[A-AS08] Multiple scale structure and their interactions in Asian monsoon system

[AAS08-11] A heavy rainfall event in the eastern Nepal Himalayas caused by multi-scale processes

*Hatsuki Fujinami¹, Hidetaka Hirata³, Koji Fujita², Hironari Kanamori¹, Yota Sato², Masaya Kato¹, Rijan B. Kayastha⁴ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.Graduate School of Environmental Studies, Nagoya University, 3.Faculty of Data Science, Rissho University, 4.School of Science, Kathmandu University)

Keywords:Asian monsoon, the Himalayas, heavy precipitation

In order to better understand summer precipitation in the high-altitude area and mechanisms of its variability, we are conducting international collaborative research on precipitation in the Himalayas (HiPRECS: Himalaya precipitation study). We set up a rain-gauge network along Rolwaling valley, eastern Nepal Himalayas, leading to the Trakarding-Trambau glacier system in the middle of May in 2019. Six rain gauge stations are located at different altitudes, covering from ~1,000 m to ~4,200 m above sea level. We successfully observed precipitation at all the stations from the middle of May to early October in 2019. At Dongang station (~2,800 m above sea level.), daily precipitation reached 112.4 mm on 8 July, which was heaviest daily rainfall in all the six rain-gauge stations in 2019 summer (June–September). We examined the multiscale process causing the heavy rainfall event. A monsoon low developed close to Nepal from 7 July to 10 July. Precipitation systems of the low and southerlies in the eastern side of the low provided a favorable environment for heavy rainfall around Rolwaling valley. The event was mainly caused by intense precipitation during 1900–2300 LT on 8 July. A meso-scale precipitation system passed over Dongang in the period. Southeasterlies in the eastern side of the low and surface heat flux from the Himalayan slope generated the meso-scale precipitation system. Before the development of the monsoon low, northward propagating boreal summer intraseasonal oscillation (BSISO) and westward propagating quasi-biweekly intraseasonal oscillation (QBW) enhanced a low-level cyclonic shear zone around the Gangetic plain south of the Himalayas, which provided a suitable environment for the development of the monsoon low around the regions. Thus, the heavy rainfall event in the eastern Nepal Himalayas came from a series of multi-scale processes associated with intraseasonal oscillations, low-pressure systems, diurnal cycle and topography.