Anu Dudhia

Abstract:

Water vapour and methane are two of the target gases retrieved operationally in near-real-time (NRT) from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat. Water vapour and methane are chemically linked, since methane oxidation is the main source of water vapour in the stratosphere. The oxidation of methane eventually produces water vapour and molecular hydrogen such that the sum H2+H2O+2CH 4 is approximately constant with altitude. Assuming that the mixing ratio of molecular hydrogen is constant with altitude in the lower stratosphere, we would also expect the quantity H2O + 2CH4 to be reasonably constant in this region, providing a useful internal validation test. Here we assess the quality of water vapour and methane profiles from the ESA NRT retrieval, based on MIPAS data from July 2002 until March 2004. Monthly means of MIPAS profiles are used to examine differences from climatology, and look for any biases or long-term trends. Comparisons are also made with measurements from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS).

Abstract:

Spectrally resolving infrared instruments such as the Michelson Interferometer for Passive Atmospheric Sounding generate measurements at a far greater rate than can be accurately modelled within a retrieval. For limb-viewing instruments the usual approach is to select microwindows: subsets of the spectrum of a few cm-1 width which maximise the information on the target species while minimising the contributions from potential error sources such as emissions from other molecules. Here, the algorithm used to select microwindows for ESA's Level 2 processing of MIPAS data is outlined, together with some examples of its application.

Abstract:

The isotopic ratios of stratospheric ozone can be used as valuable tracers for many processes in the atmosphere. Here we compare stratospheric enrichments of heavy ozone obtained from two limb sounders: MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), on board Envisat, SMR (Sub-Millimetre Radiometer), on board the Odin satellite.

Abstract:

Operationally only pressure, temperature and six significant trace gases are retrieved by ESA from MIPAS data. However, information on many other species is also present in the spectra. We apply a variety of techniques and our own retrieval model to retrieve the concentration of three other species: SO 2,OCS and SF6. Sulphur Dioxide (SO2) is an acidic gas with both natural and anthropogenic sources that is rapidly converted to sulphuric acid and hence sulphate aerosols in the atmosphere. Carbonyl Sulphide (OCS) is produced naturally at the ocean surface and by biomass burning and, through stratospheric oxidation, it is thought to be the main contributor to non-volcanic stratospheric sulphate aerosols. Sulphur Hexafluoride (SF 6) is almost entirely anthropogenic in its origins and shows steady year-on-year increases making it useful for age of air and tracer studies. We anticipate the good global coverage and continuity of data will make MIPAS useful for the determination of changes and trends in the quantity and distribution of these species both natural and anthropogenic. In this paper we summarise the current progress that has been made in the retrieval of these important sulphur-containing species. We show some preliminary zonal mean fields and briefly outline the methods applied.

Abstract:

The isotopic composition of stratospheric water vapour depends on the sources of water vapour and the temperature and precipitation history of the stratospheric air. Isotopic measurements therefore have potential to aid invesigation of dehydration in the polar vortex, of stratospheric/tropospheric exchange, of upward transport in the tropics and of subsidence at polar latitudes. Here we compare measurements of H216O, HDO and H218O from two satellite limb-sounding instruments: the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), an infra-red limb sounder on Envisat, and the Sub-Millimetre Radiometer (SMR), a microwave limb sounder on board the Odin satellite.