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Yu-Kun Qian, Chang-Xia Liang, Shiqiu Peng, Shumin Chen, and Sihua Wang

Abstract

A horizontal map of the upper-level forcing index (ULFI) is constructed to show the possible influence of upper-level large-scale environmental flow on the intensity change of tropical cyclones (TCs). The ULFI includes three commonly used diagnostics, that is, 200-hPa eddy flux convergences of both relative (REFC) and planetary angular momentum (PEFC), as well as axisymmetric absolute vorticity as a denominator that rescales the strength of the eddy forcings similar to the outflow-layer inertial stability. A simple procedure is adopted to convert these storm-relative components and the ULFI into Eulerian horizontal maps. Applications of this index map to three selected TC cases clearly demonstrate the process of upper-level TC–environment interaction: when a TC moves into a region of high (low) index, significant upper-level asymmetric forcing is exerted on the TC, leading to the strengthening (weakening) of the TC’s axisymmetric outflow and then possibly its intensity. As such, the horizontal map of ULFI not only provides a quantitative way of estimating the strength of upper-level asymmetric forcing at each grid point, but also serves as an indicator showing where the possible intensity change of a TC would occur under the influence of upper-level environmental flow. The index is thus recommended to be used in future studies of TC–environment interaction.

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Shiqiu Peng, Yineng Li, Xiangqian Gu, Shumin Chen, Dongxiao Wang, Hui Wang, Shuwen Zhang, Weihua Lv, Chunzai Wang, Bei Liu, Duanling Liu, Zhijuan Lai, Wenfeng Lai, Shengan Wang, Yerong Feng, and Junfeng Zhang

Abstract

A real-time regional forecasting system for the South China Sea (SCS), called the Experimental Platform of Marine Environment Forecasting (EPMEF), is introduced in this paper. EPMEF consists of a regional atmosphere model, a regional ocean model, and a wave model, and performs a real-time run four times a day. Output from the Global Forecast System (GFS) from the National Centers for Environmental Prediction (NCEP) is used as the initial and boundary conditions of two nested domains of the atmosphere model, which can exert a constraint on the development of small- and mesoscale atmospheric perturbations through dynamical downscaling. The forecasted winds at 10-m height from the atmosphere model are used to drive the ocean and wave models. As an initial evaluation, a census on the track predictions of 44 tropical cyclones (TCs) during 2011–13 indicates that the performance of EPMEF is very encouraging and comparable to those of other official agencies worldwide. In particular, EPMEF successfully predicted several abnormal typhoon tracks including the sharp recurving of Megi (2010) and the looping of Roke (2011). Further analysis reveals that the dynamically downscaled GFS forecasts from the most updated forecast cycle and the optimal combination of different microphysics and PBL schemes primarily contribute to the good performance of EPMEF in TC track forecasting. EPMEF, established primarily for research purposes with the potential to be implemented into operations, provides valuable information not only to the operational forecasters of local marine/meteorological agencies or international TC forecast centers, but also to other stakeholders such as the fishing industry and insurance companies.

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