Combining the independent pixel and point-spread function approaches to simulate the actinic radiation field in moderately inhomogeneous 3D cloudy media

A fast method is presented for gaining 3D actinic flux density fields, Fact, in clouds employing the Independent Pixel Approximation (IPA) with a parameterized horizontal photon transport to imitate radiative smoothing effects. For 3D clouds the IPA is an efficient method to simulate radiative transfer, but it suffers from the neglect of horizontal photon fluxes leading to significant errors (up to locally 30% in the present study). Consequently, the resulting actinic flux density fields exhibit an unrealistically rough and rugged structure. In this study, the radiative smoothing is approximated by applying a physically based smoothing algorithm to the calculated IPA actinic flux field.

Research highlights► A fast method is presented for gaining 3D actinic flux density fields in clouds. ► We use Independent Pixel Approximation and parameterize horizontal photon transport. ► Radiative smoothing is approximated by a physically based smoothing algorithm. ► Significant improvement of the spatial distribution of actinic radiation was found. ► The required computation time is much reduced compared to a full 3D calculation.