Validation of a new vector version ofValidation of a new vector version of

the 6S radiative transfer code for the 6S radiative transfer code for

atmospheric correction of MODIS dataatmospheric correction of MODIS data

AFRL Transmission MeetingJune 16th, 2005

Svetlana Y. Kotchenova1, Eric F. Vermote1 & Raffaella Materesse2

1Department of Geography, University of Maryland, USA

2Department of Physics, University of Bari, Italy

Radiation polarization

2

A new version of 6S, which accounts for radiation polarization, has been developed.

The real situation

account for polarization

Vector codes

{ I, Q, U, V }

Scalar codes

{ I, 0, 0, 0 }

The artificial situation

no polarization

Degrees of polarization

90-100% - molecular atmosphere at 900 scattering angle

30% - small aerosol particles

5-25% - snow

0-40% - ice

0-15% - sand

2-23% - vegetation

unpolarizedradiation

polarizedradiation

Second Simulation of a Satellite Signal in the Solar Spectrum

15

6S is based on the method of successive orders of scattering approximations. Besides its extensive applications in the radiative transfer studies, it is the basic code used for MODIS atmospheric correction.

The scalar version of 6S (6SV4.1):

• accurate simulations of satellite and plane observations

• accounting for elevated targets

• use of Lambertian and anisotropic surfaces

• calculation of gaseous absorption

The vector version of 6S (6SV1.0B):

• all the features of the old version

• accounting for radiation polarization

• calculation of highly asymmetric phase functions

• possibility to change the number of calculation angles and layers

3

Validation of the new version of 6S

molecular atmosphere

vector mode(with polarization)

Monte Carlo(with polarization)

the new versionof 6S

scalar mode(no polarization)

molecular atmosphere

aerosol atmosphere

Coulson’stabulated values

DISORTSHARM

C O M P A R I S O N

aerosol atmosphere

MODTRAN

C O M P A R I S O N

Deutsch

4

Scalar mode – a molecular atmosphere

• geometry: SZA={0.0, 10.0, 23.07, 45.0, 58.67, 75.0} + a wide range of VZA + AZ={0.0, 90.0, 180}

• optical thickness: = 0.1 (λ ≈ 0.53 μm), = 0.3445 (λ = 0.4 μm)

• ground reflectance: = 0.0 (black soil), = 0.25 (Lambertian)

Agreement is better than 0.015%

5

Scalar mode – an aerosol atmosphere

• model: continental (70% of dust, 30% of water-soluble, and 1% of soot)

• geometry: SZA={0.0, 10.0, 23.07, 45.0, 58.67, 75.0} + a wide range of VZA + AZ={0.0, 90.0, 180}

• waveform: λ = 0.694 μm• optical thickness: = {0.210, 0.778}

Agreement is better than 0.08%

Vector mode – Monte Carlo

6

VZA / AZ 0 - 22.5 22.5 - 45.0 45.0 - 67.5 67.5 - 90.0 90.0 - 112.5 112.5 - 135.0 135.0 - 157.5 157.5 - 180.00 - 15.0 -0.02 -0.04 0.06 0.08 0.06 0.00 0.17 0.14

15.0 - 21.4 0.25 0.16 0.20 -0.13 0.20 0.24 0.21 0.0921.4 - 26.6 0.15 0.01 0.03 0.17 0.25 0.09 0.13 0.0626.6 - 31.1 0.41 0.20 0.21 0.13 0.01 0.26 0.02 0.1931.1 - 35.3 0.02 0.22 0.36 -0.03 0.30 0.24 0.24 0.1135.3 - 39.2 -0.06 0.08 0.37 0.09 0.23 0.20 0.12 0.0939.2 - 43.1 0.15 -0.02 0.11 0.13 0.15 0.17 0.19 0.1343.1 - 46.9 -0.04 0.11 0.20 -0.04 0.17 0.07 -0.01 0.2246.9 - 50.8 0.22 0.18 0.14 0.15 0.19 0.14 0.07 0.1450.8 - 54.7 0.25 -0.12 0.15 0.15 0.12 0.23 0.18 0.2654.7 - 58.9 0.08 0.01 0.09 0.14 0.16 0.17 0.13 0.0358.9 - 63.4 0.00 0.11 0.02 0.17 0.16 0.12 0.18 0.0163.4 - 68.6 -0.15 -0.04 0.02 0.12 -0.01 0.13 -0.13 0.0468.6 - 75.0 -0.14 -0.17 -0.25 -0.26 -0.07 -0.19 [ -0.44 ] -0.42

0.14Nadir viewing

6S (with polarization) vs. Monte Carlo (1 billion photons)

SZA = 23.0, = 0.35, = 0.0Difference, %

• atmosphere: purely molecular

• optical thickness: = 0.35

• geometry: SZA = {0.0; 23.0; 57.0} + {VZA, AZ}

• ground surface: black soil

Agreement is better than 0.45%

Vector mode – Coulson’s tabulated values

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• atmosphere: purely molecular

• geometry: a wide range of {SZA, VZA, AZ}

• optical thickness: = 0.1 (λ ≈ 0.53 μm), = 0.25 (λ ≈ 0.44 μm)

• ground reflectance: = 0.0 (black soil), = 0.25 (Lambertian)

Agreement is better than 0.21%

Validation of the new version of 6S - Conclusions

8

• The new vector version of 6S, which accounts for radiation polarization,

has demonstrated good agreement with the Monte Carlo code and Coulson’s

tabulated values for a wide range of geometrical and atmospheric conditions.

