What favors convective aggregation and why?

Abstract

The organization of convection is ubiquitous, but its physical understanding remains limited. One particular type of organization is the spatial self‐aggregation of convection, taking the form of cloud clusters, or tropical cyclones in the presence of rotation. We show that several physical processes can give rise to self‐aggregation and highlight the key features responsible for it, using idealized simulations. Longwave radiative feedbacks yield a “radiative aggregation.” In that case, sufficient spatial variability of radiative cooling rates yields a low‐level circulation, which induces the upgradient energy transport and radiative‐convective instability. Not only do vertically integrated radiative budgets matter but the vertical profile of cooling is also crucial. Convective aggregation is facilitated when downdrafts below clouds are weak (“moisture‐memory aggregation”), and this is sufficient to trigger aggregation in the absence of longwave radiative feedbacks. These results shed some light on the sensitivity of self‐aggregation to various parameters, including resolution or domain size.

An Observational View of Relationships Between Moisture Aggregation, Cloud, and Radiative Heating Profiles

“…The enhanced cooling occurs in the dry regions of the domain, reinforcing the moist-static energy gradient between moist and dry regions. Modeling studies suggest that this radiative effect acts as a positive feedback contributing with other processes to maintain the organization of convection in moist areas (Muller and Bony 2015). It is important here to draw a distinction between the initiation and maintenance of the aggregated state.…”

“…Stein et al (2017) found that low-level cloud fraction increases along with their aggregation metric. Modeling results also suggest that low-level clouds are crucial for the onset of convective aggregation and are one of the several processes that help maintain an established aggregated state (Muller and Bony 2015). Figure 8 shows that the low cloud The mean vapor variability (a) as a function of mean rain rate within a 5°grid box.…”

“…This analysis averages data across the various aggregation states while retaining information of the covariability of moisture and cloudiness within each domain. Because SST variation across each 5°box is relatively small, this is analogous to energy-budget analyses used to diagnose the mechanisms leading to self-aggregation (Bretherton et al 2005; Geophys (2017Geophys ( ) 38:1237Geophys ( -1254Geophys ( 1249 Emanuel 2014; Muller and Bony 2015) although open boundary conditions suggest that inferring circulations from this stratification is unwise. Within each rain rate/SST regime, the moistest areas have the highest cloud fraction at all levels, while the driest show very little cloud in the middle atmosphere and enhanced low-level cooling.…”

“…This picture is consistent with modeling results showing preferential convection in the moist regions that can be maintained (or caused) by an enhanced LW radiative cooling in the dry region (c.f. Figure 2 in Muller and Bony (2015)). …”

“…High clouds are less frequent in aggregated states, allowing increased longwave (LW) radiative cooling to space. The precise details of convectively aggregated states are still being explored (Muller and Bony 2015) but the frequent finding that aggregation increases with sea surface temperature suggests a stabilizing feedback on climate reminiscent of the 'Iris hypothesis' (Lindzen et al 2001), albeit through different mechanisms than originally proposed (Mauritsen and Stevens 2015;Bony et al 2016).…”