New paper: investigations on debris-covered glaciers

Glaciers covered by debris – rocks, dirt, silt, and sand – are common in the Himalayas. Depending on who’s counting (and where you are looking), debris covers nearly 25% of the total glacierized area in the region.  Experiments and previous studies have shown that really thin debris enhances melt, but that anything over 2 cm thick insulates the ice melt.  But what is the net effect of debris cover on glacier melt rates? Our recently published (open access) paper in the Cryosphere tries to answer this question.

 

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Khumbu Glacier (center) is debris covered. So is the bottom 2/3 of Changri Nup Glacier, located to the west. Everest is at the far right of this Landsat scene.

 

Unfortunately, the answer is not so easy to obtain. Traditional mass balance stake measurements are (a) difficult to install and maintain on debris-covered glaciers, and (b) impossibly biased towards locations where it is possible to drill. You could look at surface elevation changes over part of the glacier with either photogrammetry, UAV, or satellite (we use all three), but if you do this you also need to consider the emergence velocity (or increase in elevation) of the glacier as it flows downhill. On any given point in the ablation zone, the total surface elevation change is a function of both emergence and melt. And to estimate the mean emergence velocity, you need to measure the ice flux through a cross-section of the glacier.

 

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Rates of surface elevation change at Changri Nup Glacier for different periods and data sources: (A) 2011 – 2014 (photogrammetry); (B) 2011 – 2015 (photogrammetry and UAV); (C) 2009 – 2014 (satellite and photogrammetry)

 

Christian Vincent and Patrick Wagnon, French glaciologists from Laboratoire de Glaciologie et Geophysique (LGGE) and Institut de Recherche pour le Development (IRD), have collected multiple datasets over 4 years to estimate the mass gain and loss over the debris-covered Changri Nup Glacier. I’d remind you that debris-covered glaciers at 5400 m of elevation are not among the easiest places to work.

But together with a team of co-authors they have measured surface velocities and surface melt rates with ablation stakes; developed digital elevation models from photogrammetry in 2011 and 2014, from unmanned aerial vehicle surveys in 2015, and from high-resolution satellite data in 2009; measured ice depths with ground-penetrating radar, and mapped ground control points and elevation profiles with differential GPS.

 

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The lead author C. Vincent uses a differential GPS to measure a ground control point for UAV flights over the clean Changri Nup.

 

And the overall result: melt rates on the debris-covered glacier are about 60% less than what they would be if the glacier was free of debris. Ice cliffs and ponds enhanced melt locally, but not enough to offset the overall reduction in melt caused by the debris. The surface mass balance (in m of water equivalent, or m w.e.) over the debris-covered tongue, inferred from average surface lowering of -0.81 m w.e./yr and an average emergence velocity of +0.37 m w.e./yr, is -1.21 m w.e./yr. If the glacier were debris-free, we would expect to see an average mass balance rate of -3.00 m w.e./yr.

This field-based study provides strong evidence that the ‘debris-cover anomaly’ (where satellite data show that debris-covered glaciers appear to be lowering at the same rate as clean-ice glaciers) is an artifact. It also shows that, in this location at least, the effects of ponds and ice cliffs are minimal.

Why is this important? If debris-covered ice (low-angle and thick) occupies 25% of the total glacierized area, it probably contains an even greater percentage of the total ice volume. Better estimates of the net insulating effect of debris will help us improve simulations of future ice loss, and its impacts on water resources downstream.

 

This is a re-post of a recent blog by Joseph M. Shea.

Martian crater officially named Langtang

To honor Langtang, the village that was tragically destroyed by an avalanche triggered by the Nepal earthquake of April 2015, we proposed to name a Martian crater after Langtang. The idea was initiated by colleague Tjalling de Haas, who investigates debris flows and land-forms on Mars. The request was officially approved by the International Astronomical Union and Langtang will now forever be remembered, even on Mars. The crater has a diameter of 12 km and interesting enough contains glacial land-forms and the moraines of the Last Mars Glacial Maximum and the debris fans formed after the glaciers melted are clearly visible.

 

The Langtang Crater

The Langtang Crater

 

 

 

 

 

 

 

 

 

 

 

Debris flows in the Langtang Crater

Debris flows in the Langtang Crater

 

 

 

 

AWS on Ice

[Joe is a Senior Glacier Hydrologist at the International Centre for Integrated Mountain Development in Kathmandu, Nepal]

True fact: there have been not one but two workshops dedicated specifically to the installation of automatic weather stations (AWS) on glaciers.

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The newly-installed AWS at Yala Glacier. We didn’t get these views when we did the work. (Photo credit: Jitendra Bajracharya)

One of the biggest unknowns in how glaciers will respond to climate change are the meteorological conditions and melt rates at the glacier surface, and how these conditions relate to data from standard observation networks and/or climate reanalysis products. But setting up precise sensors on a surface that can move, melt, and be buried by snow – sometimes all of these in the same day – is a big challenge. Unfortunately, for all challenges (including drinking milk upside down through a straw) you either learn by experience (AKA “mistakes”), or you learn from the experiences of others. For some reason I’ve tended to go with the former.

Our recent AWS installation at Yala Glacier is another attempt to obtain a year-long record of meteorological conditions at 5350 m in the Himalayas. At this altitude, temperatures are rarely above zero and the melt of snow or ice is basically controlled by the radiation balance at the surface (see below for a more technical discussion). So our station will record radiation received and emitted or reflected by the surface, air temperature and relative humidity, wind speed and direction, and surface height changes from melt and snowfall.

