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Shortwave Radiation Options in the WRF Model

Shortwave Radiation Options in the WRF Model. An oh-so fascinating study of the Dudhia , Goddard and RRTMG shortwave schemes. Radiation in the WRF. Current Schemes: All single column, 1-D schemes – each column treated independently

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Shortwave Radiation Options in the WRF Model

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  1. Shortwave Radiation Options in the WRF Model An oh-so fascinating study of the Dudhia, Goddard and RRTMG shortwave schemes

  2. Radiation in the WRF • Current Schemes: • All single column, 1-D schemes – each column treated independently • Good approximation if vertical depth is much less than horizontal scale • Radiation schemes resolve atmospheric heating from: • Radiative flux divergence • Surface downward longwave and shortwave radiation [for ground heat] • Shortwave radiation: • Includes wavelengths of solar spectrum • Accounts for absorption, reflection and scattering in atmosphere and on surfaces • Upward flux dependent on albedo • In atmosphere, determined by vapor/cloud content, as well as carbon dioxide, ozone and trace gas concentrations

  3. Dudhia Scheme ra_sw_physics = 1 • Based on Dudhia 1989, from MM5 • Uses look-up tables for clouds from Stephens 1978 • Version 3 has option to account for terrain slope and shadowing effects on the surface solar flux • Simple downward integration of solar flux, which accounts for: • Clear air scattering • Water vapor absorption [Lacis and Hansen, 1974] • Cloud albedo and absorption

  4. Goddard Schemera_sw_physics = 2 • Based on Chou and Suarez 1994 • Includes 11 spectral bands • Different climatological profiles available for numerous ozone options • Considers both diffuse and direct solar radiation in 2-stream approach, accounts for scattering and reflection

  5. RRTMG Schemera_sw_physics = 4 • Uses MCICA [Monte Carlo Independent Column Approximation] method of random cloud overlap – statistical method to resolve sub-grid scale cloud variability • Finer resolution runs usually associated with WRF model means that clouds will most likely take up the entire grid space [binary clouds], in which case MCICA will not work.

  6. Temperature

  7. Relative Humidity

  8. Zonal Winds

  9. Meridional Winds

  10. Vertical Winds

  11. Top of Atmosphere RadiationLongwave Radiation Upward

  12. Top of Atmosphere RadiationLongwave Radiation Upward Differences

  13. Surface RadiationLongwave

  14. Surface RadiationLongwave Differences

  15. Surface RadiationShortwave

  16. Surface RadiationShortwave Differences

  17. Surface RadiationLongwave Radiation Upward

  18. Surface RadiationLongwave Radiation Upward Differences

  19. Surface RadiationLongwave Radiation Downward

  20. Surface RadiationLongwave Radiation Downward Differences

  21. Surface Heat FluxGround Heat

  22. Surface Heat FluxGround Heat Differences

  23. Surface Heat FluxSensible Heat

  24. Surface Heat FluxSensible Heat Differences

  25. Surface Heat FluxLatent Heat

  26. Surface Heat FluxLatent Heat Differences

  27. Significant Variations and Conclusions • Goddard Scheme (ra_sw_physics=2) initialized differently and gave the most extreme values • Most variations were insignificant, other than mid-level drying in RRTMG scheme. • Much larger flux differences arise if clouds are sparse or absent during peak diurnal heating • Surface fluxes • Clear sky conditions – algorithmic differences in handling gaseous absorption/emission of longwave radiation and extinction of shortwave radiation • Differences in initial concentrations of trace gases • Differences in allowable cloud fractions

  28. Resources • “Assessment of Radiation Options in the Advances Research WRF Weather Forecast Model”, Iacono and Nehrkorn • “A Description of the Advanced Research WRF Version 3”, Skamarocket al.

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