Inter-calibration of the SEVIRI solar bands against MODIS Aqua, using Deep Convective Clouds as transfer targets Sébastien Wagner, Tim Hewison In collaboration with D. Doelling and D. Morstad (NASA) Contact: sebastien. wagner@eumetsat. int EUMETSAT Conference Sopot, 3 -7 Sept 2012
Outline 1. Context (GSICS) 2. Current calibration system (for the reflective bands) 3. Inter-calibration with MODIS using DCCs a) Main steps b) Some results 4. Conclusions and future work EUMETSAT Conference Sopot, 3 -7 Sept 2012
Context. . . GSICS • What is GSICS? – Initiative of CGMS and WMO – Effort to produce consistent, well-calibrated data from the international constellation of Earth Observing satellites • What are the basic strategies of GSICS? – Improve on-orbit calibration by developing an integrated inter-calibration system (GEO-LEO, LEO-LEO) – Best practices for prelaunch characterisation (with CEOS WGCV) • Benefits: – – – Improve consistency between instruments Reduce bias in Level 1 and 2 products Provide traceability of measurements Retrospectively re-calibrate archive data Better specify future instruments Slide : 3 EUMETSAT Conference Sopot, 3 -7 Sept 2012 EUMETSAT CNES JMA NOAA CMA KMA ISRO NASA WMO USGS NIST JAXA ROSHYDROMET IMD ESA
Context. . . GSICS principles • Systematic generation of inter-calibration products • For Level 1 data from passive satellite sensors (weather, climate and other environmental applications) • To compare, monitor and correct the calibration of monitored instruments to community references • By generating calibration corrections (NRT / Re-analysis applications) with specified uncertainties • Through well-documented, peer-reviewed procedures • Based on various techniques to ensure consistent and robust results • Delivery to users • Free and open access • Adopting community standards • To promote • Greater understanding of instruments’ absolute calibration • More accurate and more globally consistent retrievals • Inter-operability for more accurate environmental, climate and weather forecasting products EUMETSAT Conference Sopot, 3 -7 Sept 2012 TRACEABILITY / UNBROKEN CHAINS OF COMPARISONS
GSICS in context SCOPE CM Level 1 b/c Data (Radiances) Corrections NWP centres Developers of Lev 2/3 products. . . GOS Users GSICS Statement of Needs EUMETSAT Conference Sopot, 3 -7 Sept 2012
What about the SEVIRI solar band calibration (GOS box)? • 4 solar channels ( VIS 06 / VIS 08 / NIR 16 / HRVIS ) • Specifications for the solar channel calibration? è SEVIRI = 10% accuracy + 5% long-term stability è Achieved by vicarious calibration SEVIRI Solar Channel Calibration syst. (Govaerts et al. , TGRS 2004) • Reference = RTM simulations of Top-Of-Atmosphere radiances (evaluated against wellcalibrated polar-orbiting instruments) • Comparison with TOA measured signal above desert and ocean targets • Currently: è Meteosat 8: launch = 28/08/2002 operation = 19/01/2004 è Meteosat 9: launch = 21/12/2005 operation = 18/07/2006 è Meteosat 10: launch = 05/07/2012 VIS 06 HRVIS Slide : 6 NIR 16 VIS 08 EUMETSAT Conference Sopot, 3 -7 Sept 2012
Using Deep Convective Clouds as transfer target for inter-calibration of SEVIRI with Aqua/MODIS • DCCs = invariant targets • Available all around the Earth (not the case for desert for example) seen by all GEO satellites • Well characterized and easily detectable (infra-red threshold) • Higher part of the dynamic range • Seen by well calibrated instruments on board polar satellites calibration transfer targets Þ Outcome: 1. Improved calibration and long term monitoring of the instruments 2. Consistent calibration across the GEO satellite constellation • Within the GSICS framework: • ATBD as provided by D. Doelling et al. , 2011 (https: //gsics. nesdis. noaa. gov/wiki/Home) • Use of MODIS Aqua as a reference • Within EUMETSAT, two-fold approach: 1. Implementation of an inter-calibration algorithm based on the use of DCCs for VIS 06 2. Development of a vicarious calibration algorithm to be added to the current SSCC system (absolute calibration + drift monitoring) EUMETSAT Conference Sopot, 3 -7 Sept 2012
Main steps of the method as in Doelling et al, 2011 1. DCC identification • Time: for MET-9 (0. 0 Lat / 0. 0 Lon) 11: 00 < t < 13: 00 • Geometry: • Lat / Lon between 0. 0 and +/- 20. 0 degrees (with respect to the SSP) • SZA and VZA < 40 degrees • DCC identification threshold using the MODIS 11 m band SEVIRI 10. 8 m (BT<205 K) • Spatial homogeneity (over boxes of pixels in the “ 11 m” BT + in the “ 0. 6 m” radiances) 2. Conversion from counts to overhead sun + spectral transformation to account for Spectral Response Function differences between MODIS and SEVIRI • Use of an Angular Distribution Model (Hu model, Hu et al. 2004) • Use of correction factors as given by the GSICS ATBD (Doelling et al, 2011) 3. Construction of the Probability Density Functions on a monthly basis 4. Derivation of the gain from the calibration equation: Slide : 8 EUMETSAT Conference Sopot, 3 -7 Sept 2012
Example of DCCs tracking with MODIS and SEVIRI as implemented Aqua/MODIS 11 m Met-9/SEVIRI 10. 8 m Test on the view zenith angle Missing DCCS due to test on VZA MODIS Aqua (01/04/2010 – 13: 00) Slide : 9 EUMETSAT Conference Sopot, 3 -7 Sept 2012 SEVIRI Met-9 (01/04/2010 – 12: 57)
Preliminary results for a few cases. . . SEVIRI Met-9 DCC counts – 0. 6 m MODIS Aqua DCC radiance – 0. 6 m Shift in the PDFs + double peaking for 07/2008 0. 57 Statistical uncertainty u / mean < 0. 2% 2008 2011 1. 2 % 0. 56 0. 55 With 0. 54 Sampling pb, but not only. . . Slide : 10 Derived gain for Julys 0. 53 -2. 8% 1. 7 % 2010 2009 0. 52 0. 51 EUMETSAT Conference Sopot, 3 -7 Sept 2012 -5. 7 % 2012 What is wrong? ? ?
Conclusions • Implementation of the GSICS ATBD still on-going (uncertainty analysis still missing) • Limited to the VIS 06 band • Still some issues to be solved • Questions: 1. What about the other visible channels (VIS 08 and HRVIS)? 2. What about Rapid Scan Data? (limited spatial coverage) Once problems are solved Development of a vicarious calibration algorithm to be added to the current SSCC system (absolute calibration + drift monitoring) Slide : 11 EUMETSAT Conference Sopot, 3 -7 Sept 2012
Thank you Slide : 12 EUMETSAT Conference Sopot, 3 -7 Sept 2012
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