How does the diurnal cycle of incoming solar radiation affect self-aggregation of convective clouds?

This study investigates the impact of the diurnal cycle of incoming solar radiation on the spontaneous organization of convective clouds, hereafter self-aggregation. We run 3D cloud-resolving simulations in the RCE framework with interactive sea surface temperature (SST). SST is allowed to interact with the atmosphere using a slab ocean ( H = 1 - 200 meters) with a fixed mean but locally varying temperature. The self-aggregation of deep clouds starts with the appearance of dry patches that grow in size while getting drier, and confine the moist convention into a small fraction of the domain, consistent with previous studies of self-aggregation.

Interactive SST has been confirmed to decelerate or prevent self-aggregation. However, our finding shows that including the diurnal cycle reduces the impact of slab depth on the self-aggregation so that the aggregation proceeds much faster for shallower slabs (1,2 or 5 meters). For deeper slabs (50 and 200 meters) the self-aggregation progress is negligibly affected by the diurnal cycle. The accelerated self-aggregation with shallow slabs is found to be related to the mechanism by which the dry patches are triggered.

The triggering of dry patches is typically assumed to be a random process; however, we find that, especially with shallow ocean slabs, the dry patches form in places of cold pools. In other words, the lower tropospheric and boundary layer dryness induced by cold pools as well as surface temperature cooling by cloud shading can persist long enough to ensure a divergent flow, which was found to be important for self-aggregation. With shallow slabs, the negative SST anomaly under the cold pools thermally enhances the radiatively driven night time divergent flow and dries the boundary layer rapidly. The negative moisture anomaly persists even during daytime when the surface warms in dry regions and ensures a divergent flow, however weak, that then leads to the formation of dry patches in the following days. This process significantly accelerates the appearance of first dry patches. Moreover, this mechanism results in the occurrence of self-aggregation for shallow slabs (H=1 or 2 meters ) for which the self-aggregation does not proceed with constant solar radiation in our simulations. The enhanced divergent flow does not play a role for deep slabs as the SST anomalies are very small. Once the self-aggregation is triggered, its progress becomes negligibly affected by the diurnal cycle.