The use of operational ice charts for evaluating passive microwave ice concentration data

More than 1380 regional Canadian weekly sea-ice charts for four Canadian regions and 839 hemispheric U. S. weekly sea-ice charts from 1979 to 1996 are compared with passive microwave sea-ice concentration estimates using the National Aeronautics and Space Administration (NASA) Team algorithm. Compared with the Canadian regional ice charts, the NASA Team algorithm underestimates the total ice-covered area by 20.4% to 33.5% during ice melt in the summer and by 7.6% to 43.5% during ice growth in the late fall. The wide range in performance occurs because some Canadian regions such as the western Canadian Arctic are only partly in the marginal sea-ice zone while the Hudson Bay and the East Coast regions are entirely in the marginal sea-ice zone, being completely ice free in summer. Compared with the U. S. National Ice Center hemispheric chart series, the average underestimation is 18.6% in summer. During other times of the year the differences are less than 7.8%. The magnitudes of the underestimation during ice melt and ice growth periods with respect to the Canadian regional charts are higher than found in other studies in the literature. The difference in performance of the NASA Team algorithm when compared with the Canadian regional ice charts and hemispheric charts is attributed to: 1) passive microwave data are used in preparing the hemispheric charts so comparison with the hemispheric charts is not independent; 2) the Canadian regional charts are in the marginal ice zone where sea-ice melt or growth conditions occur over a large percentage of the region and last for several weeks to a month; and 3) the tie points used to calibrate the NASA Team algorithm are tuned to perform best over the hemisphere rather than over individual regions. If the Canadian regional ice charts can be accepted as correct, then these results suggest exercising caution in using passive microwave ice concentration data in the marginal ice zone where ice melt and ice growth conditions are a major component of the sea-ice regime.