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H2O Total Column

Table of Contents H2O Total Column

Water vapour is calcualted with the AMC DOAS Method (Noël et al., 1999). The AMC-DOAS algorithm is based on the well-known Differential Optical Absorption Spectroscopy (DOAS) approach (Platt, 1994) which has been modified to handle effects arising from the strong differential absorption structures of water vapour. The general features of this modified DOAS method are that

1. saturation effects arising from highly structured differential spectral features which are not resolved by the measuring instrument are accounted for, and

2. O2 absorption features are fitted in combination with H2O to determine a so-called air mass factor (AMF) correction which compensates to some degree for insufficient knowledge of the background atmospheric and topographic characteristics, like surface elevation and clouds.

The main equation of the Air Mass Corrected DOAS method is given by:


equ 6-1



I,I0 Earthshine radiance and solar irradiance
P Polynomial to correct for broadband contributions (resulting e.g. from Rayleigh and Mie scattering or surface albedo)
tO2 Optical depth of O2
CV Vertical column amount of water vapour
b,c Spectral quantites describing saturation effect and absorption

c contains the effective reference absorption cross section and the air mass factor. The scalar parameter a is the above mentioned AMF correction factor. The quantities tO2, b, and c are determined from radiative transfer calculations performed for different atmospheric conditions and solar zenith angles. CV and a are then derived from a non-linear fit. The error of the vertical column is calculated from the covariance matrix also resulting from the fit. CO Total Column

Carbon monoxide retrieval from SCIAMACHY nadir observations is rather challenging: Only channel 8 from 2259 to 2386 nm features CO absorption signatures, albeit very weak and superposed by stronger absorption lines of concurrent gases, i.e. H2O and CH4. Additionally, an ice layer on the detector modifies the measured signal. Even worse, degradation of the detector increasingly reduces the number of reliable pixels, i.e. only about 50 of 1024 pixels in channels 8 are useful for CO retrieval.

The forward model is based on the MIRART (Modular InfraRed Atmospheric Radiative Transfer) line-by-line code, developed for arbitrary observation geometry, instrumental field-of-view and spectral response functions (Schreier and Schimpf, 2001). Molecular absorption cross sections are calculated using spectroscopic line parameters from the Hitran, Geisa and other databases, together with optional continuum corrections (continuum corrections to the absorption coefficient are supported). Derivatives of transmission and/or radiance spectra are obtained by means of automatic differentiation. MIRART has been extensively verified by intercomparisons with other codes, e.g. in the framework of the EU study AMIL2DA.

The relation between forward model F and measured signal I is


where image is the optical depth along the entire line-of-sight (Sun-ground-satellite) for the reference atmosphere, image is the wavenumber and image is the spectral response function. The state vector x to be retrieved comprises the column density scaling factors image, the slit function half width image, the surface reflectivity (albedo) r and the baseline correction b. Note that the reflectivity r and the baseline b enter the forward model linearly and the least squares problem can be reduced to a separable nonlinear least squares problem. For the solution of the least squares problem, BIRRA uses solvers provided in the PORT Optimization Library based on a scaled trust region strategy. BIRRA provides the option to use a least squares with simple bounds (e.g., non-negativity) to avoid unphysical results.



Retrieval Setting Summary

Level 1b-c Settings
Calibration All calibrations except polarisation and radiometric
SMR A0 (Sun over ASM diffuser without radiometric calibration)
Main Settings
Fitting Interval 2324.4 - 2335.0 nm
Absorbers Fitted CO, CH4 , H2O
Polynomial Degree Albedo 2
Slit function Gaussian
Proxy for xCO CH4 CH4 Total Column


The CH4 retrieval uses two spectral windows in channel 6. As a proxy correction to take into account the effect of clouds and transmission changes CO2 is used, since its variations are small compared to methane. As for CO two columns can be found in the product, the total column and the total column corrected by the CO2 proxy.


Retrieval Setting Summary

Level 1b-1c Settings
Calibration All calibrations except polarisation and radiometric
SMR A0 (Sun over ASM diffuser without radiometric calibration)
Main Settings
Fitting Interval 1557.18 - 1594.13 nm & 1628.93 - 1670.56 nm
Absorbers Fitted CO2, CH4 , H2O
Polynomial Degree Albedo 2
Slit function Gaussian
Proxy for xCO CO2


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