INTDT     Default interpolation type code for vertical interpolation to
           potential temperature surfaces (*VPOT) (one INTEGER value, default
           is INTDT = 2).  This interpolation type is used for all fields not
           named in the exceptions list <TINTXL>. Specifying a value for this
           ICP nullifies the default for <TINTXL>.  See <TINTXL> for examples.
           The valid interpolation type codes are as follows.

           0 - no interpolation
           1 - linear in sigma
           2 - linear in natural log of sigma (default)
           3 - linear in natural log of natural log of (1000*sigma + 2.72)