4 Summary and Discussion

This study examines the impact of radiative interactions on the spatial organization of convection, the distribution of clouds and relative humidity, and extreme daily precipitation events. We show that suppressing radiative interactions does not alter the mean circulation much but reduces the degree of convective aggregation. The reduction in the degree of aggregation is associated with an increase in high cloud coverage and relative humidity in the free troposphere and a decrease in the boundary layer. These differing responses of cloud and humidity are found to be sensitive to radiative interactions and, to some extent, independent of variations in the degree of aggregation.
Additionally, a less aggerated state due to suppressed radiative interactions is associated with a decrease in the frequency of extreme precipitation events due to a reduction in the dynamic contribution, i.e., a weakening of the strongest updrafts. At regional scales, suppressing radiative interactions diminishes the horizontal gradient of radiative cooling from dry to moist regions, weakening the upgradient transport of energy and reducing the degree of aggregation. However, the mean ITCZ width shows little difference with and without radiative interactions.
While our results show that the impact of radiative interactions on the degree of aggregation and extreme precipitation is not overwhelmed by the presence of realistic SST gradients, it is worth mentioning that the model response to suppressed radiative interactions shows considerable spatial variability. Overall, stronger signals are found in the tropics than those at mid to high latitudes. Signals over sea surfaces are also more robust than those over land ones. These spatially heterogeneous responses indicate that local environment may play a role in determining the extent to which radiative interactions impact extreme weather extremes and convective aggregation. In addition, our simulations are performed with prescribed SSTs, which essentially excludes the possible impact of changes in the degree of aggregation on SSTs. It remains unclear whether and how changes in the degree of aggregation affect SSTs, and if yes, how changes in SSTs would feedback onto interactions between radiation and convection. Future studies may explore these issues using fully coupled GCMs.