So what is CryoSat validation?
Yesterday we ended the last ice station for the CryoSat validation group during a beautiful evening. The levelling team spent the last hours by the lead on the remote side of the floe, helped by two wonderful polar bear guardesses. The surrounding ice cover was deformed into astounding formations, in the horisont you could see blue castles and pointed obelisks. The mirage of Svalbard was bathing in the distant sun.
Everything was unique, about to disappear soon during the melting season, and the scenery was also changing all the time. We wondered how a 10-ton massive slab of ice, which was tilted to form a 5 m high mountain, had changed place from one side of the station to almost opposite during one day.
So what was this all about CryoSat and validation? CryoSat satellite carries a radar altimeter, which is a complicated device for measuring a simple thing, the distance from the satellite to what happens to be underneath. In our case that is ice floes and water, and if the satellite is accurate enough it will then find the altitude difference between the two, that is, the sea ice freeboard. Then just convert the freeboard to total thickness with the Archimedes law. Sounds really simple, but as the satellite altimeter is a complicated thing the getting of the freeboard out of its data is kind of complicated matter as well. The main thing is that it is not operating like a handheld laser distance meter but each pulse illuminates a larger area, about the size of a regular ice floe. How the pulse echo received by the satellite looks like depends on how the floe looks like: is flat, rough or covered by huge ridges, how thick is the snow cover, what are the snow properties.
Validation is checking that the altimeter and the rules for calculating the freeboard really work. Ideally this means that the ice team and the satellite would measure the same floe at the same time. As this is difficult to realise, a similar altimeter carried by an aircraft can be flown across the site - this was done during the first longer ice station of the cruise. Another possibility is to leave a drifter on the station so that its track can be followed and hope that CryoSat will fly over it before the conditions on the floe change too much.
As CryoSat is about freeboard, one main thing for the field team is to measure the floe freeboard. This was done by two Finns who had the right mentality for such utterly monotonous work. Myself (the other case) I asked once from our polar bear guard which she thought is more boring, watching for polar bears or our measurements and she replied probably the latter. They were really straightforward - you mark a long line, march it straight and forward and measure the surface elevation every few meters. This is done with a laser levelling device, where a rotating laser is sending a horisontal beam all over the place. A Finn walking the line has a pole with the sensor, and when the sensor feels the beam it utters high-pitch beep that sounds irritating even over hundreds of meters. With the same snow thickness is measured so that the elevation of ice surface under the snow is got, as the satellite is supposed to see the ice surface through the snow - in good conditions. But if the snow layer is thick, moist or both the altimeter pulse scatters back in part from the snow layers already. That's why others in the ice group were measuring snow properties from pits. Measurements of ice thickness and density, on the other hand, can be used to parameterise the conversion of freeboard to total thickness. And finally the helicopter EM has extended the local fine scale measurements to regional coverage.
When you start studying something this often generates an interest that was not there in the beginning. Snow thickness and surface elevation are not held as very sexy variables among sea ice scientist, they are something for 2nd year students to measure during the first field course. And there is no salt in snow - we are no bloody hydrologists are we? But our impression is now that above surface there is a realm of snow and not of ice. At some stations there was no ice at all - it was all depressed under water surface. The highest freeboard was 10 cm for the poor CryoSat to measure. The 20-50 cm thick snow cover was dominating everywhere, and in accumulations by ridge sails the snow could be 1.5 m thick.
As you walk ten hours a day along straight lines repeating the same simple antics every few meters you have time to think. To think how the uneven snow load bends the floes into a new shape, how it affects the thermodynamics, how it even may be one reason making the floes fragment. So after what first appeared as a bit boring regular validation campaign we return home with our bags full of new ideas and hypotheses, some of which can be studied already with the comprehensive data set collected during the cruise. Its possibilities really extend far beyond the CryoSat objectives. And for what can not be answered we already have great plans for the next cruise opportunity.
Making long lines has the benefit that you may get little further from the ship and its constant dunk-dunk noise. This was even nicer yesterday when I finally realised - for the very last hours of the cruise - that the laser can be well used also without the beeping. In silence the icescape comes out in all its magnificent formations and one notices how the floe on the other side of the lead is slowly moving and hears the faint squeakings contacting floes are making. But polar bears (today we saw two males having a short fight) make no sounds, and we would like to thank all the bear guards, esp. guardesses, that have been helping us, often apparently willingly for some odd reason.