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HELIOSAT-II

HELIOSAT-II. Sophia Antipolis, 17 octobre 2003. What is Heliosat?. Irradiation (energy). Raw image. Why Heliosat-II?. Heliosat 0 (Cano et al. 1986) Heliosat-I (customised versions) Need to Improve accuracy Improve reliability Ease implementation. Achievements of Heliosat-II.

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HELIOSAT-II

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  1. HELIOSAT-II Sophia Antipolis, 17 octobre 2003

  2. What is Heliosat? Irradiation (energy) Raw image

  3. Why Heliosat-II? • Heliosat 0 (Cano et al. 1986) • Heliosat-I (customised versions) • Need to • Improve accuracy • Improve reliability • Ease implementation

  4. Achievements of Heliosat-II • No more parameter to tune (to be checked) • Easier to implement • Reliability in irradiation assessment More explicit physical modelling • Improvement possible and easy More accurate

  5. Accuracy (relative RMSE) Type Irradiation (Wh m-2) Type Irradiance (W m-2) Hourly values 100 Hour 100 Daily values 600 Day 25 5-days sum of daily irradiation 2500 5-days 20 10-days sum of daily irradiation 3500 10-days 15 Monthly mean of hourly irradiation 50 Month 12 Monthly mean of daily irradiation 300 Single pixel. Hi-Res images. 30 and 60 stations WMO in Europe

  6. How it works ? Same principle than Heliosat-0 and other methods (e.g., DWD) t(i,j) - tg(i,j) nt(i,j) = ------------------- tcloud - tg(i,j) n is an attenuation: 1 – transmittance, of TOA irradiation => Irradiation at ground level

  7. How it works ? (2) Kc = f(n) G = Kc Gc n  -0,2 Kc = 1,2 -0,2  n  0,8 Kc = 1-n 0,8  n  1,1 Kc = 2,0667 - 3,6667 n+1,6667 n² n  1,1 Kc = 0,05

  8. Prerequisite - Reflectance Given a radiance L, the reflectance is given by

  9. Prerequisite – Linke turbidity factor TL : optical turbidity of the clear-sky (aerosols and WV) Transmittance  exp[ - k TL dRayleigh ] TL = 1 => pure atmosphere TL = 5 => polluted atmosphere Europe, TL  3 – 3.5

  10. Prerequisite – Basic modelling Downwelling Sensor Clear sky – Broadband Lsat = Tatm Lg + Latm Latm = path radiance rsat = Tatmrg + ratm Tatm = Tdown Tup Latm Tup Tdown Reflected Lg 

  11. Atmospheric Reflectance (2) tsat(i,j) = tatm(qS, qv,) + tg(i,j) Tt(qS) Tt(qv) Latm = (Dc / p) (I0met / I0) (<cosqV> / cosqV)0,8 r* t(i,j) = [ tsat(i,j) - ratm(qS,qv,)] / T(qS) T(qv)

  12. km 50 vis = Latm[W.m-2.sr-1] Modelling Modtran Modelling as a function of Dc Ground Albedo, rg

  13. General scheme Irradiation Atmosphere rsat Cloud index n Gh ratm Calib. coeff I0met Reflectance rg Linke TL Elevation z

  14. Calibration and reflectance Calibration coefficients rsat Lsat I0met calibration calcul_albedo

  15. Atmospheric reflectance Latm I0met ratm Elevation z calcul_Latm Linke TL calcul_albedo

  16. Cloud index rsat r* calcul_n rc rg n n ratm Linke TL Elevation z calcul_rho_rhoc glitter rg

  17. Irradiation Correction_Kc Kc Kc Gh n calcul_Kc Calcul_Gh Linke TL Elevation z

  18. Influence of Albedos Assessment • Albedos of ground rg and clouds rc are important parameters n = r - rg rc - rg if =10 % { small for overcast skies [2 - 5 %] large for clear skies [20 - 50 %] Dn   [10 - 20 %] n if Accuracy on Gh is linked to that of n Drg Drc = 10 % rc rg Drg has a influence Drc has a systematic influence upon n DGh  GcDn  Definition of albedoes is very important

  19. Ground albedo (1) r* t(i,j) = [ tsat(i,j) - ratm(qS,qv,)] / T(qS) T(qv) ratm and T(qS) and T(qv)are computed from the clear-sky model. Here, the ESRA model

  20. Cloud Albedo From Taylor and Stowe (1984, JGR), using maxima of time-series of r* teff(i,j) = 0.78 –0.13 [1 - exp(-4 cos(qS)5] tcloud(i,j) = [ teff(i,j) - tatm(qS,qv,)] / T(qS) T(qv) tcloud(i,j) > 0.2, otherwise tcloud(i,j) = 0.2 and tcloud(i,j) < 2.24 reff(i,j), otherwise tcloud(i,j) = 2.24 teff(i,j)

  21. HelioClim Data since 1985 and on-going Covering the whole field-of-view of Meteosat, except limits Available through the SoDa service http://www.soda-is.com

  22. VALIDATION Two years of ground measurements over Europe and Africa (90 sites) for 1994 and 1995 Cell size: 5' arc angle IRRADIANCE (W m-2) Daily average. 30 949 values. Correlation: 0.94 Mean value: 192 — Bias: -1 (0 %) — RMSE: 35 (18 %) Monthly average. 1 005 values. Correlation: 0.96 Mean value: 195 — Bias: -2 (1 %) — RMSE: 23 (12 %)

  23. This Summer • More validation. • 324 000 values from 1985 to 1990 • 162 000 values from 1991 to 1993 • 126 000 values after 1994 (out previous ones) • Then routine validation in co-operation with Meteo-France • Preparation of the operational chain for Meteosat Second Generation, in cooperation with DLR, Eumetsat and other European partners

  24. Long Wave Radiation Algorithms exist for computing downward and upward LW radiation from SW and other parameters. May be used in conjunction with HelioClim Typical errors (RMSE) are: L Downward LW: 20 - 25 W/m2 L Upward LW: 25 – 30 W/m2 (depends strongly on the wind) R Radiative Balance: 25 – 30 W/m2 R = L - L + I (1-r)

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