The role of the upper-level vertical wind shear (VWS) on the rapid intensification (RI) of super typhoon Lekima (2019) is investigated with a high-resolution numerical simulation. Our simulation shows that under moderate upper-level easterly VWS, the tilting-induced convective asymmetry is transported from the initially downshear quadrant to the upshear quadrant and wrapped around the storm center by the cyclonic flow of the storm while moving inward. This process enhances upward motions at the upshear flank and creates upper-level divergent flow. As such, the establishment of outflow acts against the environmental flow to reduce the VWS, allowing vertical alignment of the storm. The organized outflow plays an important role in sustaining the inner-core deep convection by modulating the environmental upper-level thermal structure. Accompanying deep convective bursts (CBs), cold anomalies are generated in the tropopause layer due to the adiabatic cooling by the upward motion and radiative process associated with the cloud anvil. Physically, cold anomalies at the tropopause locally destabilize the atmosphere and enhance the convections and the secondary circulation. The CBs continue to develop episodically through this process as they wrap around the storm center to form a symmetric eyewall. The results suggest that deep convections are capable of reducing the upper-level VWS, promoting the development of upper-level outflow. Lekima overcame the less favorable environment and eventually intensified to become a super typhoon.

In this study, super typhoon Lekima (2019) with an atypical rapid intensification (RI) episode is investigated by conducting a pair of experiments. In the control experiment, the model reproduces reasonably well the evolution characteristics of convective activity and intensity changes. That is, active downshear-left convection moved counterclockwise to the upshear flank. In the sensitivity experiment without the cloud-radiation feedback (CRF), the simulation fails to capture the observed upshear deep convection and the RI process. Our analyses suggest that the cloud–radiation interaction acts as positive feedback between the tilting-induced convective asymmetry (TCA) and outflow channel. On the one hand, the radiative process will induce upper (lower) cooling (warming) above (within) the cloud anvil of the outflow layer. This thermodynamical pattern locally destabilizes the upper troposphere and is conducive to enhance the deep convection. On the other hand, the enhanced deep convection provides the energy source to promote the upper divergent flows. The stronger divergent flow acts efficiently to block the vertical wind shear (VWS) and leads to a stronger outflow channel. This CRF assists in the development of a thicker and more radially extensive outflow than that CRF-off simulation. This study further confirms the outflow blocking effect, which gains insights on the evolution of upshear-left asymmetric convection and its role in the atypical RI event.

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In this study, the formation of an idealized multiple tropical cyclones (MTCs) event within a monsoon trough (MT) region is investigated by using the weather forecasting model (WRF_ARW). Sensitivity experiments are conducted by specifying different initial conditions. It is revealed that both dynamical and thermodynamical conditions associated with the MT are important in triggering MTCs event over the western North Pacific basin. In the composite active years, which have a strong MT with higher ambient moisture, an MTCs event is produced. In contrast, no MTCs event occurs in the inactive years, which confirms the previous observational study. The possible pathway for the formation of MTCs event is proposed. In the active years, under favourable moist environments, the first TC is generated faster through the greater barotropic kinetic energy conversion. Once the first tropical cyclone (TC) is generated, the energy dispersion induced low‐level Rossby wave train acts as a precursor to the second TC. Furthermore, the upper‐level asymmetric outflow jet acts as a dynamical forcing to induce vertical motion, which builds up a favourable environmental condition for a second TC development. This work provides some insights into the formation of MTCs event.

In this study, the Advanced Weather Research and Forecasting (WRF-ARW) model is used to investigate possible influences of a predominantly upper-level easterly wave (EW) on Typhoon Megi's (2010) sharp northward turn on 20 October, 2010 after passing over the Philippines. Observational analysis indicates that an upper-level EW with a cold-cored structure was located to the east of Megi. This EW moved westward along with Megi and modified the large-scale environmental flow around the typhoon, thus affecting its movement. In a control experiment, the sharp northward turn that was observed was captured well by a simulation. The retreat of the subtropical high contributed directly to the poleward steering flow for Megi. Sensitivity experiments were conducted by filtering out the synoptic-scale (3-8-day) signals associated with EWs. In the absence of the upper-level EW, the simulation showed that Megi would not have made a sharp northward turn. Two mechanisms are proposed regarding the impact of the easterly wave on Megi. First, an upper-level EW may have impacted the environmental flows, allowing Megi to move at a slower westward speed so that it entered the eastern semicircle of the nearby monsoon gyre where an enhanced southerly steering flow then led to the typhoon making a sharp northward turn. Second, the diabatic heating and associated cyclonic vorticity induced by the middle-level (around 400 hPa) convergence may have eroded the western flank of the subtropical high in the western North Pacific, causing an eastward retreat of the high-pressure system. The present modeling approach provides a reasonable assessment of the contribution of upper-level wave disturbances to sudden changes in tropical cyclones (TCs).