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Hydrodynamics of Cyclogenesis from Numerical Simulations



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Kannan, Veeraraghavan 


Cyclogenesis is referred to as a transition of low-pressure disturbances on the tropical ocean into a more symmetric warm-core cyclone. This has been studied in detail for the past 100 years. However, no laboratory analogue for tropical cyclones has been discovered to date. Thus, the fundamental understanding of cyclogenesis remains challenging and limited as one has to rely on satellite imagery, which has disadvantages. This calls for further research to develop a simple model that can be replicated in the laboratory to better understand cyclogenesis. The main advantage of using a simple model is that the birth of a tropical cyclone from a quiescent environment can be continuously tracked, and the various hydrodynamic processes involved in its genesis can be better understood in a confined domain. This understanding can help in developing better forecasting models. The rotating Rayleigh-Benard convection (RRBC) model has been very effective and widely used in numerical simulations and by experimentalists to understand complex geophysical flows. Therefore in this work, the same RRBC model is used to carry out 3D numerical experiments in a shallow cylindrical domain filled with Boussinesq fluid. In atmospheric flows, the large eddies are responsible for turbulent transport since they contain most of the turbulent kinetic energy. Thus, the Large Eddy Simulation (LES) technique that explicitly resolves the larger eddies and models the effects of smaller eddies is appropriate for atmospheric flows and is used in this work to simulate a tropical cyclone-like vortex. This thesis attempts to tackle the problem of cyclogenesis purely based on hydrodynamics, neglecting the effects of stratification, thermodynamics and external wind shear. Therefore, the resulting large-scale vortex simulated is referred to as a tropical cyclone-like vortex. This thesis explores the capability of the RRBC model in simulating tropical cyclone-like vortex and understanding cyclogenesis in detail. The first part of the thesis focuses on fixing suitable boundary conditions and finding the parameter space necessary for forming a tropical cyclone-like vortex. This is because a tropical cyclone-like vortex has not been observed in a 3D RRBC model. This is a continuation of the previous studies, which focussed on simulating tropical cyclone-like vortex with axisymmetric approximation. The numerical experiments are then performed for a wide parameter range spanning several orders of magnitude. The simulated vortex is compared quantitatively with an actual tropical cyclone. It was found that the distinct structure of the vortex, namely the eye, eyewall and spiral bands, as well as the force balance of the simulated tropical cyclone-like vortex, is in good agreement with an actual tropical cyclone.

In the second part of the thesis, the data obtained from the simulation are analysed to understand the cyclogenesis in the simple model. The timescale for the cyclogenesis phase is proposed by taking advantage of simulating the vortex from a quiescent initial state. The cyclogenesis phase is the time difference between the start of the spinup of the cyclonic vortex and the vortex is fully evolved in the flow. The time for the start of the spinup of the cyclonic vortex is directly proportional to Ω−1√Ret, where Ω is the background rotation rate and Ret is the turbulent Reynolds number. The vortex fully evolves at Ekman spinup time, proportional to Ω−1/√Et, where Et is the turbulent Ekman number. During this period, unique spatial features appear in the large-scale vortex, namely, the eye, eyewall and spiral bands, and the vortex simultaneously becomes more intense. The proposed cyclogenesis timescale in the simple model agrees well with the real tropical cyclone track data from the United States Military's Joint Typhoon Warning Centre (JTWC) database. In addition, the simulated data are used to understand the energy exchange between the large-scale symmetric vortex and the asymmetric spiral bands in a tropical cyclone-like vortex by looking at the energy budget. Finally, the reason behind the formation and sustenance of tropical cyclone-like vortex in the RRBC setup is studied. The spontaneous onset of barotropic instability during the start of the spinup of the cyclonic vortex is seen as a plausible reason for the formation of a tropical cyclone-like vortex in the computational model used for this work.





Swaminathan, Nedunchezhian


Geophysical Fluid Dynamics, Large Eddy Simulation, Rotating Rayleigh Benard Convection, Tropical Cyclones


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
V. Kannan. was funded by the Cambridge India Ramanujan Scholarship from the Cambridge Commonwealth, European and International Trust and SERB, Government of India.