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Version 1.2, 25 February 2005 APPLICATIONS OF METEOSAT SECOND GENERATION (MSG) CONVERSION FROM COUNTS TO RADIANCES AND FROM RADIANCES TO BRIGHTNESS TEMPERATURES.

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Presentation on theme: "Version 1.2, 25 February 2005 APPLICATIONS OF METEOSAT SECOND GENERATION (MSG) CONVERSION FROM COUNTS TO RADIANCES AND FROM RADIANCES TO BRIGHTNESS TEMPERATURES."— Presentation transcript:

1 Version 1.2, 25 February 2005 APPLICATIONS OF METEOSAT SECOND GENERATION (MSG) CONVERSION FROM COUNTS TO RADIANCES AND FROM RADIANCES TO BRIGHTNESS TEMPERATURES AND REFLECTANCES Author:D. Rosenfeld (HUJ) daniel.rosenfeld@huji.ac.il Contributors:I. Lensky (HUJ), J. Kerkmann (EUM), S. Tjemkes (EUM) Y. Govaerts (EUM), HP. Roesli (MeteoSwiss)

2 Version 1.2, 25 February 2005 Conversion from Counts to Radiances R = CAL_offset + CAL_slope * Count (1) R = spectral radiance in mWm -2 sr -1 (cm -1 ) -1 CAL_offset = offset constant between the pixel count and the physical radiance extracted either from the on-board calibration (for IR channels) or from other sources (e.g. SEVIRI Solar Channel Calibration (SSCC) for solar channels). The units are mWm -2 sr -1 (cm -1 ) -1 CAL_slope = linear calibration coefficient extracted either from the on-board calibration (for IR channels) or from other sources (e.g. SEVIRI Solar Channel Calibration (SSCC) for solar channels). The units are mWm -2 sr -1 (cm -1 ) -1 Count = binary pixel value (pixel count, between 0 and 1023) More info: www.eumetsat.de, MSG/MSG Documentation (under Quick Links)/ Level 1.5 Data Format Description (MSG/ICD/105) The relation between the binary pixel value and the physical radiance is fully defined for each spectral band by the relation:

3 Version 1.2, 25 February 2005 Conversion from Counts to Radiances Detailed structure of the Level 1.5 Header: the calibration info (slope, offset) can be found in the radiometric processing sub-header !

4 Version 1.2, 25 February 2005 The image header itself is in the prologue file coming with each MSG image. Examples of filenames for prologue files are: HRIT Prologue File:H-000-MSG1__-MSG1________-_________-PRO______-200502180815-__ LRIT Prologue File:L-000-MSG1__-MSG1________-_________-PRO______-200502180845-__

5 Version 1.2, 25 February 2005 Conversion from Radiances to Brightness Temperatures In the MSG-MPEF the following analytic relation between the equivalent brightness temperatures (T b ) and the SEVIRI radiances (R) is adopted: With:C 1 = 1.19104 10 -5 mW m -2 sr -1 (cm -1 ) -4 C 2 = 1.43877 K(cm -1 ) -1 c = central wavenumber of the channel A, B coefficients (see next slide) More info: www.eumetsat.de, MSG/Data Products & Services/Image Data/Calibration/ (2)

6 Version 1.2, 25 February 2005 Conversion from Brightness Temperatures to Radiances Viceversa, the analytic relation between the radiances (R) and the equivalent brightness temperatures (T b ) for the MSG infra- red channels is given by equation (3): With:C 1 = 1.19104 10 -5 mW m -2 sr -1 (cm -1 ) -4 C 2 = 1.43877 K(cm -1 ) -1 c = central wavenumber of the channel A, B coefficients (see next slide) More info: www.eumetsat.de, MSG/Data Products & Services/Image Data/Calibration/ (3)

