the valley.