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Nadine GobronInstitute for Environment and Sustainability of EC-JRC, 21020
Ispra (VA), Italy
Radiative Transfer Modeling - EO LDAS
EO-LDAS Team: Jeff Settle, Thomas Kaminski, Philip Lewis, Sietse Los, Peter North,
Tristan Quaife, Jon Styles.
JRC: B. Pinty & J.-L. Widlowski
2
EO-LDAS
Radiances TOC or TOA Land Properties
Process Model
RT Models
EO-LDAS (Simplest View)
Complexity depends:
1) Spatial resolution
2) number of land state
variables of interest
3
Vertical radiative coupling between geophysical media
Atmosphere
')',()',(~),()],,(~[4
dzIzzIz sOe
Vegetation
Soil
Upper limit (1)
RTE
Lower Limit (1)
Upper limit (2)
RTE
Lower Limit (2)
Upper limit (3)
RTE
Lower Limit (3)
ZA
ZV
ZS
Extinction coefficient Differential scattering coefficient
In each medium, the transfer of radiation may be represented
by the following approximate equation:Lev
el o
f im
ple
men
tati
on
TOA
TOC
Pinty B. and Verstraete M. M. (1998) `Introduction to Radiation Transfer Modeling in Geophysical Media', in From Urban
Air Pollution to Extra-Solar Planets, Vol. 3, Chapter IV, Edited by C. Boutron, 67--87.
4
Atmosphere
Vegetation
Soil
•Non oriented small scatterers
•Infinite number of scatterers
•Low density turbid medium
•Oriented finite-size scatterers
•Finite number of scatterers
•Dense discrete medium
•Oriented small-size scatterers
•Infinite number of clustered scatterers
•Compact semi-infinite medium
Vertical radiative coupling between geophysical media
(1D view)
RT 1D - no 3D
Pinty B. and Verstraete M. M. (1998) `Introduction to Radiation Transfer Modeling in Geophysical Media', in From Urban
Air Pollution to Extra-Solar Planets, Vol. 3, Chapter IV, Edited by C. Boutron, 67--87.
5
Atmosphere
Vegetation
Soil
Za
ZV
ZS
),(
azI)(),( 0 ota IzI
0),(
zI
The description of the interaction of a radiation field with a
layered geophysical medium implies the solution of radiation
transfer equations and the specification of appropriate
boundary conditions
),(]||
exp[)(),(0
,
bada
tatv zIzIzI
'''
,
'
2
||)(),,(1
),(
dzIzzI babavba
'''
,
'
2
||)(),,(1
),(
dzIzzI bvbvsbv
Vertical radiative coupling between geophysical media
Pinty B. and Verstraete M. M. (1998) `Introduction to Radiation Transfer Modeling in Geophysical Media', in From Urban
Air Pollution to Extra-Solar Planets, Vol. 3, Chapter IV, Edited by C. Boutron, 67--87.
6
Model Assumption
),,( 'bav z
Parametric
2-Stream
1-D
3-D
Rahman, H., M. M. Verstraete, and B. Pinty (1993) ' Coupled surface-atmosphere reflectance (CSAR) model. 1. Model
description and inversion on synthetic data ', Journal of Geophysical Research, 98, 20,779-20,789.
10
Spatial Resolution ? Process Model ? Coupling RT atmos. ?
Parametric
2-Stream
1-D
3-D
N/A directly
N/A directly (fluxes)
Which type of RT model?
Option?
discussion …
),,( 'bav z
11
3-D versus 1-D
True <LAI> =2.0
3-D heterogeneous system
True <LAI> =2.0True <LAI> =2.0
3-D heterogeneous system
True <LAI> =2.0
Direct transmission at 30 degrees Sun zenith angle,
0.596
)(3
LAITdirect
D
Direct transmission at 30 degrees Sun zenith angle,
0.596
)(3
LAITdirect
D
1-D system representation
True <LAI> =2.0
1-D system representation
True <LAI> =2.0
0
12
exp)(LAI
LAITdirect
D
Direct transmission at 30 degrees Sun zenith angle,
= 0.312
Effects induced by internal variability of LAIPinty, B., N. Gobron, J.-L. Widlowski, T. Lavergne and M. M. Verstraete (2004) `Synergy between 1-D and 3-D radiation
transfer models to retrieve vegetation canopy properties from remote sensing data', Journal of Geophysical Research, Vol.109,
D21205 10.1029/2004JD005214.
12
Parametric
2-Stream
1-D
3-D
Model Assumption
Gobron, N., B. Pinty, M. M. Verstraete and Y. Govaerts (1997) ' A semi-discrete model for the scattering of light by vegetation
', Journal of Geophysical Research, 102, 9431-9446.
