I.6.2. ->VPOT - Vertical interpolation to potential temperature surfaces Primary ICPs: [TEMPLEV] [FIELDcn] Secondary ICPs: Fields on Model surfaces can be vertically interpolated to an arbitrary set of potential temperature surfaces. This interpolation uses the same technique, and has basically the same (independent) options as the interpolation to press- ure surfaces (*VPRS): the ICP is used to request this option and specify the isentropic surfaces, while , , , and are used to specify the default interpolation type, lower boundary treatment, number of levels to copy, and the field exceptions list, respectively. Inter- polation to isentropic surfaces, however, has an additional, unique option: to control the handling of multiple isentropic surfaces (i.e., the case where a requested potential temperature surface occurs at more than one elevation within a given vertical column). Isentropic interpolation has the same surface pressure availability require- ments as pressure interpolation. In addition, the derived field THETA (poten- tial temperature) (*CDFL) must be explicitly requested for each Case being processed ( and/or ). THETA is defined for computation types 11 and 12 only, which means that it can only be computed from history tape input. This implies that if input is from a Processor Save Tape, isentropic vertical interpolation can only be done if THETA is on the Save Tape. Note, however, that a Save Tape may be translated into a history tape in a prelimi- nary jobstep (*OHST); THETA can then be computed from the translated history tape as long as both temperature (T) and surface pressure (PS) are available. The field THETA is not interpolated onto potential temperature surfaces, and is unconditionally deleted immediately after other fields are interpolated. If subsequent processing of potential temperature on potential temperature sur- faces is necessary (e.g., in order to generate a field containing potential temperature for use in a derived field expression), THETA may be copied to a user-defined derived field prior to interpolation (*UDFL). Fields interpolated to potential temperature surfaces may be output to any type of Processor Save Tape (*SAVT), including history tapes. When a history tape containing data on potential temperature surfaces is input to the Proces- sor, the ICP should be assigned the value 'THETA'. For fields interpolated to potential temperature surfaces, the associated level word contains the potential temperature in degrees Kelvin, followed by a 'T' suffix. When requesting vertical cross-section plots (*PMEX) (*PLAX), it may be desirable to override the default logarithmic scaling for the vertical coordinate . Only fields on sigma surfaces can be vertically interpolated; requesting both isentropic and pressure interpolation results in a fatal error. As with pressure interpolation, isentropic interpolation is linear with respect to a transformed sigma vertical coordinate, with the form of the trans- form determined by the interpolation type code. This method requires knowledge of the vertical variation of THETA in order to determine a transformed sigma value on the requested potential temperature surfaces. It is assumed that THETA varies linearly with respect to the natural log of sigma (i.e., within a given sigma layer), regardless of the interpolation type code specified for any field. If a requested potential temperature surface is outside of the range of THETA in a particular column, then sigma on this surface is computed by ex- trapolating THETA above the highest (or below the lowest) level available, but only if the highest (or lowest) available layer is thermodynamically stable. Otherwise all fields are blocked (*DEFS) for that particular surface. All fields are also blocked on any theta surface whose equivalent sigma value is greater than 5 (this may occur for very low potential temperatures, especially in the tropics).