7 Version 1.2, 25 February 2005 Conversion from Radiances to Brightness Temperatures Values for the central wavenumber (in cm -1 ), and the parameters A, and B (in K) for the analytic relationship between radiance and equivalent brightness temperature for the thermal IR SEVIRI channels on MSG-1: Channel No.Channel ID c A B 04IR3.92569.0940.99593.471 05WV6.21598.5660.99632.219 06WV7.31362.1420.99910.485 07IR8.71149.0830.99960.181 08IR9.71034.3450.99990.060 09IR10.8930.6590.99830.627 10IR12.0839.6610.99880.397 11IR13.4752.3810.99810.576

8 Version 1.2, 25 February 2005 Conversion from Radiances to Reflectances for VIS Channels REFL(i) = 100 * R (i) / TOARAD (i ) / cos(TETA) i=1, 2, 3, 12 (4) REFLReflectance [in %] for channel i, i = 1, 2, 3, 12 Rmeasured Radiance [in mW m -2 ster -1 (cm -1 ) -1 ] for channel i, i = 1, 2, 3, 12 TOARADsolar constant at Top of the Atmosphere [in mW m -2 ster -1 (cm -1 ) -1 ] for channel i, i = 1, 2, 3, 12 TETAsolar zenith angle (to be calculated from date, time, lat, lon); for twilight condition (i.e. TETA > 80°) TETA is set to 80° to avoid problems inumber of the channel (1 = VIS0.6; 2 = VIS0.8; 3 = NIR1.6; 12 = HRV) TOARAD (i=1, VIS0.6) = 20.76 / ESD**2 TOARAD (i=2, VIS0.8) = 23.24 / ESD**2 TOARAD (i=3, NIR1.6) = 19.85 / ESD**2 TOARAD (i=12, HRV) = 25.11 / ESD**2 ESD is the earth-sun-distance (in Astronomical Units), which varies during the year according to the following equation: ESD (JulianDay) = 1.0 - 0.0167 cos ( 2  (JulianDay - 3) / 365) (5) A simple way to calculate the reflectances for channels VIS0.6, VIS0.8 NIR1.6 and HRV is:

9 Version 1.2, 25 February 2005 Conversion from Radiances to Reflectances for VIS Channels An IDL (Interactive Data Language) tool to calculate brightness temperatures and reflectances is offered at: www.eumetsat.de Data, Products and Services Useful Programs and Tools (under Quick Links) SEVIRI Pre-processing Toolbox

10 Version 1.2, 25 February 2005 Conversion from Radiances to Reflectances for Channel IR3.9 REFL = 100 * (R_tot - R_therm) / (TOARAD - R_therm) (6) with: REFLReflectance [in %] for channel IR3.9 R_totmeasured total Radiance [in mW m -2 ster -1 (cm -1 ) -1 ] for channel IR3.9 R_thermCO2-corrected, thermal component of Radiance [in mW m -2 ster -1 (cm -1 ) -1 ] for channel IR3.9 TOARADCO2-corrected, solar constant at Top of the Atmosphere [in mW m -2 ster -1 (cm -1 ) -1 ] for channel IR3.9 During daytime, Channel IR3.9 receives energy both from emitted thermal radiation and from reflected solar radiation. One possibility to calculate the reflectance for channel IR3.9 is:

11 Version 1.2, 25 February 2005 Conversion from Radiances to Reflectances for Channel IR3.9 The CO2-corrected thermal component of the radiance in Channel 04 (IR3.9) can be estimated from the IR10.8 channel by equation (7): Using equation (2) to calculate the brightness temperature for channel IR10.8 and equation (3) to convert this temperature back to radiance (using the coefficients A and B for channel IR3.9 !) R3.9_corr is the CO2 correction factor to account for the attenuation of the emitted thermal radiation by CO2 absorption (see next slide). R_therm = R(IR3.9, BT(IR10.8)) * R3.9_corr (7)

12 Version 1.2, 25 February 2005 Conversion from Radiances to Reflectances for Channel IR3.9 The CO 2 correction factor of IR3.9 total radiation, R3.9_corr, can be estimated using the IR10.8 and the IR13.4 brightness temperatures: (8)

