1. Megha-Tropiques Workshop, Ahmedabad, October 20051 ScaRaB / Megha-Tropiques: objectives, description, calibration and radiance-to-flux conversion ScaRaB: Scanner for Radiation Budget M. Viollier LMD/IPSL/CNRS with LMD and CNES contributors

2. Megha-Tropiques Workshop, Ahmedabad, October 20052 ScaRaB Objectives to observe simultaneously the radiation fluxes and the water cycle components (water vapor, clouds, precipitation,…)  studies of the water and energy balance in the Tropics  mean accuracy of 10 Wm -2 is sought for the instantaneous fluxes to extend the time-series of data from broadband and well calibrated radiometers (ERBE, ScaRaB, CERES)  direct survey of climate parameters  mean accuracy of 5 Wm -2 is sought for the regional monthly means, and up to 2 Wm -2 for zonal monthly means

3. Megha-Tropiques Workshop, Ahmedabad, October 2005 3 Instantaneous Water and Radiation Elements Precipitable water (mm) SSM/I Radiation (W/m 2 ) ScaRaB Indian Ocean : March, 19, 1999 Rainrate (mm/h) SSM/I Source: R. Roca, M. Desbois

4. Megha-Tropiques Workshop, Ahmedabad, October 2005 4 Some key requirements - precise spectral response of the broadband channels (in the SW-Shortwave and LW- Longwave domains) - absolute radiometric calibration, 1% (LW), 2% (SW), compared to about 5% for most other SW radiometers - robust and qualified data processing for levels 2 & 3: to resolve the triple sampling issue (viewing angles, space and time averaging) and to provide detailed description of the observed scenes.

5. Megha-Tropiques Workshop, Ahmedabad, October 20055 ScaRaB = SW and LW broadband radiometer measuring the whole spectral ranges of the solar reflected radiation (SW), and of the longwave (LW) emission from the surface and atmosphere. Longueur d’onde (micromètre) SW response T response

6. Megha-Tropiques Workshop, Ahmedabad, October 20056 2 main and 2 auxiliary channels CHANNELDescriptionSpectral IntervalFilter 1Visible (VIS)0,55 — 0,65 µmInterferential 2Solar or SW0,2 — 4 µmSilice Filter 3Total (T)0,2 — 100 µmNo filter 4Infrared window (IRW)10,5 — 12,5 µmInterferential The day-time LW radiance is obtained by difference between the Total and SW channels, Llw =Lt – A’ Lsw A’ depends on the spectral response of T and SW channels

7. Megha-Tropiques Workshop, Ahmedabad, October 20057 ScaRaB history: 2 launches and 16 months of data from Russian polar orbiters. Below: 7 orbits of ScaRaB/Resurs -Total Channel

8. Megha-Tropiques Workshop, Ahmedabad, October 20058 Instrument (Resurs Model) ScaRaB Channel: pyroelectric detectors (LiTaO3) 1 Aluminium mirror shopped modulation (16 hz)

9. Megha-Tropiques Workshop, Ahmedabad, October 20059 Two challenges for ScaRaB/MegaTropics To keep the high degree of absolute calibration (1-2%) To improve the radiance-to-flux conversion (with no imager aboard)

10. Megha-Tropiques Workshop, Ahmedabad, October 200510 moyenne 20°S-20°N, Flux LW Anomalie par rapport à la période 85-89 (Source : Wielicki et al, Science, 2002) À comparer au  1% de précision

11. Megha-Tropiques Workshop, Ahmedabad, October 200511 Broad-band radiometer calibration: Ground characterization (spectral and spatial response of detectors, gain determination) On-board calibration (blackbodies and lamps) Geophysical cross-calibration with 3 channels (Total, SW and WIR) Inter-Comparison with other BBR when simultaneously in space (for example ScaRaB and CERES/TRMM in March 1999)

12. Megha-Tropiques Workshop, Ahmedabad, October 2005 12 Intercomparaison ScaRaB-CERES 1999 (Radiances) ( 2 periods, 19-21 Jan. et 3-6 March 1999) Difference SW : 1.5% ± 1%, LW : 0.7% ± 0.1% Haeffelin, M., B. Wielicki, J.-Ph. Duvel, K. Priestley, and M. Viollier, 2001, Geophys. Res. Let. 28(1), 167-170 SWLW dayLW night

13. Megha-Tropiques Workshop, Ahmedabad, October 200513 geophysical cross-calibration Analysis of very cold bright daytime cloud scenes over tropical convective regimes, for which the TW signal is dominated by SW reflection and the LW component can be estimated independently from the IRW radiance yields agreement at the 1% level Publication: Duvel, J.-Ph., and P. Raberanto, A geophysical cross-calibration approach for broadband channels: Application to the ScaRaB experiment, 2000, J. Atmos. Oceanic Technol. 17, 1609-1717 Equivalent to the CERES 3 channels consistency method

