ABSTRACT
For data analysis of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) atmospheric limb emission spectroscopic experiment on Environmental Satellite microwindows, i.e., small spectral regions for data analysis, have been defined and optimized. A novel optimization scheme has been developed for this purpose that adjusts microwindow boundaries such that the total retrieval error with respect to measurement noise, parameter uncertainties, and systematic errors is minimized. Dedicated databases that contain optimized microwindows for retrieval of vertical profiles of pressure and temperature, H2O, O3, HNO3, CH4, N2O, and NO2 have been generated. Furthermore, a tool for optimal selection of subsets of predefined microwindows for specific retrieval situations has been provided. This tool can be used further for estimating total retrieval errors for a selected microwindow subset. It has been shown by use of this tool that an altitude-dependent definition of microwindows is superior to an altitude-independent definition. For computational efficiency a dedicated microwindow-related list of spectral lines has been defined that contains only those spectral lines that are of relevance for MIPAS limb sounding observations.
ABSTRACT
In atmospheric Fourier transform spectroscopy so-called microwindows are usually analyzed for retrieval of trace constituents rather than the spectrum as a whole. These microwindows, which are sets of consecutive spectral grid points, contain one or more prominent transitions of the target species, whereas it is desirable for the signal of interfering species to be minimum. An objective, quantitative method is presented to optimize the microwindow boundaries with respect to random errors, signal of interfering species, other parameter and systematic errors and to select optimum microwindows with respect to their associated retrieval errors. Case studies for N(2)O microwindows are performed for a spaceborne limb emission experiment to assess the dependence of the optimum microwindow width on the retrieval concept.
ABSTRACT
It is generally accepted that multiple scattering is important for evaluating backscatter lidar signals in the case of moderate or high optical depths and large receiver fields of view. On one hand, multiple scattering must be considered in inverting signals to obtain backscatter coefficients; on the other hand, it offers the opportunity to derive microphysical parameters of the scattering medium. Bissonnette developed a numerical code for the propagation of a continuous-wave laser beam through an atmosphere including multiple scattering. His model is also applicable to a backscatter lidar approximatively.
In this paper we investigate if the assumptions on which his backscatter lidar application is based are valid for typical atmospheric situations. It is found that for small and moderate optical depths, a prerequisite for the backscatter lidar application is fulfilled: second-order iterations of the solution to the radiative transfer equation can indeed be neglected as proposed by Bissonnette.
Furthermore, we propose an improvement of the simulation for limited fields of view that significantly alters the radial dependences of the backscattered signals. Essentially, on-axis backscattered signals are increased and the profiles tend to be somewhat narrower near the optical axis. The dependence of the radiative distribution on the phase function of the scattering medium, the optical depth, and on the field of view of the receiver is also changed. The modifications only slightly increase the computer time. Examples for typical atmospheric situations are shown, and proposals for intercomparisons with other models and measurements are made.
