# Presentation: 871819

## Polarization in Earth Remote Sensing: Aerosol monitoring, and more

- François-Marie Bréon
- Laboratoire des Sciences du Climat et de l’Environnement
- Visiting GSFC until August 29th

## Overview of POLDER-Parasol saga

- POLDER-1 onboard the ADEOS platform (1996-1997).  Solar panel failure terminated operations (8 months of continuous acquisition)
- POLDER-2 onboard the ADEOS-2 platform (2003).  Solar panel failure terminated operations (7 months of continuous acquisition)
- Parasol onboard a CNES microsat satellite.  Fly in formation with the Aqua Train.  Near-continuous acquisition since March 2005 (3+ year worth of data)
- Eight spectral bands from 440 (blue) to 910 (near IR) nm
- Bi-dimensional CCD matrix. ≈2400x1800 km IFOV
- Spatial Resolution ≈6 km
- Three “Polarized bands”: 440/490, 670, 865 nm.

**Notes:**
Parasol is very similar to POLDER.  Some slight changes in the spectral bands.  Long axis of the CCD matrix is cross-track for POLDER, along track for Parasol.
It yields a lower daily coverage of the Earth, but a better directional sampling.

## POLDER Multidirectional acquisition

- Principal Plane

- Backscattering

- Nadir

- Multispectral measurements acquired every 20 sec

- Phase
- angles

- View Angles

- Perpendicular Plane

- 14 POLDER views

**Notes:**
The animation shows a sequence of POLDER acquisition.  This animation demonstrates how a pixel on the Earth is observed from up to 14 different directions as the satellite
 flies over (16 for Parasol).  Some directional effects are apparent.  The strongest is the glint over the Mediteranean area, but one may also observe directional effects over France and Africa (the Hot Spot is visible.
Left image shows the limit of the swath (blue) and limits of an IFOV (red)
Bottom right shows a typical directional coverage.  Red circles are view angles by step of 20 degrees.  Blue is phase angle by step of 10 degrees.
Mention that, the next day, the instrument is at a different place in the sky, which provides another cut in the directional space.  After a few days, the directional space within 60 degrees
of view angles is fully covered with, in particular, a good sampling of the principal plane.

## Parasol polarization measurements

- Three successive measurements with polarizer turned by step of 60°
- Inversion of radiometric model yield linear polarization parameters [I, Q, U]
- Three spectral bands 490, 670, 865 nm

- Rough correction for molecular scattering.  Three colour composites

**Notes:**
We had problems with the polarized band at 443 nm on POLDER.  It is 490 nm on Parasol.
The bottom images are 3 color composites in total (left) and polarized (right) light.  It indicates that the information content of polarization is different than that of total light.
Some people use SQRT(Q^2+U^2) to define the polarize radiance.  I rather use -Q to keep the information on the polarization direction (parallel or perpendicular).  There is little information in U that is close to zero.
All images and graphs in my presentation are -Q.  The polarized reflectance defined this way is positive when polarization is perpendicular to the plane of scattering, negative when it is parallel to the plane of scattering.

## A few examples of Directional/Spectral / Polarized signatures

- 3 color composites in total light
- 443-670-865 nm

- 3 color composites in polarized light
- 443-670-865 nm

- Directional signatures.
- Total light

- Directional signatures.
- Polarized light

- Location of swath
- and image

- Directional sampling
- View angle
- Phase or Glint angle

*Simple formula for polarized reflectance*

**Notes:**
We will now show a few examples of signatures acquired with POLDER.  The format is always the same with images to the left, corresponding graphs of directional signatures at the center,
Location of images a top-right, and directional sampling corresponding to the graphs a bottom right.
If lack of time, drop this graph together 

## Sunglint over Mediterranean Sea

***s*=36°

- Wave slope

**Notes:**
Large directional signature of sunglint.  Red lines on images show circles of off-glint angles.  Reflectance is maximum for an off glint angle close to zero (not exactly zero).  A simple formula, based on Cox&Munk model approximates the reflectance and/or polarized reflectance of sunglint.
Not that, off glint, the blue (443 nm) reflectance is larger than that of green (670 nm) and red (865 nm) due to molecular scattering.  True both for reflectance and polarized reflectance

## Vegetation (Amazonian)

**Notes:**
In total light, one clearly sees the directional effect of the surface.  Maximum in backscattering. With my definition of Hot Spot, one does NOT see the hot spot here.
Polarized reflectance shows a minimul at backscattering.  Signal is dominated by molecular scattering
Formula shows a simple model for the polarized reflectance of vegetated surfaces;

