Glacier ablation and temperature indexed melt models in the Nepalese Himalaya

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 ( T a ) 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 ( SW net ) 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.

The Western Tibetan Vortex as an Emergent Feature of Near‐Surface Temperature Variations

“…Another cause of the glacier growth can thus be an increase in precipitation (e.g., Cannon et al, 2014;de Kok et al, 2018). The surface energy balance can change across glaciers (e.g., Azam et al, 2014;Bonekamp et al, 2019;Litt et al, 2019) and might not match that of the reanalysis data, which is tuned to match observed quantities at a coarser scale. It might be worthwhile to investigate whether direct correlations between wind fields and precipitation in a region exist in a similar way to the WTV, to gain more understanding of precipitation in northwestern HMA.…”

“…To understand individual glacier behavior, detailed surface energy balance studies will give more insight into glacier melt. The surface energy balance can change across glaciers (e.g., Azam et al, 2014;Bonekamp et al, 2019;Litt et al, 2019) and might not match that of the reanalysis data, which is tuned to match observed quantities at a coarser scale. However, large-scale trends likely influence all glaciers within a region such as WKSK to some extent.…”

“…The question then remains what drives the near-surface temperatures in northwestern HMA. These low correlations are common when the largest terms of the energy balance have similar magnitudes (e.g., Litt et al, 2019). We performed correlations between total net radiation and near-surface temperatures from ERA5 to see how strongly nearsurface temperatures are related to surface fluxes.…”

“…The turbulent fluxes from ERA5, and the sum of radiation and turbulent fluxes, also do not give high correlations in these periods for northwestern HMA. These low correlations are common when the largest terms of the energy balance have similar magnitudes (e.g., Litt et al, 2019). However, ERA5 data does suggest that in the ablation season of the glaciers, net radiation is the main driver of temperature fluctuations.…”

Measurements, models and drivers of incoming longwave radiation in the Himalaya

“…The energy balance over glacier and snow surfaces, which determines the melt, is generally dominated by the shortwave and longwave radiative fluxes (e.g., Azam et al ., ). During daytime, the net shortwave radiation is the major energy flux term, whereas in cloudy conditions above fresh snow, as well as during the night, net longwave radiation becomes the most important driver for melt (Müller, ; Granger and Gray, ; Bintanja and van den Broeke, ; Litt et al ., ). Clouds do not only decrease the shortwave radiation that reaches the surface but they also have the potential to emit more longwave radiation towards the surface (LW in ) than the gaseous atmosphere due to their larger optical thickness, giving them a higher effective emissivity.…”