We have a pretty accurate 2D view of the Arctic sea ice, and some clues with regards to its third dimension: thickness. It's the thickness of the ice that determines the influence of atmospheric conditions on the ice pack, and is thus a crucial factor in the amount of sea ice that covers the Arctic Ocean during any given time, especially towards the end of the melting season. But also during the freezing season sea ice thickness plays an important role as an intermediary between sea surface and atmosphere. The thinner the ice is, the more heat and moisture can be transferred from one to the other, which in turn influences atmospheric patterns.
image on the top right, courtesy of NSIDC
The importance of accurate sea ice thickness measurements is not lost on the scientific community. We've had ICESat, we eagerly await the end of the calibration phase of the CryoSat-2 mission, and in the meantime another handy tool for measuring ice thickness has been devised by scientists from the University of Hamburg, using a passive microwave sensor aboard the ESA's Soil Moisture and Ocean Salinity (SMOS) satellite.
images retrieved from the SMOSIce wiki (with permission of L. Kaleschke)
The news was announced two months ago in an ESA press release, but has now been followed up by a research paper that has just been accepted for publication in Geophysical Research Letters: Kaleschke, L., X. Tian-Kunze, N. Maaß, M. Mäkynen, and M. Drusch (2012), Sea ice thickness retrieval from SMOS brightness temperatures during the Arctic freeze-up period.
From the abstract:
The Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) on board the European Space Agency’s (ESA) Soil Moisture and Ocean Salinity (SMOS) mission for the first time measures globally Earth’s radiation at a frequency of 1.4 GHz (L-band). It had been hypothesized that L-band radiometry can be used to measure the sea ice thickness due to the large penetration depth in the sea ice medium. We demonstrate the potential of SMOS to derive the thickness of thin sea ice for the Arctic freeze-up period using a novel retrieval algorithm based on Level 1C brightness temperatures.
The SMOS ice thickness product is compared with an ice growth model and independent sea ice thickness estimates from MODIS thermal infrared imagery. The ice thickness derived from SMOS is highly consistent with the temporal development of the growth simulation and agrees with the ice thickness from MODIS images with 10 cm standard deviation. The results confirm that SMOS can be used to retrieve sea ice thickness up to half a meter under ideal cold conditions with surface air temperatures below -10◦ C and high-concentration sea ice coverage.
And from the conclusion:
[S]everal retrieval uncertainties remain that should be considered in future studies. Despite these uncertainties our analysis provides clear evidence for a maximum of the retrievable ice thickness [0.5 m]. SMOS obtains daily coverage of the polar regions with a resolution of about 35 km x 35 km which is suitable for several applications of a sea ice thickness product. We expect the greatest benefit during the cold freeze-up period in Autumn when extensive areas of thin sea ice control the ocean-atmosphere heat exchange, which is important for weather and climate, as well as for operational marine applications.
This quote from the above mentioned ESA press release from last December also explains the usefulness of this new technique:
The information on sea-ice thickness is complementing that delivered by ESA’s CryoSat. Carrying a radar altimeter, CryoSat uses a different method of measuring sea ice – the height of the ice protruding above the water.
SMOS can offer daily coverage over the polar seas, while CryoSat provides higher spatial-resolution data and orbits very close to poles, thereby giving extra coverage.
The puzzle is in the process of being solved.