Regimes of Convective Self-Aggregation in Convection-Permitting Beta-Plane Simulations

Abstract

The spontaneous self-aggregation (SA) of convection in idealized model experiments highlights the importance of interactions between tropical convection and the surrounding environment. The authors have shown that SA fundamentally changes with the background rotation in previous f-plane simulations, both in terms of the resulting forms of organized convection, and the relative roles of the physical feedbacks driving them. This study considers the dependence of SA on rotation in one large domain on the β-plane, introducing an additional layer of complexity. Simulations are performed with uniform thermal forcing and explicit convection. Focuses include statistical and structural analysis of the convective modes, process-oriented diagnostics of how they develop, and resulting mean states. Two regimes of SA emerge within the first 15 days, separated by a critical zone where f is analogous to 10-15° latitude. Organized convection at near-equatorial values of f primarily consists of convectively-coupled Kelvin waves. Wind speed-surface enthalpy flux feedbacks are the dominant process driving moisture variability early on, then clear-sky shortwave radiative feedbacks are strongest in wave maintenance. In contrast, at higher f, numerous tropical cyclones develop and co-exist, dominated by surface flux and longwave processes. Tropical cyclogenesis is most pronounced at intermediate f (analogous to 25-40°), but are longer-lived at higher f. The resulting modes of SA at low f differ between these β-plane simulations (convectively-coupled waves) and prior f-plane simulations (weak tropical cyclones or non-rotating clusters). Otherwise, these results provide further evidence for the changing roles of radiative, surface flux, and advective processes in influencing SA as f changes, as found in our previous study.

A simple dynamical model linking radiative-convective instability, convective aggregation and large-scale dynamics

“…One possibility is that this corresponds to our f > f stab , with formation of tropical cyclones being a distinct process occurs that occurs at larger f value in the CRM, but which is simply absent in our model. On the β-plane (Carstens and Wing, 2023) identify the dominant structures that arise out of convective aggregation at low latitudes as convectively coupled Kelvin waves, whereas in our model the structures are clearly moisture-mode in character, with quasi-steady dynamical fields. In future work it would be interesting to investigate further whether there were are parameter regimes in which our model also shows moisture-modified Kelvin waves as the dominant low-latitude structures.…”

The Impact of Rotation on Tropical Climate, the Hydrologic Cycle, and Climate Sensitivity

“…Moisture, clouds, and precipitation for CAM5 and CAM6 in RCE were shown to organize from homogeneous initial conditions into aggregated structures (Reed et al., 2021). Carstens and Wing (2022) and Carstens and Wing (2023) showed a large range of aggregated states that result from a latitude‐dependent Coriolis parameter in cloud resolving simulations. In our RRCE cases, despite the presence of TCLVs (see Section 5), aggregation, as measured by the subsidence fraction (fraction of the domain with downward vertical velocity at 500 hPa, averaged daily) is reduced relative to RCE (Table 1, Table S9 in Supporting Information ), and the mean RH can be 10%–20% larger (Figures S1 and S2 in Supporting Information ).…”

Intensification of daily tropical precipitation extremes from more organized convection

“…In an effort to fill in this blind spot of the global modeling, researchers have begun using storm-resolving models (SRMs), which are distinguished by their use of a ca 3-km grid mesh to represent the transient dynamics of the precipitating convective systems responsible for the most extreme precipitation (20)(21)(22)(23)(24). Despite the model settings being idealized, important progress has been made in understanding the characteristics of convective storms, particularly how storms are initiated and maintained (25)(26)(27).…”