14. Megha-Tropiques Workshop, Ahmedabad, October 200514 Plans for ScaRaB on MeghaTropiques Reference blackbody (emissivity > 0.99) for channels T,IR The high level of radiometric stability found with ScaRaB 1 and 2 and the geophysical cross-calibration method allow to simplify the original calibration system in the SW domain. We then plan to reduce the number of lamps Improvement to check the SW calibration and to detect long term drift of the relative spectral responses of the SW and total channels in the SW domain This improvement consists in in-flight direct inter- comparisons of both channels by switching the silica filters, owing to a new definition of the filter wheel.

15. Megha-Tropiques Workshop, Ahmedabad, October 200515 Accurate radiometric calibration continues to be a difficult task. Examples: The calibration of the ERBS non-scanning radiometer has been re-examined (Wong et al., 2005) A small drop of the SW flux estimates from CERES/Terra between 2000 and 2004 raises questions GERB: the calibration is partly finalized, specifically in the SW domain.

16. Megha-Tropiques Workshop, Ahmedabad, October 2005 16 Angular corrections or radiance-to-flux conversion

17. Megha-Tropiques Workshop, Ahmedabad, October 200517 Radiance-to-flux conversion Radiance TOA flux estimate SAT

18. Megha-Tropiques Workshop, Ahmedabad, October 200518 Angular Dependence Model (ADM) Flux with where is the ADM for the ‘j’ scene

19. Megha-Tropiques Workshop, Ahmedabad, October 200519 ERBE – CERES ADM Source: N. Loeb et al., CSTM

20. Megha-Tropiques Workshop, Ahmedabad, October 200520 The radiation anisotropy ERBE ADM (Suttles et al., 1988) with 4 cloud categories The new CERES angular model from NASA (Loeb et al., 2004), adjusted to detailed cloud description Both are statistically based Improvements are expected from studies with POLDER (see presentation by F. Parol) and MISR data

21. Megha-Tropiques Workshop, Ahmedabad, October 200521 The CERES ADM cannot be directly applied for ScaRaB/Megha-Tropiques because it is based on the MODIS imager data

22. Megha-Tropiques Workshop, Ahmedabad, October 200522 Several ways to study 1/4 1 - By improving the use of the visible and infrared auxiliary channels Inspired from Stubenrauch et al (JAM, 1993) for LW and Chang et al (JAM, 2000) for SW. (preliminary study carried out by LMD and Noveltis in 2002, some improvements/ERBE, but they are not yet decisive)

23. Megha-Tropiques Workshop, Ahmedabad, October 200523 Several ways to explore 2/4 2- The neural network approach (Loukachine and Loeb, JAOT, 2003)

24. Megha-Tropiques Workshop, Ahmedabad, October 200524 Several ways to explore 3/4 3- The combination with geostationary data according to the GERB experience (Harries et al, 2005) but its application depends on the location accuracy (both ScaRaB and GEOs)

25. Megha-Tropiques Workshop, Ahmedabad, October 200525 Several ways to explore 4/4 Any consistent combination with related space instruments and atmospheric data. Other suggested methods from our colleagues are welcome and they will be discussed as far as possible.

26. Megha-Tropiques Workshop, Ahmedabad, October 2005 26 CONCLUSION Radiance-to-flux conversion: the most efficient method from several studies (by comparison to archived results from CERES, POLDER, MISR or GERB) should finally be selected for the operational data processing. Radiometric Calibration: it is necessary to maintain the efforts for the ground characterizations, and for the in-flight calibration and instrument analysis (possible overlaps with CERES/Aqua or ERBS/NPOESS ???)

27. Megha-Tropiques Workshop, Ahmedabad, October 2005 27 Back-up slides

28. Megha-Tropiques Workshop, Ahmedabad, October 200528 geophysical cross-calibration On-board calib. Geophys. Approach SW gain Duvel, J.-Ph., and P. Raberanto, A geophysical cross-calibration approach for broadband channels: Application to the ScaRaB experiment, 2000, J. Atmos. Oceanic Technol. 17, 1609-1717 0.8% Instrument temperature

29. Megha-Tropiques Workshop, Ahmedabad, October 200529 Orbites et fauchées de ScaRaB/MT

30. Megha-Tropiques Workshop, Ahmedabad, October 200530 Matrices Mensuelles Jour Heure locale d’observation Latitude: 20° (5 heures observées)Latitude 10° Nombre d’observations par régions de 2.5°x2.5° Observations de 5 à 7 heures locales (2 pour Terra ou Aqua)