Qingqing Li

Rapid intensification of tropical cyclones poses a great challenge due to their highly nonlinear dynamics and inherent uncertainties. Conventional statistical dynamics and artificial intelligence prediction models typically rely on static parameterization schemes, which limits their ability to capture the non‐stationary error structure in the intensity evolution. To address this issue, we propose a physically‐inspired covariate adaptive conformal prediction framework that dynamically adjusts uncertainty quantification by incorporating process information such as intensity and evolutionary stage. Our approach not only surpasses state‐of‐the‐art models in point prediction accuracy but also delivers physically consistent and interpretable forecast intervals, establishing a more process‐aware framework for probabilistic prediction of extreme weather events.

This study utilizes coastal radar observations to investigate eyewall formation processes during Typhoon Cempaka's (2021) rapid intensification. Two distinct types of inner rainband governed the evolution of eyewall formation: (a) strain‐generated bands forming inside the radius of maximum winds (RMW) through cellular convection, and (b) vortex Rossby wave‐coupled bands developing outside the RMW via convergence‐driven convection. When strain‐generated bands dominated near the RMW, a mean tangential wind tendency budget indicates that enhanced convection inside the RMW contributed to net spin‐up at lower levels, promoting RMW contraction. Meanwhile, the prevalence of wave‐coupled bands beyond the RMW led to significant wind acceleration near and outside the RMW, primarily driven by symmetric contributions. The axisymmetrization of these inner rainbands ultimately resulted in eyewall formation. These radar‐based findings underscore the critical role of inner rainbands in eyewall formation, contrasting with previous studies that emphasize outer rainbands.

Two idealized numerical experiments are conducted to address secondary eyewall formation (SEF) of tropical cyclones (TCs) in positive and negative TC-relative environmental helicity and the underlying dynamics. No SEF is observed in the positive environmental helicity experiment, whereas SEF arises in the negative environmental helicity experiment. The results indicate that an asymmetric SEF process is observed in the negative environmental helicity, with the upshear-left development of convection initially arising near the edge of the inner core and subsequently becoming azimuthally symmetric to form an outer eyewall. However, in contrast with previous findings, a confluent region in the lower layers gives rise to the upshear-left convection initiation. The midlevel dry intrusion, linked to the shear, triggers an intensified descending inflow below the stratiform sector of the principal rainband. Conversely, no such inflow occurs in the positive environmental helicity experiment due to the misalignment between the midlevel dry intrusion and the stratiform portion of the principal rainband. The descending inflow confluences with the supergradient wind-forced outflow from the inner core. A new finding of the study is that tropospheric humidification in the upshear-right quadrant serves to promote the convective axisymmetrization process prior to the onset of SEF in the negative environmental helicity. In particular, the “showerhead effect” markedly contributes to midlevel humidification in the upshear-right quadrant. As the upshear-right troposphere near the edge of the inner core becomes moist, the axisymmetrization process is rapidly accomplished, ultimately leading to the formation of the secondary eyewall in the negative environmental helicity. Significance Statement: Our study investigated how environmental helicity (a measure of wind rotation with altitude) influences tropical cyclone secondary eyewall formation. Using numerical simulations, we compared two scenarios: one with positive helicity (winds veering with height) and another with negative helicity (winds shifting counterclockwise with height). The secondary eyewall developed exclusively under negative helicity conditions through an asymmetric formation process. We observed that midlevel moisture buildup—caused by the “showerhead effect”—helped the process of axisymmetrization during secondary eyewall formation. These findings clarify the physical mechanisms behind secondary eyewall formation in complex sheared environments. Future research should validate these model results through observational studies.

A numerical simulation was conducted in this study to investigate the physical processes governing the unusual rapid intensification (RI) of Typhoon Mekkhala (2020) under strong vertical wind shear. The vortex tilt exhibited a gradual decrease from approximately 70 to below 10 km over a period of 6 h prior to and following the onset of the RI. Nevertheless, due to substantial midlevel ventilation, the storm intensity exhibited only slight variations in the early phase of the vortex alignment, 6 h prior to the RI onset. The RI occurred subsequent to the precipitation within the inner core becoming increasingly axisymmetric as the stratiform clouds developed cyclonically. The precipitation axisymmetrization was found to be closely associated with inner-core humidification in the left-of-shear and upshear quadrants, occurring 3 h before the RI onset. As previously documented, horizontal vapor advection greatly influenced the moistening process. A new finding of this study was that the “showerhead effect,” namely, moistening due to sublimation from stratiform clouds and evaporation from stratiform rain, could be predominantly conducive to the humidification of the inner core compared with horizontal vapor advection. Another noteworthy finding in the tangential wind budget was that the eddy radial vorticity flux contributed negatively and positively during the pre-RI and RI of Mekkhala, respectively, in conjunction with the dominant positive contribution resulting from the mean radial vorticity flux. Significance Statement: Rapid intensification has been a long-standing part of the challenge of tropical cyclone forecasting. This study aims to understand the dynamics of the rapid intensification of a typhoon in the presence of strong vertical wind shear, which is generally known to be unfavorable for rapid intensification. The results suggest that evaporation of stratiform precipitation could contribute more substantially to the humidification in the inner core than horizontal advection of moisture. In this circumstance, inner-core precipitation axisymmetrization and rapid intensification occurred. These results will hopefully shed more light on how the tropical cyclone community perceives the effects of humidification near the tropical cyclone core on tropical cyclone rapid intensification.

