RFM Driver Table Sections

*TAN Section

08MAY22

Type
Primary Section#6

Description
Definition of ray-path. The original RFM was for limb-viewing so the section was used to specify TANgent height, but now used as a generic section header for specifying some aspect of ray path which depends on viewing geometry (See RFM Viewing Geometries).

Format
Generally: Multiple fields, arbitrary order, except for
*DIM Section/TAB Flag - see [specific page].
SectionFlagsFieldTypeDescriptionRange
(any) (any) TANFIL C200 Name of a Data File
Limb-viewing
*TAN TAN R Actual (refracted) tangent heights [km] Within atmospheric altitude range
*GEO GEO R Projected (geometric) tangent heights [km] -Radius of Curvature:top of atmosphere
*ELE ELE R 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 Airmass factors (=sec(zenith angle)) [dimensionless], for plane-parallel atmospheres ≥1.0 (1=vertical path)
*LEV FLX LEV R Altitude levels [km] for flux calculation outputs. Within atmospheric altitude range .atm file
*LEN HOM LEN R Path lengths [km] for homogeneous path calculations. >0
" UNITS=UNITS C2 Units for homogeneous path length km, m, cm or mm
*DIM TAB See [specific page] for record structure.
Type: I=Integer; R=Real; D=Double Precision; Cn=character string, length n.

Notes
  1. TANFIL: The type of field is identified by first using the FORTRAN INQUIRE statement to see if it is a Data File, in which case the file is opened and the numbers read from the file as if they were directly inserted within the section at that point.
  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 TEMSPA in phyadj_dat.f90 (modified using the *PHY section). This provides negligible radiance in the infrared or shorter wavelengths.
  3. Any limb paths which intersect the surface (defined as the lowest altitude in the atmospheric profile) it is also necessary to enable the SFC Flag. This is always mandatory for NAD and FLX (although not FLX+ZEN) geometries which, by definition, include surface terms.
  4. Although TAN values themselves must always lie within the atmosphere (= surface value is allowed) it is possible that the FOV convolution causes the 'surface' to be seen, in which case the SFC Flag must also be enabled.
  5. 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 -(Radius of Curvature), corresponds to a nadir view.
  6. 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. 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.
  7. 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.
  8. The use of *TAN, *ELE or *GEO section labels also has an impact on the interpretation of the data in the .fov file.
  9. 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.
  10. 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.
  11. 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.
  12. *LEN section: by default the RFM defines all lengths in km (this can be changed by setting UNITAN in tancom_dat.f90). However, RFM v5.10 onwards allows an additional PARAM=VALUE entry in the *LEN section to redefine the length units as m, cm or mm.
  13. *LEN: the path length can be used to scale the dimensionless optical depth (OPT) spectrum χ to have values numerically equivalent to spectrum of absorption coefficient in [m2 mol-1]. The path length s is related to optical depth χ by
    χ = k ρ s
    where ρ is the absorber density. The aim is to set ρs =1 mol m-2, so that, numerically, χ=k.

    Expanding ρ, for standard atmospheric pressure and temperature 296 K, assuming a volume mixing ratio of 1 ppmv

    s = (1 mol m-2 ) ( R T )/( v p )
    = (1 mol m-2 ) ( 8.3145 J mol-1 K-1 x 296 K) / ( 10-6 x 1013.25 Pa )
    = 2.4289 x 106 m = 2428.9 km

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
Homogenenous path, specifying length in cm
*LEN
UNITS=cm
    50.3  ! Output filenames will be, eg tra_50300.asc

Bugs
Bug#10 (Fixed v5.02)
Bug#2 (Fixed v5.01)