Nadia [email protected] Richard Fernandes & Michael Chelle

Nadia [email protected] Fernandes& Michael Chelle

Does Needle Clumping Affect Shoot Scattering and Canopy BRDF ?

Outlook Context Background Smolander Approach Objectives Impact of needle reflectance and transmittance on shoot

scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion

Outlook ContextContext Background Smolander Approach Objectives Impact of needle reflectance and transmittance on shoot


Conteoxt Monitoring the seasonal development and carbon uptake by vegetation in

relation with global climatic change. Northern vegetation (Dominant forests) represent significant carbon sink.

VEGETATION

MISR

Spectral & Directional Information

Reflectance or Radiative Transfer Models

Empirical approaches(Vegetation Indices)

Biophysical variables Leaf Area Index

fcoverfAPARalbédo

Functioning Model

PROCESSESPhotosynthesis

Growth

Energy & mass exchanges

Understand & model processes at various scales

LAI EvaluationRe

fere

nce

LAI

x10

Refe

renc

e LA

I x1

0

VGT LAI x10 VGT LAI x10

Watson Lake, Yukon Kejimikujik, Nova Scoatia

/ 1.4

Outlok Context

Background Background Smolander Approach Objectives Impact of needle reflectance and transmittance on shoot


Coniferous Forest Modeling Background

Tree scale

Shoot scale

Structure organization at various scales constrains canopy radiative transfer.

Clumping around branch

Clumping around Twig

Spatial Distribution

Stand Scale

Empirical approaches : Implicit taking into account within field measurements Reflectance & Radiative transfer models:

Turbid media models all structure captured in single element Geometrical Optical (GO): opaque crowns are described by geometric shapes (cone, spheroid, cylinder …) Geometric-Optical & Radiative Transfer GORT: Radiative transfer within crowns are considered Ray Tracing/ Radiosity: explicit placement of elements – how much complexity?

-

Shoots are typically assumed the basic element

Needle clumping parameterization

Background

Tree scale

Shoot structure variability : (specie, age, canopy depth)

Stand ScalePinus Bancksiana (Jack Pine) Picea-Mariana (Black Spruce) Pinus-sylvestris (Scot Pine)

How needle clumping affect shoot scattering ?

Shoot silhouette to total needle area ratio of the Shoot ‘STAR’: (Oker-Blom 1985) Projected Shoot area divided by total needle area Average over spherical shoot orientation : Variability [0.09 0.22]

Needle to shoot area ratio ‘ ’: (Chen and Cihlar 1995 )

Coefficient factor within GORT models and Optical LAI measurements (TRAC, hemispherical photographs,

LAI2000)

STAR

STAR41

Outlook Context Background

Smolander’s ApproachSmolander’s Approach Objectives Impact of needle reflectance and transmittance on shoot


Shoot scattering parameterization

Smolander’s Approach

Tree scale

Impact of STAR variable on Shoot scattering albedo sh: (Smolander & Stenberg 2003)

STARp

pp

sh

Nsh

shNsh

41

)(.11).()(

N: Needle albedopsh: probabilty of more than 1 interaction within

the shoot

Validation over Scot pine using “constrained” ray tracing simulations Shoot scale - Shoot phase function shows a hot spot in illumination direction - Shoot phase function averaged over all directions is bi-lambertian Canopy scale: - Spherical oriented shoots may be as bi-lambertian flat leaves with G()=2STAR. - Good agreement in nadir view

5SCALE simulation on BOREAS Old Black Spruce

Is it sufficient to reproduce BRDF ?

STAR effect at canopy scale

Smolander’s Approach

Is such shoot parameterization sufficient to reproduce Canopy BRDF ?

Outlook Context Background Smolander Approach

ObjectivesObjectives Impact of needle reflectance and transmittance on shoot


Objectives

1- What is the impact of Needle reflectance andtransmittance on shoot scattering and canopy BRDF?

2- Can we design a simple shoot with same effectiveoptical properties as a detailed shoot ?

3- What is the impact of using simplified shoot oncanopy scale BRDF?

