Latent Heat Driven Energy Dynamics in Inner Core of Super Typhoon Yagi (2024) During Rapid Intensification

The typical synoptic flow patterns and environmental factors that favor the rapid intensification (RI) of tropical cyclones (TCs) in the South China Sea (SCS) have been identified based on all TCs formed in the SCS between 1981 and 2011. The quantity RI is defined as the 24-h increase in maximum sustained surface wind speed by 15 m s−1 as in previous studies, which is close to the 95th percentile of 24-h intensity change of all SCS samples excluding those after landfall. There are 4.9% (2.3%) of tropical depressions (tropical storms) that experienced RI. No typhoons satisfied the RI threshold. Six low-level synoptic flow patterns favoring RI have been identified based on 18 RI cases. In the monsoon season very few TCs experience RI due to large vertical wind shear (VWS). Most RI cases occurred in the postmonsoon season when the midlatitude troughs often penetrated into the SCS whereas the southwesterly monsoon flow is still strong in the southern SCS. Compared with those of non-RI cases, the mean initial conditions of RI cases include weak VWS and relatively strong forcing from midlatitude troughs. Several criteria of significant environmental factors for RI are statistically identified based on all TC samples. It is found that 16 non-RI TCs fitted in the RI flow patterns but only two of them satisfy all the criteria, suggesting that a combination of the synoptic flow pattern and the environmental factors can be used to predict RI in the SCS. In addition, two RI cases involving TC–trough interaction are analyzed.

Analysis and Prediction of Tropical Storm Pakhar at National Center for Hydro-Meteorological Forecasting of Viet Nam

This study investigated the evolution of Typhoon Hato (1713) and analyzed the inner dynamics of its rapid intensification. The moving path of Hato was controlled by the southwestern part of the western Pacific subtropical high which extended westward significantly. Due to a relatively steady leading steering flow, the trajectory of Hato showed a straight ESE to WNW direction, leading to disastrous rainfall in southern Guangdong Province on 23 August before and after the landfall of the typhoon. In addition, Typhoon Hato exhibited a rapid intensification period from 3 to 13 o'clock on 23 August, with central wind speeds increasing from 35 to 48 m/s. This study mainly examined the inner dynamics of the development of Hato, especially its rapid intensification period. The regression analysis results showed that Hato was generated and developed under a TD‐type‐like wave. The coupled convection and vortex provided a favorable environment for the development of Hato. The analysis of the barotropic eddy kinetic energy (EKE) and latent heat flux further demonstrated this scenario. The results showed that the regions with a positive EKE tendency and strong latent heat release were consistent with the reach of and shifted westward with Hato. The EKE conversion was enhanced significantly on 23 August, with a maximum intensity observed over the northern South China Sea and southern Guangdong Province. The mean kinetic energy was largely converted to EKE, which, together with the great latent heat release, led to the rapid intensification of Hato.

Diabatic Heating and Convective Asymmetries During Rapid Intensity Changes of Tropical Cyclones Over North Indian Ocean

NCEP–NCAR reanalysis data are used to identify large-scale environmental flow patterns around western North Pacific tropical storms with the goal of finding a signal for those most favorable for rapid intensification, based on the hypothesis that aspects of the horizontal flow influence tropical cyclone intensification at an early stage of development. Based on the finding that intensification rate is a strong function of initial intensity (Joint Typhoon Warning Center best track), very rapid, rapid, and slow 24-h intensification periods from a weak tropical storm stage (35 kt) are defined. By using composite analysis and scalar EOF analysis of the zonal wind around these subsets, a form of the lower-level (850 mb) combined monsoon confluence–shearline pattern is found to occur dominantly for the very rapid cases. Based on the strength of the signal, it may provide a new rapid intensification predictor for operational use. At 200 mb the importance of the location of the tropical storm under a region of flow splitting into the midlatitude westerlies to the north and the subequatorial trough to the south is identified as a common criterion for the onset of rapid intensification. Cases in which interactions with upper-level troughs occurred, prior to and during slow and rapid intensification, are studied and strong similarities to prior Atlantic studies are found.

The total (i.e., convective and stratiform) latent heat release (LHR) cycle in the eyewall region of Hurricane Opal (October 1995) has been estimated using observations from the F-10, F-11, and F-13 Defense Meteorological Satellite Program Special Sensor Microwave/Imagers (SSM/Is). This LHR cycle occurred during the hurricane’s rapid intensification and decay stages (3–5 October 1995). The satellite observations revealed that there were at least two major episodes in which a period of elevated total LHR (i.e., convective burst) occurred in the eyewall region. During these convective bursts, Opal’s minimum pressure decreased by 50 mb and the LHR generated by convective processes increased, as greater amounts of latent heating occurred at middle and upper levels. It is hypothesized that the abundant release of latent heat in Opal’s middle- and upper-tropospheric region during these convective burst episodes allowed Opal’s eyewall to become more buoyant, enhanced the generation of kinetic energy and...

