Impact of an Interactive SST on the Convective Aggregation Process

The spontaneous aggregation of convective clouds over a moist portion of the domain is ubiquitous in cloud resolving model simulations. This phenomenon significantly reduces the domain mean total water vapor and enhances the outgoing long radiation. In this study we use the system of atmospheric modeling (SAM) in a radiative-convective equilibrium (RCE) setup in order to investigate the impact of an interactive sea surface temperature (SST) on the aggregation progress. We use a slab ocean (with depth of 5, 10 and 50 m) with constant target SST to which the domain mean SST is relaxed. Our results show that, consistent with previous studies, an interactive SST delays the aggregation with a larger impact for a shallower slab. This effect is enhanced for a smaller target SST.

The aggregation proceeds by the expansion of non-convective dry areas. Before aggregation, dry areas are associated with warmer surface due to enhanced short-wave radiation. During and after the aggregation, a single large dry patch develops and is associated with a colder surface. This cooling is due to a reduction in downwelling long-wave radiation and to enhanced latent heat flux due to drier boundary layer. The edge of the dry patch has warm SST anomaly forming a ring of warm water around it that favors divergence of low-level moist air from the dry patch and accelerates dry patch expansion. This is favored by a positive surface pressure anomaly (PSFC) in the dry patch.

Therefore, at first, the warm SST anomaly opposes the divergent flow from dry regions, opposing the aggregation. Then the cold SST anomaly that develops in dry regions increases the divergent flow and favors the dry patch expansion. For a small ocean slab, the warm SST anomaly that develops in the dry areas at early times inhibits the dry patch expansion and can significantly delay the beginning of aggregation.