MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is operating on board of the ENVISAT satellite and is acquiring for the first time high spectral resolution middle infrared emission limb sounding spectra of the Earth atmosphere from space. The measurement capabilities make it possible to determine every 75 sec. the vertical profile of several atmospheric trace constituents, during both day and night with an almost full coverage of the globe. This leads to a three dimensional measurement of the atmospheric composition. In order to handle the large data flow, an optimized code for the Level 2 near real time analysis of MIPAS data was developed by an international consortium of scientists under an ESA contract and was implemented in the ENVISAT Ground Segment. The code is designed to provide, in an automated and continuous mode, atmospheric vertical profiles of temperature and pressure, as well as of concentrations of O3, H2O, CH4, HNO3, N2O and NO2, in the altitude range from 6 to 60 km. The "commissioning phase," in which verification and validation of the instrument and of the analysis code are performed, is still in progress, but some preliminary results have been obtained. The first examples of the MIPAS near real time Level 2 data products, consisting of retrieved profiles and auxiliary data that characterize the measurement accuracy and resolution, are shown.
Retrieval methods for limb-scanning measurements usually adopt the assumption of horizontal homogeneity for the parcel of atmosphere sounded by the analyzed observations. For along-track observations from an orbiting platform, the dimension of the probed parcel of atmosphere may approach 2000 km. Horizontal homogeneity assumption in the inversion algorithms induces an error on the retrieved atmospheric parameters. Two questions arise from these considerations: 1) how to characterize these errors, 2) how to avoid this assumption if the size of the induced errors is not acceptable. In order to answer these questions, an innovative forward and retrieval model (geo-fit) was developed which does not use horizontal homogeneity assumption. In this approach the radiative transfer is made through a two-dimensional inhomogeneous atmospheric field. The retrieval algorithm is based on the simultaneous analysis of all the limb-scanning measurements relating to a given orbit. This feature allows to gather information from several contiguous limb-scanning sequences on a target atmospheric parameter at a given location, and therefore to improve the trade-off between accuracy and horizontal resolution. The obtained results show that the horizontal homogeneity assumption induces a significant systematic error on the retrieved atmospheric parameters, especially in the presence of strong horizontal gradients.
The capability of remote-sensing measurements of the stratosphere in the submillimetric spectral region is presented. The analysis is discussed of spectra recorder by a balloon-borne Fourier Transform spectrometer that measures the emission of the atmosphere in the submillimetric with the limb-scanning observation technique. Two problems, that have been studied with submillimetric spectroscopy, are presented in detail: the measurement of HBr and the diurnal variability of hydroxyl radical.
An ESA supported study was carried out for the development of an optimized code for near real time retrieval of altitude profiles of pressure, temperature (p, T) and volume mixing ratio (VMR) of five key species (O3, H2O, HNO3, CH4 and N2O) from infrared limb sounding spectra recorded by MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), which will be operated on board ENVISAT-1 satellite. The implemented model is based on the Global Fit approach, i.e. all the limb-scanning spectra are simultaneously fitted, and on the analysis in narrow spectral intervals (microwindows). The trade-off between run time and accuracy of the retrieval was optimized from both the physical and mathematical point of view, with improvements 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 as follows: noise error on temperature less than 2 K at all the altitudes covered by the standard MIPAS scan (8 - 53 km), noise error on tangent pressure less than 3%, noise error on VMR of the target species less than 5% at most of the altitudes of scientific interest covered by the standard MIPAS scan, with a total run time of less than 6 minutes on a SUN SPARC station 20.
The IR emission limb sounder MIPAS will be operated as an ESA core instrument on the ENVISAT-1 satellite. Near real time retrieval of pressure, temperature and volume mixing ratio of five key species from calibrated spectra will be performed in the Level 2 processor of the ENVISAT Payload Data Segment. In order to develop an optimized 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: (i) temperature accuracy < 2 K at all the altitudes covered by the standard MIPAS scan; (ii) tangent pressure error: < 3 percent; (iii) error on the retrieved VMR of the key species: < 5 percent at most of the latitudes 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 optimization were implemented in the radiative transfer model and in the computation of the jacobian of the retrieval.
The chemical composition of the lower stratosphere has been measured using a polarizing interferometer operating in the far infrared and submillimetric spectral region. The instrument was flown three times (in 1992, 1993 and 1994) from the NSBF balloon base (Fort Sumner, New Mexico) in coincidence with overpasses of the UARS satellite, for a total of about 50 hours of measurements. In this paper we report some of the results obtained from the data analysis made up to now.