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17 Apr 2019 | Cryosphere

HKH Science News: Conventional models for glacier melt calculation may not work in High Mountain Asia environments

A recent research undertaken by ICIMOD and partners in central Nepal between 2013 and 2017 provides a guideline for ablation modelling in High Mountain Asia (HMA) environments. Maxime Litt, lead author of the study, said, “We show that the conventional models do not consider a number of important drivers of glacier mass loss at high altitudes and such approaches have to be handled with care.”

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An automatic weather station on Mera Glacier, one of two ICIMOD research sites in Nepal. Researchers used data from six automatic weather stations installed on the two glaciers. (Photo: Emmy Stigter/Utrecht University).

The conventional approach of using temperature index models for modelling glacier ablation requires few input variables and relies on simple empirical relations. The approach is assumed to be reliable at lower elevations below 3,500 metres above sea level (masl), where the air temperature relates well to the energy inputs driving glacier melt.

At the high-elevation glaciers in the HMA, the scientists involved in the research observed that incoming shortwave radiation is the dominant energy input and a full surface energy balance model relates only partly to daily mean air temperature.

During monsoon in HMA environments, surface melt dominates ablation processes at lower elevations between 4,950 and 5,380 masl. As net shortwave radiation is the main energy input at the glacier surface, albedo and cloudiness play key roles while being highly variable in space and time. For these cases only, ablation can be calculated with a temperature index model or an enhanced temperature index model that includes a shortwave radiation scheme and site-specific ablation factors. In the ablation zone during other seasons, and during all seasons in the accumulation zone, sublimation and other wind-driven ablation processes are important for mass loss and remain unresolved through the use of temperature index or enhanced temperature index methods.

The research article concludes that empirical models using only one set of parameters for modelling the observed ablation at different sites and periods demonstrate limited performance. The lack of consistency in temperature index or enhanced temperature index parameters between sites and periods is similarly problematic. Furthermore, ablation modeled with a surface energy balance model can diverge from the observations, but since sublimation is important, a suitable value for surface roughness can solve the issue, acting as a tuning parameter.

For details, please see: https://www.nature.com/articles/s41598-019-41657-5

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