Stepwise onset of monsoon weather observed in the Nepal Himalaya

Mountain weather changes in the Nepal Himalaya were intensively examined during the 2003 monsoon onset using in situ datasets, observed by multiple automatic weather stations (AWS) distributed across the Coordinated Enhanced Observing Period (CEOP) Himalaya reference site at locations with significant differences in altitude. Onset of monsoon rainfall characterized as nighttime precipitation was initiated simultaneously at all stations with the occurrence and migration of a monsoon depression in the north of the Bay of Bengal. Satellite infrared images detected evident suppression of diurnal cloud development after the onset. About two weeks prior to the onset, the mountain weather abruptly changed such that the daytime valley wind at lower elevations weakened associated with morning clouds and the nighttime southerly wind began at upper elevations. The timing corresponded with the weakening of the westerly wind over the Himalaya due to the northward shift of the upper subtropical jet stream. The time lag between the precipitation onset and the change in the mountain weather was confirmed by 9 yr of in situ AWS data. The mechanism of nighttime southerly winds at high elevations is also discussed in relation to large-scale monsoon flow and local circulation.