*SVD Section

1. Each look-up table listed in this section is opened and the gas and spectral range specified in the file contents are read. If the gas matches one of the absorbers specified in the *GAS section, and the spectral range either matches or completely overlaps one or more of the ranges specified in the *SPC section, then the look-up table will be used. If not, then the default is to use the normal line-by-line calculation. An appropriate message appears in the rfm.log file.

2. A filename template is identified by the presence of a '*' character, which is expanded as [s]_[i] (note underscore character separating fields) where [s] is the spectral range label LABSPC and [i] is the HITRAN Index of the absorber (this follows standard nomenclature of MIPAS LUTs).

3. The LUT is only required to overlap the output spectral range: if an ILS convolution is required then a further check is performed on whether the LUT covers the additional range required by the width of the ILS. If this is not the case then a warning message is issued and the calculation proceeds on the assumption that the absorption beyond the edges of the LUT (for that species) is negligible.

4. The LUT is also checked for Max/Min limits on temperature and pressure. If the RFM calculation involves paths for that absorber beyond the table limits (apart from low pressures, which is assumed normal), a warning message is sent to the rfm.log file but the calculation proceeds using the value of the LUT at the closest edge (effectively assuming k(p,T) is constant beyond p,T edges of the table).

5. The effect of setting the qualifier NS=0 is not quite the same as using zero singular values for the 'log' type of LUT tabulation. Using 0 SVs implies a reconstructed value of log(k)=0, hence an absorption coefficient k = 1. It is assumed that this is undesirable, so if the qualifier is used to select 0 SVs, the tabulation treated as linear ('lin') in order to force k=0 (ie as if the gas were omitted completely, which is assumed to be the intended effect). If this happens, a warning message is written in the rfm.log file.

6. Some care is required when combining H2O LUTs with the CTM flag: the normal H2O continuum requires subtracting the line absorption coefficient at 25cm^-1 from line centre before combining the line shape with the continuum. This is not possible if the H2O line information comes via the LUT, so an incorrect (larger) H2O line+continuum absorption results. Solutions are:
   • Include the H2O continuum into the LUT data when it is generated (and don't use the CTM flag in the subsequent RFM calculation)
   • Same, except use H2O(NOCTM) in the *GAS section, allowing the CTM flag to be used for other continua.

*LUT ! Absorbers for microwindow CH4_0001 CS_CH4_0001_06.DAT(10) ! SVD-LUTs for CH4 limited to 10 S.V.s CS_N2O_0001_n2o01a.lut(-2) ! SVD-LUTs for N2O reduced by 2 S.V.s CS_*.DAT ! standard SVD-LUTs for other gases