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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 the predicted values of temperature, humidity, multi-layer clouds, surface long-wave emissivity and short-wave albedo, and 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 using the values of cloud fraction, an assumed multi-layer cloud overlap, and liquid, ice and snow water contents from the cloud scheme.  Solving the radiative transfer equations to obtain the radiation fluxes is computationally expensive.  So, depending on the model configuration, full radiation calculations are performed on a reduced (coarser) grid and on a reduced time frequency (about 6-10 times fewer points and at intervals of 1 hour).  Additionally, the short-wave fluxes are updated at every grid point and time-step using solar radiation values modified by path length through the model atmosphere due to the varying solar zenith angle between calls to the full radiation scheme.  The fluxes are then interpolated back to the original grid for use by surface radiation flux simulations (e.g. HTESSEL). 


 

 Fig2.1.19: Radiation absorption and emission considered in the Radiation Scheme.  Handling of long-wave radiation is complex including multiple absorption and emissions at the surface and by atmospheric moisture and cloud (possibly in several layers) and other constituents.

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