Turbulence measurements over debris-covered ice

I’m writing up a paper on some eddy covariance measurements I made a few years ago. It took a long time to get to the point of writing this study up, because initially I was a bit disappointed with the dataset as we suffered some instrumental failures that meant our intended 10 day study was reduced to 3 days due to a faulty solar power regulator. I allowed myself to be discouraged by this – which was silly as its still a great dataset!

The motivation for the study goes like this:

  • Turbulence drives atmosphere-surface exchanges of energy, momentum and mass is important for glacier surface ablation (mass loss).
  • Continued climate change is expected to cause increasing (1) importance of turbulent energy exchanges over most mountain glacier environments; (2) debris cover over the remaining mountain glaciers.
  • This is significant because turbulent theory does not apply well to glacier surfaces , which violate most of the required conditions for which turbulent theory was developed.
  • Commonly used ‘bulk’ approaches to treating turbulence have been shown to perform badly on both clean ice (Radic et al., 2018) and debris covered glaciers (Steiner et al., 2018).
  • So, there is a need to better understand some fundamental properties of turbulence over glaciers and in particular to understand the contrast between clean and debris covered ice.

We set us two sets of instruments – one over exposed glacier ice and over the debris covered ice of my main study site, Suldenferner. 

The eddy covariance instruments on the debris covered part of Suldenferner in August 2015.
The eddy covariance instruments on the bare ice part of Suldenferner in August 2015.

The semi-negative findings first:

  • The data we collected shows that single level instrumentation as shown in the photos is a bit limiting, if I had more money and instruments I’d always instal sensors at at least 2 heights above the glacier surface, but we scientists tend to have limited budgets.
  • Krypton hygrometers (measure high speed humidity) really are delicate beasts, they stop working if there is dew, dust, rain, cloud …. which all, ahem, tend to occur on glaciers.
  • Never buy cheap solar panel regulators to use with high end instrumentations – its much more disappointing when an effortful and expensive field campaign goes awry due to failure of simple components.

More interesting findings:

  • The structure of the eddies over the two types of glacier surfaces are similar except for during sunny days when buoyant convection takes off over the debris covered ice.
  • In midsummer conditions over this glacier
    • heat is almost always being delivered from the warm summer atmosphere to the colder glacier surface, except over the debris cover during clear sky daytime conditions when the debris cover surface temperature can exceed the air temperature
    • similarly, deposition of moisture onto the glacier predominates, moisture transfer is essentially only from the glacier to the atmosphere over debris cover during clear sky conditions
  • The glacier katabatic wind dominates the microclimate even though it is a very small glacier, and it gets deeper downglacier over the debris covered ablation zone.
  • The katabatic winds are degraded over the debris covered ablation zone during sunny days, and are also broken up by stronger valley scale circulation.

I’m lucky that I have a great team of scientists around me that support the fieldwork and are expert in boundary layer atmospheric studies – we will continue this work and hopefully gain new insights into turbulent processes and how they affect glaciers.

References:

Radic, V., Menounos, B., Shea, J., Fitzpatrick, N., Tessema, M. A., and Déry, S. J. (2017). Evaluation of different methods to model near-surface turbulent fluxes for a mountain glacier in the Cariboo Mountains, BC, Canada. Cryosphere 11, 2897–2918. doi: 10.5194/tc-11-2897-2017

Steiner JF, Litt M, Stigter EE, Shea J, Bierkens MFP and Immerzeel WW (2018) The Importance of Turbulent Fluxes in the Surface Energy Balance of a Debris-Covered Glacier in the Himalayas. Front. Earth Sci. 6:144. doi: 10.3389/feart.2018.00144

About lindsey

Environmental scientist. I am glaciologist specialising in glacier-climate interactions to better understand the climate system. The point of this is to understand how glaciated envionments might change in the future - how the glaciers will respond and what the impact on associated water resources and hazard potential will be.
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