|
Atmos. Meas. Tech. Discuss., 4, 6811-6844, 2011
www.atmos-meas-tech-discuss.net/4/6811/2011/ doi:10.5194/amtd-4-6811-2011 © Author(s) 2011. This work is distributed under the Creative Commons Attribution 3.0 License. |
1-D-Var retrieval of daytime total columnar water vapour from MERIS measurements
1Institut für Weltraumwissenschaften, Freie Universität Berlin, Carl-Heinrich-Becker-Weg 6–10, 12165 Berlin, Germany
2Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, 1225 W. Dayton Street Madison, WI, USA
Abstract. A new scheme for the retrieval of total columnar water vapour from measurements of MERIS (Medium Resolution Imaging Spectrometer) on ENVISAT (ENVIronmental SATellite) is presented. The algorithm is based on a fast forward model of the water vapour transmittance around 900 nm, including a correction for atmospheric scattering. It provides the water vapour column amount for cloud-free scenes above land and ocean at a spatial resolution of 0.25 km × 0.3 km and 1 km × 1.2 km, depending on whether applied to the "full resolution" or the operational "reduced resolution" mode of MERIS. Uncertainties are provided on a pixel-by-pixel basis, taking into account all relevant sources of error. An extensive validation against various sources of ground-based reference data reveals a high accuracy of MERIS water vapour above land (root mean square deviations between 1 mm and 2.7 mm), apart from a wet bias of MERIS between 5 and 10% that is found in all comparison studies. This wet bias might be caused by spectroscopic uncertainties, such as the description of the water vapour continuum. Above ocean the accuracy is reduced, due to the uncertainty introduced by the unknown atmospheric scattering. Consequently, an increased root mean square deviation of ≥5 mm was found by comparing MERIS total columnar water vapour above ocean against SSM/I and ENVISAT MWR data. An increased wet bias of 2–3 mm is found over ocean, potentially du to a not properly working atmospheric correction scheme.
Citation: Lindstrot, R., Preusker, R., Diedrich, H., Doppler, L., Bennartz, R., and Fischer, J.: 1-D-Var retrieval of daytime total columnar water vapour from MERIS measurements, Atmos. Meas. Tech. Discuss., 4, 6811-6844, doi:10.5194/amtd-4-6811-2011, 2011.