SEA-ICE MOTION FROM SATELLITE PASSIVE MICROWAVE IMAGERY ASSESSED WITHERS SAR AND BUOY MOTIONS

Observing the motion of sea ice from space is analogous to observing w ind stress over the wet oceans; both provide surface forcing for model ing ocean dynamics. Ice motion also directly provides the advective co mponent of the equations governing the mass balance of the sea ice cov er. Thus its routine observation from space would be of great value to understanding ice and ocean behavior. To demonstrate the feasibility of creating a global multidecadal ice motion record from satellite pas sive microwave imagery and to quantitatively assess the errors in the estimated ice motions, we have tracked ice every 3 days in the Arctic Ocean and daily in the Fram Strait and Baffin Bay during the 8 winter months from October 1992 to May 1993 and daily in the Weddell Sea duri ng the 8 winter months from March to October 1992. The method, which h as been well used previously, involves finding the spatial offset that maximizes the cross correlation of the brightness temperature fields over 100-km patches in two images separated in time by from 1 to 3 day s. The resulting ice motions are compared with contemporaneous buoy- a nd SAR-derived ice motions, The uncertainties in the displacement vect ors, between 5 and 12 km, are better than the spatial resolution of th e data. Both 85-GHz data with 12-km spatial resolution and 37-GHz data with 25-km resolution are tracked. These trials with the 37-GHz data are new and show quite surprisingly that the error is only about 1 km larger with these data than with the 12-km 85-GHz data, Errors are typ ically larger than average in areas of lower ice concentration; in the most dynamic regions, particularly near the ice edge in the Barents a nd Greenland Seas; and in zones of high shear. These passive microwave ice motions show a large increase in spatial detail over motion field s optimally interpolated from buoy and wind observations, especially w here buoy data are virtually absent such as near coasts and in some pa ssages between the Arctic Ocean and its peripheral seas. The feasibili ty of obtaining ice motion from the 37-GHz data in addition to the 85- GHz data should allow an important record of ice motion to be establis hed for the duration of the scanning multichannel microwave radiometer (SMMR), special sensor microwave/imager (SSM/I), and future microwave sensors, that is, from 1978 into the next millenium.