Visualizing surface roughness

Here is another thing that came out of the InnSAR Summer School on Alpine Research on Surface-Atmosphere Exchange in Mountainous Terrain: we were looking for ways to easily visualise surface roughness, and this is what we (myself, Georg Kaser (who also took the photos in this post) and Rainer Diewald) came up with:

Glacier comb

I hope you agree that it gives a pretty decent idea of the surface micro topography, abstracted into a single cross profile of bright pink blobs.

However, you can see the use of knitting needles as the vertical rods poses some problems as they are tapered and therefore do not remain perfectly vertical in the machined holes. I guess this could be turned from a visualisation tool into a measurement tool by (a) making the metal cross bar a bit deeper to help guide the vertical rods and (b) using rods that are not tapered, so that they cannot deviate from the vertical, (c) using gradations on the vertical rod to make it easier to quantify the height they have been pushed up and (d) adding a spirit level to the cross beam.

In snow studies, micro topography has been obtained by taking photographs of a black board (with a scale bar) shoved vertically into the snow so that you see the sharp contrast of the surface along the cross section set by the board (e.g. Fassnacht et al., 2009). Obviously you can’t so easily shove this type of reference board into solid ice, but an improved tool based on this funny looking ‘glacier comb’ could do the trick instead. The use of close-range scaled topography is such an accessible tool, that it might catch on!

But why do we even care about the roughness of the surface? Well, its because the surface roughness is on of the things that affects the efficiency of mixing in the layer of air immediately overlying the surface and thereby it affects the efficiency with which heat and moisture can be exchanged between the atmosphere and surface by turbulent airflow. Thats a pretty important process if you are interested in the coupling between a surface and the overlying air.

I like our ‘glacier comb’, but although previous work has related roughness profiles such as this to turbulent exchange (e.g. Munro, 1989, 1990),  I am not yet convinced that we have established good relationships between geometric roughness (by which I mean physical measurements of the surface geometry) and its impacts on turbulent exchanges between the surface and the atmosphere. Part of the problem is that measurements of turbulent exchanges of heat and moisture are tricky to do well, so data against which to compare ideas is somewhat lacking.

Anyway, back to our jazzy measurement tool – look how nicely these students can highlight the small meltwater channel on the glacier here:

Meltwater dip

References:
Fassnacht, S. R., J. D. Stednick, J. S. Deems, and M. V. Corrao (2009), Metrics for assessing snow surface roughness from digital imagery, Water Resour. Res., 45, W00D31, doi:10.1029/2008WR006986.
Munro, D.S. 1989. Surface roughness and bulk heat transfer on a glacier: comparison with eddy correlation. J. Glaciol., 35(121), 343–348.
Munro, D.S. 1990. Comparison of melt energy computations and ablatometer measurements on melting ice and snow. Arct. Alp. Res., 22(2), 153–162.

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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