Sophia Antipolis, 17 octobre 2003 HELIOSAT-II

What is Heliosat? Raw image Irradiation (energy)

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 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

Accuracy (relative RMSE) Type Irradiation (Wh m -2 ) Type Irradiance (W m -2 ) Hourly values100Hour100 Daily values600Day25 5-days sum of daily irradiation days20 10-days sum of daily irradiation days15 Monthly mean of hourly irradiation 50Month12 Monthly mean of daily irradiation 300 Single pixel. Hi-Res images. 30 and 60 stations WMO in Europe

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

Kc = f(n) G = K c G c n  -0,2K c = 1,2 -0,2  n  0,8K c = 1-n 0,8  n  1,1K c = 2, ,6667 n+1,6667 n² n  1,1K c = 0,05 How it works ? (2)

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

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

Prerequisite – Basic modelling Clear sky – Broadband L sat = T atm L g + L atm L atm = path radiance  sat = T atm  g +  atm T atm = T down T up Downwelling T up L atm  Sensor Reflected L g T down

 t sat (i,j) =  t atm (  S,  v,  ) +  t g (i,j) T t (  S ) T t (  v ) L atm = (D c /  ) (I 0met / I 0 ) ( / cos  V ) 0,8  * t (i,j) =  t sat (i,j) -  atm (  S,  v,  )] / T(  S ) T(  v ) Atmospheric Reflectance (2)

L atm [W.m -2.sr -1 ] Modelling Modtran Modelling as a function of D c vis 50 km = Ground Albedo,  g

Cloud index  atm Linke TL Elevation z Reflectance  sat  g nGh AtmosphereIrradiation I0met Calib. coeff General scheme

calibration Lsat calcul_albedo Calibration and reflectance Calibration coefficients I0met  sat

Atmospheric reflectance Latm calcul_albedo I0met  atm calcul_Latm Elevation z Linke TL

Cloud index  atm calcul_rho_rhoc Elevation z Linke TL  sat cc **  g gg n n glitter calcul_n

Irradiation calcul_Kc Elevation z Linke TL KcKc KcGh Calcul_Gh Correction_Kc n

 G h  G c  n  Albedos of ground  g and clouds  c are important parameters n =  -  g  c -  g if gg gg = 10 %    c has a systematic influence upon n Accuracy on G h is linked to that of n    g has a influence small for overcast skies [2 - 5 %] large for clear skies [ %] {  Definition of albedoes is very important cc cc = 10 % if  [ %] nn n   Influence of Albedos Assessment

 * t (i,j) =  t sat (i,j) -  atm (  S,  v,  )] / T(  S ) T(  v ) Ground albedo (1)  atm and T(  S ) and T(  v ) are computed from the clear-sky model. Here, the ESRA model

 t eff (i,j) = 0.78 – 0.13  - exp  cos  S ) 5 ]  t cloud (i,j) =  t eff (i,j) -  t atm (  S,  v,  )] / T(  S ) T(  v )  t cloud (i,j) > 0.2, otherwise  t cloud (i,j) = 0.2 and  t cloud (i,j) < 2.24  eff (i,j), otherwise  t cloud (i,j) = 2.24  t eff (i,j) Cloud Albedo From Taylor and Stowe (1984, JGR), using maxima of time-series of  *

HelioClim Data since 1985 and on-going Covering the whole field-of-view of Meteosat, except limits Available through the SoDa service

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 values. Correlation: 0.94 Mean value: 192 — Bias: -1 (0 %) — RMSE: 35 (18 %) Monthly average values. Correlation: 0.96 Mean value: 195 — Bias: -2 (1 %) — RMSE: 23 (12 %)

This Summer More validation values from 1985 to values from 1991 to 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

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: 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-  )