Wednesday, June 8, 2016

On the Formation of Sastrugi Across the Western Greenland Ice Sheet

While Thomas has been navigating our trail and repeatedly monitoring the GSSI console to record data, I, Tate, had the luxury of placing my skis in his track, which removes the fuss of steering the snow machine, and pondered the surface features of the snow.

My curiosity of Sastrugi germinated during the NASA snow working group I attended in January. During a lecture by Matthew Sturm he mentioned that their formation remains a mystery. I couldn't contain the thought, studiously raised my hand, then blurted, "Sastrugi form like migratory birds so as to minimize wind friction upon formation."

Sastrugi formations are a powerful treachery for the traversing snow machine. During the first attempt towing the radar sleds north from Raven Camp to Core Site 1, the strong Sastrugi had bent the angled steel FMCW Radar mount like a violin bow, battered the deep-cycle batteries like they were matched up at the MGM Grand for Fight Night, and ironically defeated the surface roughness laser rattling it from its perch into a deadly tumble. The GreenTrACS Team did bound off the ropes, wielding U-bolts and soldering irons in the coming rounds.

 A rare crosscut Sastruga with overhanging tip nosing towards the surface.
(Photo - Erich Osterberg)
During the lackadaisical afternoons of GPR traverses I examined a thousand km of boundless Sastrugi. I attended to the subtle and braided details of the surface. I have contrived a few principles which plausibly identify the origin and growth of the perplexing Sastrugi. May blogspot serve as the game warden to any publication poacher.

An individual Sastruga has a lifespan. There is a conception, a birth, a maturation, changes in its direction, an adulthood, and later yet a burial. It is romantic in a sense, though a jar of Nutella will appear romantic after some amount of time on the ice sheet.

I have only personal record to describe the Sastruga lifespan. But I hope to direct a study testing this hypothesis. Doctors, I may be crazed.

After the first bout with Sastrugi, I became keen to observe the spatial variation of the snow. I took note of the weather conditions, precipitation, wind events, cloud cover, and temperature. As the weeks passed by I gathered my intel and began a survey forecast for traversing the radar equipment. In the evenings after a survey with an eager jeer the team would inquire about the latest developments in my understanding of the vast Sastrugi. So I went on and told 'em.

Before Sastrugi are Sastrugi, there exists a temperature gradient and a gentle breeze to drive faceting of the snow surface. The feather-like facets have the tendency to cluster in the breeze. These clusters are the nucleation sites of the to-be Sastrugi.

When surface wind speed surpasses 10 knots the surficial snow is carried within a density current of near-laminar flow. It’s mesmerizing to watch the braided blowing of snow across the surface of the ice sheet. As a fragment of snow collides with a nucleation site, accretion of snow may occur. A snow fragment is guided left of, right of, above, and/or halted by the bunch of facets. These outcomes are dictated by the lateral and vertical incidence of the snow fragment, and the air pressure about the nucleus as a function of its size and the wind speed.

The continuous stream of flowing snow navigates the clustered facets. A baby Sastruga is born. This formation has a developed, golf-ball-size cluster and elongated, narrow ridge of snow at its tail.

The width of a linear current of blowing snow is between 10 and 30 centimeters, much wider than the forming young Sastruga. This allows for growth of the Sastruga and development of the canonical flying-V form.

With a constant wind velocity the V is broadened and a wind crust becomes pronounced. The many neighboring Sastrugi become enveloped into a larger dune formation. This feature is often braided on its surface, as the laminar flow weaves about the many neighbors.

If the direction of the wind shifts, the lesser compacted tail of the Sastrugi is scoured away creating a mogul formation within the duned Sastrugi. Rare crosscut Sastrugi form a drooping, leaf shaped cap (the tip of which nearly noses on the surface) after the lighter substrate has been scoured away.

Large dunes may form like boomerang of 5 square meters in area and 50-75 centimeters tall; I have not observed many features larger.

Across the Western Greenland Ice Sheet, several inches of snow will precipitate in events weekly to bi-weekly. The Sastrugi are laid to rest. Driving across such snow forms a rolling trail with a wavelength about that of the snow machine. "Fluff Kitten," Forrest calls it.

T'Ocho signing off.


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  2. Really informative article about the Formation of Sastrugi Across the Western Greenland Ice Sheet. It seems very interesting to see this ice-sheet. Thanks for sharing the article.

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