Explicit representation of liquid water retention over bare ice using the SURFEX/ISBA-Crocus model: implications for mass balance at Mera glacier (Nepal)

Abstract. In a warming climate, glaciers will experience increased liquid precipitation and melt, making it crucial to better understand and model the associated surface processes. This study presents a modeling approach developed to investigate the dynamic interaction between surface liquid water and bare ice using the SURFEX/ISBA-Crocus model. The implementation of the temporary retention of liquid water from rain or melt at the ice surface is described. The water is drained or can refreeze depending on meteorological conditions, directly affecting the albedo, thermal profile and glacier mass balance. This new development, tested to Mera Glacier (Nepal) shows an impact up to 6 % on the annual mass balance with contrasted effects depending on the meteorological conditions. During the pre-monsoon season, this implementation leads to greater mass loss (up to 20 %) due to surface liquid water, which enhances warming rather than compensating through refreezing. During the monsoon and post-monsoon seasons, it leads to less negative mass balance as a result of increased refreezing. Sensitivity analyses identified drainage and albedo as key model parameters. A 10 % change in stored liquid water drainage results in a 10 % change in annual mass balance. The albedo of bare ice and liquid water over ice represent the primary contributors to mass balance loss and the greatest uncertainties, making them priority targets for further investigation and improved characterization. This physically-based model development is essential for future climate projections worldwide, particularly given increasing melt, rainfall, and bare ice exposure under climate change.