## Desert

*Overestimate at large *
*phase angles*

**Notes:**
Larger reflectance than for vegetation.  Directional signatures are similar however, both for total and polarized light

## Aerosols over the Ganges Valley

**Notes:**
Reflectance due to aerosol scattering is highly polarized.  Look at the signal over the Gange valley.  This large polarization by aerosols is used to monitor aerosols
over land surfaces using the POLDER-Parasol measurements

## Biomass Burning Aerosol

**Notes:**
This is mostly to show the large polarized signal by aerosols .  The signal is very small at backscattering.  Note that one can hardly distinguish the land-ocean boundary on the polarized images
Which demonstrates, if necessary, that polarized reflectance is mostly generated within the atmosphere

## Antarctic + Cloud Bow

## Low Cloud and High Cloud

*Goloub Ph et al., JGR, 2000*
*Riedi J. et al., GRL, 2000*

**Notes:**
On this viewgraph, we show the directional and polarized signature for two targets (blue symbols on the images) that correspond to a low and high clouds.  The low cloud is made of spherical
Droplets which have a very specific directional signature for a phase angle of 40 degree (scattering angle of 140°, direction of the rainbow).  High cloud, made of ice particles, does not show such signature.
This characteristic of the directional-polarized reflectance is used in POLDER algorithms to distinguish liquid and solid phase clouds.
Riedi, J., M. Doutriaux-Boucher, P. Goloub, P. Couvert.   Global distribution of cloud top phase from POLDER/ADEOS I.   Geophys. Res. Lett., 27 (12) 1707-1710 (2000)
Goloub P, Herman M, Chepfer H, Riedi J, Brogniez G, Couvert P, Seze G: Cloud thermodynamical phase classification from the POLDER spaceborne instrument.  J Geophys. Res.-Atmos, 105 (D11) 14747-14759 (2000)

## Aerosol retrieval over the oceans

- Best fit of both radiance and polarized radiance against tabulated TOA simulated radiances
- Different number of free parameters depending on the signal level and the observation geometry (a wide range of scattering angles, including ≈150° is best)
- Two modes can vary independently
- 	• Small mode with effective radius between 0.04 and 0.13 μm (fixed variance)
- 	• Large mode with both a spherical component (Mie simulations) and a non-spherical component (empirical phase function, *Volten et al. 2001*)
- Search for model and optical thickness that fits the simulations best
- Quality indices based on the measurement-model fit and observation geometry

## Aerosol Inversion over the oceans

*Deuzé JL. et al., GRL, 1999*
*Deuzé JL. et al., JGR, 2000*

**Large to small particles**

- Spectral effect increases
- 150° arc decreases

**Notes:**
POLDER multidirectional observations have been used to measure the phase function and polarized phase functions of aerosols over the oceans.
Large aerosol particles show a maximum in the phase function for a scattering angle of ≈150° with limited spectral signature.  On the other hand, small particles do not show this150° maximum but show a large spectral signature.

Deuze JL, Herman M, Goloub P, Tanre D, Marchand A  Characterization of aerosols over ocean from POLDER/ADEOS-1.  Geophys. Res. Lett. 26 (10) 1421-1424 (1999)
Deuzé J.L., P. Goloub, M. Herman, A. Marchand, G. Perry, S. Susana, D. Tanré.  Estimate of the aerosol properties over the ocean with POLDER on ADEOS-1.  J. Geophys. Res., 105 (D12) 15329-15346 (2000)

## Main Parasol products over the oceans

- Total Optical Thickness

- Angström Coefficient

- Fine Mode Optical Thickness

- Coarse Mode Optical Thickness

- + Effective radii, info on scattering phase function and quality indices

- Sept. 2005

## A movie...

- Dark  : Low aerosol load
- Red    : Fine mode aerosols
- Green : Coarse mode aerosols
- Grey  : No data

## Identification of non spherical particles

- 150° arc indicates the presence of large, spherical particles

- Small spectral effect but no Arc:  Non spherical particles.

*Herman M. et al., JGR, 2005*

**Fraction of non-spherical particles in coarse mode**

**Notes:**
Herman M, Deuze JL, Marchand A, Roger B, Lallart P
Aerosol remote sensing from POLDER/ADEOS over the ocean: Improved retrieval using a nonspherical particle model
J. Geophys. Res., Vol. 110, No. D10S02

## Aerosol non-sphericity (2/2)

- Total Optical Thickness

- Fraction of non spherical in Coarse Mode

- Coarse mode Non-Spherical Optical Thickness

- Coarse mode Spherical Optical Thickness

- Non spherical particles downwind of dust sources.  Spherical particles where hydrated, sea-salts are expected.

