Retrieval, Validations and Interpretation of Stratospheric Water Vapor Distributions from SCIAMACHY Lunar Occultation Measurements

Abstract

Water vapor is a unique atmospheric component, its distribution directly influences the chemistry and dynamics in different parts of the atmosphere. Water vapor observations from satellite borne instruments provide information on its long term seasonal and inter-annual variabilities and are important for climate projections and predictions. In the polar stratosphere, the water vapor amounts control the polar vortex temperatures and the formation temperature of the polar stratospheric clouds (PSCs). Water vapor has a strong relationship with the circulation and transport features related to polar vortex and its amounts define the Formation and deposition of PSCs. SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) aboard Envisat launched in 2002 has observed the Earth s atmosphere in nadir, limb and solar or lunar occultation geometry covering ultraviolet, visible and near ifrared (240-2830 nm) wavelength range at moderate spectral resolution. SCIAMACHY's measurements have provided total columns as well as vertical profiles of atmospheric constituents and climate parameters relevant to the ozone chemistry, air pollution and global climate change issues, from Troposphere upto the Mesosphere. This thesis contributes to exploit the lunar occultation spectra measured by SCIAMACHY at the local night time, with a latitudinal coverage of 56-89°S, to derive vertical stratospheric water vapor number density profiles in the altitude range of 17-50 km, from 2003 till 2010. The water vapor distributions are retrieved using the spectral window 1350-1420 nm in the near infrared region of the SCIAMACHY spectra. SCIATRAN version 3.0, a radiative transfer model and an inversion scheme based on optimal estimation (OE), is optimized and adapted for the retrieval. Within the inversion scheme, the residuals between the measured differential optical depths and the ones calculated by the forward model are in the order of 0.5%. The radiative transfer computation is performed using correlated-k method employing the exponential sum fitting of transmission function (ESFT) approximation instead of the detailed but computationally costly line by line (LBL) model. Since LBL is the most precise representative of reality, the ESFT database (pressure, temperature and coefficient grids) is optimized with an objective to achieve the closest agreement between the two approaches and to obtain a high quality retrieval product. The retrieval is observed to be highly sensitive to the numbers of coefficients in the ESFT grid. Extensive sensitivity studies and optimizations are performed for the key input parameters as slit function, signal to noise ratio, Tikhonov parameter and the climatological profiles to select their optimal values in the retrieval setting. To evaluate the quality of the retrieved lunar occultation water vapor product, validations are performed with collocated measurements from the satellite occultation instruments ACE-FTS and HALOE and the instruments measuring in limb geometry, MLS and MIPAS. In addition SCIAMACHY limb observations are also used for comparison. SCIAMACHY lunar occultation and ACE-FTS measurements agree within 7% on the average. With HALOE, the difference is around 5%. The validation of the coincident lunar occultation and the MLS measurements is exceptionally good with an agreement of 1.5-4% whereas that with MIPAS is in the range of 10%. The comparisons of the lunar occultation product with all the four instruments are found to be well within their reported biases. With the SCIAMACHY limb water vapor measurements, the differences are of the order of 4%. The validation results show that an excellent SCIAMACHY lunar occultation water vapor product is obtained. The dataset of water vapor distributions from SCIAMACHY lunar occultation measurements is expected to facilitate the understanding of physical and chemical processes in the southern mid-latitudes, the dynamical processes related to polar vortex and on the formation of PSCs, which can be studied through our product right at their onset as observed. The product will add as the southern hemispheric measurement coverage to the SCIAMACHY long term global water vapor time series.