Anu Dudhia

Abstract:

An optimized code to perform the near-real-time retrieval of profiles of pressure, temperature, and volume mixing ratio (VMR) of five key species (O(3), H(2)O, HNO(3), CH(4), and N(2)O) from infrared limb spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) experiment on board the European Space Agency (ESA) Environmental Satellite ENVISAT-1 was developed as part of a ESA-supported study. The implementation uses the global fit approach on selected narrow spectral intervals (microwindows) to retrieve each profile in sequence. The trade-off between run time and accuracy of the retrieval was optimized from both the physical and the mathematical points of view, with optimizations in the program structure, in the radiative transfer model, and in the computation of the retrieval Jacobian. The attained performances of the retrieval code are noise error on temperature <2 K at all the altitudes covered by the typical MIPAS scan (8-53 km with 3-km resolution), noise error on tangent pressure <3%, and noise error on VMR of the target species <5% at most of the altitudes covered by the standard MIPAS scan, with a total run time of less than 1 min on a modern workstation.

Abstract:

A new method for evaluating momentum balance in the mesosphere using radar and satellite data is presented. This method is applied to radar wind data from two medium frequency installations (near Adelaide, Australia and Christchurch, New Zealand) and satellite temperature data from the Improved Stratospheric and Mesospheric Sounder (ISAMS). Because of limitations in data availability and vertical extent, the technique can only be applied to evaluate the momentum balance at 80 km above the radar sites for May 1992. The technique allows the calculation of the residual terms in the momentum balance which are usually attributed to the effects of breaking gravity waves. Although the results are inconclusive above Adelaide, this method produces values of zonal and meridional residual accelerations above Christchurch which are consistent with expectation. In both locations it is apparent that geostrophic balance is a poor approximation of reality. (This result is not dependent on a mismatch between the radar and satellite derived winds, but rather is inherent in the satellite data alone.) Despite significant caveats about data quality the technique appears robust and could be of use with data from future instruments.

Abstract:

As an ESA-provided payload instrument on board ENVISAT the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) will routinely acquire atmospheric limb emission spectra in the middle infrared region. Due to its high radiometric sensitivity and spectral resolution capabilities a large number of middle atmospheric constituents can be detected that are of primary interest in the various disciplines of atmospheric research. The ENVISAT ground segment concept foresees generation and near real time dissemination of MIPAS data products up to the level of pressure (p), temperature (T) and volume-mixing ratio (VMR) profiles for the high priority target species O3, H2O, CH4, N2O, HNO3. The basic algorithms have been defined and are currently being verified in various ESA supported studies. End-to-end tests based on simulated instrument raw data have been carried out, using prototype versions of the Level 1 B (processing from raw data up to calibrated radiance spectra) and Level 2 algorithm components (retrieval of vertical p, T and VMR profiles). Runtime checks have been performed for the critical profile retrieval modules. For a reference scenario (p, T plus five trace gas retrievals for a single, 16 heights elevation sequence; one iteration per retrieval), a total runtime of approx. 150 s has been achieved on an IBM RS/6000 model 397.

Abstract:

We discuss the intercomparison between the Karlsruhe Optimized and Precise Radiative transfer Algorithm (KOPRA) and the Reference Forward Model (RFM) codes, which have been designed for analysis of MIPAS-ENVISAT data. The purpose of this intercomparison is to validate the KOPRA algorithm, i.e. to identify and to remove possible errors in the KOPRA (or RFM) code and to quantify the reason of remaining differences. Similar comparisons between the MIPAS Optimised Forward Model (OFM) and the RFM as well as between KOPRA and the RFM have already been performed(Ref.(1,2)). To be able to relate on these results, this validation is similarly organised: we perform subsequently more complex tests of ray-tracing, integrated column amounts, homogeneous and limb path calculations of unapodised, apodised and held-of-view (FOV) convolved spectra, using the same isolated CO2 line as well as the same six MIPAS microwindows. Additionally we compare modelling of CO2 line-mixing non-local thermodynamic equilibrium (NLTE), trace gas continua and cross-section spectra.The KOPRA-RFM residuals are below a quarter of the noise-equivalent spectral radiance (NESR) for the isolated CO2 line as well as for the MIPAS microwindows, i.e. KOPRA fulfils the acceptance criteria requested for the OFM. In most cases the deviations are even clearly below 1 nW/(cm(2) sr cm(-1)), that is more than one order of magnitude below the acceptance threshold. This is valid for unconvolved as well as for ALS (apodised line shape) and FOV convolved spectra. There is also good agreement in modelling of the H2O-, O-2- and N-2-continua and of CO2 line-mixing. Larger deviations of up to several nW/(cm(2) sr cm(-1)) occured for NLTE calculations on the basis of "default" atmospheric profiles with vertical resolution of I or 2.5 km. These differences were found to be due to different layer-averaging of the vibrational temperatures and could be considerably reduced by calculations with a higher vertical resolution of 250 m. Cross-section spectra agree well, if the tabulated data are given independent of pressure, e.g. for ClONO2 and N2O5, and cover the atmospheric temperatures. Due to different temperature extrapolation the deviations increase up to 10 nW/(cm(2) sr cm(-1)) for atmospheric temperatures outside the measuring range. The RFM is not yet adjusted to cross-sections tabulated for non-equidistant temperatures and for atmospheric pressures, like CFC-data in the HITRAN96 database. If these data are used, larger differences arise, e.g, up to 30 nW/(cm(2) sr cm(-1)) between CFC-12 spectra. Avoidance of interpolation by performing homogeneous path calculations for p,T of one of the tabulated cross-section datasets reduces the deviations to below 0.5 nW/(cm(2) sr cm(-1)).