Irradiance calculation over mountainous areas in the reflective spectral domain: comparison with an accurate radiative transfer code

A new model has been developed to estimate irradiance at ground level over a rugged terrain in the reflective spectral domain in order to be used in an hyperspectral inversion code. Modtran4 allows to calculate atmospheric parameters over a flat scene which are then used to estimate the four components of irradiance over a mountainous area (direct, diffuse, reflected and coupling irradiance). This method have been compared with an accurate radiative transfer code called AMARTIS. Simulations are done at three wavelengths and for two solar configurations over a relief composed of two hills and flat terrain. Irradiances obtained with our model are in good agreement with this reference code except in shadow areas in the SWIR. Our model is also compared with a currently used model developed by Sandmeier whose results are worse than our model's results. Current relative errors of our diffuse, reflected and coupling irradiance calculation model do not have much influence on total irradiance in most of the cases. This influence become significant for high beam incidence angles where Digital Elevation Model errors can be much more important.