Radiation balance

First, the measured radiation (as provided by each station that measured both ISWR, TA and RH) is interpolated over the domain:

1. At each station that provides radiation the splitting coefficient (into direct and diffuse components) and an atmospheric loss factor are computed by comparing the measured radiation with the potential radiation;
2. The splitting coefficient Md and the atmospheric loss factor Corr are spatially interpolated with an Inverser Distance weighting algorithm;
3. At each cell, the clear sky potential radiation is computed by (Iqbal, 1983), split into direct and diffuse components using its spatially interpolated value and corrected by the spatially interpolated atmopsheric loss factor. The direct component is also projected on the local slope.

Then the effects of the terrain on the radiation field are accounted for. These are twofold: first the topography can cast shade on some parts of the domain and second some radiation can be reflected by the terrain on some other cells (see Radiation balance).

Topographical shading

This handles the shading of cells by the topography. The position of the sun is computed and the cells that don't have a direct view of the sun only receive the diffuse fraction of the global radiation while the cells with direct view of the sun receive both the direct and diffuse components. Please keep in mind that when looking for the horizon (in order to compute shading), the search will stop at cells that are set to nodata in the dem. This means that nodata cells on the border of the domain of interest will prevent searching for shading outside of the domain while nodata cells within the domain should be as much as possible avoided.

Terrain radiation

The second effect is computed when the key Terrain_Radiation is set to true (by default it is set to false) in the [EBalance] section of the configuration file and is handled by a choice of various algorithms, as chosen in the configuration file with the key Terrain_Radiation_Method that can take any of the following choices:

• SIMPLE : a very basic (but fast) guess, see TerrainRadiationSimple
• FULL : a parallelized implementation of a radiosity algorithm, see TerrainRadiation
• HELBIG : the original radiosity implementation, not parallelized, see TerrainRadiationHelbig
• PETSC : a parallel implementation relying on PETSC for added performances, see TerrainRadiationPETSc

[EBalance]
Terrain_Radiation = true
Terrain_Radiation_Method = SIMPLE