ABSTRACT
This study fits within the ongoing activities aimed at filling the lack of an operational IR limb sounder after the ENVISAT fault. Notably, we report the performance of a possible evolution of the MIPAS experiment. The strategy proposed for the new experiment (that we denote as MIPAS2k) is derived from the PREMIER infrared limb sounder (IRLS) and relies on both 1D array detector technology and reduction of the spectral resolution to achieve dense atmospheric sampling. We define observation parameters and report, as an example, the performance obtained by MIPAS2k when measuring O3fields. The information load (IL) analysis was exploited to assess the sensitivity of MIPAS2k and to select optimal spectral intervals for retrieval tests on simulated observations of the new experiment. The results of the IL analysis suggest a new approach to the retrieval strategy (denoted as full-2D) in which the unknown parameter is no longer an element of the altitude profile but the constant value taken by the atmospheric quantity within a parcel (denoted as "clove") of the 2D discretization. We demonstrate that the clove homogeneity assumption generates errors that are below the spectral noise of MIPAS2k when an appropriate clove thickness is used. Full-2D retrievals have been carried out on MIPAS2k simulated observations corresponding to a high resolution model atmosphere. We report a test case on O3 VMR in which the retrieval precision is better than 5% between 20 and 40 km and better than 30% in the upper troposphere-lower stratosphere. We test the ability of MIPAS2k to reconstruct a fine O3 structure present in the model atmosphere and we show how this structure would have been represented by MIPAS when measuring the same scenario. We have estimated the spatial resolution of MIPAS2k products by means of the perturbation approach that, in simulated retrievals, can be adopted to evaluate the averaging kernel of the retrieval parameters. For O3 we have found the estimates of 200 km and 2.5 km for the horizontal and vertical resolutions respectively.
Subject(s)
Atmosphere/chemistry , Environmental Monitoring/methods , Models, Theoretical , SpacecraftABSTRACT
We present a new retrieval model designed to analyze the observations of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), which is on board the ENVironmental SATellite (ENVISAT). The new geo-fit multitarget retrieval model (GMTR) implements the geo-fit two-dimensional inversion for the simultaneous retrieval of several targets including a set of atmospheric constituents that are not considered by the ground processor of the MIPAS experiment. We describe the innovative solutions adopted in the inversion algorithm and the main functionalities of the corresponding computer code. The performance of GMTR is compared with that of the MIPAS ground processor in terms of accuracy of the retrieval products. Furthermore, we show the capability of GMTR to resolve the horizontal structures of the atmosphere. The new retrieval model is implemented in an optimized computer code that is distributed by the European Space Agency as "open source" in a package that includes a full set of auxiliary data for the retrieval of 28 atmospheric targets.
ABSTRACT
The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a limb-scanning spectrometer that has operated onboard the Environmental Satellite since the end of March 2002. Common features of limb-scanning experiments are both high vertical resolution and poor horizontal resolution. We exploit the two-dimensional geo-fit retrieval approach [Appl. Opt. 40, 1872-1875 (2001)] to investigate the possibility of improving the horizontal resolution of MIPAS measurements. Two different strategies are considered for this purpose, one exploiting the possibility (offered by the geo-fit analysis method) for an arbitrary definition of the retrieval grid, the other based on the possibility of saving measurement time by degrading the spectral resolution of the interferometer. The performances of the two strategies are compared in terms of the trade-off between the attained horizontal resolution and the retrieval precision. We find that for ozone it is possible to improve by a factor of 2 the horizontal resolution, which in the nominal measurement plan is approximately 530 km. This improvement corresponds to a degradation of the retrieval precision, which on average varies from a factor of 1.4 to 2.5, depending on the adopted spectral resolution.
