In the wave spectra, all frequency/direction components with values less than some (field-dependent) threshold are set to 'missing' when encoded in GRIB;  values larger than this threshold are encoded as log10 of the actual value to an accuracy of 9 bits (bitsPerValue=9).

To interpret these values correctly, the user should do:

if (value == missingValue) then
    value = 0.0 
    value = 10.0**value  ! 10 to the power of the value