Cloud inhomogeneity and broadband solar fluxes

Abstract

Simplified representations of spatially inhomogeneous (three-dimensional (3-D)) clouds in radiative transfer models provide systematic errors when calculating solar broadband radiative fluxes. An example is the neglect of horizontal photon transports as it is the case for the independent column approximation (ICA). The present work tries to quantify and interpret these errors on the basis of a large set of 3-D mixed phase cloud scenarios with 3-D varying extinction coefficients, scattering phase functions, and single-scattering albedos. The cloud cases result from a mesoscale atmospheric circulation model with detailed cloud microphysics. Domain-averaged cloud radiative fluxes are calculated by means of a Monte Carlo radiative transfer model. Depending on cloud type and solar zenith angle (SZA) the differences between 3-D and ICA results range from +20 W m-2 to -30 W m-2 for the upward reflected fluxes and from +10 W m-2 to -7 W m-2 for the absorbed fluxes. The mean (averaged over all cloud realizations) errors of the ICA-based upward fluxes vary between 5 W m-2 overestimation at 15°SZA and 6 W m-2 underestimation at 75°SZA. The ICA underestimates the absorbed flux by ∼1-2 W m-2 for most SZA except for 75°. It is found that neglecting the horizontal variability of the absorption and scattering properties of the cloud hydrometeors leads to a general underestimation of solar broadband absorption by as much as 15 W m-2 with average values between 4 W m-2 at small SZA and 1 W m-2 at large SZA.