| In this study, an approach combining a dynamical initialization scheme and the large-scale spectral nudging is proposed to achieve improved numerical simulations of tropical cyclones (TCs), including track, structure, intensity, and their changes, based on the Advanced Weather Research and Forecasting (ARW-WRF) model. The effectiveness of the approach has been demonstrated with a case study of Typhoon Megi (2010). The ARW-WRF model with the proposed approach realistically reproduced many aspects of Typhoon Megi in a7-day long simulation. In particular, the model simulated quite well not only the storm track and intensity changes but also the structure changes before, during, and after its landfall over Luzon Island in the northern Philippines, as well as after it reentered the ocean over the South China Sea (SCS). The results from several sensitivity experiments demonstrate that the proposed approach is quite effective and is ideal for achieving realistic simulations of real TCs and thus is useful for understanding the TC inner-core dynamics, and structure and intensity changes.Typhoon Megi (15W) was the most powerful and longest lived tropical cyclone (TC) over the western North Pacific in2010. While it shared many common features of TCs that crossed Luzon Island in northern Philippines, Megi experienced unique intensity and structure changes Megi experienced rapid intensification (RI) over the warm ocean with high ocean heat content and decreasing environmental vertical shear. The onset of RI was triggered by convective bursts (CBs), which penetrate into the upper troposphere, leading to the upper-tropospheric warming and the formation of the upper-level warm core. In turn, CBs with their roots in the eye in the boundary layer were buoyantly-driven with slantwise convective potential energy (SCAPE) accumulated in the eye region. During RI, convective area-coverage in the inner-core region was increasing while the updraft velocity in the upper troposphere and the number of CBs were both decreasing. In addition, different from the majority of TCs that experience RI with a significant eyewall contraction, the simulated Megi, as the observed, rapidly intensified with no eyewall contraction and even experienced an inner-core size increase. The lack of eyewall contraction and the inner-core size increase were attributed to the binary interaction between the typhoon vortex and a low-level large-scale depression in which Typhoon Megi was embedded and diabatic heating in active spiral rainbands that enhanced by the binary interaction.The increased surface friction and reduced surface entropy flux both weakened the storm and led to the inner part of the original eyewall to contract and collapse after Megi moved over Luzon Island. The large new eyewall formed as a result of the axisymmetrization of two outer spiral rainbands after the storm core crossed Luzon Island and entered the SCS. Several sensitivity experiments demonstrate that the overall eyewall evolution and the reappearance of the original eyewall after the simulated storm core entered the SCS were largely affected by the mesoscale-mountain over western Luzon Island. With the mesoscale-mountain replaced by flat surface over Luzon Island, the inner eyewall survived over Luzon Island and the size of the new, large eyewall was about25%smaller than that in the control experiment, suggesting that the large eyewall formation was partially attributed to the terrain over the western Luzon Island. |
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