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GERB thermal flux regional bias: detection and correction by comparison with CERES

GERB thermal flux regional bias: detection and correction by comparison with CERES. S. Dewitte Royal Meteorological Institute of Belgium. Overview. Methodology Night time GERB CERES comparison Day time GERB CERES comparison MS5 MS7 comparison Validation ADM corrections. Methodology.

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GERB thermal flux regional bias: detection and correction by comparison with CERES

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  1. GERB thermal flux regional bias: detection and correction by comparison with CERES S. Dewitte Royal Meteorological Institute of Belgium

  2. Overview • Methodology • Night time GERB CERES comparison • Day time GERB CERES comparison • MS5 MS7 comparison • Validation ADM corrections

  3. Methodology • Radiative flux at the top of the atmosphere: F (W/m2) • Satellite observations: radiances L (W/m2sr) • Satellite viewing zenith angle qvz F = p L (qvz) / R(qvz) • GERB: fixed qvz • Validation GERB fluxes: comparison with CERES fluxes with variable qvz

  4. Used data • GERB: ARG fluxes, SEVIRI as imager, Version 2 • CERES FM3: RAPS or GERB mode or special scan, use of inflight calibration • 1-6/2004 • use of night and day data for thermal fluxes • CERES data is colocated to nearest GERB ARG pixel

  5. Regional difference – night time

  6. GERB – CERES difference fit • Impose linear variation with qvz :f(qvz)=(52.5°- qvz)/ 52.5° • Fit difference as function of GERB flux:Fgerb-Fceres=a(Fgerb). f(qvz)+b(Fgerb)a(Fgerb) = GERB nadir error • Corrected GERB flux = Fgerb - a(Fgerb) . f(qvz)

  7. Difference versus flux – night time

  8. All scenes – night time Before correction After correction

  9. Daytime LW comparison

  10. Day and night time LW nadir error

  11. MS7 – MS5 difference fit • OLR estimate from MS5: Fms5OLR estimate from MS7: Fms7 • Comparison for July 2004 • Fit difference as function of MS7 flux:Fms7-Fms5=a(Fms7). f(qvz,ms7) -b(Fms7).f(qvz,ms5)+c(Fms7)a(Fms7) = MS7 nadir errorb(Fms7) = MS5 nadir error

  12. Without correction Cold cloud correction

  13. Full correction

  14. Conclusions • Theoretical thermal ADM errors are identified from comparsions of • Night time GERB / CERES FM3 • Day time GERB / CERES FM3 • MS5 / MS7 • The remaining differences are • within +/- 15 W/m2 over hot desert • within +/- 5 W/m2 elsewhere

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