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

In order to investigate the accuracy of simplification in modeling the radi ative transfer in those solar spectral regions with major impacts on bio-or ganisms, i.e., the UVA (0.32-0.4 mum); the UVB (0.28-0.32 mum): and the pho tosynthetically active radiation (PAR, 0.4 0.7 mum): radiative transfer cal culations with varying treatments of cloud geometries (plane-parallel homog eneous (PPHOM), independent column approximation (ICA), and three-dimension al (3-D) inhomogeneous) have been performed. The complete sets of atmospher ic information for 133 cloud realizations are taken from the three-dimensio nal nonhydrostatic mesoscale atmospheric model (GESIMA). A Monte Carlo radi ative transfer model (GRIMALDI) has been developed that simulates scatterin g and absorption for arbitrarily three-dimensional distributions of cloud h ydrometeors, air molecules, and water vapor. Results are shown for domain-a veraged direct and total transmission (and so, implicitly, diffuse transmis sion) at the ground surface. In the UVA the PPHOM assumption leads to an un derestimation ill direct (total) downward flux by as much as 43 (28) W m(-2 ) which is about 49% (32%) of the incoming irradiation, whereas results bas ed 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.