Glacier ablation and temperature indexed melt models in the Nepalese Himalaya

Maxime Litt,Walter Immerzeel,Jakob Steiner,Emmy Stigter,Joseph Shea,Patrick Wagnon,Inka Koch

doi:10.1038/s41598-019-41657-5

Abstract

Temperature index (TI) models are convenient for modelling glacier ablation since they require only a few input variables and rely on simple empirical relations. The approach is generally assumed to be reliable at lower elevations (below 3500 m above sea level, a.s.l) where air temperature (Ta) relates well to the energy inputs driving melt. We question this approach in High Mountain Asia (HMA). We study in-situ meteorological drivers of glacial ablation at two sites in central Nepal, between 2013 and 2017, using data from six automatic weather stations (AWS). During the monsoon, surface melt dominates ablation processes at lower elevations (between 4950 and 5380 m a.s.l.). As net shortwave radiation (SWnet) is the main energy input at the glacier surface, albedo (α) and cloudiness play key roles while being highly variable in space and time. For these cases only, ablation can be calculated with a TI model, or with an Enhanced TI (ETI) model that includes a shortwave radiation (SW) 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 also contribute to mass loss, and remain unresolved with TI or ETI methods.

Highlights

In High Mountain Asia (HMA), present and future glacier down wasting and retreat will lead to changes in seasonal and spatial patterns of meltwater contributions to streamflow[1,2]
Meteorological controls on glacier ablation in High Mountain Asia (HMA) were studied using in-situ meteorological data collected at two glaciers in Nepal
A reliable estimate of surface α is essential, since surface energy balance (SEB) changes are controlled by the changes in net shortwave radiation

Summary

Introduction

In High Mountain Asia (HMA), present and future glacier down wasting and retreat will lead to changes in seasonal and spatial patterns of meltwater contributions to streamflow[1,2]. By implicitly assuming that surface melt is the dominant ablation term, hydrological and glaciological models often use temperature-index (TI) or enhanced temperature-index (ETI) approaches to quantify total ablation Such models conveniently rely only on near-surface air temperature, as well as a degree-day factor (TFTI) which depends on the state of the surface i.e., ice or snow, and assume that melt is a linear function of the sum of the difference between the daily mean air temperature Ta and an air temperature threshold (TTI)[4,5]. At high elevation glaciers, such as those found throughout High Mountain Asia (HMA), incoming shortwave radiation (SWinc) is the dominant energy input[7] and the SEB relates only partly to daily mean Ta, so numerous studies use ETI in place of TI.

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Conclusion