Chapter 2. Radiation

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Table of contents

Chapter 1. Overview

Chapter 2. Radiation

Chapter 3. Turbulent diffusion and interactions with the surface

Chapter 4. Subgrid-scale orographic drag

Chapter 5. Convection

Chapter 6. Clouds and large-scale precipitation

Chapter 7. Land suface parametrization

Chapter 8. Methane oxidation

Chapter 9. Climatological data

REFERENCES

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2.4 Horizontal interpolation

As stated in the introduction, the cost of the radiation scheme described in the previous sections is prohibitive if it were used to compute the radiative fluxes at every time step and every grid point of the model.

In order to cut down the computing costs, the full radiation scheme is only used every 3 hours ("full radiation time steps") at every fourth grid point, and a spatial and temporal interpolation thus provides the relevant interaction of the shortwave radiative fluxes with the solar zenith angle at every time step and every grid point.

To do so we define an effective transmissivity

at each of the model level such that:

(2.58)

where

is the solar (shortwave) flux and

is the solar flux at the top of the atmosphere. The values of

is kept constant between full radiation time steps and the net fluxes are recomputed at every time step, using (2.58) with the correct solar angle for every grid point.

The interpolation is done only in the zonal direction because of the strong meridional variation of the radiative processes and of the internal organisation of the model code. For each row, variables relevant to the input of the radiation calculation are transformed to a coarser subgrid via simple sampling. On the coarser grid so defined, fluxes are then evaluated as described in the previous sections. Finally, effective solar transmissivities and longwave fluxes are computed and transformed back to the full resolution of the model via cubic interpolation.

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05.04.2002