MIPAS new measurement scenario: enhanced vertical resolution and regularization

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is operating on ENVISAT since March 2002. After two years of nearly continuous limb-scanning measurements (from July 2002 to March 2004) at the end of March 2004 the instrument was stopped due to problems with the mirror drive of the interferometer. Operations with reduced maximum path difference, corresponding to both a lower spectral resolution (0.0625 cm-1 instead of 0.025 cm-1) and a shorter measurement time (1.8 sec instead of 4.5 sec), were resumed on January 2005. In order to exploit the reduction in measurement time, a change in the measurement scenario was implemented. Compared with the original measurement scenario, the new scenario adopts a finer vertical limb scanning step in the region of the troposphere and lower stratosphere. If the retrieval is done at the tangent altitudes, the use of a limb scanning step smaller than the width of the instrument field of view introduces instabilities in the retrieval and requires a regularization to avoid oscillations in the retrieved profiles. A Tikhonov regularization with a novel automated procedure to determine the value of the regularization parameter has been developed for the analysis of spectra acquired in the new measurement scenario. The procedure is based on the criterion that the differences between the regularized and the non-regularized profiles should be, on average, equal to the errors of the regularized profile. This criterion leads to an analytical formula for the calculation of the regularization parameter. In this paper we present the results of this method in terms of both vertical resolution and errors of retrieved profiles and compare these performances with those obtained with the original measurement scenario.