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Shortwave Radiation Options in
the WRF ModelAn 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
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
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 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 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.
Temperature
Relative Humidity
Zonal Winds
Meridional Winds
Vertical Winds
Top of Atmosphere Radiation
Longwave Radiation Upward
Top of Atmosphere Radiation
Longwave Radiation Upward Differences
Surface RadiationLongwave
Surface RadiationLongwave Differences
Surface RadiationShortwave
Surface RadiationShortwave Differences
Surface RadiationLongwave Radiation Upward
Surface RadiationLongwave Radiation Upward
Differences
Surface RadiationLongwave Radiation Downward
Surface RadiationLongwave Radiation Downward
Differences
Surface Heat FluxGround Heat
Surface Heat FluxGround Heat Differences
Surface Heat FluxSensible Heat
Surface Heat FluxSensible Heat Differences
Surface Heat FluxLatent Heat
Surface Heat FluxLatent Heat Differences
Significant Variations and
ConclusionsGoddard 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 fluxesClear 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“Assessment of Radiation Options in the Advances Research WRF Weather Forecast Model”, Iacono and Nehrkorn
“A Description of the Advanced Research WRF Version 3”, Skamarock et al.