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.

On the Nomenclature of T8: The Birth of a NickName

Early in every trip, nearly immediately, slang names of Tate are created. This follows me however, and where-ever. Perhaps in egocentricity, I receive a particular amount of Nick-Naming. Spud, Tater, TaterSalad, DarthTater, T8. I wonder how the man named Nick reckons with Nick-Naming.

Many of these names I receive often, or always - Tater-Tot. I like all forms of my name in a saying. A particular name re-birthed on this trip. T'Ocho.

Hugo, my Ecuadorian office mate in Boise dashed the Spanish flavor upon the coder spelling T8, after reading the revolving font of my screen saver, "Windows T8". "Ahhh, T-Eight - Like T-Ocho..."

T'Ocho and Forrest share a tender moment. (photo by Erich Osterberg)

On an isolated landmass -- which speaks little Spanish -- I striped my bomber 2-liter Thermos early in the trip with duct-tape so the crew would recognize this is T8's Thermos, as we are all issued the same stainless flask by CPS. T8 is simple to write with duct tape.

I had stepped out of the kitchen tent onto an evening ice sheet. A moment later Thomas called over to me, "T'Ocho, how about we back up some data?" I was stupefied. Humorous to me, how similar people are, and how simply Thomas must have read "T8" on my Thermos, and thought, "T-Eight, like T-Ocho."

Erich spun a derivative off T'Ocho, Calling me The-Ocho. When "The" is presented before a word beginning with a vowel, the long E sound is pronounced. Though Erich would state The with both long and short pronunciations. He would to say Hey'Ocho, similar to the Spanish T'Ocho. Likely stemming from the indiscernible name callings of his Mother yelling for either him or his brother. "Did she say Hey Rick! Or Erich?"

#thisisforthehomies
#daveocho

T'Ocho out.

Some Summit Work - Done

Erich here checking in from Summit. I have to admit that I got a little choked up when we finally arrived at Summit after midnight monday morning. Pushing the "stop" button on the snow machine that last time was a long time coming - all the months/years of planning, the 5 weeks of work here on the ice, and then the 16 hours of snowmobiling on Sunday to actually make it to Summit. What a ride.

Our approach to Summit was not easy - avoiding the clean air sector and the
ICESat line south and west of the station. The diagonal gray line is the Summit Station ice runway.

It turns out that the most difficult part of our whole 1200+ km traverse was the last 10 km approaching Summit. The "clean air" sector for atmospheric measurements at Summit is towards the south (since the winds come from the south most of the time) - unfortunately, we were coming from the south too.  Snowmobiles are not allowed in the clean air sector because they're not clean. So we had to detour well west of the Station, then continue 10 km north of the Station, and eventually approach from the northwest. It was tricky because there is an "ICESat Line" also west of Summit where they validate satellite measurements of Greenland's surface elevation - and we had to avoid that area too. Everything is marked with flags - the challenge came from differentiating the "come this way!" flags from the "stay away!" flags, which were often the same color. It took Summit Camp Manager Kaija talking us in on the satellite phone like an air traffic controller - her watching our headlights from the Big House - before we were able to find our way to the station without compromising someone else's science project. You can see our tortuous path to Summit on the "where we are" breadcrumb tracker page, and in the picture above.

T'Ocho triumphantly hoists his cot after successfully collapsing it - for
the first time since April 29th. 

Today we completed the last of our major tasks here at Summit before Thursday's flight to Kanger. First, we broke down all of our gear and science sleds to sort the gear into piles - stuff going back to Dartmouth, stuff going to Boise, stuff going to Kanger, and stuff staying here. We've become awfully (too) familiar with this gear over past 5 weeks.

Before: Our sleds all loaded after arriving at Summit. Tent city is in the background on the left.

After: T'Ocho dancing on an Air Force pallet after we finished sorting and re-packing the gear.

Thomas completed his last kite survey to get a high-resolution Digital Elevation Model (DEM) of Summit Station. PI Bob Hawley was here last summer and conducted a similar survey with his drone. So Thomas will be able to compare his DEM to Bob's to see how much the glacier surface has changed around camp from drifting around the buildings.

Erich with the famed GISP2 Ice Core borehole.

I also completed our last science snowpit - number 22 of the expedition. It was a very cool experience for me. First, I got to visit the GISP2 ice core borehole, which is kind of like a baseball fan going to Fenway or Wrigley for the first time. GISP2 was one of the most important climate records ever collected - it fundamentally changed the way we understood the climate of the last ice age. My PhD advisor Dr. Paul Andrew Mayewski (UMaine) was the lead PI on that project in the early 90's, and so it was a special experience for this ice core scientist. Then, completing a snowpit here at summit - a place from where so many of my samples have come over the years (including an ice core named after my first born!) - was very satisfying. All in all, my first visit to Summit has been awesome.

The last of 22 snowpits - this time at the top of Greenland.