Experience tells us that ‘floating’ weather stations, such as tripods that simply sit on the top of the surface, don’t work so well on glaciers. The surface melts down unevenly, the station can be buried and damaged by heavy snowfall, and there is no way to get a record of surface lowering: the surface height sensor needs to be mounted at a fixed height in order to get information that makes any sense.

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Our first attempt to measure conditions on Yala Glacier, found after typhoon HudHud in October 2014.

For the new station, we used a slick tower design that can be built up in the field (full credit to Alex Jarosch and Faron Anslow; tower recipe below or see P. 52-55 here). Essentially, we connect three 2.0 m aluminum pipes vertically to make a 6.0 m tall triangular structure. Horizontal supports brace the top 2.0 m of the tower, and the bottom 4.0 m of each leg is drilled in to the ice. If you’re going to try this at home, don’t forget to stick small plastic caps on the bottom of the pipes that go in the ice. Without these, the weight of the tower would be supported on a very small surface area and it would melt into the ice – probably due to heat conduction through the aluminum. If the tower sinks into the ice during the experiment, the surface height measurements are meaningless. (Thanks, experience!)

Once the base and the tower are installed and leveled, the waterproof enclosure (which contains the battery, solar charge controller, and the datalogger) and all sensors were mounted to the tower. In the time-lapse animation shown below, you can see the clouds rolling up and down over us as we mount the sensors. In response, we shed layers and then put them back on, because the thickness of the cloud layer really affects the ‘felt’ temperature at the surface (you should really read the technical explanation below). Air temperatures during the setup hovered around 0C.

 

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The tower from above: logger box and temperature/humidity sensor is on the left, wind sensor is top right, and it looks like three people are required to mount the net radiometer (which measures shortwave and longwave radiation – really, there is no longer an excuse to not read just a little bit more in depth below).

The full installation took only half a day, and we were back drinking tea in camp by mid-afternoon (though thankfully not upside down and with straws). But getting the equipment and the tower components up there literally took a small army. We have nothing but huge gratitude and respect for Dawa Sherpa and Ngawang Sherpa who helped haul everything up the glacier, and to all the trekking agency staff who carried everything up from the trailhead at 1600 m to the basecamp at 5000 m.

[Thanks to Maxime Litt and Desiree Treichler for their help in the field, but also for the pre-field testing and programming. This is a critical step in the recipe.]

 

Glacier Station Recipe

  • 9 x 2.0 m aluminum pipe (48.25 mm OD)
  • 9 x 0.50 m aluminum pipe (48.25 mm OD)
  • 3 internal pipe connectors
  • 3 external pipe connectors
  • 3 plastic cap ends (large corks also approved)
  • 18 x 90 degree joints (48.25 mm OD)
  • Ice auger (4-5 m)
  • Plumber’s level
  • Tools
  • AWS components and all mounting hardware (!)
  • Patience
  • Reasonably good weather
  • Preparation, preparation, preparation

Radiation Balance Details

The net radiation at the surface (Q*) can be calculated from incoming and outgoing shortwave and longwave radiation:

Q* = Sin – Sout + Lin – Lout

Shortwave radiation comes from the sun: its highest at solar noon, and zero at night. But the amount of radiation reaching the surface depends on clouds and the atmospheric conditions, and the amount of shortwave radiation absorbed at the surface depends on the reflectivity (or albedo) of the surface. Brighter surfaces reflect more radiation, and have a higher albedo, which means less energy available for melt.

Longwave radiation is a mainly function of temperature: incoming longwave radiation is emitted by the atmosphere, and the earth’s surface emits longwave radiation upwards. Temperatures near the surface will be warmer on cloudy nights because the clouds both (a) emit greater longwave radiation towards the surface than a clear sky and (b) trap some of the longwave radiation emitted by the ground. Incoming longwave radiation is also a function of water vapour in the atmosphere, which affects the temperature profile.

 

Contribution to EGU General Assembly 2016

This year’s EGU General Assembly has passed and we presented a number of topics in 4 different sessions.

In a session with numerous outstanding talks on Mountain Climates on Wednesday, Joseph Shea presented initial results from an analysis of glaciological and hydrological sensitivities in modeling in the Hindukush Himalaya region.

On Thursday, Walter Immerzeel opened the session on debris covered glaciers with a solicited talk, summarizing our recent efforts in quantifying mass changes on the debris covered glaciers. During the same row of talks Jakob Steiner looked at the spatial and temporal evolution of ice cliffs and lakes in the Langtang catchment in the recent decade.

The round of talks was followed by a poster session with 20 submissions specifically on the topic of debris covered glaciers, underlining the increased attention the issue has received recently. Philip Kraaijenbrink presented his work on the monitoring of glaciers using unmanned aerial vehicles. Pascal Buri presented some progress on the distributed modeling of ice cliff backwasting in the catchment. Evan Miles provided insight into the temporal change of supraglacial lakes and how to extract this information from Landsat imagery. He also presented recent work on deriving surface roughness oft he glacier with photogrammetric analysis. The contribution to this session was rounded off by Pascal Egli’s work on deriving debris thickness from remotely sensed temperature data. Jakob Steiner also presented some insights from analysis of the development of a large alluvial fan in the Langtang catchment in a session on sediment transport in pro-glacial environments.

The week ended with Tobias Bolch presenting Silvan Ragettli’s recent work on mass balance change in the Langtang catchment during the last decade.

 

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