A correlated‐ k distribution scheme for overlapping gases suitable for retrieval of atmospheric constituents from moderate resolution radiance measurements in the visible/near‐infrared spectral region

An accurate and fast radiative transfer scheme for the simulation of the spectral radiance measurements of a new generation of multichannel moderate resolution satellite spectrometers, such as GOME/ERS‐2 (240–790 nm) and SCIAMACHY/ENVISAT‐1 (240–2385 nm), has been developed. Representative absorption cross sections of the individual molecular line‐absorbers CH 4 , CO, CO 2 ; H 2 O, N 2 O, and O 2 for relatively narrow spectral intervals (≤0.2 nm) have been determined using a combined correlated‐ k ( c ‐ k ) distribution and exponential sum fitting of transmittances (ESFT) scheme. The speed of the c ‐ k calculations mainly depends on the channel‐dependent spectral resolution of the instruments. For the measurements to be made by SCIAMACHY, this may be up to 800 times faster than line‐by‐line calculations. The agreement between c ‐ k radiances and line‐by‐line reference radiances is in the range 1–2%. The dominant error (∼1%) is attributed to the correlation assumption made when applying the k distribution method to inhomogeneous atmospheres. As the commonly used c ‐ k methods for overlapping gases are shown to result in large radiance errors at SCIAMACHY spectral resolution or do not permit individual representative absorption cross sections to be defined for each absorber separately, a new and novel method for the accurate simulation of overlapping absorption has been developed, verified, and validated. This technique enables overlapping line absorption to be considered for arbitrary spectral correlation of the monochromatic absorption cross sections of the individual gases. The new c ‐ k scheme is based on a linear mixing of two radiance terms determined assuming correlation and anticorrelation of the monochromatic absorption cross sections of the overlapping gases.