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Abstract Tropical cyclones (TCs) are often generated from preexisting “seed” vortices. Seeds with higher persistence might have a higher chance to undergo TC genesis. What controls seed persistence remains unclear. This study proposes that planetary Rossby wave drag is a key factor that affects seed persistence. Using recently developed theory for the response of a vortex to the planetary vorticity gradient, a new parameter given by the ratio of the maximum wind speed (V_max) to the Rhines speed at the radius of maximum wind (R_max), here termed “vortex structural compactness” (C_υ), is introduced to characterize the vortex weakening by planetary Rossby wave drag. The relationship between vortex compactness and weakening rate is tested via barotropicβ-plane experiments. The vortex’s initialC_υis varied by systematically varying their initialV_maxandR_maxin idealized wind profile models. Experiments are also conducted with real-world seed vortices from reanalysis data, which possess natural compactness variability. The weakening rate depends strongly on the vortex’s initialC_υacross both idealized and real-world experiments, and the initial axis-asymmetry introduces minor differences. Experiments doubling the size of seed vortices cause them to weaken more rapidly, in line with other experiment sets. The dependence of the weakening rate on initial compactness can be predicted from a simple theory, which is more robust for more compact vortices. Our results suggest that a seed’s structure strongly modulates how long it can persist in the presence of a planetary vorticity gradient. Connections to real seeds on Earth are discussed. Significance StatementThis study explores the evolution of tropical cyclone (TC) seeds, which are preexisting weakly rotating rainstorms, in a simple setting that isolates the dynamical effects of the rotating sphere. It is not clear why some seeds can persist for a longer duration and might have a higher chance to eventually undergo genesis. We proposed that a factor called “planetary Rossby wave drag” plays a crucial role in this process. To investigate this, we introduce a new parameter called “compactness” to describe how the size and intensity of a seed vortex determines how quickly it will weaken due to this drag. We conducted experiments with numerical simulations and real-world TC seeds to test our ideas. Our findings show that the initial compactness of seeds strongly influences how quickly they weaken. We have developed a formula to predict how quickly these seeds weaken based on their compactness, which is especially accurate for more compact seeds. This research helps us understand how planetary Rossby wave drag affects the persistence of a TC seed and, ultimately, how it might impact the frequency of TCs. 

Abstract Recent work found evidence using aquaplanet experiments that tropical cyclone (TC) size on Earth is limited by the Rhines scale, which depends on the planetary vorticity gradientβ. This study aims to examine how the Rhines scale limits the size of an individual TC. The traditional Rhines scale is first reexpressed as a Rhines speed to characterize how the effect ofβvaries with radius in a vortex whose wind profile is known. The framework is used to define the vortex Rhines scale, which is the transition radius that divides the vortex into a vortex-dominant region at smaller radii, where the axisymmetric circulation is steady, and a wave-dominant region at larger radii, where the circulation stimulates planetary Rossby waves and dissipates. Experiments are performed using a simple barotropic model on aβplane initialized with a TC-like axisymmetric vortex defined using a recently developed theoretical TC wind profile model. The gradientβand initial vortex size are each systematically varied to investigate the detailed responses of the TC-like vortex toβ. Results show that the vortex shrinks toward an equilibrium size that closely follows the vortex Rhines scale. A larger initial vortex relative to its vortex Rhines scale will shrink faster. The shrinking time scale is well described by the vortex Rhines time scale, which is defined as the overturning time scale of the circulation at the vortex Rhines scale and is shown to be directly related to the Rossby wave group velocity. The relationship between our idealized results and the real Earth is discussed. Results may generalize to other eddy circulations, such as the extratropical cyclone. Significance StatementTropical cyclones vary in size significantly on Earth, but how large a tropical cyclone could potentially be is still not understood. The variation of the Coriolis parameter with latitude is known to limit the size of turbulent circulations, but its effect on tropical cyclones has not been studied. This study derives a new parameter related to this concept called the “vortex Rhines scale” and shows in a simple model how and why storms will tend to shrink toward this size. These results help explain why tropical cyclone size tends to increase slowly with latitude on Earth and can help us understand what sets the size of tropical cyclones on Earth in general.