Comment on egusphere-2022-336

Abstract

Local 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 5 day period in December 2014. Model evaluation against meteorological observations from three automatic weather stations in 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. The night-time along-valley winds are weak in magnitude and flow mostly in the up-valley direction. The diurnal cycle of the winds is interrupted more during the days with large-scale northerly winds than during the westerlies which is most likely due to channelling of the above-valley winds into the valley atmosphere. Differences in the daytime up-valley winds are found between the valleys and their parts. Since 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 steep valley floor inclination (2–5 degrees) are associated with weaker and shallower daytime up-valley winds compared to the parts which have nearly flat valley floors (<1 degrees). 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 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 when comparing to the open valley entrances. Strong and shallow winds, resembling down-slope winds, are found on the lee-side slope of the barrier followed by weaker and deeper winds at the valley entrance, 20 km towards the valley from the barrier.