Rain-on-snow events over High Mountain Asia

With the ongoing global warming, rain-on-snow (ROS) events are becoming more frequent in mountainous regions around the world such as High Mountain Asia (HMA) with dramatic consequences on downstream hydrology. HMA region spans over 9 countries (e.g., China, Myanmar, Bhutan, Nepal, Bangladesh, India, Pakistan, Afghanistan, and Kyrgyzstan) and is home to billions of people who rely on its water tower. HMA is experiencing unprecedented warming at a rate twice higher than the global average. Consequently, HMA's water towers, a lifeline to many people and the largest reservoirs of freshwater outside the polar zone, are subject to ROS events which modulate snowmelt and runoff and are key contributors in influencing water availability and hazards such as floods and landslides. Therefore, quantifying changes in ROS are essential for water management and climate change mitigation strategies in the region. Because of HMA's hostile and harsh environments which are not easily accessible, ground measurements and reliable datasets necessary to study ROS events such as the spatiotemporal variations of precipitation and snow as well as other hydrological variables are difficult to obtain. To overcome this issue, in this study, we blended precipitation products coming from the best available datasets and used a snow water equivalent (SWE) reconstruction. We assimilated the SWE reconstruction into the land surface model Noah-Multiparameterization (Noah-MP) to study ROS events over HMA in the past two decades. Our study shows that because of the significant physical heterogeneity and atmospheric dynamics of HMA, ROS characteristics and their trends are region dependent. ROS events occur predominantly over the Indus, the Ganges-Brahmaputra, and the northwestern basins (Amu Darya, Syr Darya, and Ili basins). In the Indus basin, ROS, which represent ~5% of the annual precipitation and ~20% of the annual snowmelt in snow-covered areas, have an increasing trend. The increasing ROS in the Indus decreases the summer snowpack despite an increasing snowfall; therefore, ROS changes the snow dynamics of the region, with concerns of reduced water availability and increased groundwater exploitation for agricultural purposes. In the Ganges-Brahmaputra basin, ROS represent ~11% of the annual precipitation and ~60% of the annual snowmelt in snow-covered areas and are characterized by decreasing ROS trends. Because of their high occurrence and contribution to snowmelt, the decreasing ROS trends in the Ganges-Brahmaputra basins will have consequences of decreased recharge from the headwaters and exacerbated use of groundwater unless increasing trends of rainfall compensate for the decreasing snowmelt. In the northwestern basins, ROS have bidirectional trends due to elevation patterns and trends in rainfall, and they constitute ~5 to ~10% of the annual precipitation. Because of their ROS bidirectional trends, both the Indus basin and the Ganges-Brahmaputra basin ROS events consequences could be locally observed in the northwestern basins.