Twice-daily Maxima of Summer Precipitation in High-altitude Areas of the Himalayas: Two Contrasting Land Surface Effects

Abstract:

Precipitation and its variability in a glacierized high-altitude area of the Himalayas in summer (June–September) is still not well known. Here, we investigated the diurnal cycle of precipitation in summer and its mechanism in the Nepal Himalayas using hourly in-situ observation, TRMM-PR and ERA5 reanalysis datasets. We used precipitation and other meteorological elements observed by an automatic weather station on a lateral moraine beside Trakarding Glacier at 4,806 m above sea level in Rolwaling valley, eastern Nepal Himalayas for the three summers from 2016 to 2018. The composite diurnal cycle of precipitation revealed the twice-daily maxima in daytime (1400 LT) and nighttime (2300 LT) with suppressed phase around 1900 LT. Precipitation in the daytime peak was ~ 0.3 mm hr^–1 and identical to that in the nighttime peak. The daytime peak was accompanied by the acceleration of upslope wind, whereas there was no significant signal in surface wind during the nighttime. Precipitation characteristics from a 17-year (1998–2014) TRMM-PR data showed that convective-type rain was dominant during daytime while the fraction of stratiform-type rain became higher in nighttime along the slopes of the Himalayas. Vertically integrated moisture flux toward the Himalayas was indeed enhanced twice a day in daytime and nighttime. In daytime, southerly moisture flux was strengthened only over and near the Himalayas due to thermally-driven upslope winds. In contrast, southerly onshore moisture flux toward the Himalayas was accelerated across the Gangetic plain during nighttime. Nocturnal low-level jet over the Gangetic plain was responsible for the enhancement of the southerly moisture flux during nighttime. The twice-daily enhancements of southerly moisture flux caused moisture flux convergence along the slopes twice a day, providing the twice-daily maxima of precipitation. We also discuss the altitudinal variation of the diurnal cycle in the Himalayas.