A Physics-Informed Data Science Approach to Quantifying Rain-Snow Fraction Dynamics in the Central Himalayas

Sujan Bhattarai , Dhiraj Pradhananga, Bhola Nath Dhakal, Sunil Adhikary

Mountain studies worldwide have documented increases in rainfall fraction as an impact of climate change. Most mountain systems show an increasing trend in rainfall fraction due to shifting snow precipitation to rain. In Nepal, which occupies an 800 km-long belt of the Hindu Kush Himalaya, the precipitation phase trend is not well known. This study conducts a precipitation phase study in the Langtang region and investigates the response of snowfall and rainfall to recent climate variation and change. The study uses 40 years (1979-2018) of bias-corrected WFDEI climate reanalysis data and applies a physically based psychrometric energy-balance model to partition precipitation into rainfall and snowfall. The study identifies points of statistically significant changes in trends using the Exponential Weighted Average, Mann-Kendall test, Sen's slope estimator, and changepoint analysis. Changepoint detection was conducted using Pettitt's method. In addition, the study estimates transient temperature for rainfall-snowfall transition using logistic mapping. The results show that the rainfall fraction has been increasing for all timestamps (annual, seasonal, monthly) except winter nights, with changepoints occurring between 1990-2000. Post-monsoon months (October and November) showed the greatest annual increase in daytime rainfall fraction at 0.34% and 0.27%, respectively. Winter months exhibited the least changes, with no significant increase in rainfall, particularly at night. The transient temperature for rainfall-snowfall transition was identified at 1.47°C. Future research should extend this methodology across the broader Himalayan region to develop region-specific precipitation phase models, incorporate high-resolution climate projections to assess future changes in snow-rain ratios, and investigate the cascading impacts of changing precipitation phases on glacier mass balance, streamflow timing, and downstream water security.