Twice‐Daily Monsoon Precipitation Maxima in the Himalayas Driven by Land Surface Effects

Abstract

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 ∼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 ∼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.

Nocturnal Southerly Moist Surge Parallel to the Coastline Over the Western Bay of Bengal

“…Both the ERA5 and the TRMM-PR data were averaged during June-September for all years to produce summer mean diurnal (hourly) climatologies. The procedures adopted in this study were the same as in Fujinami et al (2021).…”

“…The maximum nocturnal low-level wind speed appears almost simultaneously parallel to the coastline over the western BoB (Figure S3 in Supporting Information S1). Nocturnal precipitation reaches a maximum over the southern slopes of the Meghalaya Plateau and the Himalayas during 23:00-02:00 LT because the nocturnal enhancement of the southerly moisture flux causes moisture flux convergence over their southern slopes at night (Fujinami et al, 2017(Fujinami et al, , 2021. In contrast, large moisture flux anomalies are absent in the eastern BoB.…”

“…Fujinami et al. (2021) revealed nocturnal enhancement of the vertically integrated southerly/southwesterly moisture flux extending from the eastern part of the Indian subcontinent to the western BoB, and blowing toward the foothills of the Meghalaya Plateau and the Himalayas (their Figure 12d). Generally, the nocturnal acceleration/enhancement is explained by the nocturnal jet process.…”

“…The low‐level southerly monsoon flow over the Indian subcontinent, including the Gangetic Plain, exhibits an obvious diurnal cycle (Chen, 2020; Fujinami et al., 2017, 2021; Sato, 2013; Sugimoto et al., 2021; Terao et al., 2006). The wind speed at 950–925 hPa is generally a minimum in the afternoon and reaches a maximum at night, which is in the opposite sense to a land–sea breeze circulation.…”

“…In South Asia, the monsoon westerlies pass over the Indian subcontinent and then cross the Bay of Bengal (BoB) where they gain latent heat from the surface waters (Fujinami et al., 2020) (Figure 1a). One branch of the monsoon westerlies turns northward over the BoB and then moves across the Gangetic Plain, transporting abundant moisture toward areas such as the Meghalaya Plateau and the Himalayas that are known for heavy rainfall (Fujinami et al., 2017, 2021). The sequence of low‐level monsoon flow is induced by the monsoon trough centered over the northeastern part of the Indian subcontinent.…”