MT Workshop - 17-20 October 2005 JUNE 2004 DECEMBER 2004 End of OCTOBER 2005 ? MAY 2002 ? Capabilities of multi-angle polarization cloud measurements from

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


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


~ -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


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


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


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)


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


(A)Derivation of cloud properties from POLDER

just two examples…

Interests of the multiangle polarization measurements


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


Cloud thermodynamic phase from multiangle polarization


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


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


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


(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


• 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


Multispectral integration : shortwave albedo

= 443 nm 200 – 550 nm

= 670 nm 550 – 700 nm

= 865 nm 700 – 4000 nmSW ALBEDO

3 Spectral albedo


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


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)


POLDER ERB,WV and Clouds Products

Rapid overview of the different levels of operational cloud products


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


Data ordering interface from the CNES POLDER web portal ... Web POLDER

http://polder.cnes.fr


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


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, …)

Documents

MT Workshop - 17-20 October 2005 JUNE 2004 DECEMBER 2004 End of OCTOBER 2005 ? MAY 2002 ? Capabilities of multi-angle polarization cloud measurements from