megha-tropiques workshop, ahmedabad, october 20051 scarab / megha-tropiques: objectives,...
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Megha-Tropiques Workshop, Ahmedabad, October 2005 1
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
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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
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Instantaneous Water and Radiation Elements
Precipitable water (mm) SSM/IRadiation (W/m2)
ScaRaB
Indian Ocean : March, 19, 1999
Rainrate (mm/h) SSM/I
Source: R. Roca, M. Desbois
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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.
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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
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2 main and 2 auxiliary channels
CHANNEL Description Spectral Interval Filter
1 Visible (VIS) 0,55 — 0,65 µm Interferential
2 Solar or SW 0,2 — 4 µm Silice Filter
3 Total (T) 0,2 — 100 µm No filter
4 Infrared window (IRW) 10,5 — 12,5 µm Interferential
The day-time LW radiance is obtained by difference betweenthe Total and SW channels, Llw =Lt – A’ Lsw
A’ depends on the spectral response of T and SW channels
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ScaRaB history: 2 launches and 16 months of data from Russian polar orbiters.
Below: 7 orbits of ScaRaB/Resurs -Total Channel
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Instrument (Resurs Model)
ScaRaB Channel:pyroelectric detectors (LiTaO3)1 Aluminium mirrorshopped modulation (16 hz)
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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)
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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
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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)
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Intercomparaison ScaRaB-CERES1999 (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
SW LW day LW night
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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
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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.
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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.
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Angular corrections orradiance-to-flux conversion
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Radiance-to-flux conversionRadiance TOA flux estimate
ddsincos),,(L)(F O
2
0
2/
0
O
),,(L O )(F O
O
SAT
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Angular Dependence Model (ADM)
ddsincos),,(L
),,(L),,(R
O
2
0
2/
0
OJOJ
),,(R
),,(L),,(F
OJ
OO
Flux
with
),,(R OJ where is the ADM for the ‘j’ scene
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ERBE – CERES ADM
Source: N. Loeb et al., CSTM
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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
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The CERES ADM cannot be directly applied
for ScaRaB/Megha-Tropiques because it is based on the MODIS
imager data
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Several ways to study 1/4
1 - By improving the use of the visible and infrared auxiliary channels
Inspired fromStubenrauch 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)
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Several ways to explore 2/4
2- The neural network approach (Loukachine and Loeb, JAOT, 2003)
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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)
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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.
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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 ???)
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Back-up slides
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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
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Orbites et fauchées de ScaRaB/MT
