Tropical Cyclogenesis from Self-Aggregated Convection in Idealized Numerical Simulations: Sensitivity to Planetary Vorticity

Organized convection is ubiquitous in the tropical atmosphere. One type seen in numerical simulations is self-aggregation, which arises from interactions between convection and its surrounding environment, including radiative and surface flux feedbacks. Prior studies have shown self-aggregation to take several different forms, including that of spontaneous tropical cyclogenesis in an environment of rotating radiative-convective equilibrium (RCE). This study expands upon previous work to address the processes leading to tropical cyclogenesis in this rotating RCE framework. More specifically, we use a 3-D, cloud-permitting numerical model to examine the self-aggregation of convection and potential cyclogenesis, and vary the background planetary vorticity to simulate 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.

All simulations with planetary vorticity corresponding to latitudes from 10°N poleward generate intense tropical cyclones, with azimuthal-mean tangential wind speed peaking at 80 ms^-1 or above. Time to genesis and to hurricane intensity varies widely, even within a 5-member ensemble of 20°N simulations, reflecting a degree of stochastic variability. We also show that cyclogenesis is possible in this model even at very low values of planetary vorticity, as far equatorward as 1°N. In these experiments, convection self-aggregates into a quasi-circular cluster, which then begins to rotate and gradually strengthens into a tropical storm. Other experiments at these lower latitudes, in which only the initial distribution of random convection is changed, instead self-aggregate into an elongated band and fail to undergo cyclogenesis over the 100 days of simulation time. Finally, we examine in greater detail the dynamic and thermodynamic evolution of cyclogenesis in these experiments, comparing the physical mechanisms to current theories.