## Over land...

- Rayleigh

- Aerosols

- Surface

- Why is polarization so useful for aerosol remote sensing over land ?

-  is small compared to atmospheric contribution
-  is spectrally neutral
-  is rather uniform (varies little with surface type)
-  can be roughly estimated from surface classification

## Variability of surf. polarized reflectance

_*Fan et al., 2007*_

- Scattering angle [°]

- Surf. Pol. Reflectance @865 nm

- Measurements show that the surface polarized reflectance is little variable, even over rather heterogeneous surfaces

## Aerosol reflectance is highly polarized

**Clear atmosphere (AOT=0.03)  :  the reflectance at TOA  is close to the surface values**

- AEROSOL

**Illustration for Biomass Burning Aerosols **

**Hazy atmosphere :  large aerosol contribution, ****1.0×10****-2 ****at 110°-120° ****for AOT=0.31 **

- Deuzé et al., 2001

## Aerosol inversion over land

- Based on the hypothesis that surface polarized reflectance is small and varies little
- Parameterization of the surface polarized reflectance (semi empirical model as a function of surface type and NDVI; *Nadal&Bréon, 1998*)
- Model + optical thickness estimate based on measured polarized reflectance at 670 and 865 nm.
- Works well for “small” aerosol (sulfates, biomass burning) over vegetated areas
- BUT...
- Does not work for coarse aerosols (desert dust)
- Does not work over desert or snow due to their larger polarized reflectance

*Deuzé JL et al., JGR, 2001*

- Polarized
-  reflectance

**Notes:**
Image shows a 3 color composite of polarized reflectance over India.  Large aerosol load are evidenced over the Gange valley (local pollution due to heating with biomass)

Deuze JL, Breon FM, Devaux C, Goloub P, Herman M, Lafrance B, Maignan F, Marchand A, Nadal F, Perry G, Tanre D.  Remote sensing of aerosols over land surfaces from POLDER-ADEOS-1 polarized measurements.   J Geophys. Res.-Atmos 106 (D5): 4913-4926 (2001)
Nadal F. Breon FM:  Parameterization of surface polarized reflectance derived from POLDER spaceborne measurements.  IEEE Trans Geosci. Rem. Sens. 37 (3) 1709-1718 (1999)

## Fine mode optical thickness

- Fine mode optical Thickness 550 nm

- Aerosol load by sub-micronic particles (fine mode)
- Over land: based on multi-directional polarized measurements
- Over the ocean : Uses both reflectance and polarized meas.

- Note annual cycle of biomass burning activity, pollution over China, Galapagos volcanic eruption late October 2005

## Time series against Aeronet

**(03/2005-05/2006)**

- 78 coincident days

- 70 coincident days

_*Fan et al., 2007*_

## Validation against AERONET

- Sunphotometer

- POLDER

- Optical Thickness

- Angström Coefficient

- AOT>1

- Fine mode

- Good agreement on optical thickness and Angström coefficient
- Largest scatter on small mode optical thickness [stay tuned]

- Marine Retrievals

## Validation against AERONET

- Angström coefficient

- Land Retrievals

- Optical Thickness

- Fine : r < 0.3 μm

- Fine : r < 0.4 μm

- Resuts affected by the definition of “fine” mode

## Validation against Aeronet

- Australia

- Medit. Europe

- Non Med. Europe

- India
- SE Asia

- China

- Sahel

## Parasol-MODIS comparison: 1 Trends

- Mar.05 - Feb.06
- PARA.	 MOD.
- 0.136	0.138
- 0.016	0.010
- Mar.06 - Dec.06
- PARA.	 MOD.
- 0.137 	0.140
- 0.012 	0.006

-  Calibration procedures are different
-  LUT’s are different
-  Spectral versus directional/polarization

## Parasol-MODIS comparison. 2: JJA 2006

**Tanre et al. 2008**

- MODIS

- Parasol

- Fine mode AOD

- Total AOD

## Parasol-MODIS comparison. 3: DJF 2007

- MODIS

- Parasol

**Tanre et al. 2008**

## Parasol concept for aerosol : Pros and con

- Multispectral +  multidirectional + Polarization measurements provide a lot of constrains for the aerosol model retrieval
- Depending on the target location with respect to the satellite, the range of scatterinig angle varies
- Multidirectional acquisition reduces the glint issue.
- Spatial resolution of POLDER.  Limits daily coverage in the presence of broken clouds (a problem in the tropics, in particular for Parasol afternoon views).
- Surface contribution to the measured polarized reflectance.  Although small, the surface contribution is not fully negligible
- A longer wavelength polarized channel would help constraining the surface polarization contribution.  Airborne measurements indicate that the surface polarized reflectance is spectrally neutral. => APS on glory

