Tropical Cyclone Landfall in South China: Seasonal Prediction and Climate Change Projection

熱帶氣旋登陸華南之季度預報及氣候推算

Student thesis: Doctoral Thesis

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Award date27 Dec 2017

Abstract

As a tropical cyclone (TC) makes landfall, the damage it causes largely depends on its intensity. The purpose of this study is to develop a numerical modelling system to simulate/ predict/project seasonal TCs making landfall in the South China coast, so that their intensities may be better predicted. The modelling system contains a regional climate model (RegCM) developed by the Abdus Salam International Centre for Theoretical Physics and the Weather Research and Forecasting mesoscale model (WRF). In this thesis, sensitivity tests on the model configurations, seasonal forecast predictability and climate projection throughout the twenty-first century are presented.

Five sensitivity tests have been performed to determine the optimal configurations of the modelling system: the RegCM model version, the RegCM model domain, the WRF horizontal resolution and physical packages. It is found that the newer version of the RegCM model (RegCM4) fails to reproduce a realistic East Asia summertime monsoon and therefore RegCM3 is adopted. For the model domain, inclusion of the Tibetan Plateau or the Australian continent has no significant impact on the RegCM3 simulation of TC landfalls in South China. Also, the two WRF physical scheme combinations—the WSM- 6 scheme (Hong and Lim, 2006) for microphysics and Tiedtke (1989) for cumulus, and Ferrier et al. (2002) and Kain (2004) schemes—perform similarly. For the WRF horizontal resolution, grid spacing of 6 km produces stronger TCs at landfall than 9 km spacing. However, without ocean coupling, the higher resolution model generally overestimates the seasonal TC intensities at landfall and therefore the 9 km horizontal resolution is used instead.

Driven by the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) dataset, the nested modelling system is able to reproduce a reasonable TC climatology, in particular the TC intensities at landfall in South China. With the use of the modelling system, the root mean square error of the annual power dissipation index (the sum of cubes of TC maximum sustained winds at landfall) is reduced by 15% from the parent global model. It is further shown that the strength and extent of the subtropical high in the East China Sea are crucial to simulate TC landfalls in South China.

The modelling system is further applied to the Hadley Centre Global Environment Model version 2 - Earth System (HadGEM2-ES) global model outputs for future climate projection of landfalling TC activity in South China. The projection shows a northward migration of TC activity in the western North Pacific (WNP), and subsequently a 35% decrease in the number of TCs making landfall in South China by the end of the twentyfirst century. However, these landfalling TCs are projected to be more intense. Enhanced anticyclonic flow in the South China Sea in the low- and mid- troposphere, together with smaller vertical wind shears basin-wide are attributed to such changes.

This thesis demonstrates the skill of the modelling system to simulate seasonal TC landfall activity. It is capable of seasonal predictions and/or projections of future changes of TC landfall activity in South China.