Impact of different definitions of clear-sky flux on the longwave cloud radiative forcing: NICAM simulation results

Using one month of cloud-resolving NICAM general circulation model simulations, we examined the impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing. It is because the satellite-like cloud-free composite preferentially sample drier conditions relative to the all-sky mean state that used for the conventional clear-sky flux calculation in the model. The drier bias is evident for the cloud-free composite, causing an overestimate of the longwave cloud radiative forcing by about 10%, compared to the NICAM simulated longwave cloud radiative forcing based on all-sky mean state. Those water vapor contributions up to 10% of the total should be added to make the conventional model-generated cloud forcing comparable with satellite measurements.