RFM Driver Table Sections

*TAN Section: Tangent Heights (or other ray path definitions)

19JUL17

Type
Primary Section#6

Description
Contents of section defined according to section label and flags: (NB: 'limb-viewing' here refers to any mode using a full ray-trace calculation for a curved atmosphere, ie any mode not using the HOM, FLX, NAD, ZEN or TAB Flags).
*TAN can also be used as the default section label for all options unless *GEO or *ELE are specifically required since in the other cases the definition of this section's contents are determined by the Flags.

Format
Generally: Multiple fields, arbitrary order, except for
*DIM Section/TAB Flag - see [specific page].
SectionFlagsFieldTypeDescriptionRange
(any) (any) FILTAN C*200 Name of a .tan file
*TAN TAN R*4 Actual (refracted) tangent heights [km] (Lowest:Highest) *HGT levels in first .atm file
*GEO GEO R*4 Projected (geometric) tangent heights [km] -(Radius of Curvature):(top *HGT level after refraction)
*ELE ELE R*4 Elevation angles [deg] relative to observer horizontal,
usually negative=viewing below the horizontal
Observer outside atmosphere: -90:(top *HGT level after refraction)
Observer within atmosphere: -90:+90
ZEN Flag: 0.1:+90
NAD Flag: -90:-0.1
*SEC NAD or
ZEN
SEC R*4 Airmass factors (=sec(zenith angle)) [dimensionless], for plane-parallel atmospheres >= 1.0 (1=vertical path)
*LEV FLX LEV R*4 Altitude levels [km] for flux calculation outputs. (Lowest:Highest) *HGT levels in first .atm file
*LEN HOM LEN R*4 Path lengths [km] for homogeneous path calculations. >0
*DIM TAB See [specific page]

Notes
  1. The type of field is identified by first using the FORTRAN INQUIRE statement to see if it is a .tan filename (FILTAN). in which case the file is opened and the numbers read from the file - or else read the value directly from the field itself, treated as a character string.
  2. For limb or zenith paths with 'space' as the background, a cosmic background temperature 2.7 K is assumed, set by parameter TEMSPA_DEF in file phycon.inc (TEMSPA in phyadj_dat.f90 for RFM v5 (modified using the *PHY section)
  3. For limb paths which intersect the surface (defined as the lowest altitude specified in the first *HGT profile in the *ATM Section rather than 0km), it is also necessary to enable the SFC Flag so that the surface parameters are specified. This is always mandatory for NAD and FLX (although not FLX+ZEN) geometries which, by definition, include surface terms. Although TAN values themselves must lie within the atmosphere it is possible that the FOV convolution causes the 'surface' to be seen, in which case a fatal error results unless the SFC Flag is enabled.
  4. The *GEO Section label can be used to specify rays which intersect the surface (in which case the 'refracted' tangent height is meaningless). The minimum geometric tangent height, given by to -(Radius of Curvature), corresponds to a nadir view.
  5. The *ELE Section label requires the OBS flag to be enabled since an observer altitude has to be specified in order to calculate the projected path. However, this is the only way to specify an 'upward-viewing' path in a spherical atmosphere since tangent height, refracted or otherwise, has no meaning in this case.
  6. Tangent heights, elevation angles and path lengths must generally be distinct to 5 significant figures so that they give distinct output filenames (eg 1m below 100km, 0.001 deg elevation, but note: 1 km above 100km) otherwise a 'Repeated Tan.Hgt' fatal error message results.
  7. The use of *TAN, *ELE or *GEO section labels also has an impact on the interpretation of the data in the .fov file.
  8. With the GRA Flag enabled (limb-viewing with horizontal gradients), the location of the tangent point is either determined by the observer location PSI=PSIOBS (OBS Flag enabled) or taken at the reference profile, PSI=0.0 as defined in the *ATM section.
  9. Note that there are two different types of *LEV sections, the one here is a primary section and requires the FLX Flag to be enabled. The other definition of the *LEV section is a secondary section linked to the LEV Flag which generates intermediate output levels for a ray path calculation. The *FLX and LEV Flags are mutually exclusive so there should be no ambiguity in the interpretation of the *LEV section.
  10. With the *LEV/FLX option if LEV does not correspond to an existing atmospheric profile level a new level is inserted. This may not be desirable when also using the MTX option so a warning is issued in that case.
  11. *LEN section: path length s [m] is related to dimensionless optical depth χ (generated with OPT flag) by
    χ = k ( v p s ) / ( R T )
    where k is the absorption coefficient [m2 kmole-1], v is the volume mixing ratio [ppv], p is the pressure [Pa], R is the molar gas constant [J K-1 kmole-1] and T is the temperature [K]. So specifying a homogeneous path pressure of 1013.25 Pa, temperature 296 K, and VMR 1 x 10-6 (ie 1 ppmv), a length
    s = ( R T )/( v p ) = ( 8314.472 x 296 ) / ( 1 x 10-6 x 1013.25 ) = 2.4289 x 109 m = 2.4289 x 106 km
    would give an optical depth spectrum numerically identical to the absorption coefficient spectrum k [m2 kmole-1]. However, since the RFM only allows path lengths up to 10 000 km, a suggested value is 2428.9 km to give [m2 mole-1] (or else increase the VMR to 1000 ppmv).

Examples
Normal use as list of actual tangent heights for limb path calculations
*TAN                
     paths.tan      ! list of tangent heights from a standard scan pattern.
    40 41 42        ! additional paths at 40, 41, 42 km,
Nadir-viewing calculations for plane-parallel atmosphere (NAD Flag).
*SEC
    1  ! Direct nadir view only
Set of elevation angles for a balloon observations (OBS Flag)
*ELE
     -90,  -1.5, -0.5, 0.0, 0.5, 1.5  ! [deg] nadir, plus views towards limb

Bugs
Bug#117 (Fixed v4.36)
Bug#85 (Fixed v4.31)
Bug#83 (Fixed v4.31)
Bug#69 (Fixed v4.28)
Bug#2 (Fixed v4.02)