Sequential and joint retrieval of trace gas profiles Original Research Article

A common approach within the retrieval of atmospheric trace gas VMR profiles is the prior determination of pressure and temperature followed by sequential retrievals of various species. This, however, can lead to degraded results if errors from previously retrieved profiles propagate into succeeding computations. An alternative is given by the simultaneous (i.e. joint) retrieval of species combinations if the joint retrieval leads to smaller retrieval errors. During the development of a level 2 offline processor for the MIPAS instrument (an infrared limb sounding instrument to be flown on the ENVISAT mission in 2001), a number of tests based on a preliminary set of spectral microwindows were conducted that show characteristic effects resulting from sequential versus joint retrieval. We describe typical examples of our retrieval sequencing, notably the basic retrieval scheme, sources of error propagation, cases where a joint retrieval is advantageous — in particular, a combination of temperature retrieval with trace gas determination, issues of computational efficiency and memory requirements during joint retrievals, as well as the impact of microwindows.