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Abstract

Most atmospheric models consider radiative transfer only in the vertical direction (1D), as 3D radiative transfer calculations are too costly. Thereby, horizontal transfer of radiation is omitted, resulting in incorrect surface radiation fields. The horizontal spreading of diffuse radiation results in darker cloud shadows, whereas it increases the surface radiation in clear sky patches (cloud enhancement). In our recently published study, we provided a proof-of-concept for a simple method to account for the horizontal transfer of diffuse radiation. We compared the surface radiation of Large-Eddy Simulations (with 1D radiative transfer) with observations from the Baseline Surface Radiation Network station in Cabauw, the Netherlands, for two summer days on which shallow cumulus clouds formed. We spatially filter the simulated surface diffuse radiation field with a Gaussian filter, which is conceptually simple and computationally efficient. Without the filtering, cloud enhancements are not captured, and the probability distribution of global radiation is unimodal, whereas the observed distribution is bimodal. After filtering, the probability distribution of global radiation is bimodal and cloud enhancements are simulated, in line with the observations. Additionally, we showed that the width of the filter can be parameterized as a linear function of for example, the cloud cover. In ongoing work, we focus on generalizing our parameterization for the width of the filter based on a larger set of cumulus days. In addition, we incorporated the direct effect of aerosols on radiation.