Validation of MODTRAN 5.3 sea surface radiance computations

Transcription

1 Validation of MODTRAN 5.3 sea surface radiance computations Vincent Ross*, Denis Dion** et Daniel St-Germain** * With AEREX Avionique Inc., ** With DRDC Valcartier ITBM&S, Toulouse France. June 27th 211

2 Contents Introduction The importance of a sea surface BRDF in radiative transfer Current model shortcomings MODTRAN 5 v? The sea surface BRDF model Analytical formulation Coupling in MODTRAN Other modeling considerations Aerosols, MBL thermodynamic modeling, etc. The MIRAMER campaign Experimental and modeling uncertainties Results Conclusion 1of 13

3 Introduction Why is the sea surface reflectance important? Background Thermal emissions Direct solar reflections Indirect solar reflections Foreground Contributes to atmospheric radiance Radiative coupling 2of 13

4 Introduction Why is the sea surface reflectance important? 3of 13

5 Current situation with MODTRAN No full BRDF coupling up to MODTRAN 4 No sea surface BRDF up to MODTRAN 5 v2 Only basic aerosol models, limited user input Inadequate for accurate horizontal path refraction MODTRAN 5 v? Fully coupled analytical sea surface BRDF SAP (Spectral Aerosol Profile) input Refracted path input Currently in beta testing stage, soon to be released. }DRDC-Valcartier participation 4of 13

6 The sea surface BRDF Wave facet angle weighing Probability of specular reflection Wave facet hiding Fresnel reflectance f ψ, ψ ( ) s r = π r ψ, ψ p( ζ) W( ζ, Ψ ) H ( ζ, Ψ ) ( ) s r r ζ r 3 4 z ( )[ 1 ( ) ( )] n Un Ur +Λ vr +Λ vs cosθr cos s θ Bistatic wave shadowing Ross, V., D. Dion, and G. Potvin, Detailed analytical approach to the Gaussian surface bidirectional reflectance distribution function specular component applied to the sea surface, J. Opt. Soc. Am. A Opt. Image Sci., 22, (25). 5of 13

7 Coupling to MODTRAN BRDF is coded in FORTRAN in MODTRAN 5 Fourier moments are computed Input in DISORT DISORT multiple scattering uses BRDF as a lower boundary condition 6of 13

8 Other modeling considerations Marine aerosols are computed using MEDEX Well suited for the Mediterranean Input in MODTRAN using the SAP input MBL (marine boundary layer) thermodynamic profiles are computed using the DRDC AP module Monin-Obukhov similarity theory Refracted optical path are input Sea surface statistical properties: Elfouhaily et al. Fetch, atmospheric stability 7of 13

9 The MIRAMER campaign From May 13 th to May 18 th In the Mediterranean sea near Toulon, France 2 Cedip Jade (Flir ATS) cameras on board the Atalante ship μm ( μm filter for glint) μm Good environmental characterization Radiosondes (2-3 day) Local meteorological measurements Air and sea temperature Wind speed/direction Relative humidity Visibility meter (aerosols) Aeronet station nearby (Toulon) Solar pyranometer (solar irradiance) 8of 13

10 Experimental and modeling uncertainties Experimental 9of 13 Image calibration and limited dynamic range Can reach 2% but probably lower (4-5%) Horizontal variations (temperature, etc.) not measured Temperature +/- 1 o Wind gusts Bulk vs. skin temperature +/- 1 o Modeling Slope statistics values 8% between models Aerosol modeling Multiple reflections Cirrus clouds Radiance (W/m2/ster) difference (%) maximum minimum ATAL 18 (1262) BIII Apparent temperature ( o C)

11 Results Non glint Midwave ATAL 95 (1159) BII Simulation (-.227 W/m 2 /ster) Apparent temperature ( o C) ATAL 17 (1255) BII Simulation (-.113 W/m 2 /ster) Apparent temperature ( o C) ATAL 19 (1267) BII Simulation (-.99 W/m 2 /ster) Apparent temperature ( o C) σ err =.56% σ err =.19% σ err =.76% Longwave ATAL 89 (1116) BIII Simulation (-1.39 W/m 2 /ster) σ err = 1.76% Apparent temperature ( o C) ATAL 17 (1255) BIII Simulation (-.133 W/m 2 /ster) σ err = 1.14% Apparent temperature ( o C) ATAL 19 (1267) BIII Simulation (+.265 W/m 2 /ster) σ err = 1.42% Apparent temperature ( o C) 1 of 13

12 Results Glint B B ATAL 145 (1416) BII reference image A A Relative azimuth (degrees) ATAL 145 (1416) BII vertical 1.5 ATAL 145 (1416) BII horizontal Simulation (-.5 W/m 2 /ster) 1.5 Simulation (-.5 W/m 2 /ster) Relative Azimuth (degrees) (Note: Cirrus cloud modeled using Aeronet AOD data) 11 of 13

13 Results Glint B B ATAL 64 (92) BII reference image A A Relative Azimuth (degrees) 7 6 ATAL 64 (92) BII vertical Measurement Simulation (+.2 W/m2/ster) Horizon correction 6 5 ATAL 64 (92) BII horizontal Measurement Simulation (+.2 W/m2/ster) Horizon correction Relative Azimuth (degrees) 12 of 13 Ross, V., Dion, D., "Sea surface slope statistics derived from Sun glint radiance measurements and their apparent dependence on sensor elevation, J. Geophys. Res., 112, C915, (27)

14 Conclusion A radiatively coupled sea surface BRDF is important in maritime environment radiative transfer MODTRAN 5 v? will introduce a coupled sea surface BRDF And many other useful features Soon to be released Validation against MIRAMER radiometric images shows promise Simulations and measurements agree well within experimental uncertainties No systematic errors 13 of 13

15 The authors would like to thank the ONERA for generously providing their measurements for this validation. Special thanks to Sandrine Fauqueux (ONERA) for answering questions about the data and to Stéphane Langlois (ONERA) for enlightening discussions on the calibration uncertainties. We would also like to underline the notable contributions of Gail Anderson (US Air Force Research Laboratories) and Alexander Berk (Spectral Sciences Inc.) for their great help in implementing the BRDF in our version of MODTRAN 5.