*RTV Section

28FEB25
Type
Mandatory Section#5

Description
Retrieval grid, and list of parameters to be retrieved.
The retrieval grid is defined in terms of indices (1: NLEV, increasing in altitude) of the internal atmospheric grid established in the *LEV section.

Format
Single record defining grid level indices for retrieval, then multiple fields of single type, any order.
Structure
IDXGRD Retrieval Grid Indices, single record
RTV1[i:j:k] RTV2[i:j:k] ... Retrieval Parameters [retrieval grid subsets], multiple records
... RTVn[i:j:k]

Field Type ValueDescriptionUnitsRestrictions
IDXGRD[1] Cn * Retrieve profile on all grid levels Only item in record
0 Column retrievals Only item in record
i Retrieve on level i 1:NLEV
i:j Retrieve on all levels i:j inclusive 1:NLEV
i:j:k Retrieve on levels i:j with step k 1:NLEV
RTV Cn TEM Temperature [2] K
PRE Pressure [2] hPa Not on *PRE levels
[gas] Molecule VMR[5] ppmv ∈ {List of gases}
AEROSOL Aerosol [4] km-1
SFCTEM Surface Temperature K
Parameters retrieved independently for each microwindow[3]
CTM Continuum[4] km-1
OFF Radiometric Offset nW/(cm2.sr.cm-1)
SCA Radiometric Scale Factor
LIN Radiometric Linear Variation
QAD Radiometric Quadratic Variation
Type: Cn=character string, length n.

Notes
  1. IDXGRD lists the indices of the forward model grid (defined in the *ALT/*PRE/*TAN section), on which the retrieved profiles are represented (although individual species may be retrieved on subsets of this grid). Index 1 represents the lowest altitude level. Apart from the special cases of * and 0 this can be a list of increasing integers, a range expression (i:j or i:j:k), or any mix as long as these increase monotonically.

  2. The hydrostatic constraint is assumed whenever temperature (TEM) and pressure (PRE) are retrieved together in limb mode, or if temperature is retrieved on pressure surfaces in nadir mode.

  3. Parameters retrieved independently for each microwindow and therefore not included in the output unless the MWO Flag is set. Most of these are calibration adjustments relating the forward model (f) to the measurement (y) as
    y = OFF + f × ( 1 + SCA ) × ( 1 + LIN×ν + QAD×ν2 )
    where ν is a 'normalised' wavenumber, varying from −0.5 to +0.5 across the microwindow. All four parameters default to zero if not retrieved, and have zero as their a priori unless specified otherwise *APR section)

  4. AEROSOL differs from CTM in that a single profile is retrieved for all microwindows, rather than an independent profile for each microwindow separately. Effectively AEROSOL is treated just like any other retrieved gas. Note that the a priori covariances for AEROSOL and CTM differ, so even if AEROSOL is retrieved for a single microwindow over the same altitude range as CTM the results will differ.

  5. [gas] Molecules can either be specified by the chemical formula, HITRAN index or (for CFCs) 'F' number. Internally these are are stored as formulae or (for CFCs) 'F' number, all lower case.

Examples
Nadir-viewing (IASI)
*RTV 0 ! Column retrieval SFCTEM NH3 H2O ! Joint surface temperature, NH3 & H2O columns
Limb-viewing (MIPAS)
*RTV * ! Retrieve on all (17 here) tangent heights TEM PRE CTM[6:17] OFF ! Joint pT, MW continuum (scans 6-17 only) and offset