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Shortwave Radiation Options in the WRF Model An oh-so fascinating study of the Dudhia, Goddard and RRTMG shortwave schemes.

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Presentation on theme: "Shortwave Radiation Options in the WRF Model An oh-so fascinating study of the Dudhia, Goddard and RRTMG shortwave schemes."— Presentation transcript:

1 Shortwave Radiation Options in the WRF Model An oh-so fascinating study of the Dudhia, Goddard and RRTMG shortwave schemes

2 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

3 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

4 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

5 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.

6 Temperature

7 Relative Humidity

8 Zonal Winds

9 Meridional Winds

10 Vertical Winds

11 Top of Atmosphere Radiation Longwave Radiation Upward

12 Top of Atmosphere Radiation Longwave Radiation Upward Differences

13 Surface Radiation Longwave

14 Surface Radiation Longwave Differences

15 Surface Radiation Shortwave

16 Surface Radiation Shortwave Differences

17 Surface Radiation Longwave Radiation Upward

18 Surface Radiation Longwave Radiation Upward Differences

19 Surface Radiation Longwave Radiation Downward

20 Surface Radiation Longwave Radiation Downward Differences

21 Surface Heat Flux Ground Heat

22 Surface Heat Flux Ground Heat Differences

23 Surface Heat Flux Sensible Heat

24 Surface Heat Flux Sensible Heat Differences

25 Surface Heat Flux Latent Heat

26 Surface Heat Flux Latent Heat Differences

27 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

28 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.


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