UC Davis

Tropical cyclones (TCs) are among the world’s most intense and feared storms. What physical processes lead to cyclogenesis remains the most mysterious aspect of TC physics. Uncertainties in understanding and forecast of TCs hinders effective planning and risk mitigation by society. This work explores the processes that organize random convection into a TC in Rotating Radiative- Convective Equilibrium (RRCE) simulations to try and gain insight into the processes that turn a pre-existing disturbance into a TC on Earth..

We first study spontaneous TC genesis in RRCE using cloud-resolving simulations over an f plane with constant sea surface temperature. Previous studies proposed that spontaneous TC genesis requires either radiative or surface-flux feedbacks. To test this hypothesis, we perform mechanism-denial experiments, switching off both feedback processes in numerical simulations. We find that TCs can self-emerge even without radiative and surface-flux feedbacks. Although these feedbacks accelerate the genesis and impact the size of the TCs, TCs in the experiments without them can reach similar intensities as those in the control experiment. We show that TC genesis is associated with increased available potential energy (APE) and that convective heating dominates APE production. Our result suggests that spontaneous TC genesis may result from a cooperative interaction between convection and circulation and that radiative and surface-flux feedbacks accelerate the process. Furthermore, we find that increasing the planetary rotation favors spontaneous TC genesis.

Second, we ask, why is TC genesis possible without radiative and surface-flux feedbacks? Thir- teen 3D cloud-resolving simulations show that the moisture-convection (MC) feedback can effectively lead to spontaneous TC genesis and intensification without radiative and surface-flux feedbacks. In the MC feedback, a moister environment favors new deep convective events that further moisten the environment, leading to aggregation of deep convection. The impact of the MC feedback on TC genesis and intensification occurs in two distinct time scales: a short time scale set by detrainment moistening the environment (a few hours) and a long time scale (17 days) due to subsidence drying. The hours-long time scale of detrainment suggests that the MC feedback is an efficient process relevant to TC genesis in the real world.

Finally, we implement a GPU-capable convective parameterization into a flexible high-performance shallow water model using open-source software tools. This system permits running economic simulations of convective systems with and without rotation with linear or non-linear dynamics. We show that this model is capable of reproducing features of organized convection in much more complex 3D models at a fraction of the cost. We further show that simple scaling arguments can predict some geometric characteristics of the organized steady state, showcasing the usefulness of simple models to improve our understanding of tropical convection.