Ground-based Assessment of the V8 Reprocessing of ESA's Envisat MIPAS Geophysical Data Products

Infrared observations of the Earth's limb by the MIPAS instrument on Envisat have contributed a unique data record to study physical and chemical processes in the atmosphere between 2002 and 2012. MIPAS spectra contain the information to infer accurate vertical profiles of air pressure, temperature and volume mixing ratio of more than 20 trace gases from cloud top well into the mesosphere. Over the past few years the chain of ESA's operational Level-1b and Level-2 processors was further developed and it now entered a phase of comprehensive testing and validation by the MIPAS Quality Working Group. This upgraded MIPAS chain is labelled V8 and it includes, e.g., (a) updated spectroscopic data, (b) revised non-linearity coefficients and an improved pointing correction model (Level-1b), and, (c) the handling of horizontal inhomogeneities in the retrieval of geophysical quantities (Level-2). Such changes potentially have an important impact on the quality of the Level-2 data products: their bias, precision or long-term stability, and their dependence on geophysical parameters. Here, we present the results of a delta-validation study of the altitude registration and of five primary MIPAS Level-2 data products (temperature, O3, HNO3, CH4 and N2O). Our analyses are based on comparisons of ~10% of the MIPAS data record to co-located ground-based observations by ozonesonde, temperature and ozone lidar, microwave radiometer and FTIR instruments operating within monitoring networks contributing to WMO's Global Atmospheric Watch (GAW), such as the Network for the Detection of Atmospheric Composition Change (NDACC) and the Southern Hemisphere ADditional OZonesonde programme (SHADOZ). We performed comprehensive studies of the structure of MIPAS bias and short-term variability in the spatial (vertical, latitudinal) and in the temporal domain at various scales. Estimates of these quality indicators on the partial data record have proven in the past to be reliable and they reflect what is later on obtained once the entire mission is reprocessed. These first results for the upcoming V8 reprocessing are then compared to those obtained for earlier MIPAS processor chains in order to verify whether the Level-2 data quality evolves according to expectations.