Meteorology, debris cover and hydrology of Himalayan glaciers

The 'Third Pole' refers to the Hindu Kush-Himalaya, which spans around 4.3 M km² across Afghanistan, Bangladesh, Bhutan, China, India, Myanmar, Nepal, and Pakistan. It contains more snow and ice than anywhere outside the polar regions, is home to the world's highest mountains, and is the source of ten major river systems. The snow, ice and rivers provide water for 210 million people, and sustain indirectly around 1.3 billion people - one fifth of the worlds' population - living in downstream basins.

Climate change is a major concern, with recent warming significantly higher than the global average. It is important, therefore, to understand the controls on weather and climate of the region, how this is affecting the region's glaciers, many of which are covered in debris in the lower sections, and how this determines the flow and supply of water in streams downstream. We are coordinating three interlinked projects to address these issues.

Meteorology

Understanding the controls on weather and climate of the region is severely hampered by a lack of meteorological observations and weather and climate modelling. To address this knowledge gap, we have embarked on a three-year measurement and modelling project to improve understanding of basic meteorological processes operating in the region. Our focus is on the Khumbu Valley, Nepal; one of the steepest valleys in the region, and containing one of the region's largest glaciers.

We are performing a series of modelling experiments using the high resolution (1 km) Weather Research and Forecasting Model (WRF) to investigate the effects of topography on radiation fluxes, air temperatures, wind patterns, humidity, cloud build-up and precipitation. Satellite and ground-based data are being used to test the model. The model will be used to better understand how snowfall, snow and ice melt, and rainfall affect the river regimes of the region.

Debris-cover

Compared to the relatively debris free glaciers in most parts of the world, many glacier tongues in the Hindu Kush-Himalaya are covered with rubble as a result of steep slopes, and regular rockfalls, landslides and snow avalanches occurring near their headwalls and around their margins. Comparatively little is known about the role of debris on the energy and mass balance of such glaciers. Debris thickness is a key control on ice-melt rate because it modulates energy transfer. It is highly variable at the glacier-scale and between glaciers. As a result, it has implications for water supply, glacier evolution, and Glacier Lake Outburst Flood (GLOF) risk.

Several sub-debris melt models exist, and replicate sub-debris melting with reasonable success. Likewise, models exist for the hydrological response of glacierised catchments to changing climate, the evolution of debris-covered glaciers, and the development of supraglacial lakes. However, there is a lack of debris thickness data, at every scale, because taking measurements is often difficult. This means the likely future impacts of debris thickness on melting, for large geographical areas, cannot currently be quantified.

We have recently embarked, therefore, on a project to investigate the potential of using ground penetrating radar (GPR) to map sediment thickness across debris-covered glaciers. We collected GPR, meteorological and thermal camera data during field season on Lirung Glacier, Langtang, Nepal in 2015 and Ngozumpa Glacier, Khumbu, Nepal in 2016. A key finding is that a radar frequency of 900MHz is ideal for detecting debris thickness, which varies from a few centimetres to over several metres and is often related to surface slope and curvature. The data collected will be supplemented by satellite thermal data and an energy balance modelling methodology to map the debris thickness of Himalayan glaciers to an accuracy that has so far not been possible.

Hydrology

In addition to the role of debris, comparatively little is known about the effects of the associated surface ponds, lakes and adjacent near-vertical bare ice cliffs on the energy and mass balance, and the hydrology of glaciers in the Himalaya. To this end, we have been working on Lirung Glacier, Nepal for the last four years to monitor important meteorological and hydrological processes occurring on, within and beneath the debris; within ponds and lakes; and against ice cliffs. Our work has been aided by the collection of very high resolution digital elevation models (DEMs) of the glacier surface using photographs taken from an unmanned aerial vehicle (UAV) or drone. The data are being used to develop numerical models of ice melt associated with specific debris covers, ponds and cliffs, which can be scaled up to model the mass balance of the entire glacier tongue. Results suggest that debris typically retards glacier melt, but the cliffs and ponds are associated with high melt rates. Furthermore, the ponds absorb energy from the atmosphere and deliver relatively "warm" water into the glacier's interior where it melts the glacier ice internally, contributing to surface collapse, and the maintenance of the hummocky surface and further pond and cliff development in a positive feedback mechanism.