13 Version 1.2, 25 February 2005 Conversion from Radiances to Reflectances for Channel IR3.9 TOARAD = 4.92 / ESD**2 * cos(TETA) * exp[-(1-R3.9_corr)] * exp [-(1-R3.9_corr) * cos(TETA) / cos(SAT)] (9) ESD earth-sun-distance (in Astronomical Units), see equation (4) TETAsolar zenith angle (to be calculated from date, time, lat, lon); for twilight condition (i.e. TETA > 80°) TETA is set to 80° to avoid problems SATsatellite zenith angle 4.92 / ESD**2solar constant [in mW m -2 ster -1 (cm -1 ) -1 ] in channel 4 without CO2 correction exp[-(1-R3.9_corr)] is the CO2 attenuation of the reflected solar radiation from cloud to satellite exp[-(1-R3.9_corr)] * cos(TETA) / cos(SAT)) is the CO2 attenuation of the solar radiation from the sun to cloud The CO2-corrected, solar constant at the Top of the Atmosphere in Channel 04 (IR3.9) can be estimated from:

14 Version 1.2, 25 February 2005 CO2 Correction of Brightness Temperature of Channel IR3.9 T4_CO2corr, the CO2-corrected brightness temperature at IR3.9, can be estimated using the IR10.8 and the IR13.4 brightness temperatures: T4_CO2corr = ( BT(IR3.9) 4 + Rcorr ) 0.25 (10) Where: Rcorr = BT(IR10.8) 4 - (BT(IR10.8) -  T_CO2) 4 And:  T_CO2 = (BT(IR10.8) - BT(IR13.4)) / 4.

15 Version 1.2, 25 February 2005 I.Temperature difference between surface and air mass at about 850 hPa (  T_CO2 is very large for hot desert surfaces during daytime (see next slide)) II.Height of the cloud (  T_CO2 is small for high clouds (see next slide)) III.Satellite viewing angle (so called "limb cooling" effect,  T_CO2 is large for large satellite viewing angles) IV.Differences in surface emissivity at 10.8 and 13.4  m CO2 Correction of Brightness Temperature of Channel IR3.9 In equation (10), the CO2-correction of BT(IR3.9) depends non-linearly on  T_CO2, the difference between IR10.8 and IR13.4, which depends on (in order of priority):

16 Version 1.2, 25 February 2005 MSG-1,  T_CO2 [i.e. (BT(IR10.8) - BT(IR13.4)) / 4.], Range = 0 / +10 K,  =1.0 2 March 2004, 12:00 UTC 2 March 2004, 24:00 UTC CO2 Correction of Brightness Temperature of Channel IR3.9

17 Version 1.2, 25 February 2005 IR3.9 Solar Reflectance: Summer Example IR3.9 Brightness Temperature IR3.9 Reflectance Difference IR3.9 - IR10.8 Range = 243 K / 333 K,  =1.0 Range = 0 / 25 %,  =1.7 Range -5 / +65 K,  =1.0 (for comparison) MSG-1, 13 June 2003, 12:00 UTC

18 Version 1.2, 25 February 2005 IR3.9 Solar Reflectance: Summer Example... close up view IR3.9 Brightness Temperature IR3.9 Reflectance Difference IR3.9 - IR10.8 Range = 243 K / 333 K,  =1.0 Range = 0 / 25 %,  =1.7 Range -5 / +65 K,  =1.0 (for comparison) MSG-1, 13 June 2003, 12:00 UTC

19 Version 1.2, 25 February 2005 MSG-1, 26 January 2004, 10:00 UTC IR3.9 Solar Reflectance: Winter Example IR3.9 Brightness Temperature IR3.9 Reflectance Difference IR3.9 - IR10.8 Range = 243 K / 303 K,  =1.0 Range = 0 / 20 %,  =1.0 Range 0 / +50 K,  =1.0 (for comparison)

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