Optimized forward model and retrieval scheme for MIPAS near-real-time data processing

Abstract

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 six key species (O 3 , H 2 O, HNO 3 , CH 4 , N 2 O and NO 2 ) from calibrated spectra will be performed in the Level 2 processor of the ENVISAT Payload Data Segment. An ESA supported study was carried out for the development of an optimized (with respect to speed and accuracy) retrieval algorithm suitable for the implementation in MIPAS Level 2 processor. In the framework of this study, an optimized forward / retrieval code was implemented based on the global fit approach. In this approach all the spectra of a limb-scanning sequence are simultaneously fitted in order to correctly account for error correlations in the altitude domain. Besides, only spectral intervals which are sensitive to the retrieved parameters are analyzed by using a microwindow approach. This also minimizes interferences of spectral signatures from atmospheric species with unknown concentration. Finally, a sequential retrieval of the target species VMR profiles is performed. The trade-off between run time and accuracy of the retrieval was optimized from both the physical and mathematical point of view, with optimizations in the program structure, in the radiative transfer model and in the computation of the retrieval Jacobian. The attained performances of the retrieval code are: noise error on temperature < 2 K at all the altitudes covered by the typical MIPAS scan (8-53 km with 3 km resolution), noise error on tangent pressure < 3 % , noise error on VMR of the target species < 5 % at most of the altitudes 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 1 minute on a modern work-station.

MIPAS: an instrument for atmospheric and climate research

“…The operational MIPAS level 2 data processor was conceived by an international consortium which developed the scientific prototype, named Optimized Retrieval Model (ORM, Ridolfi et al, 2000), used for the operational implementation. The main challenge of the ORM has been the requirement of performing the complex operation of mathematical inversion in near real time (NRT) for a large number of data.…”

“…While the ORM retrieves atmospheric temperatures and mixing ratios of constituents on an altitude grid defined by the tangent altitudes of the actual limb scan (Ridolfi et al, 2000), other algorithms use a fixed altitude grid, which in case of the RCP is much finer than the MIPAS tangent altitude spacing (1 km vs. 3 km) (von Clarmann, 2003b). Most of the processors either rely on the engineering information on the MIPAS elevation pointing (e.g.…”

“…In the operational processing temperature and pressure are retrieved from CO 2 spectral signatures assuming hydrostatic equilibrium (Ridolfi et al, 2000). Total temperature errors as a function of height are estimated by Oxford University.…”

Volcanic ash detection with infrared limb sounding: MIPAS observations and radiative transfer simulations

“…The vertical MIPAS field of view is about 3 km. For retrieval of MIPAS measurements, it is often assumed to be trapezoidal with edge lengths of 2.8 and 4 km (Ridolfi et al, 2000;Hurley et al, 2011). The field of view at the tangent point extends about 3 km in the vertical direction and 30 km perpendicular to the line of sight.…”

Characterization of model errors in the calculation of tangent heights for atmospheric infrared limb measurements

“…Our implementation is however planned for inclusion in the ESA retrieval model for the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) routine data processing (Ridolfi et al, 2000;Raspollini et al, 2006Raspollini et al, , 2013, which assumes that all limb scans lie in the orbit plane. In a forthcoming evolution of this algorithm, the atmosphere will be represented with a 2-D discretization in the orbit plane and will be considered constant in the direction perpendicular to this plane.…”