# Presentation: 645308

## Atmospheric Correction around AERONET sites: Development of Global Land Validation Dataset 

**PI:*** A. Lyapustin; ***co-I:*** Y. Wang *(GEST UMBC/NASA GSFC)
**Collaborators:** *B. Holben, J. Privette *(GSFC); *J. Martonchik, D. Diner *(JPL)

**GSFC, October 5, 2000 (ETM+)**

**Challenges of SR Validation**
- Heterogeneous surface.
- Spatial resolution (1 km) *vs* scale of surface variation	(20 m) (thousands of measurements).
- Spectral differences between ground-based and 	space-borne instruments.
- Ground-based measurements require atmospheric	correction (complex experiments).

**Research Objective:**
- 	***Development of Advanced Atmospheric Correction Algorithm for MODIS***
**Linked to:**
- 	***Development of Validation Dataset of Surface Reflectance over Land***

## ASRVN - AERONET-based Surface Reflectance Validation Network

**ASRVN - AERONET-based Surface Reflectance Validation Network**

_**THEORETICAL BASIS**_** **
-  3D Radiative Transfer (**Lyapustin & Knyazikhin, *****Appl. Opt.*****, 2001; 2002**)
-  variable anisotropic surface;
-  arbitrary spatial resolution;
-  semi-analytical, accurate and fast due to parameterizations.
-  Accurate Modeling of Gaseous Absorption
-  Inversion with *MRPV*MISR and *LSRT*MODIS BRF Models
_**PRODUCTS**_
-  BRF, Albedo (spectral & SW broadband)
-  Surface Radiative Fluxes, PAR
_**EXPECTED BENEFITS**_
-  Validation of BRF & Albedo over Heterogeneous Surfaces
-  MODIS-MISR Data Fusion
-  Calibration Analysis
- Vicarious calibration
- Cross-calibration of different sensors
- Detection of calibration trend based on a time 	series of surface reflectance.

- _MISR_
-        MODIS
-               ETM+
-                      SeaWIFS
-                             VIIRS

## Study Area:

**1. Mongu (Zambia)**
***(15*****o*****15’S, 23*****o*****09’E), H=******1,104 m***
**2. GSFC (USA)**
***(39*****o*****03’N, 76*****o****88*****’W), H=50****** m***

_**Albedo Time Series**_
**(Mongu, 2003) **
-     ***Vis. (RGB) and NIR (grey)***
**from**
***MISR Collection 4 Data***

## Analysis of BRF

*Solid lines* – MISR radiance/BRF
*Squares* – ASRVN BRF (MRPV algorithm)
*Triangles* – model BRF calculated with the best-fit parameters (MRPV)

*MISR retrieves *_*correct BRF shape*_*.*
*MISR BRF is *_*less anisotropic*_* than the ASRVN BRF in the Vis.*
*	**    **(*analysis*: in IEEE TGARS Special Issue on Land Product Validation)*

## Analysis of Albedo

*MISR correctly reproduces the *_*time series*_* of albedo.*
*MISR albedo is on average *_*lower*_* by about 0.005 in the green and red bands. *

- MISR AOT

- AERONET

- MISR

- ASRVN

## Analysis of Albedo Spatial Distribution

*MISR correctly reproduces *_*spatial distribution*_* of albedo. *
*An albedo *_*mosaic*_* may develop due to MISR aerosol retrievals at 17.6 km resolution.*
*ASRVN albedo does not have model-dependent distortions from the variable land cover.*

## Summary of Results (local analysis)

*1.  MISR BRF and albedo products are **generally *_*accurate*_* in both *_*clear*_* and *_*hazy*_* atmospheric conditions.*
*2.  MISR correctly reproduces the *_*time series*_* and *_*spatial distribution*_* of albedo. *
*3.  MISR BRF on average is *_*less anisotropic*_* than actual BRF in the visible bands.*
- 	The difference is greatest in the blue band, decreases with wavelength, and it is negligible in the near-IR band. This discrepancy most likely originates in 1) MISR aerosol retrieval algorithm over heterogeneous land, which tends to select an aerosol model that benefits the spectrally invariant shapes of surface BRF; 2) MISR surface HDRF retrieval algorithm where the iteration loop that removes the diffuse atmospheric transmittance is currently turned off.
*4.  Our initial results suggest that the MISR surface albedo is on average *_*lower*_* than the ASRVN albedo by about 0.005 in the green and red bands. *
**BRF Model (LSRT vs MRPV)**
*1.  MRPV model fits BRF shapes slightly better than LSRT model, except in the blue band.*
*2.  The AC algorithm based on LSRT model is much faster and more robust (due to linearity). *
*3.  The albedos from the two models are generally similar, with the average difference ****q****0.005. The difference ****q is spatially homogeneous but site-dependent (function of aerosol absorption). *

_**Summary of Results (**__***local***__** analysis)**_

## What is Next ...

**MISR**
*ASRVN is receiving and processing operational MISR collection 5 data (0.5 Yr in 2000, and since Dec. 2004) for all AERONET stations *(*166*)*. We plan to carry on MISR validation analysis on the regional and then global scales.*
*The web-interface with data analysis tools is under development.*
**ETM+**
*The ASRVN_ETM+ processing algorithm is being finalized (1 month).*
*ASRVN_ ETM+ products will become available this summer.  *
**MODIS**
*Cooperating with Goddard Earth Sciences Data & Information Services Center (G. Leptouch, D. Ouzounov) to produce MODIS Collection 5 subsetted and aggregated data for ASRVN. *
*Operational ASRVN_ MODIS products will become available this fall.*