The agreement is better than 0.45% for Monte Carlo and 0.22% for Coulson’s.

• The new vector version of 6S, used in scalar mode, has demonstrated good

agreement with the scalar codes SHARM and DISORT: better than 0.015% for

a molecular atmosphere and better than 0.08% for an aerosol atmosphere.

• The observed difference is not of concern, as it is much less than the 2%

accuracy of raw MODIS top-of-atmosphere reflectance data.

Effects of polarization – a molecular atmosphere

9Error can be more than 10%

• geometry: SZA={0.0, 10.0, 23.07, 45.0, 58.67, 75.0} + a wide range of VZA + AZ={0.0, 90.0, 180}

• optical thickness: = 0.1 (λ ≈ 0.53 μm), = 0.3445 (λ = 0.4 μm)

• ground reflectance: = 0.0 (black soil)

Effects of polarization – an aerosol atmosphere (1)

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0

0.02

0.04

0.06

0.08

0.1

0.05 0.11 0.26 0.58 1.30 2.94 6.64 15.00

dV/d

lnr,

(μ

m3/μ

m2)

radius, μm

SSA = 0.9237asym = 0.585

Average aerosol volume size distribution(from AERONET measurements)

The scalar (no polarization) and the vector (with polarization) versions of 6S have

been compared for a ‘biomass burning smoke’ aerosol model. This model is a typical

pattern produced by forest fires over the Amazonian tropical forest region in Brazil.

Reference - O. Dubovik et al., J. Atmos. Sci., 59, pp.590-608, 1996

Effects of polarization – an aerosol atmosphere (2)

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0

1

2

3

4

5

6

0 10 20 30 40 50 60 70 80 90

VZA, degrees

Ab

so

lute

dif

fere

nc

e, %

6S (with polarization) vs. 6S (no polarization)‘biomass burning smoke’, λ = 0.67 μm, = 0.72804,

SZA = {0.0; 11.48; 23.07; 32.86; 58.67}, AZ = {0; 90.0; 180.0}

Error can be up to 5%

Retrieval of ocean surface reflectance (1)

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MODIS AQUA data, collected over the Hawaii islands, have been corrected using the

new version of the 6S code (with polarization) and AERONET measurements collected

at Lanai island. The corrected data were compared with surface reflectances measured

by MOBY (the Marine Optical Buoy System) just above the ocean surface.

The MOBY data

dates = {January 2, February 1, February 10,

September 3, September 19, October 6,

October 22; 2003}

λ = {412; 443; 490; 530; 550; 667; 678} nm.

*MOBY

Retrieval of ocean surface reflectance (2)

13

y = 1.0223x - 0.0007

R2 = 0.9868

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.01 0.02 0.03 0.04 0.05

measured reflectances

mo

dis

re

fle

cta

nc

es

measured reflectances

AQ

UA

re

flect

an

ces

Corrected MODIS AQUA data vs. MOBY data

The agreement between the corrected AQUA and the MOBY surface reflectances was

0.001 to 0.002 for the 400-550 nm region.

Effects of polarization - Conclusions

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• Ignoring the effects of radiation polarization leads to large errors in

calculated top-of-atmosphere reflectances. The maximum relative error is more

than 10% for a purely molecular atmosphere and is up to 5% for a purely

aerosol atmosphere.

• Accounting for radiation polarization is extremely important for atmospheric

correction of remotely sensed data, especially those measured over dark

targets, such as ocean surface or dark dense vegetation canopies.

Release of the new version of 6S

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• Code: ftp://kratmos.gsfc.nasa.gov/pub/eric/6S

• Web Interface: http://6s.ltdri.org

• Manual: in preparation

(the manual for the scalar version is at ftp://kratmos.gsfc.nasa.gov/pub/6S)

• Questions: skotchen@kratmos.gsfc.nasa.gov

• Paper: in preparation

(S. Kotchenova, E. Vermote, R. Matarrese, & F. Klemm, ‘Validation of a new vector

version of the 6S radiative transfer code for atmospheric correction of MODIS

data’, to be submitted to IEEE in July 2005.)

Future plans

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• Further ‘theoretical’ validation of the new version of 6S

- comparison with Monte Carlo for aerosol and mixed atmospheres

- inclusion of anisotropic surfaces

- refinement of the code

• Calculation of new Look-Up Tables for the MODIS (Collection 5) atmospheric

correction algorithm

• Further ‘experimental’ validation of the new version of 6S

- retrieval of ocean surface reflectance and comparison with MOBY data

- comparison with ground-measured surface reflectances

• Use of the new version of 6S to potentially improve the inversion of AERONET

measurements (depends on funding). Collaboration with O. Dubovik