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Matrices Mensuelles Jour Heure locale d’observation
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 1 0 11 0 50 0 66 0 61 0 28 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 14 0 51 0 63 61 0 28 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 0 17 52 0 63 0 60 0 27 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 20 0 54 0 65 0 59 24 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 24 0 54 67 0 57 0 23 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 24 6 0 57 0 70 0 57 0 23 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 23 7 0 57 0 67 55 0 21 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 25 8 58 0 64 0 52 0 17 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 25 0 9 59 0 63 0 53 15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 26 0 10 60 64 0 51 0 13 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 28 0 60 11 0 67 0 50 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 30 0 60 12 0 70 49 0 8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 31 62 0 13 73 0 46 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 33 0 63 0 14 68 0 45 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 35 0 63 65 15 0 43 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 35 0 66 0 65 16 0 43 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 38 0 66 0 68 17 42 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 39 64 0 67 0 18 40 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 40 0 67 0 64 0 19 38 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 42 0 68 65 0 37 20 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 42 0 65 0 65 0 35 21 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 43 0 65 0 63 34 0 22 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 46 68 0 63 0 32 0 23 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 0 47 0 70 0 63 0 32 0 24 0 0 0 0 0 0 0 0 0 0 0 0 0 0 8 0 49 0 68 62 0 31 0 0 25 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 50 0 67 0 62 0 28 0 0 26 0 0 0 0 0 0 0 0 0 0 0 0 0 13 0 51 0 64 0 61 18 9 0 0 27 0 0 0 0 0 0 0 0 0 0 0 0 0 15 0 51 63 0 60 0 26 0 0 0 28 0 0 0 0 0 0 0 0 0 0 0 0 18 0 53 0 64 0 60 0 25 0 0 0 29 0 0 0 0 0 0 0 0 0 0 0 0 20 0 55 0 67 0 58 23 0 0 0 0 30 0 0 0 0 0 0 0 0 0 0 0 0 22 20 37 73 0 56 0 23 0 0 0 0 31 0 0 0 0 0 0 0 0 0 0 0 23 0 57 0 70 0 55 0 24 0 0 0 0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 1 17 71 0 42 0 14 0 32 0 69 38 0 0 0 0 0 0 0 0 0 0 0 0 20 2 0 73 0 41 0 14 31 0 71 0 35 0 0 0 0 0 0 0 0 0 0 0 0 23 3 0 73 39 0 16 0 34 0 73 0 32 0 0 0 0 0 0 0 0 0 0 0 23 0 4 74 0 39 0 16 0 37 73 0 31 0 0 0 0 0 0 0 0 0 0 0 0 26 0 5 74 0 36 14 0 37 0 74 0 29 0 0 0 0 0 0 0 0 0 0 0 0 27 74 6 0 35 0 16 0 36 0 74 26 0 0 0 0 0 0 0 0 0 0 0 0 28 0 72 7 0 36 0 16 39 0 74 0 27 0 0 0 0 0 0 0 0 0 0 0 0 31 0 72 8 36 0 16 0 39 0 74 0 24 0 0 0 0 0 0 0 0 0 0 0 33 0 71 0 9 33 0 16 0 40 74 0 23 0 0 0 0 0 0 0 0 0 0 0 0 35 0 68 0 10 32 16 0 42 0 74 0 21 0 0 0 0 0 0 0 0 0 0 0 0 38 68 0 33 11 0 16 0 43 0 72 15 0 0 0 0 0 0 0 0 0 0 0 0 39 0 66 0 33 12 0 16 44 0 71 0 12 0 0 0 0 0 0 0 0 0 0 0 0 43 0 65 31 0 13 16 0 46 0 68 0 9 0 0 0 0 0 0 0 0 0 0 0 45 0 63 0 30 0 14 18 0 47 67 0 5 0 0 0 0 0 0 0 0 0 0 0 0 49 0 61 0 30 21 15 0 51 0 65 0 2 0 0 0 0 0 0 0 0 0 0 0 0 51 61 0 27 0 21 16 0 51 0 63 0 0 0 0 0 0 0 0 0 0 0 0 0 51 0 58 0 23 0 21 17 53 0 58 0 0 0 0 0 0 0 0 0 0 0 0 0 0 55 0 57 22 0 21 0 18 54 0 58 0 0 0 0 0 0 0 0 0 0 0 0 0 58 0 54 0 21 0 22 0 19 57 55 0 0 0 0 0 0 0 0 0 0 0 0 0 0 60 0 52 0 21 23 0 58 20 0 52 0 0 0 0 0 0 0 0 0 0 0 0 0 0 64 51 0 21 0 26 0 61 21 0 50 0 0 0 0 0 0 0 0 0 0 0 2 0 67 0 49 0 21 0 30 62 0 22 48 0 0 0 0 0 0 0 0 0 0 0 0 5 0 67 0 48 18 0 30 0 64 0 23 43 0 0 0 0 0 0 0 0 0 0 0 0 9 68 0 46 0 15 0 30 0 66 17 24 26 0 0 0 0 0 0 0 0 0 0 0 12 0 70 0 44 0 15 32 0 67 0 40 25 0 0 0 0 0 0 0 0 0 0 0 0 15 0 73 43 0 15 0 32 0 68 0 39 26 0 0 0 0 0 0 0 0 0 0 0 20 0 73 0 42 0 16 0 30 0 71 36 0 27 0 0 0 0 0 0 0 0 0 0 0 22 0 73 0 40 16 0 33 0 72 0 33 0 28 0 0 0 0 0 0 0 0 0 0 0 23 74 0 39 0 16 0 36 0 72 0 31 0 29 0 0 0 0 0 0 0 0 0 0 26 0 74 0 39 0 14 0 36 73 0 28 0 0 30 0 0 0 0 0 0 0 0 0 0 26 0 74 35 0 14 0 36 0 73 0 27 0 0 31 0 0 0 0 0 0 0 0 0 29 0 73 0 35 0 16 0 37 0 74 27 0 0 0
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)