The decay of wind speed in tropical cyclones
File(s)
Author(s)
Li, Min
Type
Thesis
Abstract
The decay of tropical cyclones (TCs), characterized by a weakening of TC wind speed, is a key stage prior to landfall and is therefore essential for risk management in coastal regions. A full description of the decay of global TCs over the ocean using a new modified empirical model (the β model) has been established, which captures the decay feature with time using two characteristic parameters: a scale parameter α (the decay scale) and a shape parameter β (which determines whether the decay rate decelerates or accelerates over time). Global fits indicate that most TCs (about 60%) show accelerating decay (β > 1) over the ocean and that decay scales are larger at lower latitudes. Next, the mechanisms behind the decay of a well-developed TC moving to the Equator are investigated using idealized simulations. Under reduced latitude, a deeper inflow layer and strengthened downdrafts in the outer rainbands enhance the entrainment of mid-level dry air into the boundary layer. This causes a decrease in boundary layer θ e and suppresses the deep convection in the eyewall, leading to TC weakening. Last but not least, large synthetic TC datasets are generated by simulating the decay of TC wind speed from the point of lifetime maximum intensity (LMI) in a stochastic model (IRIS) for reliable landfall risk assessment. Using this framework, the increased intensity of North Atlantic hurricanes from 1979 to 2024 is analysed. The model attributes the increasing trend in LMI (0.08 m s−1 yr−1) to comparable contributions from rising potential intensity and southward shifts in TC tracks, with little sensitivity to changes in relative intensity to date. The model also predicts a southward shift in landfall (-0.10 ◦ yr−1), which is hard to detect.
Version
Open Access
Date Issued
2026-02-06
Date Awarded
2026-06-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Toumi, Ralf
Sponsor
Imperial College London
Vodafone Group Plc
Natural Environment Research Council (Great Britain)
Lighthill Risk Network Firm
Grant Number
NE/W009587/1
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)