Convection On the Edge

Abstract

Deep convection over tropical oceans often appears intensified at the edge of convectively active regions, both in idealized studies and in observations. This edge intensification of convection is studied in detail here, using the steady state of a radiative-convective equilibrium study, marked by a single convective cluster with deep convection intensified at the edge of this cluster. The cause for edge intensification and its dependence on the cluster area is investigated by comparing the spatial distribution of deep convection to different variables known to be important for convection. Analysis of the simulation suggests that the edge is marked by an increased probability for the triggering of convection rather than by stronger updrafts. In particular, while the edge of the moist region is not thermodynamically more favorable, we find strong surface convergence and therefore dynamical lifting at this edge. The surface convergence is shown to result from two opposing flows. On the one hand, there is, as expected from previous radiative-convective equilibrium simulations, a low-level inflow directed toward the moist region. On the other hand, there is a positive density anomaly at the surface which is the result of continuously forming cold pools within the convectively active region, creating a super-cold-pool. As the velocity of the low-level inflow approximately matches the potential propagation speed of the super-cold-pool boundary, these opposing flows explain the presence of strong convergence at the edge of this region. Whether the resulting lifting induces the formation of deep convection is shown to depend on the large-scale instability. Key Points:• Precipitation tends to intensify at the edge of the convectively active region if the region is large enough • Edge intensification in a RCE simulation with a single convective cluster cannot be explained by spatial variations of CAPE or moisture • Edge intensification in this case is linked to the formation of a super-cold-pool in the convective cluster balanced by a low-level inflow

Impact of a Mixed Ocean Layer and the Diurnal Cycle on Convective Aggregation

“…Windmiller and Hohenegger ( 2019) emphasized the role of these cold pools, collectively forming a so-called super cold pool, and suggested that in their simulations, the suppression of convection in the super cold pool region by negative buoyancy anomalies lead to the enhanced convection being found at the boundaries of the moist zone, much the same mechanism as suggested by Nakajima and Matsuno (1988) in an early study of organization. The role of the interactive ocean used in their simulations was discounted by Windmiller and Hohenegger (2019), but the strong impact on surface fluxes in the organized state would also act to enhance convection on the moist region edges, as also seen in Shamekh et al (2020). The development of zones of maximum SST between the convective area and the dry regions are emphasized in Hovmöller plots, which rank the SST as a function of the TCWV percentile (Fig.…”

New Critical Length for the Onset of Self‐Aggregation of Moist Convection

“…Note that for the onset of CSA, the existence of at least one such place is sufficient. Eventually, these opposite flows become balanced in the aggregated case, as described by Windmiller and Hohenegger (2019).…”

Convective self–aggregation in a mean flow

“…Additional simulation days for UB4 (until day 30) corroborate that UB4 reaches a quasi-equilibrium state (not shown). The strong fluctuation around the mean is due to the oscillating features of aggregation (Bretherton et al, 2005;Windmiller and Hohenegger, 2019;Patrizio and Randall, 2019). This fluctuation hinders our ability to unambiguously distinguish between a slow propagation speed and a stationary one, although its amplitude is comparable to the one with no mean wind (UB0).…”