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MT Workshop - 17-20 October 2005
JUNE 2004
DECEMBER 2004 End ofOCTOBER 2005 ?
MAY 2002
?
Capabilities of multi-angle polarization cloud measurements from satellite : POLDER results
POLDER and PARASOL are very similar instruments
Frédéric Parol and the POLDER “ERB and Clouds”science team
Laboratoire d ’Optique Atmosphérique / CNRS
MT Workshop - 17-20 October 2005
Outlines
Context and rationale : Cloud, radiation and
climate
The POLDER instrument
Interests of multi-angle polarization
measurements
Cloud properties
Earth radiation balance
POLDER “ERB,WV and Clouds” operational
products
Conclusion and perspectives
MT Workshop - 17-20 October 2005
~ -50Wm-2 in the SW
~ +30 Wm-2 in the LW
Context and rationale : cloud, radiation and climate How clouds impact the
Earth’s radiative energy balance
Net cloud radiative forcing
-20 W.m-2
MT Workshop - 17-20 October 2005
Change in the TOA Cloud Radiative Forcing associated with a CO2 doubling
is still uncertain
Context and rationale : cloud, radiation and climate
(IPCC 2001 ; from Le Treut and Mc Avaney, 2000)
Effect of instantaneous CO2doubling
Variation of emitted LW : 4 W.m-2
positive feedback : increases the warming
negative feedback : reduces the warming
MT Workshop - 17-20 October 2005
Context and rationale : cloud, radiation and climate• Small changes in clouds are potentially important for climate change
• “Probably the greatest uncertainty in future projections of climate arises from clouds and their interactions with radiation” (IPCC 2001)
• Clouds affect radiation through their height, their thickness and their radiative properties
• Satellite instruments allow for deriving and monitoring the radiation fluxes and/or the cloud properties (cloud cover, phase, temperature…) at the global scale
MT Workshop - 17-20 October 2005
POLDER concept and capabilities(POLarization and Directionality of the Earth’s
Reflectances)o POLDER instrument LOA/CNESo Japanese platforms Adeos 1 – Adeos 2and CNES micro satellite Myriad platformo Missions
o POLDER 1 : Nov 1997 – June 1997o POLDER 2 : Jan 2003 – Oct 2003o PARASOL : since Dec 2004
Main technical features
• Wide field of view lens ± 43° along track,
± 51° cross track 2400km x 1800km
• Filter wheel8 spectral bands (443, 490, 565, 670, 763, 765, 865, 910 nm) + 1020nm for PARASOL
Some are polarized
• CCD detector array242 x 288 pixels
Spatial resolution
- full resolution (~ 6x6
km2)
- coarse resolution
[superpixel scale] (~
20x20 km2)
MT Workshop - 17-20 October 2005
The multi-directional viewing capability of POLDER
Up to 14 viewing angles per pixel for a single satellite pass
θ1
θ2
t1
t2
France 97/01/13
MT Workshop - 17-20 October 2005
(A)Derivation of cloud properties from POLDER
just two examples…
Interests of the multiangle polarization measurements
MT Workshop - 17-20 October 2005
Multiangle polarisation measurements and Cloud Phase
Variations of cloud polarized reflectance with scattering angle exhibits very different features depending on particle shapes (liquid droplets vs ice crystals). The cloud phase algorithm uses 4 major differences between these angular signatures.
Riedi et al, GRL, 2000
140°180°
liquidice
MT Workshop - 17-20 October 2005
Cloud thermodynamic phase from multiangle polarization
MT Workshop - 17-20 October 2005
Multiangle measurements : a tool for testing cloud (microphysical) models
(1/3)
Departure from the model are coded in spherical (or diffuse) albedo
reflectances
Optical thickness
Spherical albedo
MT Workshop - 17-20 October 2005
Overcast conditions over ocean
0.01 - 0.05
0.25 - 0.30
0.10 - 0.15
0.15 - 0.20
0.20 - 0.25
0.05 - 0.01
0.30 - 0.35
0.35 - 0.40
> 0.40
Liquidwater clouds
Scattering angle
(deg.)
Scattering angle
(deg.)
