A proposed algorithm for line-of-sight wind retrieval from MIPAS level 1 spectra
Claude Camy-Peyret(1), Sebastien Payan(1), Pascal Prunet(2), Emmanuel Dufour(2), Roger Brugge(3), Alan O’Neill(3), Jean Giroux(4) and Philippe Lagueux(4)
(1) LPMA, Universite Pierre et Marie Curie, Tour 13, Case 76, 75252 PARIS Cedex 05, France
(2) Noveltis, 2, Avenue de l'Europe, 31520 Ramonville-Saint-Agne, France
(3) DARC, University of Reading, PO Box 243, Earley Gate, Reading RG6 6BB, United Kingdom
(4) Telops, 4940, rue Pierre-Georges-Roy, G3A 1V7, Canada
Because of their high spectral resolution (0.035 cm-1 unapodised), MIPAS spectra contain potential information on the Doppler shifts due to wind along the line-of-sight (LOS) in a given limb scan sequence. The intrinsic absolute stability of the MIPAS wavenumber scale is not granted to the required precision, however. The present ESA operational spectral calibration algorithm is relying on the position of the atmospheric emission lines themselves (assuming no wind effect and in a small range of tangent altitudes where lines are neither too broadened by pressure effects, nor too weak).
An algorithm has been proposed that just relies on the relative stability of the wavenumber scale within a given limb scan sequence (usually 17 spectra recorded from about 65 to 10 km in approximately 75 s) to derive a vertical LOS relative wind profile. This LOS wind information is derived by an appropriate algorithm called ANA (for analytical method because an explicit formula is used) producing a wavenumber scaling factor between consecutive spectra in a given limb scan. This scaling factor is derived from a number of optimised wind spectra windows spanning the 5 bands covered by MIPAS. Because of the relative nature of the process, the absolute interband/interwindow calibration is not needed and an optimum weighted average value of the scaling factors can be derived with weights (and an associated occupation matrix) representing the impact of radiometric noise in each window w and for each pair of tangent heights. The average scaling factor is then converted to a relative LOS wind difference and, assuming the LOS wind is known at one level, an absolute LOS wind profile can be provided.
The algorithm has been developed (through an ESA contract study), coded and tested on simulated spectra (to check the performances of the method and to establish the noise amplification error factors). We are in the process of testing the algorithm on a set of selected MIPAS spectra recorded in correlation with standard meteorological radio-soundings (providing T, u and v as a function of P). Preliminary results of this study will be presented including comparisons with ECMWF and sonde results, showing that a precision reaching the target of 5 m/s could be expected in the altitude range 20 to 35 km.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,