Outlook Context Background Smolander Approach Objectives

Impact of needle reflectance and transmittance Impact of needle reflectance and transmittance on shoot scattering and canopy BRDF on shoot scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion

Method: Detailed Shoot (S)Impact of needle reflectance and transmittance …

Twig length 7.7 cm

Twig width 0.3 cm

Needle length 2.85 cm

Needle width 0.092 cm

Needle Number 190

Total Needle area 199.28 cm2

Needle-twig angle 40.5

STAR 0.104

Shoot structure: Scot pine

Main Assumptions • Needle as rectangular box defined by its width and length• Twig as decahedron with given width and length• Same needle number is fixed on each decahedron face • Needle-Twig angle is constant

Shoot scattering: Forward ray tracing simulation

Optical properties A RED B A NIR BTwig reflectance 0.05 0.05 0.45 0.45

Needle reflectance 0.05 0.062 0.45 0.65Needle transmittance 0.05 0.028 0.45 0.25

PARCINOPY(Chelle 1997)

Scot pine shoot

Optical properties

• Directional scattering • Reflectance coefficient• Transmittance coefficient • Albedo

Illumination direction s

Black soil

Shoot ScatteringImpact of needle reflectance and transmittance …

Directional Radiation scattering (N=N )

NIR

SH=0.74

RED

SH=0.056

Hot spot in principal plane around illumination direction

Shoot albedo is quite similar whatever the illumination direction while reflectanceand transmittance coefficients show some differences

N-N Needle effect (NIR)Impact of needle reflectance and transmittance …

Shoot albedo is quite insensitive Reflectance and transmission coefficients are sensitive up +5% & -13%

LAI=1.28

LAI=2.56

Canopy scale Random shoot distribution with spherical inclination Increase of backscattering and decrease of forward scattering in the principal plane No effect around nadir view direction

Shoot scale (illumination s=45)

N-N Needle effect (RED)Impact of needle reflectance and transmittance …

Canopy scale

LAI=1.28

Random shoot distribution with spherical inclination Increase of backscattering and decrease of forward scattering in the principal plane No effect around nadir view direction

LAI=2.56

Shoot albedo is quite insensitive Reflectance and transmission coefficients are sensitive up 7%

Shoot scale (illumination s=45)

SummaryIllumination direction

NIR(=) RED(=) Smolander RED NIR RED NIR

0 0.49

0.28

0.036

0.021

0.057 0.770.04

40.7945 0.3

80.33

0.026

0.03 0.056 0.71

75 0.42

0.33

0.03 0.024

0.054 0.75

Table4: Reflectance, transmittance and albedo computed for an ‘Accurate’ shoot with illumination directions around (0 45 75 ) considering N=N and Smolander’s albedo.

Shoot albedo is quite similar whatever the illumination direction while reflectanceand transmittance coefficients show some differences Smolander parameterization agrees with ours in the NIR but not in RED

(differences in shoot structure design)


scattering and canopy BRDF A simple shoot parameterizationA simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF Conclusion

Method: Simple Shoot A simple shoot parameterization

Decahedron shoot (SD) Projected shoot silhouette area (As) conserved by reducing width (2.88cm) Convex ‘non self shadowing’ element with surface area A=4*As Decahedron sides are lambertian Optical properties of each side are equal to detailed shoot one averaged on

illumination directions (with N=N)

Flat leaf (SF) Lambertian Leaf area equals to 4*As. Optical properties equal to detailed shoot one averaged on illumination directions

Optical properties RED NIR

Reflectance 0.031 0.43

Transmittance 0.025 0.31

Illumination angles 0; 45; 90

SD Shoot ScatteringA simple shoot parameterization

Directional Radiation scattering

NIRSD=0.74; SD=0.46;

SD=0.28

RED

SD=0.052; SD=0.032; SD=0.020

Hot spot in principal plane around illumination direction

Albedo is quite close from detailed shoot albedo Some differences in reflectance and transmittance coefficients Backscattering enhancement occurs over a large view directions