Tomorrow (wed) we will finish labeling our gear for the shipment home, and tomorrow night we'll give a short presentation to the Summit staff and other scientists about our project, and premier Forrest's video of our expedition. We'll be sure to post the video to the blog here once it's in polished form. Gabe hopes to be back in the air wednesday recovering the caches at the Core 3 and Core 5 sites after being grounded today due to weather. We're looking forward to reuniting with him Thursday night in Kanger.

One last note - posted on the bulletin board here at Summit are two photos of Bob from the FIRST Summit Station winter-over crew in 1997-1998. Bob is in the bottom left of both photos. Legend.

Our very own Bob Hawley (bottom left in both) - a legend here at Summit.

Made it to Summit!!

After a 16 hour day of snowmobiling 400 km from core 7, we finally made it to Summit at about 1 am last night. It was incredibly surreal first seeing something on the horizon besides snow and clouds, a man-made building standing well above the snow as a beacon of hope for some warmth and relaxation. The whole day was spent alternating between using our thumbs and palms on the throttle, standing vs sitting on the machines, listening to books on tape or podcasts, and trying to stay awake as we stared at more snow and sastrugi than one could possibly imagine. Once we got to Summit, we happily clambered into the Big House and ate some delicious leftovers, guzzled a bunch of water (without having to melt that snow first), relaxed in the warm comfortable room, and finally clambered into bed, exhausted, at 3 am.

Before: April 28 (photo - Forrest McCarthy)

After: June 5 (photo - Forrest McCarthy)

After waking up this morning we relaxed in the Summit buildings, ate some fresh fruit and drank real, non-powdered, milk, and enjoyed not having to huddle on a tote box when eating as the wind howls against the tent. Most of the crew showered and put on clean clothes that we had shipped up to Summit from Kanger, nearly 5 weeks ago. The atmosphere inside was quite happy and engaged as we recounted tales to the awestruck staff and scientists at Summit, confused as to who these smelly-beardy-goofballs were that appeared in the middle of the night without a plane. For us, it was quite bizarre to be surrounded by other people (remember that it had just been us 5 for the past 5 weeks straight, with only one flock of  birds and a few overhead planes to break the solitude) who didn't laugh at our inside jokes or suddenly burst into song at the mention of Pitch Perfect. Not only were there other people, but the Big House has couches (so much softer than our snowmobile cushions), laundry machines, internet, electricity, and a small smattering of plants growing in the windowsill.

Tate dismantling the science sled in front of the Summit Big House

Erich happily moving into an Arctic Oven in Tent City, Summit

I volunteered (or was voted off the ice sheet - still not sure) to hop on a small Twin Otter plane to pick up the caches we had left behind at Cores 3, 5 and 7. So only 12 hours after arriving at Summit I was off again to dig out our ice cores and load the empty fuel barrels into the plane. I've never before had a plane filled with only the pilot, co-pilot, and myself, so I felt like a rock-star as we covered that same 400 km in 1.5 hours that had taken us 18 hours the day before. We carefully dug up the ice core boxes and loaded them into the plane - knowing that it was a race against time before they turned into a very expensive puddle on the floor. We miraculously took off with an extra few thousand pounds of gear and headed back to Kangerlussuaq, flying over spectacular melt ponds and crevasses that we had avoided on our snowmobile traverse. I was ecstatic that I saw a few crevasses at the exact locations that my algorithm predicted they would be!

Trying to figure out how all that gear (plus 5 ice core boxes) will fit into that tiny plane

The view from the back of the plane, where I was happily squished for the two hour flight back to Kanger

Upon arriving in Kanger we were met with a team of Polar Field Services staff, who quickly and carefully whisked the ice core boxes into a large freezer (filled with ice core boxes from other scientists around the Greenland Ice Sheet), moved our gear to the warehouse, and started to dry everything that had been covered in snow and ice for the past 5 weeks. I glanced at the 1000 unread emails, 1000 new GroupMe messages, 100 Facebook notifications, few dozen texts, and even a voicemail from my doctor from a month ago before deciding that it could all wait another few days. I then attempted to email Erich and Bob that everything went well when, suddenly, the smell of my own clothes and 5-week-unshowered-body overwhelmed me and I had to put the phone down to jump in the shower. Never before has the feeling of warm water and soap made my skin feel so amazing.

Looking at crevasses from the Twin Otter that we happily avoided during the traverse

My first site of land at the edge of the Greenland Ice Sheet

I'm off to bed here in Kanger even though the rest of the group is still up at Summit for the next three days. It was very bittersweet leaving the group this afternoon even though I'll see them again on Thursday when they fly down here with the rest of the gear. Tomorrow I'll briefly head back up to the ice sheet to pick up the rest of the cached ice cores and fuel drums when the Summit team organizes gear and packs it up to be shipped back to the states.

Sending warm, showered, well-fed greetings from Kangerlussuaq, Greenland