Tropical Cyclogenesis From Self‐Aggregated Convection in Numerical Simulations of Rotating Radiative‐Convective Equilibrium

Abstract

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...

Examining the Role of Cloud Radiative Interactions in Tropical Cyclone Development Using Satellite Measurements and WRF Simulations

“…The leading contribution from longwave radiation to the increase of CIMSE within the circulation center at the early stage of simulations indicates that longwave radiation may be the primary factor in regulating TC genesis. This conclusion is consistent with previous modeling studies which suggested that longwave radiation is the leading contributor for tropical convective systems at their early stage (Carstens & Wing, 2020;Muller & Romps, 2018;Smith et al, 2020;Wing & Cronin, 2016;Wing & Emanuel, 2014…”

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Tropical Cyclone Frequency

“…Such "rotating RCE" simulations allow for an investigation of fundamental controls on TC formation, absent complications from other disturbances, varying environmental conditions, or surface heterogeneities. The formation of individual TCs has been investigated in this setting (Boos et al, 2016;Carstens & Wing, 2020;C. A. Davis, 2015;Muller & Romps, 2018;Ramsay et al, 2020;Yang & Tan, 2020;Wing et al, 2016), as have the statistics of a population of TCs ("TC world"; Cronin & Chavas, 2019;Held & Zhao, 2008;Khairoutdinov & Emanuel, 2013;Reed & Chavas, 2015;Zhou et al, 2014Zhou et al, , 2017.…”

Tropical Cyclone Precipitation Response to Surface Warming in Aquaplanet Simulations With Uniform Thermal Forcing

“…When RCE simulations include rotation, TCs spontaneously develop. TC genesis, intensification, and structure have been studied using a variety of RCE model configurations, including small, f-plane domains that contain single TCs (e.g., Carstens & Wing, 2020;Knutson & Tuleya, 2004;Nolan et al, 2007;Ramsay et al, 2020; and larger domain simulations with multiple TCs (e.g., Chavas & Emanuel, 2014;Held & Zhao, 2008;Zhou et al, 2014). However, these model configurations simulate TCs on a flat, cartesian surface and therefore do not include the impacts of Earth's spherical geometry on TCs, namely the beta effect (Chan, 2005;Merlis & Held, 2019).…”