McClear: a new model estimating downwelling solar radiation at ground level in clear-sky conditions

A new fast clear-sky model called McClear was developed to estimate thedownwelling shortwave direct and global irradiances received at ground levelunder clear skies. It is a fully physical model replacing empiricalrelations or simpler models used before. It exploits the recent results onaerosol properties, and total column content in water vapour and ozoneproduced by the MACC project (Monitoring Atmosphere Composition andClimate). It accurately reproduces the irradiance computed by the libRadtranreference radiative transfer model with a computational speed approximately10⁵ times greater by adopting the abaci, or look-up table, approachcombined with interpolation functions. It is therefore suited forgeostationary satellite retrievals or numerical weather prediction schemeswith many pixels or grid points, respectively. McClear irradiances werecompared to 1 min measurements made in clear-sky conditions at severalstations within the Baseline Surface Radiation Network in various climates.The bias for global irradiance comprises between −6 and25 W m⁻². The RMSE ranges from 20 W m⁻² (3% of the meanobserved irradiance) to 36 W m⁻² (5%) and the correlationcoefficient ranges between 0.95 and 0.99. The bias for the direct irradiancecomprises between −48 and +33 W m⁻². The root mean square error (RMSE) ranges from33 W m⁻² (5%) to 64 W m⁻² (10%). The correlationcoefficient ranges between 0.84 and 0.98. This work demonstrates the qualityof the McClear model combined with MACC products, and indirectly the qualityof the aerosol properties modelled by the MACC reanalysis