SEMIANALYTICAL CLOUD RETRIEVAL ALGORITHM AND ITS APPLICATION TO DATA FROM MULTIPLE OPTICAL INSTRUMENTS ON SPACEBORNE PLATFORMS: SCIAMACHY, MERIS, MODIS, SeaWiFS AND GOME Alexander A. Kokhanovsky, Vladimir V. Rozanov Wolfgang von Hoyningen-Huene, John P. Burrows Institute of Remote Sensing, Bremen University P. O. Box Bremen, Germany
PARAMETERS to be retrieved: 1. Cloud optical thickness /5-100/ 2. Cloud top height /0.5-10km/ 3. Cloud cover /0-1/ 4. Cloud albedo/ / 5. Liquid water path / / 6. Thermodynamic phase /ice, water or mixed clouds/ 7. Average size of droplets/crystals
PHYSICAL BASIS Bi-spectral measurement of TOA reflectance to get cloud optical thickness (visible), cloud droplet radius (IR) Spectrally resolved measurements (approximately, 50 spectral points in the oxygen A-band nm) to get CTH, CBH The ratio R(1550)/R(1670) for the ice/water cloud discrimination
Exponential approximation
Table 1. Characteristics of selected space instruments, related to measurements of the backscattered light in the oxygen A-band ( ). InstrumentPlatformYearSpectral interval, Spectral resolution, Spatial resolution, km 2 GOMEERS *320 SCIAMACHYENVISAT *60 MERISENVISAT *0.3 or 1.1*1.1 GLIADEOS-II *0.25 and 1.0*1.0 POLDERADEOS-II *7.0
Typical SCIAMACHY spectra in the oxygen A-band Cloud top height determination from SCIAMACHY
VALIDATION MULTIPLE TECHNIQUES
Cloud top height determination from GOME data using oxygen A-band information as compared to ATSR-2 IR retrievals (ERS-2 satellite)
CONCLUSION SACURA has great potentials in cloud remote sensing
Acknowledgements K. Bramstedt, M. Buchwitz R. de Beek, K.-U. Eichmann W. Lotz, T. Nauss S. Noel, A. Rozanov H. Schroeter, M. Vountas European Space Agency DLR(grant 50EE0027) and NASA
The retrieval techniques 1: The look-up-table approach
2: The semi-analytical approach
Optical characteristics of clouds Trishchenko and Liu, 2001 Number of cases Optical thickness ISCCP data Surface observations Kokhanovsky, 2003
The physics behind the semi-analytical approach SatelliteSun 1 2
The cloud optical thickness determination
The accuracy of the semi-analytical asymptotic theory
Cloud top height determination from a satellite The geometry of the problem
Nadir observation, solar zenith angle – 20 degrees Cloud optical thickness –20. The accuracy of the forward model Cloud top height determination from a satellite
The accuracy of the forward model Cloud top height determination from a satellite
The physical principle behind the retrieval
Preliminary results ! Cloud geometrical thickness determination from a satellite
Table 1. Characteristics of selected space instruments, related to measurements of the backscattered light in the oxygen A-band ( ). InstrumentPlatformYearSpectral interval, Spectral resolution, Spatial resolution, km 2 GOMEERS *320 SCIAMACHYENVISAT *60 MERISENVISAT *0.3 or 1.1*1.1 GLIADEOS-II *0.25 and 1.0*1.0 POLDERADEOS-II *7.0
Typical SCIAMACHY spectra in the oxygen A-band Cloud top height determination from SCIAMACHY
Cloud top height determination from GOME data using oxygen A-band information as compared to ATSR-2 IR retrievals (ERS-2 satellite)
Conclusions Most of important cloud parameters can be retrieved using spectral top of atmosphere reflectance measured by SCIAMACHY. The calibration of the instrument should be improved for this, however. Cases of inhomogeneous clouds can bias retrieval results considerably. This should be clarified in future research. The information on clouds obtained on a global and regional scale should enhance our studies of climate change, including anthropogenic influences on cloud microphysical and optical properties. JGR, 2003, 2004; IEEE TGRS, 2004; JQSRT, 2004; IEEE TGRS Letters, 2004
Acknowledgements K. Bramstedt, M. Buchwitz R. de Beek W. von Hoyningen-Huene S. Noel, M. Vountas ---- European Space Agency DLR(grant 50EE0027)