Spatio-Temporal Analysis of Valley Wind Systems in the Complex Mountain Topography of the Rolwaling Himal, Nepal

The diurnal, seasonal, and spatio-temporal characteristics of local wind systems in a steep mountain valley in Nepal are analyzed with the identification of valley wind days (VWDs). Distributed across the Rolwaling Himal valley in Nepal between 3700 and 5100 m a.s.l. at eight automated weather stations (AWSs), meteorological data between October 2017 and September 2018 were examined. VWDs were classified by means of ERA5 reanalysis data and in situ observations, employing established thresholds using precipitation, solar radiation, air pressure, and wind speed data at different pressure levels. Thus, overlying synoptic influences are highly reduced and distinctive diurnal patterns emerge. A strong seasonal component in near-surface wind speed and wind direction patterns was detected. Further analyses showed the diurnal characteristics of slow (approximately 0.5-0.9 m s(-1)), but gradually increasing wind speeds over the night, transitional periods in the morning and evening, and the highest averaged wind speeds of approximately 4.3 m s(-1) around noon during the VWDs. Wind directions followed a 180 degrees shift with nocturnal katabatic mountain winds and inflowing anabatic valley winds during the daytime. With AWSs at opposing hillsides, slope winds were clearly identifiable and thermally driven spatio-temporal variations throughout the valley were revealed. Consequently, varying temporal shifts in wind speed and direction along the valley bottom can be extracted. In general, the data follow the well-known schematic of diurnal mountain-valley wind systems, but emphasize the influence of monsoonal seasonality and the surrounding complex mountain topography as decisive factors.