Influence of the grain shape on the albedo and light extinction in snow

Transcription

1 Influence of the grain shape on the albedo and light extinction in snow Q. Libois 1, G. Picard 1, L. Arnaud 1, M. Dumont 2, J. France 3, C. Carmagnola 2, S. Morin 2, and M. King 3 1 Laboratoire de Glaciologie et Géophysique de l'environnement 2 Météo France, Centre d'etudes de la Neige 3 Royal Holloway, University of London, Egham

2 Context SSA determination based on reflectance measurement - In the field (POSSSUM, DUFISSS) - Remote sensing (MODIS) DUFISSS (Gallet et al., 2009) POSSSUM (Arnaud et al., 2011) Relationship between albedo and SSA is independent of shape

3 Context Does grain shape influence optical properties of natural snow? Representation by spheres of equal-ssa is fine Better than equal-v or equal-s spheres (Grenfell and Warren, 1999) 10% error in albedo (Neshyba et al., 2003 ; Grenfell et al., 2005) Albedo/SSA relation is shape-dependent Retrieved SSA is 40% greater if fractals are used (Kokhanovsky and Zege, 2004) 20% uncertainty in SSA retrieval (Picard et al., 2009) Shape has few impact (Gallet et al., 2009) Spheres

4 Context Does grain shape influence optical properties of natural snow? Representation by spheres of equal-ssa is fine Better than equal-v or equal-s spheres (Grenfell and Warren, 1999) 10% error in albedo (Neshyba et al., 2003 ; Grenfell et al., 2005) Albedo/SSA relation is shape-dependent Retrieved SSA is 40% greater if fractals are used (Kokhanovsky and Zege, 2004) 20% uncertainty in SSA retrieval (Picard et al., 2009) Numerical studies Shape has few impact (Gallet et al., 2009) Experimental study Spheres

5 Representation of snow particles in radiative transfer models Natural snow

6 Representation of snow particles in radiative transfer models Natural snow θ Snow phase function : unknown Radiative Transfer Equation

7 Representation of snow particles in radiative transfer models Natural snow Influence of shape θ Absorption coefficient Snow phase function : unknown Radiative Transfer Equation

8 Representation of snow particles in radiative transfer models Natural snow? Can snow be represented by a simple and unique geometrical shape? Spheres? (Wiscombe and Warren, 1980 ; Painter et al., 2009) Ice particles in clouds? (Garrett et al., 2001 ; Kokhanovsky and Zege, 2004)

9 Objective Evaluate the influence of shape on SSA retrieval for natural snow Approach 1. Ray tracing calculations 2. Reflectance and light extinction measurements in the field Radiative transfer modeling

10 Simplification of the Radiative Transfer Equation Absorption coefficient Phase function Simplification for weakly absorbing media and geometrical optics (Kokhanovsky, 2004) Phase function g G σ a B

11 The parameters g G and B Two shape parameters Asymmetry parameter g G Measures the forward scattering Absorption enhancement parameter B Measures the lengthening of photon path inside the grain > >

12 Solution of the RTE based on the AART theory Case of a semi-infinite snowpack (Kokhanovsky and Zege, 2004) Albedo αi 0 I 0 z Asymptotic Flux Extinction Coefficient

13 Solution of the RTE based on the AART theory Case of a semi-infinite snowpack (Kokhanovsky and Zege, 2004) Albedo αi 0 I 0 z Asymptotic Flux Extinction Coefficient

14 Solution of the RTE based on AART theory Case of a semi-infinite snowpack (Kokhanovsky and Zege, 2004) Shape EXTINCTION Ice Absorption Coefficient ALBEDO Albedo is shape dependent Size What values of B ang g G are representative of natural snow?

15 Objective Evaluate the influence of shape on SSA retrieval for natural snow Approach 1. Ray tracing calculations 2. Reflectance and light extinction measurements in the field Radiative transfer modeling

16 Numerical determination of g G and B Calculations performed using Monte Carlo Ray Tracing - Kokhanovsky and Macke (1997) - SNOWRAT (Picard et al., 2009) Sphere B=1.25 B in [1.25 ; 2.1] Spheres Great forward scattering Weakly enhance absorption

17 Numerical determination of g G and B Change of representation Sphere Measurable optical quantities

18 Numerical determination of g G and B Albedo Extinction Spheres are not representative of classical geometrical shapes for extinction Extinction more sensitive to shape than albedo Spherical assumption for albedo B/(1-g G ) equals that of spheres

19 Objective Evaluate the influence of shape on SSA retrieval for natural snow Approach 1. Ray tracing calculations 2. Reflectance and light extinction measurements in the field Radiative transfer modeling

20 Experimental determination of B Semi-infinite homogeneous layer Extinction Density Reflectance (1-g G ) is multiplied by SSA g G cannot be determined by such optical methods

21 Experimental determination of B Semi-infinite homogeneous layer Large range of B : [0.9 ; 10] Measurements Uncertainty???

22 Experimental determination of B Multi-layer snowpack Reflectance α(z) Density ρ(z) Radiative transfer model (TARTES) - Two-stream model - B and g G I mod (z) I obs (z) Extinction measurement B optimization

23 Experimental determination of B Inputs of the model 1 Density 2 Reflectance (ASSSAP) 3 Extinction profile (OO Maya2000 Pro)

24 Experimental determination of B Example : Lacs Roberts, French Alps I(z,λ=780nm) Spheres Optimal B = 1.6 (B s = 1.25)

25 Summary of B values Literature Spheres This study Theory B B in [0.8 ; 2.0] experimentally Values found experimentally are in the range of numerical calculations Large range of B values

26 Summary of B values Probably Unrealistic Spheres Literature This study Theory B B in [0.8 ; 2.0] experimentally Values found experimentally are in the range of numerical calculations Large range of B values

27 Conclusions and Perspectives Presentation of a theoretical and experimental framework to get B B variable from a type of snow to another with 0.8<B<2.0 Extinction more sensitive to grain shape than reflectance Need accurate measurements Need measurements on various snow types correlation with B? What about g G? Independent SSA measurements Knowing B and g G is essential for SSA retrieval but also for snow and climate models

28

29 Sensitivity of reflectance to grain shape LARGER B LONGER photon path IN THE GRAINS More absorption SHORTER photon paths IN THE SNOWPACK Less penetration Reflectance is less sensitive to shape than extinction