Determination of direct normal irradiance including circumsolar radiation in climate/NWP models

Abstract

An issue in the determination of the direct component of solar radiation involving the circumsolar contribution in numerical weather prediction (NWP) and climate models is discussed. The direct solar radiation has multiple definitions in different applications, but this ambiguity has not always been clearly identified and has therefore sometimes caused confusion. The confusion arises from a fact that the photons that are scattered into the direct solar beam path by atmospheric particles are no different from the non‐scattered photons travelling in the same direction. Therefore there is a question whether these scattered photons should be included in the direct flux calculation or whether they should be excluded. Different definitions of the direct solar radiation in a wide range of applications have been discussed by Blanc et al. (2014). We further discuss it in terms of its relation to NWP and climate models. The short‐wave radiation schemes used in NWP and climate models can be classified into two groups: those that calculate the direct solar radiation without including scattering contributions and those which include the scattering contribution in terms of the delta‐Eddington approximation. We assess the two treatments by comparing modelled direct solar radiation with observations. It is found that the use of delta‐Eddington scaling results in positive errors in the direct solar flux at the surface, while neglecting the delta‐scaling leads to negative errors. The important result is that the positive error from using the delta‐Eddington scaling is at least 1 order of magnitude larger than the absolute negative error due to neglecting it. In order to include properly the scattering contribution in the direct flux calculation, a simple parametrization for dust aerosol is developed which can be used to consider the scattering contribution due to the particular case of dust aerosol to the direct solar radiation within the circumsolar region.