Dep
art
ure
fro
m t
he
mod
el
Ice clouds
10µm-spheres
10µm-spheres
Multiangle measurements : a tool for testing cloud microphysical models (2/3)
What's happened when all clouds are assumed to be composed of spherical droplets ?
Parol et al, IEEE, 1999Doutriaux-Boucher et al, GRL, 2000
MT Workshop - 17-20 October 2005
Multiangle measurements : a tool for testing cloud microphysical models (3/3)
The discrepancy observed for ice clouds decreases when the Inhomogeneous Hexagonal crystal Model
(IHM) is used
MT Workshop - 17-20 October 2005
(B) Multiangle visible measurements : TOA SW albedo (and SW flux) retrievals
Radiance (or reflectance)
measurement
Flux (or albedo)
Theoretical ADM
(RT calculations)
Statistical ADM
(observations)
Interests of the multiangle polarization measurements
MT Workshop - 17-20 October 2005
• Cloud optical thickness is retrieved under up to 14 directions
• Directional parameters (optical thickness and cloud albedo) are calculated at 443 nm (or 490nm), 670 nm and 865 nm.
1. The theoretical approachMultiangle multispectral
measurements
MT Workshop - 17-20 October 2005
Multispectral integration : shortwave albedo
= 443 nm 200 – 550 nm
= 670 nm 550 – 700 nm
= 865 nm 700 – 4000 nmSW ALBEDO
3 Spectral albedo
MT Workshop - 17-20 October 2005
Juin 2003
POLDER ALBEDO (x 100)
POLDER-CERES differences , 20°S-20°N mean : -0,57 (-2,6%)
Monthly mean shortwave albedo
Viollier et al, GRL, 2002
MT Workshop - 17-20 October 2005
2. The statistical approach : ADMs based on satellite-derived cloud properties
0.5 0.7 0.9 1.1 1.3
(a) Optical Depth < 2.5
(b) Optical Depth 18-40
60o30o0o30o60o
60o30o0o30o60o
=0o
(fwd)
=0o
(fwd)=180o
(bwd)
=180o
(bwd)
R(θo,θ,)
),,(L O ϕθθ )(F Oθ
),,(R
),,(L),,(F
OJ
OO ϕθθ
ϕθθπ=ϕθθ
Radiance measurement TOA flux
Rj(θ0,θ,) is the ADM for the scene type « j »
(Loeb et al, JAS,2000)
MT Workshop - 17-20 October 2005
POLDER ERB,WV and Clouds Products
Rapid overview of the different levels of operational cloud products
MT Workshop - 17-20 October 2005
Products availabilityPOLDER1 : November 1996 to June 1997POLDER2 : April 2003 to October 2004PARASOL : since Dec 2004
Level1 : calibrated georeferenced data Level2 : daily products – one file per orbit swathThe level 2 (orbital swath) products contain about thirty non-directional parameters and ten directional parameters (for each of the 14 viewing directions) Level3 : monthly global productsThe level 3 (monthly) products contain about forty parameters Joint Atmosphere product (selected daily and monthly parameters)This product contains combined cloud and aerosol parametersProjection Lat-Lon 1080*2160 ( 1/6° )HDF file format
MT Workshop - 17-20 October 2005
Data ordering interface from the CNES POLDER web portal ... Web POLDER
http://polder.cnes.fr
MT Workshop - 17-20 October 2005
Conclusion POLDER is a very nice research instrument with still no equivalent
It provides :
- Usual “standard” cloud parameters :° Cloud cover,° Cloud Optical thickness,° Cloud pressure,° etc….
- Original parameters :° Cloud phase index° Information on shape and size of cloud
particles
+ Angular variability
MT Workshop - 17-20 October 2005
Conclusion/PerspectivesPossible contribution of POLDER (and the A-train) to the
MT mission :
- to establish a climatology of cloud properties in the
Tropics
frequency of occurrence of liquid water clouds/ice
clouds
discrimination between thick cirrus and semi-
transparent cirrus
multiangle measurements could be analyzed in order
to investigate regional variability of ice cloud
particles
others…
- to evaluate the existing ADMs or/and to construct new
ADMs based on multi-satellite-derived cloud properties
- To be discussed with the ERB people (M. Viollier, …)