Trends and spatial variations of rain-on-snow events over the high Mountain Asia

Rain-on-snow (ROS) events can greatly affect the snow process and cause severe snowmelt-related hazards. It is important to monitor the spatiotemporal distribution of ROS events over the ungauged High Mountain Asia (HMA). This study investigated the spatiotemporal variability of ROS events over the HMA and its potential influencing factors from 1981 to 2020 based on stand-alone Noah-MP land surface model simulations forced by hourly HARv2 reanalysis dataset. The results demonstrated that ROS activity occurred more frequently in the higher-elevation (2500-4000 m and 5500-6000 m a.s.l) regions of the Tianshan Mountains, Pamir, eastern Hindu Kush, Himalayas, and the western Hengduan Shan, with an annual maximum ROS frequency exceeding 15 days and a maximum intensity reaching 40 mm concentrated in spring and summer. ROS frequency experienced a significant decrease in the high-elevation (3000-4500 m a.s.l) regions of the eastern Hindu Kush, West Himalaya, and western Hengduan Shan with a rate exceeding 1.5 days/decade. The decrease in ROS frequency could be explained by a shifting of precipitation type from snowfall to rain driven by dramatic warming and resulting in a decline in snowfall and shortened snow cover persistence, particularly in spring and summer. On the contrary, significantly increasing trend mainly prevailed in the high-elevation (5000-6000 m a.s.l) regions of Transhimalaya and East Himalaya, exceeding 0.9 days/decade. The attribution of the increased ROS frequency was related to the significant increase in summer precipitation over the Inner Tibetan Plateau, along with an absolute increase in snowfall, which was beneficial to snow accumulation with frequent transition of the precipitation phase. The above outcomes improve our understanding of ROS events and highlight the importance of ROS in extreme snowmelt events and water resources management under a warming climate.