ICEC
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The (sea) ice concentration (ICEC) was estimated from SMR (78-87) and 
SSM/I (post SMR) data when those instruments were available.  Before SMR, 
the ice concentration was obtained from the Joint Ice Center (see MODEL).  
The ice concentrations were then converted into ice-free and ice-covered 
grid points using a 55% threshold for use by the forecast model.  
Unfortunately the monthly mean/variance ICEC is based on the 0/1 fields.  
Thus, a monthly mean ice concentration of 0.7 implies that grid point 
was ice covered 70% of the days.  No disagreement that treating sea ice 
better would be cheap and beneficial.

The sea-ice concentrations, before they were converted into a 2-value field, 
can be obtained from Bob Grumbine of NCEP (robert.grumbine@noaa.gov).