Evaluation of Passive Microwave Ice Concentrations using digitized operational Ice Charts

Over 1380 regional Canadian weekly sea ice charts for 4 Canadian regions and 839 hemispheric United States weekly sea ice charts are compared with passive microwave sea ice concentration estimates using the NASA Team, NASA team2 and ARTIST algorithms over the period: 1979 to 1996.

For the Canadian regional ice charts, the NASA team algorithm underestimates total ice covered area by 20.4 % to 33.5% during ice melt in the summer and 7.6 % to 43.5% during ice growth in the late fall. The wide range in performance occurs because some Canadian regions such as Hudson Bay and the East Coast regions are entirely in the marginal sea ice zone being ice free in summer while the western Canadian Arctic region is only partly in the marginal sea ice zone. For the United States hemispheric chart series, average underestimation is 18.6% in summer. Other times of the year the algorithm performs well. The large difference between the Canadian and United States charts is attributed to: 1) the Canadian regional charts cover more of the marginal ice zone and as a result, a high percentage of the regional charts represent sea ice growth and sea ice melt conditions when the algorithms are known to perform poorly; 2) passive microwave data is not used in preparing the regional Canadian charts however it is used to some extent in preparing the hemispheric charts. This biases the comparisons for the hemispheric charts; 3) by using appropriate tie points for open water, first-year ice and multi-year ice, the NASA team algorithm is calibrated to perform best for the entire hemisphere but not necessarily as well for individual regions.

The magnitude of the underestimation during ice melt and ice growth periods is higher than other studies found in the literature. Although no long-term trend in the underestimation between the ice charts and the passive microwave data was found, these results may have implications for sea ice cover trends using passive microwave during minimum sea ice extent when melt ponds cover a large percentage of the sea ice surface. Certainly these results suggest caution in using the 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.