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Mengyuan Ma and Tim Li


Typhoon Lan (2017) was one of the largest tropical cyclones (TC) in the western North Pacific Ocean (WNP), and it was developed in a low-frequency (10–90-day filtered) large-scale cyclonic vortex environment. The physical mechanism responsible for the TC’s unusual size was investigated through idealized numerical experiments with the Weather Research and Forecasting Model. Sensitivity experiments showed that the low-frequency cyclonic circulation played an important role in modulating the TC size through the following three processes. First, it weakened the background vertical wind shear and provided a favorable condition for a more rapid growth of Lan. Second, it strengthened a vorticity aggregation process through enhanced background vorticity. As a result, a stronger and more organized TC core was quickly set up, which strengthened the TC intensity and expanded its size. Third, it enhanced the total surface wind speed and surface latent heat flux, strengthening convective instability in the outer region through increased moisture. The development of the outer rainband expanded the radial profile of diabatic heating, leading to greater low-level inflow and tangential wind acceleration in the outer region and thus a large TC size.

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