In a modeled environment of rotating radiative-convective equilibrium (RCE), convective self-aggregation may take the form of spontaneous tropical cyclogenesis. We investigate the processes leading to tropical cyclogenesis in idealized simulations with a three-dimensional cloud-permitting model configured in rotating RCE, in which the background planetary vorticity is varied across f -plane cases to represent a range of deep tropical and near-equatorial environments. Convection is initialized randomly in an otherwise homogeneous environment, with no background wind, precursor disturbance, or other synoptic-scale forcing. We examine the dynamic and thermodynamic evolution of cyclogenesis in these experiments and compare the physical mechanisms to current theories. All simulations with planetary vorticity corresponding to latitudes from 10 • -20 • generate intense tropical cyclones, with maximum wind speeds of 80 m s −1 or above. Time to genesis varies widely, even within a five-member ensemble of 20 • simulations, indicating large stochastic variability. Shared across the 10 • -20 • group is the emergence of a midlevel vortex in the days leading to genesis, which has dynamic and thermodynamic implications on its environment that facilitate the spin-up of a low-level vortex. Tropical cyclogenesis is possible in this model at values of Coriolis parameter as low as that representative of 1 • . In these experiments, convection self-aggregates into a quasicircular cluster, which then begins to rotate and gradually strengthen into a tropical storm, aided by strong near-surface inflow that is already established days prior. Other experiments at these lower Coriolis parameters instead self-aggregate into a nonrotating elongated band and fail to undergo cyclogenesis over the 100-day simulation. Plain Language SummaryDespite decades of research on tropical cyclones, we still do not have a universal agreement on how they form. Current theories agree that some sort of disturbance must exist beforehand, but our knowledge of the processes leading to a surface-based cyclone remains limited. To address this, we examine idealized numerical simulations in which convection is allowed to spontaneously cluster together on its own due to interactions between clouds, moisture, and radiation. Using this framework, we obtain a complete view of the tropical cyclone formation process, including the formation of the precursor disturbance. New to this study is the use of lower values of background rotation to simulate the formation of hurricanes at lower latitudes. Overall, simulations are run to represent latitudes from 0.1 • -20 • . Every simulation corresponding to latitudes between 10 • and 20 • produces a major hurricane, a few days after a vortex emerges a few kilometers aloft and affects its surrounding environment. Some simulations at 1 • and 2 • lead to formation of a weaker tropical cyclone, after cloud s have first organized into one circular cluster. In other low-latitude cases, this cluster of storms is instead a long band and fa...

Understanding the interplay between moist convection and its surrounding environment is critical to our broader understanding of tropical climate (Bony et al., 2015). Among the consequences of this convection-environment interaction are the spatial organization of clouds and precipitation, which affects radiation budgets and other features of the climate system (Khairoutdinov & Emanuel, 2010;Wing, 2019). Moreover, organized convection is impactful on shorter timescales, contributing to much of the extreme weather of the tropics. It takes form across all spatial scales, ranging from mesoscale convective systems (Houze, 2004), to synoptic-scale waves and tropical cyclones (TCs;Emanuel, 2003), to planetary-scale features like the Madden-Julian Oscillation (Madden & Julian, 1971;Zhang, 2005). These systems are often coupled to circulation patterns, and influence weather and climate in other parts of the world.

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.

Organized convective systems like tropical cyclones (TCs) (Emanuel, 2003) produce much of the extreme weather in the tropics, and influence large-scale patterns of circulation, moisture, and radiation (Khairoutdinov & Emanuel, 2010;Silvers & Robinson, 2021;Wing et al., 2020). Idealized modeling studies have improved our understanding of convective organization, including the spontaneous self-aggregation that arises due to interactions between clouds, water vapor, radiation, and circulations (e.g., Held et al. (1993); Bretherton et al. (2005); Wing et al. (2017); Carstens and Wing ( 2022)). Similar feedbacks are also relevant in the development of TCs (Ruppert et al., 2020;Wing et al., 2016;Wu et al., 2021). Radiation feedbacks have long been thought to contribute to the diurnal cycles of tropical convection (Gray & Jacobson Jr., 1977), and more recently, TC precipitation, structure, and intensity (e.g., Dunion et al. ( 2014)).To quantify feedbacks on convective organization, Wing and Emanuel (2014) developed a budget equation for the spatial variance of column-integrated moist static energy (MSE). MSE consists of contributions by temperature (c p T), gravitational potential energy (gz), and water vapor (L v q v ):Under weak temperature gradients, MSE variance is a proxy for moisture variability associated with organized convection. Chen et al. ( 2019) suggested that TC rapid intensification is preceded by an increase in inner-core column-integrated MSE, and therefore, increased MSE spatial variance. A strong relationship between MSE variance and TC intensity was also found in both climate models and idealized simulations (Wing, 2022;Wing et al., 2019). Dropsondes from aircraft reconnaissance sample profiles of all inputs to MSE with fine vertical resolution. Prior analysis of dropsonde data has produced valuable information on TC structure,