Mesoscale convective systems (MCSs), the primary drivers of extreme rainfall over the Pearl River Delta (PRD) urban agglomeration, are strongly influenced by synoptic circulations and local geographical environments, including water bodies and topography. However, the urban impact on MCS rainfall under various synoptic backgrounds remain inadequately understood. Using a 20‐year high‐resolution MCS tracking database and self‐organizing map clustering, three typical backgrounds for MCSs, namely weak monsoon‐like, strong monsoon‐like, and low‐pressure system (Types‐1 to 3), impacting the PRD are identified. These backgrounds exhibit pronounced disparities in MCS tracks and temporal variations as well as rainfall distributions. Urban heat island (UHI) significantly alters the spatial patterns under Types‐1 and 2. Specifically, under weak UHI condition, MCS rainfall typically occurs offshore in the morning and shifts inland in the afternoon driven by the land‐sea breeze. However, UHI modifies the low‐level thermal structure, leading to anomalous convergence and instability, which causes morning rainfall to concentrate near coastal cities, while afternoon rainfall expands further inland to the northern rural region. Additionally, the strong southwesterly winds associated with Type‐2 enhance the interaction between topography and urban impact, resulting in even higher rainfall anomalies (+28.9%) over the northeastern region. The findings highlight the crucial role of urban impact and their synergistic effect with synoptic backgrounds and other land surface processes on MCSs.

This study analyzes microphysical signatures relevant to the short‐cycle lightning activity in the inner core of Typhoon Hato (2017) before landfall in China. Observations reveal that the lightning bursts were accompanied by enhanced inner‐core convection with a behavior cycle of about 3 hr. The coupling between wavenumber‐2 vortex Rossby waves (VRWs) and shear‐forced convective asymmetries resulted in a local updraft enhancement. Furthermore, supercooled liquid water droplets invigorated a striking enhancement of graupel via riming immediately above the freezing level, further enhancing charge separation and lightning generation outside the eyewall. Furthermore, when similar phase‐locking was associated with other propagating VRWs, graupel and updraft volumes were significantly boosted, leading to short‐cycle lightning outbreaks in the inner core.

Editorial: Landfalling tropical cyclones: Physical processes, forecasting, and impacts

Idealized numerical experiments show that stronger shear is more favorable for tropical cyclone secondary eyewall formation (SEF) than weaker shear amid a moist‐tropical environment when the shear magnitude is smaller than 15 m s⁻¹. In particular, the larger the shear, the earlier the SEF occurs. A stationary banding complex (SBC) prevails before SEF in the large shear experiments, suggesting that SBCs are helpful in the SEF. Furthermore, a well‐organized stratiform sector is associated with the SBC. With increasing shear, the shear‐forced outflow also increases in the upper layers, transporting more moisture and icy particles outward from the inner core and favoring the development of the stratiform sector. The diabatic cooling of the stratiform sector strengthens the underlying descending radial inflow, forcing updrafts immediately outside the inner core. The subsequent axisymmetrization of the updrafts leads to SEF.

Microphysical signatures relevant to the asymmetric convection in the outer eyewall of Super Typhoon Lekima (2019) before its landfall in China were analyzed from ground‐based radar observations. The results indicate that the microphysical characteristics varied in quadrants. In the upshear‐left outer eyewall, the convection was deepened on the inner edge due to the phase locking between vortex Rossby waves (VRWs), and warm rain was the predominant process contributing to the enhancement of rainfall. In contrast, the strongest precipitation occurred on the outer edge of the upshear‐right outer eyewall. In this region, exuberant riming processes and graupel formation prevailed above the melting layer due to the strongest and more outward tilted updrafts induced by the phase locking between the VRWs. A good deal of graupel melted into raindrops, along with significant accretion processes, produced intense rainfall with larger drop sizes.