Scattering comparisonA simple shoot parameterization

Directional scattering intercomparison (NIR)

• Detailed shoot ( Red line)• Decahedron shoot (Black line)• Lambertian leaf (Dashed line)

s=75, incidence plane

s=0, incidence plane

s=75, perpendicular plane

s=0, perpendicular plane

Both detailed and SD shoots show quite similar scattering profiles

Existence of less shadowing for decahedron shoot induces slow decrease far away from illumination direction less anisotropy in perpendicular plane due to the uniformity of shadow distribution

SummaryIllumination direction

NIR RED

NIR RED0 0.51 0.25 0.03

570.018

70.76 0.054

45 0.447

0.275

0.0306

0.0202

0.72 0.0508

75 0.416

0.317

0.0295

0.023 0.73 0.0525

Average 0.458

0.28 0.032

0.0206

0.738

0.052

Table7: Reflectance, transmittance and albedo computed for‘Decahedron’ shoot with illumination directions around (0 45 75 ).


scattering and canopy BRDF A simple shoot parameterization

Impact of simple shoot parameterization on Impact of simple shoot parameterization on canopy BRDFcanopy BRDF Conclusion

Shoot Canopy BRDFImpact of simple shoot parameterization on BRDF

Canopy Characteristics 3 types: detailed shoot (S), Decahedron shoot(SD), Flat shoot (SF) Scene dimensions: 0.5mx0.5mx1m deep (infinite boundary) LAI=[0.64, 1.28, 2.56, 5.12, 10.24]; Random distribution Spherical inclination distribution (SD) canopy have the same positions and inclination than (S) canopy (SF) shoot is described by tree surfaces randomly distributed (3D structure )

Radiative transfert simulation

Surface opt. properties RED NIR

Detailed (S) 0.05 0.05 0.45 0.45

Decahedron (SD) 0.031 0.025 0.43 0.31

Flat (SF) 0.031 0.025 0.43 0.31

PARCINOPY(Chelle 1997)

Shoot canopy

Optical properties

• Directional scattering • Reflectance coefficient• Transmittance coefficient • Albedo

Illumination direction

(45)

Black soil

(SD) and (S) BRDF comparison (NIR)

Impact of simple shoot parameterization on BRDF

• Detailed shoot ( symbol)• Decahedron shoot (line)

scattering contributions (NIR)Impact of simple shoot parameterization on BRDF

Multiple ( Bleu)Single (Red)

• Detailed ( symbol)• Decahedron (line)


(SD) and (S) BRDF comparison (RED)



Scattering contribution (RED)Impact of simple shoot parameterization on BRDF

• Detailed ( symbol)• Decahedron (line)


(SD) and (S) comparisonImpact of simple shoot parameterization on BRDF

REDNIR

Multiple scattering regime is quite different Compensation between multiple and single scattering in (SD) canopy induces canopy reflectance and transmittance similar to (S) canopy in NIR Discrepancies remains in RED due to the absence of this compensation

Upward and downward hemispherical canopy fluxes

(SF) and (S)BRDF comparison



NIR

RED

NIR

RED

Scattering contribution (RED)Impact of simple shoot parameterization on BRDF




scattering and canopy BRDF A simple shoot parameterization Impact of simple shoot parameterization on canopy BRDF

ConclusionConclusion

Conclusion 1- N-N effect

At shoot scale, no effect in albedo but some differencesin directional scattering and reflectance and transmittance coefficients

At canopy scale, N-N has significant effect around hot spot direction(Implication on LAI and clumping index estimation).

2- Can we design a simple shoot? Convex volume albedo is quite close with some differences in transmittance and

reflectance coefficients Convex volume describes directional scattering better than flat leaf

3- Impact of a simple shoot on canopy BRDF? Convex volume: - is inconvenient in RED due to a change in canopy clumping. - doesn’t well describe the canopy radiative regime. Simplification of shoot structure as lambertian flat leaf show significant

discrepancies (using ray tracing over a simplified structure is not realistic :Models assessment).

Lab measurements

Nadia [email protected] Richard Fernandes & Michael Chelle

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