An optimized forward and retrieval model for MIPAS near real time data processing

The infrared emission limb sounder MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) will be operated as an ESA core instrument on the ENVISAT-1 satellite. Near real time retrieval of pressure, temperature (p,T) and volume mixing ratio (VMR) of five key species (O-3, H2O, N2O, CH4, and HNO3) from calibrated spectra will be performed in the Level 2 processor of the ENVISAT Payload Data Segment. In order to develop an optimized (with respect to speed and accuracy) retrieval algorithm suitable for the implementation in MIPAS Level 2 processor, an ESA supported study is being carried out. In the framework of this study, an optimized forward / retrieval code based on the global fit approach was implemented. In this approach all the spectra of a limb-scanning sequence are simultaneously fitted, so that error propagation in the altitude domain is avoided. The attained accuracy performances of the retrieval code are the following: Temperature error < 2 K at all the altitudes covered by the standard MIPAS scan (8-53 km) Tangent pressure error: < 3% Error on the retrieved VMR of the key species: < 5 % at most of the altitudes of scientific interest covered by the standard MIPAS scan. The run-time required to perform p,T and VMR retrieval of the five MIPAS target species from a limb-scanning sequence of 16 limb-views is less than 6 minutes on a SUN SPARCstation 20. The most effective code optimizations were implemented in the radiative transfer model and in the computation of the jacobian of the retrieval.