# Presentation: 202731

## Validation of the 6S radiative transfer code for atmospheric correction of MODIS data

**MODIS ST Meeting, Land Discipline Breakout**

**March 24****th****, 2005**

**Eric F. Vermote **&** Svetlana Y. Kotchenova**
**Department of Geography, University of Maryland**

## Radiation polarization

**2**

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

## Validation of the new version of 6S

**Validation of the new version of 6S**

## Scalar mode – a purely molecular atmosphere

**Scalar mode – a purely molecular atmosphere**

-    **geometry**: a wide range of {SZA, VZA, AZ}
-    **optical thickness**:  = 0.1 (λ ≈ 0.53 μm),  = 0.3445 (λ** **= 0.4 μm),  = 0.5 (λ** **≈ 0.36 μm)
-    **ground reflectance**:  = 0.0 (black soil) and  = 0.25 (Lambertian)

## Scalar mode – a purely aerosol atmosphere

**5**

**Scalar mode – a purely aerosol atmosphere**

**6S (no polarization) **vs.** SHARM**
-  = 0.72804, AZ = {0; 90.0; 180.0}, SZA = {0.0; 11.48;  23.07; 32.86;  58.67}

-    **model**: 70% dust + 30% water-soluble
-    **waveform**:  λ = 0.694 μm

-    **geometry**: a wide range of {VZA, SZA, AZ}
-    **optical thickness**:   = {0.072804, 0.72804}

## Vectorial mode – Monte Carlo

**6**

## Vectorial mode – Coulson’s tabulated values

**7**

-     **atmosphere**: purely molecular
-     **geometry**: a wide range of {SZA, VZA, AZ}
-    **optical thickness**:  = 0.1 (λ ≈ 0.53 μm) and  = 0.5 (λ** **≈ 0.36 μm)
-    **ground reflectance**:  = 0.0 (black soil) and  = 0.25 (Lambertian)

## Validation of the new version of 6S - Conclusions

**8**

-   **The new vectorial version of** **6S**, which accounts for **radiation** **polarization**,    has demonstrated good agreement with **Monte Carlo** and Coulson’s    tabulated values for a wide range of geometrical and atmospheric conditions.    The agreement is better than 0.5% for **Monte Carlo** and 0.35% for Coulson’s.
-  **The new vectorial version of** **6S**, used in the scalar mode, has demonstrated   good agreement with the scalar code **SHARM**: better than 0.3% for a purely   molecular atmosphere and better than 0.35% for a purely 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 purely molecular atmosphere

**9**

## Effects of polarization – a purely aerosol atmosphere (1)

**10**

- The scalar** (no polarization)** and the vectorial **(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 – a purely aerosol atmosphere (2)

**11**

**6S (with polarization) **vs.** 6S (no polarization)**
**‘****biomass burning smoke’, ****λ**** = 0.67 ****μ****m, ******** = 0.72804,****SZA = {0.0; 11.48;  23.07; 32.86;  58.67}, AZ = {0; 90.0; 180.0}**

## Retrieval of ocean surface reflectance (1)

**12**

- 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 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.

## Retrieval of ocean surface reflectance (2)

**13**

- 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

**14**

-   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.
-   Account 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.

## Future plans

**15**

-   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
-   Use of the new version of 6S for the inversion of AERONET measurements    (depends on funding). Collaboration with O. Dubovik
-   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
-   Calculation of new Look-Up Tables for the atmospheric correction algorithm

**16**