Water vapor measurements in the stratosphere at 936 nm by stellar occultations with GOMOS/Envisat.
Jean-Loup Bertaux(1), Alain Hauchecorne(1), Nadège Montoux(1), Francis Dalaudier(1), Laurent Blanot(2) and Jean-Claude Lebrun(1)
(1) LATMOS, BP3, 91371 Verrieres-le-Buisson, France
(2) ACRI-ST, LATMOS, BP3, 91371 Verrieres-le-Buisson, France
The evolution of water vapor in the lower stratosphere is still a case for controversy. The GOMOS instrument on board ENVISAT contains a high resolution spectrometer for the measurement of O2 at 760 nm and H2O at 936 nm, with a resolution of ~0.05 nm per pixel.
GOMOS is the first instrument measuring these species by the technique of stellar occultation, which is protected in principle from long term drift, an essential feature for long term monitoring of a possible trend.
Nine stars are sufficiently bright to provide useful measurements for stratospheric H2O. However, the data retrieval has been plagued by a severe pixel-to-pixel non-uniformity of the CCD detector, coupled to the motion of the star on the CCD caused by scintillation. The PRNU (including sub-pixel structure of the response) is about ~20 %, while absorption features are of the order of a few % in the lower stratosphere.
New developments, using special motions of the star on the CCD, have allowed determining a much better PRNU and sub-pixel response, and the retrieval of reasonable values (3 to 6 ppmv) in the lower stratosphere. These new results will be presented, along with the prospects for long term trend studies.