Clouds increase the complexity of atmospheric radiative transfer processes, particularly for aerosol retrievals in clear regions in the vicinity of clouds. This study focuses on identifying mechanisms responsible for the enhancement of nadir reflectance in clear regions in the vicinity of cumulus clouds, and quantifies the relative importance of each mechanism. Using cloud optical properties and surface albedo derived from ASTER and MODIS, we performed extensive Monte Carlo simulations of radiative transfer in two cumulus scenes in a biomass burning region in Brazil. The results show that the scattering of radiation by clouds, followed by upward Rayleigh scattering by molecules above cloud top over clear gaps is the dominant mechanism for the enhancement of visible reflectance in clear regions in boundary layer cumulus field over dark surfaces with aerosols trapped in the boundary layer. The Rayleigh scattering contributes ~80% and ~50% to the total enhancement for wavelengths 0.47µm (with aerosol optical thickness 0.2) and 0.66µm (with aerosol optical thickness 0.1), respectively. Out of the total contribution of molecular scattering, ~90% arises from the clear atmosphere above cloud top height. The mechanism is valid for a large range of aerosol optical thickness (up to 1 in this study) for 0.47µm, and for aerosol optical thickness up to 0.2 for 0.66µm. Our results provides a basis to develop simplifications for future aerosol remote sensing from satellite.