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
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 Scheme
ra_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 2stream approach, accounts for scattering and
reflection
RRTMG Scheme
ra_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 Radiation
Longwave
Surface Radiation
Longwave Differences
Surface Radiation
Shortwave
Surface Radiation
Shortwave Differences
Surface Radiation
Longwave Radiation Upward
Surface Radiation
Longwave Radiation Upward Differences
Surface Radiation
Longwave Radiation Downward
Surface Radiation
Longwave Radiation Downward Differences
Surface Heat Flux
Ground Heat
Surface Heat Flux
Ground Heat Differences
Surface Heat Flux
Sensible Heat
Surface Heat Flux
Sensible Heat Differences
Surface Heat Flux
Latent Heat
Surface Heat Flux
Latent Heat Differences
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
“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.