LBTDT     Default lower boundary treatment code for vertical interpolation to
           potential temperature surfaces (*VPOT) (one INTEGER value, default
           is LBTDT = 4). This lower boundary treatment 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 lower boundary treatment codes are as follows.

           1 - use the surface level if available, extrapolate below the sur-
               face if necessary
           2 - use the surface level if available, block points below ground
               elevation
           3 - don't use the surface level, extrapolate below the lowest free
               atmospheric level if necessary
           4 - don't use the surface level, block points below the lowest free
               atmospheric level (default)