Shortwave Radiation Options in the WRF Model An

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 MM 5 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 2 stream 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.