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 . Specifying a value for this
ICP nullifies the default for . See 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)