On the accuracy of the independent column approximation in calculating the downward fluxes in the UVA, UVB, and PAR spectral ranges

Abstract

In order to investigate the accuracy of simplification in modeling the radiative transfer in those solar spectral regions with major impacts on bio‐organisms, i.e., the UVA (0.32–0.4 μm), the UVB (0.28–0.32 μm), and the photosynthetically active radiation (PAR, 0.4–0.7 μm), radiative transfer calculations with varying treatments of cloud geometries (plane‐parallel homogeneous (PPHOM), independent column approximation (ICA), and three‐dimensional (3‐D) inhomogeneous) have been performed. The complete sets of atmospheric information for 133 cloud realizations are taken from the three‐dimensional nonhydrostatic mesoscale atmospheric model (GESIMA). A Monte Carlo radiative transfer model (GRIMALDI) has been developed that simulates scattering and absorption for arbitrarily three‐dimensional distributions of cloud hydrometeors, air molecules, and water vapor. Results are shown for domain‐averaged direct and total transmission (and so, implicitly, diffuse transmission) at the ground surface. In the UVA the PPHOM assumption leads to an underestimation in direct (total) downward flux by as much as 43 (28) W m −2 , which is about 49% (32%) of the incoming irradiation, whereas results based on the ICA are almost identical to the 3‐D case, except for convective clouds where the error in the UVA for direct (total) downward flux reaches 5 (2) W m −2 , or 6% (2%) of the incoming solar irradiation.