*LEN Section

RFM Driver Table Sections: Ray Path
12APR25
Type
Primary Section#6, alternative name for *TAN section when HOM flag is used

Description
Define homogeneous path length and (optionally) path length units if not km.

Format
Multiple fields, any order, one optional PARAM=VALUE field.
Field Type Description Units Range
TANFIL C200 Name of a .dat file [1]
LEN R Path length km >0
UNITS=UNITS C2 Units for path length[2] ∈ {'km', 'm', 'cm', 'mm'}
Type: R=Real; 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 file (assumed to be a .dat 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. This can be used in combination with actual numerical fields.

  2. UNITS: By default path lengths are defined in km (set by UNITAN in tancom_dat.f90), but the *LEN section accepts a PARAM=VALUE pair to redefine the units, eg UNITS=CM (upper or lower case, but no spaces either side of the '=' sign).

  3. For radiative (rather than the usual transmittance) calculations for a homogeneous path, the SFC Flag & *SFC Section may be used to set 'backplate' conditions. Otherwise it is assumed (effectively) that the far end of the path is space (emitting with the Cosmic Background temperature TEMSPA, 2.7 K).

  4. Setting LEN = 2428.9 km with a concentration of 1 ppmv generates an optical depth spectrum (OPT) χ with numerical values equivalent to absorption coefficient in m2/mole.

    The derivation is as follows: optical depth χ is related to absorption coefficient k by
    χ = k ρ s
    where ρ is the absorber density and s is the path length. The aim is to set ρs =1 mole/m2, so that, numerically, χ=k.

    Expanding ρ= vp/(RT), at temperature T=296 K and standard atmospheric pressure p, assuming a volume mixing ratio of v=1 ppmv (keep the concentration low to return air-broadened k rather than self-broadened)

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

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

Bugs
[none recent]