Publications

Papers stemming from this project so far:

• Boxall, K., Willis, I., Giese, A. and Liu, Q., 2021. Quantifying Patterns of Supraglacial Debris Thickness and Their Glaciological Controls in High Mountain Asia. Frontiers in Earth Science, v. 9, p.657440-. doi:10.3389/feart.2021.657440.
• Potter, E.R., Orr, A., Willis, I.C., Bannister, D. and Wagnon, P., 2020. Meteorological impacts of a novel debris‐covered glacier category in a regional climate model across a Himalayan catchment. Atmospheric Science Letters, doi:10.1002/asl.1018.
• Miles, E.S., Willis, I.C., Buri, P., Steiner, J.F., Arnold, N.S. and Pellicciotti, F., 2018. Surface pond energy absorption across four Himalayan glaciers accounts for 1/8 of total catchment ice loss. Geophysical Research Letters, doi:10.1029/2018GL079678.
• Nicholson, L., McCarthy, M., Pritchard, H. and Willis, I.C., 2018. Supraglacial debris thickness variability: impact on ablation and relation to terrain properties. The Cryosphere, doi:10.5194/tc-12-3719-2018.
• Potter, E., Orr, A., Willis, I.C., Bannister, D. and Salerno, F., 2018. Dynamical Drivers of the Local Wind Regime in a Himalayan Valley. Journal of Geophysical Research, doi:10.1029/2018JD029427.
• Chudley, T.R. and Willis, I.C., 2018. Glacier surges in the north-west West Kunlun Shan inferred from 1972–2017 Landsat Imagery. Journal of Glaciology, doi:10.1017/jog.2018.94.
• McCarthy, M., Pritchard, H., Willis, I. and King, E. 2017. Measurements of debris thickness on Lirung Glacier, Nepal, using ground-penetrating radar. Journal of Glaciology. doi:10.1017/jog.2017.18.
• Miles, E.S., Steiner, J., Willis, I., Buri, P., Immerzeel, W.W., Chesnokova, A. and Pellicciotti, F., 2017. Pond dynamics and supraglacial-englacial connectivity on debris-covered Lirung Glacier, Nepal. Frontiers in Earth Science, v. 5, doi:10.3389/feart.2017.00069.
• Chudley, T.R., Miles, E.S. and Willis, I.C., 2017. Glacier characteristics and retreat between 1991 and 2014 in the Ladakh Range, Jammu and Kashmir. Remote Sensing Letters, v. 8, p.518-527. doi:10.1080/2150704X.2017.1295480.
• Miles, E.S., Pellicciotti, F., Willis, I.C., Steiner, J.F., Buri, P. and Arnold, N.S., 2016. Refined energy-balance modelling of a supraglacial pond, Langtang Khola, Nepal. Annals of Glaciology, v. 57, p.29-40. doi:10.3189/2016AoG71A421.
• Miles, E.S., Willis, I.C., Arnold, N.S., Steiner, J. and Pellicciotti, F., 2016. Spatial, seasonal and interannual variability of supraglacial ponds in the Langtang Valley of Nepal, 1999-2013. Journal of Glaciology, p.1-18. doi:10.1017/jog.2016.120.

Debris cover, ice cliff and surface pond on Lirung Glacier, Nepal. The mix of debris covered ice and clean ice cliffs and the role of water make the energy balance and melt processes on these glaciers exceedingly difficult to monitor and model. Photo: Evan Miles.

Camping beside Lirung Glacier, Nepal. Photo: Ian Willis.

Using ground penetrating radar to determine debris thickness on Lirung Glacier, Nepal. Photo: Ian Willis.

Successful examples of radar profiles taken along different transects on Lirung Glacier using different radar frequencies for different debris thicknesses. It was possible to pick the boundary at the debris - ice interface in all cases showing debris varies between ~0.5 and 3.5 m thick. From McCarthy et al, 2017.