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If I extract Potential Evaporation (PET, W/m$^2$) from the National Centers for Environmental Prediction (NCEP), climate reanalysis data (downloadable as netCDF files here), there are some negative values, e.g. for summer in southern Manitoba (50N, 100W), I get the attached distribution of PET values.:

enter image description here

Is this a real physical process, such as condensation (dew formation), or is it an error (e.g. should I average over these values or set them to zero)?

update requested metadata:

The metadata (contained in the files) says that the valid range is -800, 5200; here is further description of the 'pevpr' variable (provided in the data header):

  • long_name = "Monthly Mean of Potential Evaporation Rate"
  • valid_range = -800,5200 ;
  • units = "W/m^2" ;
  • add_offset = 0 ;
  • scale_factor = 1 ;
  • missing_value = -9.96921e+36 ;
  • precision = 1 ;
  • least_significant_digit = 0 ;
  • var_desc = "Potential Evaporation Rate" ;
  • dataset = "CDC Derived NCEP Reanalysis Products" ;
  • level_desc = "Surface" ;
  • statistic = "Mean" ;
  • parent_stat = "Individual Obs" ;
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@EnergyNumbers Thanks for pointing me in this direction. I looked at the metadata; I could not find much beyond what is in the file. However, the 'valid range' includes negative numbers so this technically answers my question ('yes it is valid') but the 'what does it mean' question still remains. –  Abe May 6 '13 at 1:27
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