Megha-Tropiques Workshop, Ahmedabad, October 20051 ScaRaB / Megha-Tropiques: objectives,...

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


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

Megha-Tropiques Workshop, Ahmedabad, October 2005

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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


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.


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


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


ScaRaB history: 2 launches and 16 months of data from Russian polar orbiters.

Below: 7 orbits of ScaRaB/Resurs -Total Channel


Instrument (Resurs Model)

ScaRaB Channel:pyroelectric detectors (LiTaO3)1 Aluminium mirrorshopped modulation (16 hz)


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)


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


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)


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


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


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.


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


Radiance-to-flux conversionRadiance TOA flux estimate

ddsincos),,(L)(F O

2

0

2/

0

O

),,(L O )(F O

O

SAT


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


ERBE – CERES ADM

Source: N. Loeb et al., CSTM


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


The CERES ADM cannot be directly applied

for ScaRaB/Megha-Tropiques because it is based on the MODIS

imager data


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)


Several ways to explore 2/4

2- The neural network approach (Loukachine and Loeb, JAOT, 2003)



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)



• 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


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


Orbites et fauchées de ScaRaB/MT


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)

Megha-Tropiques Workshop, Ahmedabad, October 20051 ScaRaB / Megha-Tropiques: objectives,...

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