13
Discrete canopy: 1D representation
H
Parameters in 1D representation:
Height of canopy, Size of a single leaf & Leaves orientation
(leaf angle distribution), LAI, leaf spectral values, soil
albedo (or 3-4 variables if anisotropic).
Df
Gobron, N., B. Pinty, M. M. Verstraete and Y. Govaerts (1997) ' A semi-discrete model for the scattering of light by vegetation
', Journal of Geophysical Research, 102, 9431-9446.
14
Ex: Semi-discrete model
single-collided-by-soil BRF:
Semi-discrete is a 1-D’ model using analytical & numerical methods.
K
r
r
r
rsoil
K
G
V
Vai
Gai
cos
)(1),(
cos
)(1 0
0
0 K
Ω0Ωr
ai
ai
ai
single-collided-by-leaves BRF:Ω0Ωr
1
0
0
0
0
cos
)(1
cos
)(1
coscos
)(
Ki
i
r
r
r
i
r
r G
V
Vai
Gaiai
Multiple-collided by leaves and soil BRF:
obtained with the discrete ordinates numerical method.
½-discrete splits the BRF into 3 components
Gobron, N., B. Pinty, M. M. Verstraete and Y. Govaerts (1997) ' A semi-discrete model for the scattering of light by vegetation
', Journal of Geophysical Research, 102, 9431-9446.
17
3-Dimensional problem
where the plan-parallel concept may be
inappropriate:
•Document the errors due to an oversimplification of the full
3-D situation, i.e. deviations from the 1D case.
•Explore new ways and techniques for representing, at
limited costs, the 3D nature of the medium which basically
require almost an infinity of parameters!
•Address the application issues for geophysical modeling, e.g.
the definition of new “equivalent variables”, and satellite data
interpretation, e.g. the non-uniqueness of the inverse
problem.
18
3-D Radiative Transfer Equations
3-D Models
•Ray tracing models
•Geometrical models
•Hybrid models
I(x,)G(x,)uL (x)I(x,) uL (x)
(x, )I(x, )d
4
19
Which type of RT model?
Feasibility for implementation in EO-LDAS
Parametric
2-Stream
1-D
3-D
Adjoint Code or LUT
Depend on process
model & spatial
resolution
21
RAMI evaluates models
in forward mode
RAdiative transfer Model Intercomparison
http://rami-benchmark.jrc.ec.europa.eu
Purpose:•act as common platform
for intercomparison efforts
•document uncertainties
and errors among models.
•establish protocol for RT
model evaluation.
22
• RAMI-1 (1999):– Turbid medium and discrete
– Solar domain + purist corner
• RAMI-2 (2002):– Topography + true “zoom-in”
• RAMI-3 (2005):– Birch and conifer scene
(GO models)
– Heterogeneous purist corner
– Local transmission and horizontal flux measurements
HOMogeneous HETerogeneous
RAMI-1 RAMI-2 RAMI-3
13
20
42
Fra
cti
on o
f H
ET
[%]
Num
ber
Experi
ments
RAMI-1 RAMI-2 RAMI-3
660715
980
RAdiative transfer Model Intercomparison
Pinty, B., N. Gobron, J.-L. Widlowski , S. A. W. Gerstl, M. M. Verstraete, M. Antunes, C. Bacour, F. Gascon, J.-P.
Gastellu, N. Goel, S. Jacquemoud, P. North, W. Qin, and R. Thompson (2001) 'Radiation Transfer Model Intercomparison
(RAMI) Exercise', Journal of Geophysical Research, 106, 11,937-11,956.
Pinty, B., J-L. Widlowski, M. Taberner, N. Gobron, M. M. Verstraete and the RAMI-2 Participants (2004) ̀ The RAdiation
transfer Model Intercomparison (RAMI) exercise: Results from the second phase', Journal of Geophysical Research, Vol.109,
D06210 10.1029/2003JD004252.y
Widlowski, J.-L., M. Taberner, B. Pinty, and colleagues (2007) `The third RAdiation transfer Model Intercomparison (RAMI)
exercise: Documenting progress in canopy reflectance models', Journal of Geophysical Research, Vol.112, doi:
10.1029/2006JD007821.
