Anu Dudhia

Abstract:

MIPAS, the Michelson Interferometer for Passive Atmospheric Sounding, is a mid-infrared emission spectrometer which is part of the core payload of ENVISAT. It is a limb sounder, i.e. it scans across the horizon detecting atmospheric spectral radiances which are inverted to vertical temperature, trace species and cloud distributions. These data can be used for scientific investigations in various research fields including dynamics and chemistry in the altitude region between upper troposphere and lower thermosphere. The instrument is a well calibrated and characterized Fourier transform spectrometer which is able to detect many trace constituents simultaneously. The different concepts of retrieval methods are described including multi-target and two-dimensional retrievals. Operationally generated data sets consist of temperature, H2O, O3, CH4, N2O, HNO3, and NO2 profiles. Measurement errors are investigated in detail and random and systematic errors are specified. The results are validated by independent instrumentation which has been operated at ground stations or aboard balloon gondolas and aircraft. Intercomparisons of MIPAS measurements with other satellite data have been carried out, too. As a result, it has been proven that the MIPAS data are of good quality. MIPAS can be operated in different measurement modes in order to optimize the scientific output. Due to the wealth of information in the MIPAS spectra, many scientific results have already been published. They include intercomparisons of temperature distributions with ECMWF data, the derivation of the whole NOy family, the study of atmospheric processes during the Antarctic vortex split in September∼2002, the determination of properties of Polar Stratospheric Clouds, the downward transport of NOx in the middle atmosphere, the stratosphere- troposphere exchange, the influence of solar variability on the middle atmosphere, and the observation of Non-LTE effects in the mesosphere.

Abstract:

Clouds are a source of major uncertainty in climate models - it is thus important to accurately model clouds in order to determine their properties. In this work, three cloud parameters (cloud top height, cloud top temperature and cloud extinction coefficient) are used to model the radiance measured in the MIPAS field-of-view as they represent the most obvious physical, thermodynamic and optical properties, respectively, of a cloud. Finally, this model is implemented in an optimal estimations-type retrieval of cloud top height, temperature and extinction co-efficient from real MIPAS spectra.

Abstract:

The main advantage in combining limb and nadir geometries is that it allows the stratospheric and tropospheric ozone concentrations to be separated which makes it possible to improve the tropospheric ozone retrieval. This paper presents the retrieval method which will be used to combine TES nadir measurements with MIPAS limb measurements and shows some preliminary results. The Tropospheric Emission Spectrometer (TES) is a high-spectral-resolution infrared imaging Fourier transform spectrometer operated by NASA's Jet Propulsion Laboratory(JPL). It has a spectral range from 3.2/μm to 15.4/μm and at present is mostly operating in the nadir mode. TES routinely measures temperature and concentrations of O3,H2O, CH4, CO, HNO3, and N2O. A local optimal estimation retrieval code (the MIPAS Orbital Retrieval using Sequential Estimation (MORSE)) was used to retrieve Volume Mixing Ratios (VMR's) from the low resolution ESA level IB MIPAS data and level IB TES data. A joint retrieval was achieved by using the MIPAS retrieved VMR profiles as the a priori for the TES retrieval. A similar method could be used in the future to combine MIPAS and IASI.

Abstract:

Ozone isotope data for 50O3 and 49O 3 are retrieved from MIPAS limb emission spectra. Enrichments for 50O3 range 7 to 12% in the middle stratosphere. For 49O3 most enrichments are between 7 and 10%. In stratosphere (25-40 km) these measurements are in agreement with previous measurements (e.g. FIRS) and with expectations based on laboratory measurements. Above 40 km and below 25 km, the enrichments of both 50O3 and 49O3 get larger, from 30 to 50%. MIPAS enrichments show latitude and altitude structure.

Abstract:

MIPAS is a limb viewing fourier transform spectrometer operating in the infra-red. It scans the tangent altitude range 6-68 km at a vertical resolution of approximately 3km. CO has various natural and anthropogenic sources including forest fires and industry. In the troposphere, CO is the main sink of the principal oxidising agent - the hydroxyl radical - and so reduces its capacity for the removal of other atmospheric pollutants. In the stratosphere, its intermediate lifetime makes it useful as a tracer for stratospheric motions and processes of tropospheric-stratospheric exchange. However, the retrieval of CO from a limb sounding IR instrument is complicated because of non-LTE (non-local thermodynamic equilibrium) effects. Ordinarily, the internal vibrational energy level populations are controlled by collisions between molecules and follow the translational (kinetic) energy distribution of the ambient gas (LTE). However, if collisions are infrequent, radiative processes can lead to a non-Boltzmann distribution of the internal energy level populations (non-LTE). CO is strongly affected by non-LTE in the IR down to around 40km and modelling these processes is difficult. These effects are especially problematic in the limb viewing geometry due to the long path length viewed through the upper atmosphere. However, using a joint CO, vibrational temperature (Tv) retrieval it is possible to estimate the non-LTE effects without modelling the energy level populations. Instead, we retrieve a parameterisation of the non-LTE effects, Tv, directly from the emission spectra.