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 independentlyGood approximation if vertical depth is much less than horizontal scale

Radiation schemes resolve atmospheric heating from:Radiative flux divergenceSurface downward longwave and shortwave radiation [for ground heat]

Shortwave radiation:Includes wavelengths of solar spectrumAccounts for absorption, reflection and scattering in atmosphere and on surfacesUpward flux dependent on albedoIn 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 scatteringWater 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 radiationDifferences in initial concentrations of trace gasesDifferences 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.