## Cloud Droplet Radiusfrom Multidirectional polarisation measurements

## Liquid phase clouds

- In some cases, clouds fields show specific features in polarized light for scattering angles between 140 and 170°

- 3-color composite 443-670-865 nm

- Baja
- California

- Stratocumulus
- cloud field

- Scattering angle

- Same scene in polarized light

**Notes:**
Observation of a field of stratocumulus clouds off the coast of Africa.  The left image is a classical 3 color composites, while the right image is the same in polarized light.  The white band for a scattering angle of 140° (phase angle of 40°) corresponds to the rainbow/cloudbow.  For larger scattering angles, colored features are observed that follow the phase angle.

## Many such examples...

- The position of color bands relative to the scattering angle is variable !

## Droplet polarized phase function

- Size distribution :

- Spectral signature :

- Dampening for wider size distributions:

- Rainbow

- The polarized phase function shows oscillations that explain the observed features.  Angular position of maxima and minima depend on wavelength and effective radius.  Such feature require a narrow size distribution.
- Polarized reflectance mostly generated by single scattering

*Bréon FM and Ph Goloub, GRL, 1998*

**Notes:**
Rapid explanation of the observed features.  The polarized light is mostly generated by single scattering (multiple scattering is non polarized). Thus, the measured polarized reflectance gives a near direct measure of the polarized phase function.  It is not the case in total light because multiple scattering dominates the signal in total reflectance over clouds.
The polarized phase function shows the maxima and minima that explain the observed features in polarized phase function.  However, these oscillations are apparent only for narrow size distributions.  They are damped out for winder size distributions.
The angular positions of the maxima and minima are very dependent on wavelength (which explains the colored features) and effective radius, which permits an estimate of the droplet effective radius.

Breon FM, Goloub P:  Cloud droplet effective radius from spaceborne polarization measurements.  Geophys. Res. Lett. 25 (11) 1879-1882 (1998)

## Measurement-model fit

- Scattering Angle [°]

- 443nm
- 670nm
- 865nm

*Bréon FM and M. Doutriaux Boucher, IEEE TGARS, 2005*

- Polarized Reflectance

**Notes:**
The POLDER measurements are aggregated over a 150X150 km2 area and binned as a function of scattering angle.  The inversion procedure searches for the effective radius that permits the best fit to the measurements.  The images show a few results showing the comparisons of measured and modeled (line) polarized reflectance for the 3 polarized channels.  Note that a single model fits all three bands.  Such astonishing agreement gives high confidence in the retrieved radius.
The estimate is representative of the very cloud top (first optical thickness)

## Retrieved spatial distributions of CDR

- August

- April

- June

- October

- Poor sampling because of limitations on viewing geometry, extended cloud field and narrow size distribution.
- Shows smaller droplets over continents, and in particular polluted areas

*Bréon FM and S. Colzy, GRL, 2000*

**Notes:**
Breon FM, CoBreon FM, Colzy S   Global distribution of cloud droplet effective radius from POLDER polarization measurements.  Geophys. Res. Lett  27 (24): 4065-4068 (2000)

## Comparison with MODIS

- Excellent correlation over the Oceans
- Poor correlations for small droplets [in particular found over land surfaces]
- Bias of 2 μm (POLDER < MODIS).
- 	- CDR at the very cloud are smaller than deeper in the cloud ?
- 	- Spatial heterogeneity ?
- 	- Size distribution different than assumed ?

## Slide 39

## CDR with polarization. Conclusions

- Multidirectional polarization provides an alternative (to spectral) method for the estimate of CDR
- Advantage of polarization : Measures the single scattering, which provides a near-direct measurement of the scattering phase function
- Requires specific conditions (geometry and cloud properties) so that its statistic is poor
- Measurements have shown that the CDR distribution is not as expected, in particular over stratocumulus clouds.
- Bias with MODIS.  Several hypothesis.  I “believe” bias is due to measurement depth into the cloud.
- Still not clear why MODIS shows large spatial variability in CDR when Parasol retrieval indicates a more homogeneous CDR field