Improved CH4- and N2O-retrieval using MIPAS-V8 spectra

Retrieval of CH4 and N2O from MIPAS V8-spectra using a new approach led to improvements with respect to vertical resolution, convergence rate and to the reduction of physically unrealistic strong oscillations, which occurred in V5 profiles at air-mass boundaries. For illustration of the improvement in convergence rates, we show latitude-height cross sections of CH4 along MIPAS orbit 29041, obtained from the V5 (a) and from the V8 retrieval (b). The number of not converged retrievals (vertical white stripes) has considerably decreased in the latter case. Figure (c) shows strong oscillations in V5 CH4 profiles retrieved around 75°N along MIPAS orbit 30832, which are considerably reduced in the respective V8 profiles (d).

The improvement of the vertical resolution especially shows up in enhancement of the poor resolution of the V5 data around 15 km, but also in improvements in the middle and upper atmosphere. This was achieved both by taking into account more CH4 and N2O lines in the spectral region used so far and by increasing the wavenumber range applied for analysis to 1217.0­--1337.9375 cm-1. Joint-fitting of CH4 and N2O gradients as well as use of an oscillation detection between subsequent iteration steps of the retrieval led to the improvement in convergence rates and to the removal of formerly physically unrealistic strong oscillations in various profiles (Glatthor et al., 2024).

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