*LEV Section

1. This section will create additional output spectra representing the integrated paths from the remote side of the atmosphere towards the observer. Effectively this is the same as specifying multiple observer altitudes (OBS Flag) along the ray-path within the same driver table.

2. Using this option requires a *HGT grid to be supplied in the *ATM section, although not necessarily as the primary vertical grid. For example, if the RFM is run on a pressure grid (for a plane-parallel atmosphere) the *HGT grid can simply be an increasing index: 1, 2, ...

3. If the output levels do not correspond to existing atmospheric levels, extra levels will be inserted

4. For limb-viewing paths and nadir paths with surface reflection there will generally be two output spectra calculated for each specified altitude: one corresponding to the downward integration from the top of the atmosphere to the tangent point or surface, the other corresponding to the upward integration to the observer.

5. Output spectra are created as normal spectral files with the level information following the [s][t] information in the output filename, e.g. if the tangent point is 8km and an output level at 11km is selected, then in addition to a normal radiance file rad_a08000.asc there will be two additional spectra rad_a08000_down11000.asc (corresponding to an observer at 11km viewing downwards to a tangent point at 8km) and rad_a08000_up11000.asc (corresponding an observer at 11km viewing upwards).

6. No output files are generated for levels which are not intersected by the ray path (eg specifying an output level at 14km when the tangent point is at 15km) or for which the contribution is obviously zero (eg downward integration for a nadir view with non-reflective surface)

*LEV 11 12 13 ! outputs at 11, 12 and 13 km alts.lev ! and further altitudes contained in file alts.lev