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  • Author or Editor: Johnny C. L. Chan x
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Martin L. M. Wong
,
Johnny C. L. Chan
, and
Wen Zhou

Abstract

The intensity change of past (1976–2005) tropical cyclones that made landfall along the south China coast (110.5°–117.5°E) is examined in this study using the best-track data from the Hong Kong Observatory. The change in the central pressure deficit (environmental pressure minus central pressure) and maximum surface wind after landfall are found to fit fairly well with an exponential decay model. Of the various potential predictors, the landfall intensity, landward speed, and excess of 850-hPa moist static energy have significant influence on the decay rates. Prediction equations for the exponential decay constants are developed based on these predictors.

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Wei Zhang
,
Yee Leung
, and
Johnny C. L. Chan

Abstract

This paper is the first of a two-part series of papers that employs the data-mining approach to analyze tropical cyclone (TC) movement in the western North Pacific Ocean. Part I unravels conditions under which TCs tend to recurve, and Part II uncovers conditions leading to TCs making landfall. Here in Part I, a detailed study is carried out into TC recurvature over the South China Sea and western North Pacific. The investigation focuses on the unraveling of rules governing TC recurvature hidden in TC data. The historical TC track database comprises recurving TCs and straight movers. Potential parameters affecting TC recurvature are categorized into three groups: large-scale circulation, circulations surrounding TCs, and variables characterizing TCs. The tree construction algorithm, C4.5, is applied to classify recurving and straight-moving TCs. Parameters measuring large-scale circulation patterns and characterizing TCs play significant roles in building the classification tree. Altogether, 18 rules are discovered from the processed database. Most of the 18 rules can be explained by existing theories and are supported by various empirical findings on TC recurvatures. Rules governing TC recurvature discovered by the present study contain quantitative descriptions of factors such as composite wind fields, geopotential heights, and deep-layer mean winds that are essential to the understanding, interpretation, and prediction of TC recurvatures.

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Wei Zhang
,
Yee Leung
, and
Johnny C. L. Chan

Abstract

This is the second paper of a two-part series of papers on the analysis of tropical cyclone (TC) tracks in the western North Pacific Ocean. In this paper, TC landfalls in the South China Sea and western North Pacific basins are investigated through the data-mining approach. On the basis of historical TC archives, the C4.5 algorithm, a classic tree algorithm for classification, has been employed to quantitatively discover rules governing TC landfall. A classification tree, with 14 leaf nodes, has been built. The path from the root node to each leaf node forms a rule. Fourteen rules governing TC landfall across the Chinese coast have been unraveled with respect to the selected attributes having potential influence on TC landfall. The rules are derived by the attributes and splitting values. From the classification tree, split values, such as 27°N latitude, 130°E longitude, 141°E in the west extension index, and 0.289 in the monsoon index have been shown to be useful for TC forecasting. The rules have been justified from the perspective of meteorology and knowledge of TC movement and recurvature (e.g., deep-layer mean winds and large-scale circulation). The research findings are also consistent with existing results concerning TC movement and landfall. Both the unraveled rules and the associated splitting values can provide useful references for the prediction of TC landfall over China.

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