Recent studies develop the SPeedy Algorithm for Radiative TrAnsfer through CloUd Sides (SPARTACUS) to handle the influence of horizontal RT on vertical radiative fluxes within an atmospheric column. The present study applies SPARTACUS to large eddy simulation (LES)-generated cloud fields across the stratocumulus to trade cumulus transition (STCT) regime with coarse and fine vertical resolutions. The results show that, as the vertical resolution increases, radiation simulations show increasingly stronger cloud-top longwave (LW) radiative cooling. Consequently, the sharp radiative heating gradient across the cloud layer in the LES-like resolution simulations cannot be resolved with the coarse resolution simulations. Including the horizontal RT typically enhances cloud LW radiative cooling rate by less than 10% for all the cloud fields but more significantly in the cloud fields during the STCT. The enhanced cloud LW radiative cooling also occurs in the lower cloud layer in the decoupled cumulus cloud regime.