23MODEL NAME PARTICIPANT AFFILIATION
ACRM A. Kuusk Tartu Observatory,Estonia
DART J.P. Gastellu, E.Martin CESBIO, France
drat M. Disney, P. Lewis UCL, UK
FLIGHT P. North Univ. Swansea, UK
frat P. Lewis, M. Disney UCL, UK
FRT M. Möttus, A. Kuusk Tartu Observatory,Estonia
Hyemalis R. Ruiloba NOVELTIS, France
MAC R. Fernandes CCRS, Canada
mbrf W. Qin NASA GFSC, USA
RGM D. Xie, W. Qin Beijing N. Univ., China
Rayspread T. Lavergne JRC, Italy
raytran T. Lavergne JRC,Italy
SAIL++ W. Verhoef NLR, Netherlands
½ discret N. Gobron JRC, Italy
Sprint3 R. Thompson Cox, USA
4SAIL2 W. Verhoef NLR, Netherlands
5scale N. Rochdie, R. Fernandes CCRS, Canada
2stream B. Pinty, T. Lavergne JRC, Italy
3-D models
1-D models
new in RAMI-3
RAMI-1 RAMI-2 RAMI-3
8
13
18
Num
ber
of
models
RAMI-1 RAMI-2 RAMI-3
5
10
11/13
3-D
m
odels
25
Absorption
Albedo
Transmission
Measurement Types
Measurements include
Flux quantities:
• Albedo
• Transmission
• Absorption
BRF quantities:
•Total BRF (PP+OP)
total BRF
multiple collided
single collided
single un-collided
BRF quantities:
•Total BRF (PP+OP)
•BRF components–multiple collided
–single un-collided(hit soil only once)
–single collided (hit leaves only once)
26
RT Model Intercomparison Caveat
• In general there is no absolute ‘truth’ available! Model results cannot be evaluated against some reference standard
• Laboratory data are difficult to use as reference standard due to incomplete knowledge of the exact illumination, measurement, as well as (structural and spectral) target properties.
but
• Model results can be compared against each other to document their relative differences.
• Model results can be compared over ensembles of test scenarios to establish trends/behaviours in their performance.
• Careful inspection/verification of an ensemble of model results may lead to the establishment of the “most credible solutions” as a surrogate for the “truth”.
27
Participation & Performance
HOM DIS cases
X2 uses σ=0.03<BRF>3D
Most models are indiscernible
(within 3%) from surrogate truth
More models are different (by
≥ 3%) from surrogate truth
Model performance may be
affected by spectral regimes
LAI=1
LAI=5
LAI=2
Relative Intercomparison: Χ2 statistics
EO-LDAS prototype
Widlowski, J.-L., M. Taberner, B. Pinty, and colleagues (2007) `The third RAdiation transfer Model Intercomparison
(RAMI) exercise: Documenting progress in canopy reflectance models', Journal of Geophysical Research, Vol.112, doi:
10.1029/2006JD007821.
28
Homogeneous
X2 uses σ=0.03<BRF>3D
RAMI-2RAMI-3
DiscreteHeterogeneous
RAMI-2RAMI-3
Model performance improved from RAMI-2 to RAMI-3!
Relative Intercomparison: Χ2 statistics
29
FLIGHT: structure and geometry
Diameter
Ez
x
z y
Ez
DBH
Radius
Canopy:
- Leaf Area Index(LAI)
- Crown envelopes
- Leaf angle distribution (LAD)
- Optical properties
North, Peter R. J. (1996) 'Three-Dimensional Forest Light Interaction Model Using a Monte Carlo Method', IEEE
Transactions on Geoscience and Remote Sensing, 34, 946-956
30
P1
Diameter
Ez
x
z y
Ez
DBH
Radius
Light interaction:
- Source(s)
- Sensor
- Photon paths (multiple)
- Scattering
MCRT - random sampling of photon trajectoriesNorth, Peter R. J. (1996) 'Three-Dimensional Forest Light Interaction Model Using a Monte Carlo Method', IEEE
Transactions on Geoscience and Remote Sensing, 34, 946-956
31
FLIGHT - canopy structure
North, Peter R. J. (1996) 'Three-Dimensional Forest Light Interaction Model Using a Monte Carlo Method', IEEE
Transactions on Geoscience and Remote Sensing, 34, 946-956
32
European beech
Coniferous forest
North, Peter R. J. (1996) 'Three-Dimensional Forest Light Interaction Model Using a Monte Carlo Method', IEEE
Transactions on Geoscience and Remote Sensing, 34, 946-956
FLIGHT – 3D scene
34
RAMI intercomparison
FLIGHT vs ASAS
North 1996; Pinty et al., 2001, 2003; Widlowski et al., 2008
FLIGHT BRF Validation
37
Various RT vegetation model:
Parametric: serve as proxy for BRDF over land but no direct link
with process model except if surface albedo is foreseen.
Main advantage: Can be used for solving coupled RT prb when TOA
data.
2-stream, 1-D (semi-discrete): Always effective
variables. Adjoint code
3-D: Infinity of parameters? LUT
All can be coupled with atmospheric model like 6S .
38
10 m 100 m 1000 m
zero order model (veg. parameters t-1)
dynamic vegetation model (climate
parameters)
Climate constraint model (climate
parameters+veg t-1)
> 10 km
RT 3D
RT 1D’
RT 1D
Main open issues
Where are the boundaries (and needs)?
Recommended