Walt sent me this essay unsolicited, and I think it is very useful for establishing some baseline techniques. There’s more useful information on techniques here than in the entire Catlin Arctic Survey website. UPDATE, a response has been posted at the end of the article. – Anthony
There have been several recent posts on sea ice thickness, particularly in regards to the Catlin expedition. I don’t have any direct connection to Catlin and in my research focus, I don’t anticipate using the Catlin data. I’m not responding to defend them or their methods. Thus, I can’t address details of their operation. However, from reading the posts and comments it seems like some basics on how sea ice thickness is estimated might be of interest.
Sea ice floats in the ocean. Because sea ice is a lower density than unfrozen water, it floats and a portion (~10-15% depending on density) rises above the water line, while most of the ice (~85-90%) is below the surface. The part of the ice cover above the water line is called the “freeboard”; the portion below is called the “draft”. The sum of the freeboard and the draft is the total ice thickness. There may or may not be snow on top of the ice which can add to the “snow+ice freeboard” and the “snow+ice thickness”.
A variety of techniques have been developed to obtain information about sea ice thickness. Most of these methods don’t actually directly measure thickness but estimate thickness from a related measurement. Here are some examples:
Upward Looking Sonar: Mounted on a submarine or on the ocean floor, these instruments measure the return of sound waves bouncing off the bottom of the sea ice. They measure the sea ice draft from underneath the ice. From this draft measurement, the thickness can be derived with knowledge of the ice and water density and the snow cover.
Altimeter: Compared to sonar, altimeter measure the opposite side of the ice. They measure the freeboard from above the ice, from which the total thickness is derived. The NASA ICESat is a laser altimeter, which actually measures the snow+ice freeboard, so knowing something about the snow cover is particularly important (http://www.nasa.gov/images/content/324868main_kwokfig2_full.jpg). Radar altimeters are also often used (including the European Cryosat-2 scheduled to be launched later this year); these penetrate through the snow and thus measure the ice freeboard. ICESat can take a lot of measurements over a large region of the Arctic, but there are limitations, which are discussed below. Altimeters can also be flown on airborne platforms.
Ground radar: This carried on or near the surface and sends out a radar pulse that echoes off the ice-water boundary. Thus it is an estimate of the total ice+snow thickness.
Drill holes: This is the simplest way to obtain ice thickness and it is the only direct measurement of ice thickness – drill a hole and stick measuring tape through it and you have the thickness (whether it is in units of meters, feet, or smoots [http://en.wikipedia.org/wiki/Smoot]). A variant of drill holes are the ice mass balance buoys that Steven Goddard wrote about – drill a hole and put in instrumentation to estimate thickness automatically over time.
There are errors associated with any estimate, but the errors tend to be higher the farther one is away from a direct measurement. For example, for ICESat, you need to know very precisely: (1) the altitude of the satellite above the surface, (2) the ocean surface topography [sea level isn’t constant], (3) the density of the ice and water, and (4) the density and height of the snow cover. All four of these are challenges, though by far the biggest one is #4. There just isn’t a lot of information about snow. ICESat has already provided valuable information about sea ice thickness over large regions of the Arctic and more results will be forthcoming. However, the goal is to continue to improve these estimates to make them even more useful.
This is where surface measurements, radar and drill holes are particularly valuable because they provide “ground truth” – of both ice and snow thickness. The problem with these ground measurements is that it is difficult to obtain a large number of them over a broad area. And this is particularly important for sea ice thickness, which can vary considerably over short distances. This is a limitation of the ice mass balance buoys. There are only a few within the entire Arctic and they measure thickness on a single floe. Even in the immediate vicinity, ice thickness could be quite different than that being measured by the buoy. Thus, while the buoys provide an excellent measurement of thickness at a point through the seasons, they do not provide good information on the large-scale spatial distribution of ice thickness.
