Alter sea ice extent calculation to improve drawing of timeseries

[phillips-ad]

Presently the CVDP-LE uses a where statement to set all siconc points < 15% to _FillValue. This results in timeseries stopping on plots if the value is _FillValue. Change the where statement to set values less than 15 to 0. Quick test seems to indicate no issues, but this needs to be verified with further testing.