Twice-Daily Monsoon Precipitation Maxima In The Himalayas Driven By Land Surface Effects

The diurnal cycle of precipitation is important to the hydroclimate in the Himalayas in summer; however, features of the diurnal cycle that affect precipitation from the foothills to glacierized, high-elevation areas are poorly understood. We investigated the diurnal cycle of precipitation using 3 years of in situ observations recorded close to a glacier at 4,806 m asl, and 17 years of data from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and the Integrated Multi-satellitE Retrieval for Global Precipitation Measurement (IMERG). The mechanisms that drive the diurnal cycle were examined using hourly European Center for Medium-Range Weather Forecasts Re-Analysis fifth generation (ERA5) data. In situ observations showed that the diurnal precipitation cycle has daytime and nighttime peaks, which both consist of high rainfall frequency and low rainfall intensity. In addition, twice-daily maxima exist in the TRMM PR data, particularly over two rain bands at around 500-1,000 m asl, and at similar to 2,000 m asl. Convective-type rainfall, with a lower rain-top height, occurs in the daytime, whereas stratiform-type rain, with a higher rain-top height, occurs at night, particularly over terrain at elevations above similar to 1,500 m asl. Land surface processes likely cause the two peaks in the diurnal cycle. Daytime surface heating drives upslope flows that promote condensation. At night, surface cooling over the plain to the south of the Himalayas causes low-level monsoon flows to accelerate, creating a nocturnal jet, which results in large-scale moisture flux convergence over the southern slopes.Plain Language Summary Orographically induced precipitation and meltwater from glaciers in the Himalayas feeds into the headwaters of major rivers such as the Indus, the Ganges, and the Brahmaputra, and provides water resources for the large population of South Asia. Glaciers from the central to the eastern Himalayas are sustained by summer precipitation, and the diurnal cycle is an important part of the climate system there. However, the features of the diurnal cycle, which stretches from the foothills to the glacierized high-elevation area in the Nepal Himalayas, are poorly understood. In this study, we investigated the diurnal cycle of precipitation in summer using data from 3 years of in situ observations (recorded close to a glacier at 4,806 m asl) and 17 years of space-borne precipitation radar observations. We identified twice-daily precipitation maxima, corresponding to daytime and nighttime peaks, in the diurnal cycle across much of the Nepal Himalayas. The two maxima are the result of different land surface effects. The daytime peak results from upslope flows that are driven by surface heating of the slopes, whereas the nighttime peak is caused by the acceleration of low-level monsoon flows toward the Himalayas as the surface cools over the plain to the south of the Himalayas.Key PointsTwice-daily precipitation maxima, corresponding to day and night, occur widely across the Nepal Himalayas, including in the glacierized areaA double band of rainfall that has twice-daily maxima exists over terrain at 500-1,000 and similar to 2,000 m asl on the southern slopesUpslope flows due to surface heating drive a daytime rainfall peak, and a monsoon nocturnal low-level jet likely creates a nighttime peak