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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

Shortwave Radiation Options in the WRF Model

An oh-so fascinating study of the Dudhia, Goddard and RRTMG shortwave schemes

radiation in the wrf
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
dudhia scheme ra sw physics 1
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
goddard scheme ra sw physics 2
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
rrtmg scheme ra sw physics 4
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.
significant variations and conclusions
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
resources
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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