*SFC Section

1. Since all surface parameters have default values it is possible to have the SFC flag enabled without the *SFC section. However the SFC Flag remains mandatory if any ray-paths intersect the surface (in case the user was unaware that the surface is being encountered), which is always the case for nadir viewing in a plane-parellel atmosphere (NAD, FLX flags), and may be the case for 'limb-viewing' (ie curved atmosphere) rays which intersect the lower boundary of the specified height profile (including any rays required for FOV convolution). If in doubt, add the SFC flag anyway.

2. Surface temperature can either be specified to have an absolute value TEMSFC, or a value TEMREL relative to the temperature at the base of the atmosphere. A warning message is issued if the surface temperature differs TEMSFC from the lower boundary atmospheric profile temperature by more than 10 K (set in module drvsfc_sub.f90). If neither is specified it is assumed to have the same temperature as the base of the atmospheric profile.

3. Unless moved (upwards) by the HGTSFC or PRESFC parameters, the surface is considered to lie at the the lowest altitude/highest pressure specified in the first profile loaded in the *ATM section, i.e. not necessarily at 0 km. (The absolute altitude is only significant for circular geometries when it is combined with the radius of curvature)

4. Either the HGTSFC or PRESFC parameters (but not both) can be used to adjust the surface level, in which case an extra profile level is inserted at the new values (although any original levels below the new surface are retained, e.g. in the PRF output). The default surface temperature, or value specified with TEMREL, are also adjusted appropriately

5. The NAD always results in a path which, in the absence of reflection, terminates at the surface, and therefore technically with zero transmittance, which is not useful.

   Since RFM v5.10, the TRA, ABS and OPT output spectra only refer to the direct path atmospheric component (and are therefore the same as would be obtained using the ZEN flag).

   This only applies to the downward paths using the plane-parallel atmosphere (NAD flag). Limb paths are treated as usual so will have zero transmittance if fully intersecting the surface. This is to ensure that paths which partially intersect the surface (i.e., with FOV convolution) are treated consistently.

   The radiance outputs, i.e. RAD, BBT and RJT output spectra, always include the surface emission and any reflected atmospheric emission as would be expected.

6. For flux calculations (FLX Flag) any surface reflection is always treated as diffuse (Lambertian). For other calculations, prior to RFM v5.10, surface reflections were always treated as specular (i.e., mirror-like). RFM v5.10 onwards: the default is specular (set in sfccom_dat.f90) but the RFLSFC parameter can be used to change this to diffuse.

7. EMSSFC: Spectrally varying surface emissivity values can be supplied using a standard RFM Spectral file (regularly or irregularly spaced) — see Example below. Linear interpolation is assumed between the tabulated values, and the emissivity outside the tabulated range is assumed to be constant at the edge value of the table (i.e., not extrapolated). A file containing a single emissivity value (labelled with any spectral wavenumber) is equivalent to specifying the EMSSFC value directly in the driver table

   To test whether EMSSFC is a value or a filename, the RFM first attempts to open it as a file. If this fails, a value is assumed. Consequently any fatal error message arising from an incorrect filename/path could be misleading.

*SFC TEMREL=0.0 ! Equivalent to default value EMSSFC=data.sfc HGTSFC=2.0 ! Set surface at 2km altitude

! Surface emissivity spectrum !N.Pts WNO1 WNOD WNO2 5 800.0 50.0 1000.0 0.995 0.996 0.995 0.994 0.993

! Surface emissivity spectrum !N.Pts WNO1 WNOD WNO2 5 800.0 0.0 1000.0 800.0 0.995 850.0 0.996 900.0 0.995 950.0 0.994 1000.0 0.993