*GRD Section

1. With a spectrally convolved output (ILS or AVG flags) the RFM, by default, performs high spectral resolution calculations on a regular internal grid. The GRD flag allows an irregular grid to be specified, via files in this section (the advantage being faster execution and smaller memory).

   (If you want an irregularly spaced output grid, e.g. when generating Look-Up Tables, then specify the irregular grid file in the *SPC section instead. In this case the internal and output grids are the same)

2. Each irregular grid file FILGRD listed in this section is opened and the spectral range of the file compared with all the output ranges selected in the *SPC section. If the file range completely overlaps one or more of the selected ranges it will be used for that range. If not, then the default is to use the normal line-by-line calculation. An appropriate message appears in the rfm.log file.

3. The filename template DEFGRD is optional, and identified by the presence of a '*' character. Only one such template may appear in the section. If a filename template is supplied, first all the explicitly specified files FILGRD are read, then '*' character in the template is expanded to the LABSPC label of each spectral range not yet assigned an irregular grid and, if the file exists, it is then included.

4. If there is any ambiguity over which grid is to be used (i.e. two files overlapping the same spectral range) a fatal error condition results.

5. The grid is only required to overlap the output spectral range: if an ILS convolution is required then a further check is performed on whether the grid 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 grid is negligible.

6. For limb-viewing, the Max/Min tangent altitudes listed in the grid are also compared with the set of tangent heights selected for calculation. If any of these are outside the max/min range of the grid file a warning message results but the calculation proceeds anyway.

*GRD H2O_0001.grd H2O_0002.grd ! GRD files for specific H2O microwindows ./other/*.grd ! Template for grd files for other MWs