*DIM Section

RFM Driver Table Sections: Axes for Tabulated Absorption Coefficients

21FEB24
Type
Primary Section#6

Description
Axis Dimensions for Tabulated Absorption Coefficients
(Replaces *TAN section if TAB Flag enabled)

Format
Multiple fields, specific order
FieldTypeDescription Units Range
Pressure axis data (mandatory).
either PFILE C200 Name of Data File hPa > 0
or PLV C3 Use internal profile levels 'PLV' [2]
or PCG C3 Use internal profile CG values 'PCG' [2] [8]
or NP I No. points on regular pressure axis, or >0
−(No. points) on regular ln(pressure) axis <0 [3]
P1 R Max/Min pressurefor tabulation hPa >0
P2 R Min/Max pressure for tabulation hPa >0 [4]
Temperature axis data (mandatory if VMR axis data required). [5]
either TFILE C200 Name of Data File Max(T) ≥0 [6]
or NT I No. points on regular temperature axis >0
T1 R Max/Min temperature for tabulation K Max(T) ≥0 [6]
T2 R Min/Max temperature for tabulation K Max(T) ≥0 [4] [6]
VMR Scale Factor axis data. [7]
either QFILE C200 Name of Data File % > 0
or NQ I No. points on regular VSF axis, or >0
−(No. points) on regular ln(VSF) axis <0 [3]
Q1 R Max/Min VSF for tabulation % >0
Q2 R Min/Max VSF for tabulation % >0 [4]
Type: I=Integer; R=Real; Cn=character string, length n.

Notes
  1. Tabulation axes can either be provided as a list of monotonic values (in an RFM Generic Data File), or constructed as a regular grid from min,max values and number of points. Axis values can either increase or decrease monotonically. The type of field is identified by first attempting to open it as a filename, else assume it is the No.points (an integer).
  2. For the pressure axis, there are additional options (using 'PLV' or 'PCG' - without quote marks - for PFILE rather than a filename) to create a list of pressure values from the internal profile loaded via the *ATM section.
    • PLV: use set of pressure levels directly
    • PCG: use set of Curtis-Godson equivalent pressures calculated for a vertical path
  3. For the pressure and VMR scale factor axes, a negative value for NP or NQ is interpreted as setting equal increments in log space A linear pressure axis is recommended for nadir-viewing, logarithmic for limb-viewing.
  4. Where only a single point is specifed for the axis, the second limit (P2, T2, Q2) is omitted.
  5. In the absence of any temperature axis information NT=1, T1=0 is assumed, which means that the absorption coefficients for each pressure are calculated at the temperature of the embedded profile.
  6. The Temperature axis may either be absolute (ie actual temperatures) or relative to the embedded temperature profile. The latter is assumed if the minimum axis temperature is ≤ 0, but both require a maximum temperature ≥0.
  7. The VMR Scale Factor dimension is intended to allow for self-broadening which, for the Earth's atmosphere, is only significant for large water vapour concentrations. In the absence of a VMR scale factor axis, NQ=1, Q1=100 is assumed, which means that the absorption coefficients for each pressure are calculated assuming a VMR given by the embedded profile.
  8. With 'PCG', for a pressure profile of N levels, there will be N-1 CG layers, but these are augmented with the min and max pressure profile values, hence N+1 pressure axis values. This option is intended to create a set of pressure axis points as close as possible to the actual values that will be required by the RFM when modelling a plane-parallel atmosphere, minimising the interpolation error in the p-domain.

    Note that the CG values are those evaluated for only the first absorber listed in the *GAS section, so if creating TAB files for multiple absorbers, it is recommended that each is calculated using a separate driver file to ensure the most appropriate CG pressure axis for each.

Examples
*DIM PCG 15 180 320 ! CG p values, T from 180, 190 ... 320 K
*DIM pvalues.txt ! file containing pressure axis values 11 -50 50 ! relative temperatures -50,-40 ...+50 K -3 50 200 ! VMR profile scaled by 0.5, 1.0 and 2.0

Bugs
[none recent]