Radiation

The radiation spectrum is divided into a long-wave part (thermal infrared) and a short-wave part (solar radiation).  The radiation scheme (ecRad) performs computations of the short-wave and long-wave radiative fluxes using:

  • predicted values of temperature, humidity, multi-layer clouds, surface long-wave emissivity and short-wave albedo. 
  • monthly-mean climatologies for aerosols and the main trace gases (CO2, O3, CH4, N2O, CFCl3 and CF2Cl2).

The cloud-radiation interaction is dealt with in considerable detail.  The cloud scheme provides:

  • the values of cloud fraction.
  • an assumed multi-layer cloud overlap.
  • liquid water, ice and snow water contents.

Solving the radiative transfer equations to obtain the radiation fluxes is computationally expensive.  Full radiation calculations are therefore performed on a coarser grid (6-10 times fewer points) with reduced time frequency (hourly intervals). The number of points and time frequency depends on the model configuration.  However, the short-wave fluxes are updated at every grid point and time-step.  Solar radiation values are modified by path length through the model atmosphere due to the varying solar zenith angle. 

The radiation fluxes are then interpolated back to the original grid for use by surface radiation flux simulations (e.g. HTESSEL). 



 

 Fig2.1.5.1-1: Radiation absorption and emission considered in the Radiation Scheme.  Long-wave radiation calculations are complex.  They include multiple absorption and emissions at the surface, by atmospheric moisture and cloud (possibly multilayered), and other constituents.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)