Daytime along-valley winds in the Himalayas as simulated by the Weather Research and Forecasting (WRF) model

Abstract

Abstract. Local along-valley winds in four major valleys on the southern slope of the Nepal Himalayas are studied by means of high-resolution meteorological modelling. The Weather Research and Forecasting model is run with a 1 km horizontal grid spacing covering a 4 d period in December 2014. Model evaluation against meteorological observations from three automatic weather stations in the Khumbu Valley (one of the four valleys) shows a good agreement between the modelled and observed daily cycle of the near-surface wind speed and direction. Well-defined daytime up-valley winds are found in all of the four valleys during this 4 d period. The night-time along-valley winds are weak in magnitude and flow mostly in the up-valley direction. Differences in the daytime up-valley winds are found between the valleys and their parts. As the valleys are under similar large-scale forcing, the differences are assumed to be due to differences in the valley topographies. Parts of the valleys with a steep valley floor inclination (2–5∘) are associated with weaker and shallower daytime up-valley winds compared with the parts that have nearly flat valley floors (<1∘). In the four valleys, the ridge heights also increase along the valley, meaning that the valley floor inclination does not necessarily lead to a reduction in the volume of the valley atmosphere. This way, the dominant driving mechanism of the along-valley winds, within the valleys, could shift from the valley volume effect to buoyant forcing due to the inclination. Two of the valleys have a 1 km high barrier in their entrances between the valley and the plain. Winds at the valley entrances of these two valleys are weaker compared with the open valley entrances. Strong and shallow winds, resembling down-slope winds, are found on the leeward slope of the barrier followed by weaker and deeper winds at the valley entrance, 20 km towards the valley from the barrier. Although the large-scale flow during the 4 d period was similar to the long-term climatology, the impact of different large-scale flows on the thermally driven winds was not considered. This topic could be addressed in the future by performing a longer simulation.