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It has been suggested that the inner eyewall structure may play an important role in the secondary eyewall formation (SEF) of tropical cyclones (TCs). This study is to further examine the role of the inner eyewall structure by comparing two numerical experiments, which were conducted with the same large-scale environment and initial and boundary conditions but different grid sizes. The SEF was simulated in the experiment with the finer grid spacing, but not in the other.Comparing the eyewall structure in the simulated TCs with and without the SEF indicates that the eyewall structure can play an important role in the SEF. For the simulated TC with the SEF, the eyewall is more upright with stronger updrafts, accompanied by a wide eyewall anvil at a higher altitude. Compared to the simulated TC without the SEF, diagnostic analysis reveals that the cooling outside the inner eyewall is induced by the sublimation, melting and evaporation of hydrometeors falling from the eyewall anvil. The cooling also induces upper-level dry, cool inflow below the anvil, prompting the subsidence and moat formation between the inner eyewall and the spiral rainband. In the simulated TC without the SEF, the cooling induced by the falling hydrometeors is significantly reduced and offset by the diabatic warming. There is no upper-level dry inflow below the anvil and no moat formation between the inner eyewall and the spiral rainband. This study suggests that a realistic simulation of the intense eyewall convection is important to the prediction of the SEF in the numerical forecasting model.

Extreme updrafts (stronger than 10 m s-1) have been observed in the tropical cyclone core region, which have profound implications to tropical cyclone intensification and structure change. Since extreme updrafts in the tropical cyclone are difficult to observe, their features and the associated mechanisms for formation and influences on tropical cyclones remain poorly understood. This study presents an analysis of extreme updrafts in a strong tropical cyclone that was simulated with the large-eddy simulation technique and the finest grid spacing of 37 meters. The simulated tropical cyclone experiences the vertical wind shear of about 5 m s-1 in a typical large-scale evironment in the western North Pacific. The simulated extreme updrafts in the inner core region exhibit the high frequency at the altitudes of ~ 750 m, 6.5 km and 13 km. The extreme updrafts in the inflow and outflow layers are closely associated with the Richardson Number of less than 0.25, indicating their relationship with severe turbulence caused by strong vertical wind shears. The extreme updrafts in the middle layer are associated with the strong convective activity. The details of the structures of the extreme updrafts are discussed.

The tornado-scale vortex in the tropical cyclone (TC) boundary layer (TCBL) has been observed in intense hurricanes and the associated intense turbulence poses a severe threat to the manned research aircraft when it penetrates hurricane eyewalls at a lower altitude. In this study, a numerical experiment in which a TC evolves in a large-scale background over the western North Pacific is conducted using the Advanced Weather Research and Forecast (WRF) model by incorporating the large eddy simulation (LES) technique. The simulated tornado-scale vortex shows the similar features as revealed with the limited observational data, including the updraft/downdraft couplet, the sudden jump of wind speeds, the favorable location, and the horizontal scale. It is suggested that the WRF-LES framework can successfully simulate the tornado-scale vortex with the grids at the resolution of 37 m that cover the TC eye and eyewall. The simulated tornado-scale vortex is a cyclonic circulation with a small horizontal scale of ~ 1 km in the TCBL. It is accompanied by strong updrafts (more than 15 m s−1) and large vertical components of relative vorticity (larger than 0.2 s−1). The tornado-scale vortex favorably occurs at the inner edge of the enhanced eyewall convection or rainband within the saturated, high-θe layer, mostly below the altitude of 2 km. Nearly in all the simulated tornado-scale vortices, the narrow intense updraft is coupled with the relatively broad downdraft, constituting one or two updraft/downdraft couplets or horizontal rolling vortices, as observed by the research aircraft. The presence of the tornado-scale vortex also leads to significant gradients in the near surface wind speed and wind gusts.

The steering principle of tropical cyclone motion has been applied to tropical cyclone forecast and research for nearly 100 years. Two fundamental questions remain unanswered. One is why the effect of steering plays a dominant role in tropical cyclone motion and the other is when tropical cyclone motion deviates considerably from the steering. A high-resolution numerical experiment was conducted with the tropical cyclone in a typical large-scale monsoon trough over the western North Pacific. The simulated tropical cyclone experiences two eyewall replacement processes. Based on the potential vorticity tendency (PVT) paradigm for tropical cyclone motion, this study demonstrates that the conventional steering, which is calculated over a certain radius from the tropical cyclone center in the horizontal and a deep pressure layer in the vertical, is not literally the steering or the advection of the symmetric potential vorticity component associated with a tropical cyclone by the asymmetric flow. The conventional steering also contains the contribution from the advection of the wavenumber-one potential vorticity component by the symmetric flow. The contributions from other processes are largely cancelled due to the coherent structure of tropical cyclone circulation and thus the conventional steering plays a dominant role. The trochoidal motion around the mean tropical cyclone track with amplitudes smaller than the eye radius and periods of several hours cannot be accounted for by the effect of the conventional steering and thus the instantaneous tropical cyclone motion can considerably derivate from the conventional steering.