Drew Rothrock
What properties of sea ice does one want to observe? The most fundamental observation answers the question: Where is there ice? Ice extent is well observed by passive microwave imagers. This requires distinguishing sea ice from open water, which is a piece of cake since the emissivities and brightness temperatures (emissivity times physical temperature) differ enormously. The more difficult observation is of the small fraction of open water within a "sea-ice" pixel (or footprint, some 10's of km across). In winter the open water fraction is on the order of a percent, and the variability of the appearance of sea ice itself makes it hard to discriminate this small percentage of open water. The most difficult observation is of the fraction of "sea ice" in a pixel covered by first-year ice (ice grown since the last autumn freeze-up), or alternatively, what's left: multiyear ice (ice that's survived a summer's melt). Large errors arise from the fact that there are of course not three types, but a large variety of surface conditions. The pure type fractions obtained from satellite brightness temperatures depend strongly on how one assigns "pure type signatures" or "end points" to these three types, a choice still mired in controversy. In spite of these difficulties, the satellite passive microwave record through three satellites series provides an exquisite record of ice coverage from 1973 into the foreseeable future. This two-decade record shows the annual cycle, interannual variability and declining trends of several percent per decade. The record is complete enough that it can be used for tests of and assimilation into sea ice and global climate models.
Another important observation is how sea ice moves--which is quite a lot. It moves several kilometers in a day. One compares images made at two different times, recognizes the same "feature" or backscatter pattern in both images and sees how the location of the feature has changed. The observation relies on geometric image fidelity rather than on radiometric accuracy. Errors arise from mis-identifying the feature--tracking errors--and from mis-mapping the image onto the Earth--geolocation errors. Both come into play when one measures displacement: only the former, when one measures strain or deformation--the change in the separation between two points in the same image pair. The movement of ice across the ocean is important for the ocean's freshwater balance. Ice doesn't hold the salt in seawater, so growing ice provides extra salt to the sea surface and melting ice freshens the sea surface. Ice that forms near Siberia flows out into the Greenland Sea several years later, and this difference between where ice forms and where it melts determines the pattern of brine forcing at the ocean surface (equivalent to evaporation minus precipitation for the wet oceans). Ice deformation is important because it forms new areas of open water between jostling floes. These "leads" are sites of rapid ice growth in winter, because the -2 C water is exposed to -30 C air. And after some ice grows in these leads, later deformation tends to pile this ice up into "ridges" which are several times thicker than undeformed ice. Over half the mass of the ice cover is in these extra thick formations.
The final crucial set of observations of sea ice consists of surface temperature and surface albedo--the surface reflectivity of solar radiation. The biggest error for both arises from the difficulty in distinguishing clouds from the surface, or "cloud masking." Unlike land or the sea surface, the snow or ice surface differ from clouds only subtly, and most strongly in the near infrared in daylight. Errors in surface temperature are about 1 C. The albedo is enormously important to the surface heat balance, and varies from 0.8 (reflecting 80% of the incoming radiation) for cold snow to 0.4 for melting bare ice, down to 0.1 for water. Satellite derived surface albedos have an uncertainty of at least several percent which gives rise to uncertainties in the surface net solar radiative flux of 5 % or 5 Watts per square meter and in a climatological 3-meter-thick ice cover of 50 cm.
Contact Information:
Drew Rothrock
Applied Physics Laboratory
Campus Box 355640
University of Washington
Seattle, Washington 98195
Telephone: (206) 685-2262
email: rothrock@apl.washington.edu