Improved Water Vapour Retrievals For MERIS, OLCI And A Strategy To Fill The Gap Using MODIS And Ground Truth
Diedrich, Hannes; Preusker, Rene; Lindstrot, Rasmus; Fischer, Jürgen
Institut für Weltraumwissenschaften FU Berlin, GERMANY
Water vapour is a key climate variable: It is the most important greenhouse gas in the atmosphere, accounting for about 60% of the natural greenhouse effect for clear skies. In the lower troposphere, condensation of water vapour into precipitation provides latent heating which dominates the structure of tropospheric diabatic heating. Satellite observations provide near-global coverage and thus represent an important source of information over the oceans, where radiosonde observations are scarce and over land, providing a higher accuracy and spatial resolution.
Within the frame of the ESA DUE GlobVapour project, an advanced algorithm for the retrieval of total columnar water vapour (TCWV) from measurements of the Medium Resolution Imaging Spectrometer (MERIS) was developed, that accounts for all influencing effects such as the temperature- and pressure-dependent gaseous absorption as well as aerosol scattering and provides realistic uncertainty estimates.
The MERIS retrieval scheme has recently been extended to enable reliable retrievals over water surfaces, by properly taking into account the bi-directional reflectance distribution function (BRDF) of the wind-roughened ocean surface. The such improved water vapour retrieval can serve to significantly improve the atmospheric correction of MERIS measurements over ocean, needed for ocean colour remote sensing.
Unfortunately, due to the loss of control over ENVISAT in April 2012, a continuous observation of TCWV was interrupted. Nevertheless, the MERIS retrieval is being adapted to the three water vapour absorption channels of the MODerate Resolution Imaging Spectroradiometer (MODIS) to fill the gap in the TCWV time line between MERIS and the Ocean and Land Colour Instrument (OLCI) onboard Sentinel-3 which is planned to launch in 2014.
A further improvement in accuracy over all surface types is expected from adapting the algorithm to include the additional, strongly absorbing OLCI water vapour channel at 940nm.
The presentation will focus on the algorithm improvements and global validation results for MERIS over both land and ocean. An overview of the MODIS algorithm will be shown and first validation results will be provided. Additionally, sensitivity studies regarding the information content with respect to water vapour of the OLCI channel setting are shown.