Observations and simulations of tropical cyclone rainfall
File(s)
Author(s)
Lau, King Heng
Type
Thesis
Abstract
Heavy rainfall is a primary hazard of tropical cyclones (TCs), yet its characterisation and prediction remain challenging. A key environmental control on TC rainfall is vertical wind shear (VWS), but its role in organising rainfall structure and modulating rainfall production has not been fully quantified. Moreover, the relative infrequency of TC events and the computational expense of full-physics simulations make rainfall risk assessment and climate projection difficult. This thesis addresses these gaps through observations and simulations of TC rainfall spanning sub-storm-scale asymmetries, storm-scale rainfall production, and regional-to-global risk assessment.
At the sub-storm scale, the asymmetric distribution of rainfall around the TC centre is known to be influenced by VWS, but its radial dependence has not been previously quantified. The wavenumber-1 rainfall maximum is shown to progressively rotate upwind with increasing radius, tracing a spiral pattern, likely due to a continuous decline in angular advection of air parcels with radius. The maximum downwind deflection is stable in hurricanes, potentially explained by increasing vertical ascent under strengthening angular flow.
At the storm scale, a 26-year global analysis of state-of-the-art precipitation observations reveals an unexpected finding: stronger VWS, conventionally regarded as detrimental to TCs, is associated with greater rainfall production despite reducing TC intensity. For the same TC intensity, stronger shear enables higher rain rates, larger rain areas, and greater rain volumes. These results reveal a dual role: VWS potentially mitigates wind damage while amplifying flood risk.
At the regional-to-global scale, a new parametric rain model for landfalling TCs is developed and integrated into a stochastic TC hazard model, validated against observed global return periods of rain rate, storm-total volume, and lifetime rain production over land. Climate projections for United States landfalling hurricanes indicate increased eyewall rain rates, contraction of the rain field, and greater inland and poleward penetration of rainfall under warming.
At the sub-storm scale, the asymmetric distribution of rainfall around the TC centre is known to be influenced by VWS, but its radial dependence has not been previously quantified. The wavenumber-1 rainfall maximum is shown to progressively rotate upwind with increasing radius, tracing a spiral pattern, likely due to a continuous decline in angular advection of air parcels with radius. The maximum downwind deflection is stable in hurricanes, potentially explained by increasing vertical ascent under strengthening angular flow.
At the storm scale, a 26-year global analysis of state-of-the-art precipitation observations reveals an unexpected finding: stronger VWS, conventionally regarded as detrimental to TCs, is associated with greater rainfall production despite reducing TC intensity. For the same TC intensity, stronger shear enables higher rain rates, larger rain areas, and greater rain volumes. These results reveal a dual role: VWS potentially mitigates wind damage while amplifying flood risk.
At the regional-to-global scale, a new parametric rain model for landfalling TCs is developed and integrated into a stochastic TC hazard model, validated against observed global return periods of rain rate, storm-total volume, and lifetime rain production over land. Climate projections for United States landfalling hurricanes indicate increased eyewall rain rates, contraction of the rain field, and greater inland and poleward penetration of rainfall under warming.
Version
Open Access
Date Issued
2026-01-26
Date Awarded
2026-04-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Toumi, Ralf
Sponsor
Imperial College London
Croucher Foundation
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)