Fast radiative transfer parameterisation for assessing the surface solar irradiance: The Heliosat‑4 method

Abstract

The new Heliosat-4 method estimates the downwelling shortwave irradiance received at ground level in all
\nsky conditions. It provides the global irradiance and its direct and diffuse components on a horizontal plane
\nand the direct irradiance for a plane normal to sun rays. It is a fully physical model using a fast, but still
\naccurate approximation of radiative transfer modelling and is therefore well suited for geostationary satellite
\nretrievals. It can also be used as a fast radiative transfer model in numerical weather prediction models. It is
\ncomposed of two models based on abaci, also called look-up tables: the already-published McClear model
\ncalculating the irradiance under cloud-free conditions and the new McCloud model calculating the extinction
\nof irradiance due to clouds. Both have been realized by using the libRadtran radiative transfer model. The
\nmain inputs to Heliosat-4 are aerosol properties, total column water vapour and ozone content as provided
\nby the Copernicus Atmosphere Monitoring Service (CAMS) every 3 h. Cloud properties are derived from
\nimages of the Meteosat Second Generation (MSG) satellites in their 15min temporal resolution using an
\nadapted APOLLO (AVHRR Processing scheme Over cLouds, Land and Ocean) scheme. The 15min means
\nof irradiance estimated by Heliosat-4 are compared to corresponding measurements made at 13 stations within
\nthe Baseline Surface Radiation Network and being located in the field of view ofMSG and in various climates.
\nThe bias for global irradiance is comprised between 2 and 32Wm−2. The root mean square error (RMSE)
\nranges between 74 and 94Wm−2. Relative RMSE values range between 15% and 20% of the mean observed
\nirradiance for stations in desert and Mediterranean climates, and between 26% and 43% for rainy climates
\nwith mild winters. Correlation coefficients between 0.91 and 0.97 are found. The bias for the direct irradiance
\nat normal incidence is comprised between −163 and +50Wm−2. The RMSE ranges from 160Wm−2 (29%
\nof the mean observed irradiance) to 288Wm−2 (63%). The correlation coefficient ranges between 0.67 and
\n0.87.