Abstract
Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is an infrared (IR) limb emission spectrometer on the Envisat platform. It measures trace gas distributions during day and night, pole-to-pole, over an altitude range from 6 to 70 km in nominal mode and up to 170 km in special modes, depending on the measurement mode, producing more than 1000 profiles day−1. We present the results of a validation study of methane, version V5R_CH4_222, retrieved with the IMK/IAA (Institut für Meteorologie und Klimaforschung, Karlsruhe/Instituto de Astrofisica de Andalucia, Grenada) MIPAS scientific level 2 processor. The level 1 spectra are provided by the ESA (European Space Agency) and version 5 was used. The time period covered is 2005–2012, which corresponds to the period when MIPAS measured trace gas distributions at a reduced spectral resolution of 0.0625 cm−1. The comparison with satellite instruments includes the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), the HALogen Occultation Experiment (HALOE), the Solar Occultation For Ice Experiment (SOFIE) and the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY). Furthermore, comparisons with MkIV balloon-borne solar occultation measurements and with air sampling measurements performed by the University of Frankfurt are presented. The validation activities include bias determination, assessment of stability, precision validation, analysis of histograms and comparison of corresponding climatologies. Above 50 km altitude, MIPAS methane mixing ratios agree within 3 % with ACE-FTS and SOFIE. Between 30 and 40 km an agreement within 3 % with SCIAMACHY has been found. In the middle stratosphere, there is no clear indication of a MIPAS bias since comparisons with various instruments contradict each other. In the lower stratosphere (below 25 km) MIPAS CH4 is biased high with respect to satellite instruments, and the most likely estimate of this bias is 14 %. However, in the comparison with CH4 data obtained from cryogenic whole-air sampler (cryosampler) measurements, there is no evidence of a high bias in MIPAS between 20 and 25 km altitude. Precision validation is performed on collocated MIPAS–MIPAS pairs and suggests a slight underestimation of its uncertainties by a factor of 1.2. No significant evidence of an instrumental drift has been found.
Highlights
Atmospheric methane (CH4) is the third most important greenhouse gas, after water vapor and CO2
The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) Institut für Meteorologie und Klimaforschung (IMK)/Instituto de Astrofisica de Andalucia (IAA) methane data product covers mixing ratio profiles of the period 2002–2004 when MIPAS operated in its original high spectral resolution mode (Glatthor et al, 2005), as well as data from 2002 to 2004 when MIPAS measured at reduced spectral resolution (Chauhan et al, 2009; von Clarmann et al, 2009b)
This paper reports the validation of the methane data retrieved from reduced spectral resolution measurements in nominal mode, which is version number V5R_CH4_222
Summary
Atmospheric methane (CH4) is the third most important greenhouse gas, after water vapor and CO2. The MIPAS IMK/IAA methane data product covers mixing ratio profiles of the period 2002–2004 when MIPAS operated in its original high spectral resolution mode (Glatthor et al, 2005), as well as data from 2002 to 2004 when MIPAS measured at reduced spectral resolution (Chauhan et al, 2009; von Clarmann et al, 2009b). IMK/IAA MIPAS products are characterized by uncertainty estimates, as well as vertical averaging kernels. The latter are used to estimate the altitude resolution of the retrievals.
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