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Radiative Transfer in Clouds: 3D and 1D Approaches

Radiative Transfer in Clouds: 3D and 1D Approaches. Brian Cook University of Maryland, College Park August 10, 2006. Why do we care?. 3D approaches are accurate but require much more computer time than their 1D counterparts;

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Radiative Transfer in Clouds: 3D and 1D Approaches

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  1. Radiative Transfer in Clouds: 3D and 1D Approaches Brian Cook University of Maryland, College Park August 10, 2006

  2. Why do we care? • 3D approaches are accurate but require much more computer time than their 1D counterparts; • GOAL: Determine how well 1D calculations approximate the 3D radiative fields for various scenarios, namely: • What is the magnitude of 1D errors? • Where do the errors come from?

  3. Inputs and outputs • Inputs: • Cloud field • Solar angles • Viewing angles • Surface albedo • Scattering phase function • Output: • Radiation fields

  4. Cloud/Photon Interactions Incoming Shortwave Radiation Reflected Shortwave Radiation (Forward Scattering) Reflected Shortwave Radiation (Backward Scattering) Transmitted Short Wave Radiation

  5. Cumulus Cloud field from 9 viewing angles

  6. 3D vs. 1D simulations 3D • 3D simulations allow photons to move between cloud regions with different properties • 1D simulations treat each column independently as a horizontally infinite cloud layer 1D

  7. Photon Leakage 1D 3D

  8. 3D / 1D Backward vs. Forward Scattering Bias • The direction of scatter is skewed towards forward for 1D simulations compared to 3D simulations 3D 1D • This bias is dampened at high sun • For 3D, surface reflectance increases the difference between backward and forward scattering.

  9. “Mie” vs. “Henyey-Greenstein” Phase Function If the sun is low, for both 1D and 3D simulations, R(HG) > R(Mie) for all viewing azimuth angles except back-scattering

  10. “Mie” vs. “Henyey-Greenstein” Phase Function 1D 3D RMS BACK BACK BACK HG RMS FRWD FRWD FRWD Mie Solar Zenith Angle - 60° Albedo = 0.2 For all scattering angles the 1D approximation works better for the real (Mie) phase function than for the HG one

  11. Conclusions • The 1D approximation works better for nadir and sideways viewing angles than for backward and forward viewing directions; • The 1D approximation is better for high sun than for low one. • In general, the 1D radiative transfer approximation works better for the real (Mie) phase function than for its HG counterpart;

  12. Future Research . . . • Extend analysis to more viewing and illumination conditions; • Try different cloud fields / cloud types • Compare different wavelengths of solar radiation • Add atmospheric affects

  13. Acknowledgments Special Thanks to : • Alexander Marshak • Tamas Varnai • Guoyong Wen • Christine Chiu • Cathy Newman • Larry Wharton

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