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Abstract: A tropical cyclone is a kind of violent weather system that takes place in warmer tropical oceans and spins rapidly around its center and at the same time moves along surrounding flows. It is generally recognized that the large-scale circulation plays a major role in determining the movement of tropical cyclones and the effects of steering flows are the highest priority in the forecasting of tropical cyclone motion and track. This article adopts a new method to derive the steering flow and select a typical swerving track case (typhoon Dan, coded 9914) to illustrate the validity of the method. The general approach is to modify the vorticity, geostropical vorticity and divergence, investigate the change in the non-divergent stream function, geoptential and velocity potential, respectively, and compute a modified velocity field to determine the steering flow. Unlike other methods in regular use such as weighted average of wind fields or geopoential height, this method has the least adverse effects on the environmental field and could derive a proper steering flow which fits well with storm motion. Combined with other internal and external forcings, this method could have wide application in the prediction of tropical cyclone track.
Key words: steering flow; prediction of tropical cyclone track; vorticity and divergence
CLC number: P444 Document code: A doi: 10.3969/j.issn.1006-8775.2010.02.007
(ProQuest: ... denotes formulae omitted.)
1 INTRODUCTION
The complexity of tropical cyclones (TCs) movement results from a wide variety of external and internal dynamical forcings and their interaction, but the most dominant factor is the relative vorticity in large-scale environmental flows, like a several-hundred-km radius vortex embedded in and steered by a basic surrounding flow on the scale of a thousand km [1]. Therefore, the steering flow is the main factor affecting the movement of TCs, and the steering concept has generally been accepted and applied wherever possible by forecasters in typhoon track prediction[2, 3]. However, as the actual weather circulation is very complex and contains the information of the TC itself (internal forcings), surrounding flow (external dynamical forcings) and their interactions, how to separate the TC vortex from the surrounding flow remains a challenge. Many researches have worked to extract the steering flow; the first method is to calculate the weighted average of wind observations surrounding the...