Impact of temperature field inhomogeneities on the retrieval of atmospheric species from MIPAS IR limb emission spectra

摘要

We examine volume mixing ratios (vmr) retrieved from limb emission spectrarecorded with the Michelson Interferometer for Passive Atmospheric Sounding(MIPAS) on board Envisat. In level 2 (L2) data products of three different retrievalprocessors, which perform one dimensional (1-D) retrievals, we findsignificant differences between species' profiles from ascending anddescending orbit parts. The relative differences vary systematically withtime of the year, latitude, and altitude. In the lower stratosphere theirmonthly means can reach maxima of 20% for CFC-11, CFC-12, HNO₃, H₂O,10% for CH₄ and N₂O. Relative differences between monthly means of1-D retrieval results and of the true atmospheric state can be expected toreach half of these percentage values, while relative differences in singlevmr profiles might well exceed those numbers. Often there are no physical orchemical reasons for these differences, so they are an indicator for aproblem in the data processing. The differences are generally largest atlocations where the meridional temperature gradient of the atmosphere isstrong. On the contrary, when performing the retrieval with a tomographic twodimensional (2-D) retrieval, L2 products generally do not show thesedifferences. This suggests that inhomogeneities in the temperature field, andpossibly in the species' fields, which are accounted for in the 2-D algorithmand not in standard 1-D processors, may cause significant deviations in theresults. Inclusion of an externally given adequate temperature gradient inthe forward model of a 1-D processor helps to reduce the observeddifferences. However, only the full tomographic 2-D approach is suitable toresolve the horizontal inhomogeneities. Implications for the use of the 1-Ddata, e.g. for validation, are discussed. The dependence of theascending/descending differences on the observation strategy suggests thatthis problem may affect 1-D retrievals ofinfrared limb sounders, if the line of sight of the instrument has asignificant component in the direction of the horizontal temperaturevariation.