Ideally, we’d send a few thousand people out to the Arctic and drill thousands of holes and get good sampling of thickness, but this is just not possible. Even putting out more than a few autonomous buoys are impractical because of the cost of the buoys and the fact that they only last a few years (the ice melts and the buoys are lost, though people are looking about buoys that can float and could potentially be recovered and recycled).
This is where the Catlin expedition can be particularly valuable. To have a group out on the ice taking direct measurements of thickness across a relatively large region (compared to most field expeditions) of the Arctic is something that has only rarely, if ever, been done before. It is unfortunate that the radar may not have worked as well as hoped, but that is the nature of field work, especially in harsh polar environments – things almost never go according to plan. The radar would essentially provide a continuous transect of thickness estimates over several hundred kilometers. However, the drill hole measurements taken regularly over the route will still likely be valuable.
It is also unfortunate that they are not likely to get as much data from multiyear ice as hoped because that is of greater scientific interest, but any ground truth estimates can help improve data from satellites like ICESat is useful. Their planned route looked like it would’ve taken them over ice of varied ages, but the older ice moved out of the area over the winter and, as Steven Goddard showed comparing their position with the ice age data on NSIDC’s web page, they started squarely in first-year ice. Generally, logistics for an expedition need to be planned several months in advance, long before anyone can know how and where precisely the ice will move. Like many scientific expeditions, it seems like they won’t get as much data as hoped, but ground data from the ice is so rare that every little bit helps.
As a final note, since it seems the measuring tape used by Catlin is of great interest, I’ll end with a bit of information on that. Basically, it is simply a measuring tape, but with a collapsible metal flange at the end of the tape. The weight pulls the tape down through the hole to the bottom of the ice. Then you pull the tape taught and the flange opens and catches on the bottom of the ice. You make your measurement, then pull hard on the tape and the flange collapses and you can pull it up through the drill hole. Since such tapes with flanges are relatively specialized, there aren’t many places to get one. One place is Kovacs Ice Drilling Equipment
NSIDC has a gauge from Kovacs and it has units of meters and feet, on opposite sides of the tape. I would guess that the Catlin tape is similar, but I don’t want to jump to conclusions.
Response to Dr. Meier by Steven Goddard.
First, I want to thank Dr. Meier for his candid explanation of how Catlin landed on first year ice, and how ice is measured. As always, he has treated our concerns seriously and that is very much appreciated.
Dr. Meier said that the ice “can vary considerably over short distances” and the Catlin web site has said “the team systematically seeks out flatter ice.” That implies to me that there is a geographical bias to the data which makes the entire data set suspect. (That might be analogous to having a temperature set where a disproportionate percentage of the thermometers were located in Urban Heat Islands.) If I were traveling across the Arctic pulling a 100Kg sledge in -40 degree weather, I would certainly seek out the flattest ice, as they have done.
The Catlin team has reported “Snow thickness, measured by the team during the first 2 weeks of March, shows an average snow depth of around 11 centimeters. Since then the average has risen to around 16cm.” Four to six inches of snow hardly sounds like a serious problem in estimating ice thickness in metres. They also said “March snow depths in this area should be 32‐34 cm on multi‐year ice.” If snow thickness is less than expected, does that imply that the satellites may be slightly underestimating the thickness of the ice?
If the multi-year ice shifted over a period of several months ahead of the expedition launch, why was the Catlin team seemingly surprised upon their arrival to find first-year ice? NSIDC knew it was first year ice in February. This reminds me of Lewis Pugh’s attempt to kayak to the North Pole, at a time when NSIDC maps showed the route blocked by 600 miles of ice.
It sounds like the new European satellite Cryostat-2 will provide the desired ice thickness data, without any geographical bias or concern about snow thickness. Speaking as a former amateur explorer, I certainly appreciate and admire the adventurous nature and grit of the Catlin team. However, I don’t see that there is a lot of scientific value to their ice measurement efforts – particularly given their stated disposition towards arriving at a seemingly pre-determined result.