Tropical cyclones (TCs) can undergo offshore rapid intensification (RI) shortly before making landfall over the South China Sea (SCS). In this study, the inner rainbands distribution of both RI and non-RI landfall TCs (LTCs) in the SCS during 2015–2020 is examined based on a multi-source merged precipitation dataset. It is found that those RI LTCs exhibit a relatively higher averaged rain rate in the inner core region than that of non-RI LTCs. Both offshore RI cases and non-RI LTCs appear to have an increasing tendency of averaged rain rate after landfall, with the rain rate peak of the RI cases a few hours earlier than that of non-RI cases. By defining an axisymmetric index, the inner rainband evolution of both offshore RI cases and non-RI ones are further discussed. For both categories, most of the axisymmetric rainfall is concentric around the center and over 70% axisymmetric rainfall dominates the inner core region within three times of radius of maximum wind speed (RMW). It is shown that there is an obvious inwards shrinkage of axisymmetric rainfall for both offshore RI and non-RI cases. Analysis of typical RI and non-RI LTCs (1713 Hato and 1714 Pakhar) also shows an increasing rain rate of the inner rainbands soon after landfall, with larger amplitude for RI example than non-RI case. The inner rainbands of 1713 Hato show that a clockwise propagation with maximum enhancement happened at the down-shear left-hand side a few hours after landfall.

This study investigates the modulation of tropical cyclone (TCs) rapid intensification (RI) over the western North Pacific by the Antarctic Oscillation. There is a significant inverse relationship between basinwide RI number during July–November from 1982 to 2020 and the simultaneous Antarctic Oscillation index. During positive Antarctic Oscillation years, RI occurrence is significantly suppressed over the main RI region (10°–20° N and 125°–150° E) and slightly enhanced over the South China Sea. By contrast, during negative Antarctic Oscillation years, RI is significantly enhanced over the main RI region and slightly suppressed over the South China Sea. The Antarctic Oscillation influences western North Pacific RI occurrence mainly through modulation of the large-scale dynamic environment. During positive Antarctic Oscillation years, increases in 850–200 hPa vertical wind shear and decreases in both low-level vorticity and upper-level divergence all suppress RI occurrence over the main RI region, while thermodynamic variables (e.g. TC heat potential, maximum potential intensity and low-to-middle level relative humidity) show mostly weak changes. These changes in dynamic factors can be linked to a low-level anomalous western North Pacific anticyclone triggered by the positive phase of the Antarctic Oscillation and a low-level anomalous western North Pacific cyclone generated by a negative phase of the Antarctic Oscillation.

The convective pattern of tropical cyclones (TCs) relative to vertical wind shear (VWS) is an important indicator of rapid intensification (RI), yet significant variability exists among RI TCs, which can make effective predictions of RI onset challenging. In this study, a self-organizing-map clustering method is employed to classify shear-relative convective patterns at RI onset. Four distinct clusters are identified: one fairly symmetric pattern with a compact core (SYM, 26% of all cases) and three asymmetric patterns with peak convection upshear left (USL, 25%), downshear left (DSL, 32%), and downshear right (DSR, 17%). An objective asymmetry index reveals that RI in SYM and USL follows significant precipitation axisymmetrization, whereas DSL and DSR experience RI after sharp increases in asymmetric precipitation. Environmental analyses indicate that SYM intensifies under lower VWS and resultant weaker ventilation. USL experiences moderate VWS, with precipitation axisymmetrization favored by increased right-of-shear latent heat flux (LHF) associated with an anomalous large-scale gyre. DSL, with relatively stronger VWS, experiences asymmetric convective intensification favored by increased midlevel relative humidity, LHF, upper-level divergence, and lower-level inflow downshear or left of shear. DSR, despite moderate VWS, has the most favorable environment, and its unique convective distribution at RI onset is influenced by an anomalous large-scale vorticity dipole downshear, a right-of-shear low-level mean flow, and the associated LHF anomalies. These findings highlight the environmental origins of the diverse convective patterns of RI TCs, underscoring the need to account for intercase variations in convective and environmental properties in future statistical models. Significance Statement The convective pattern of tropical cyclones (TCs) relative to vertical wind shear plays a crucial role in their rapid intensification (RI). Despite various reported shear-relative convective patterns, a systematic classification has yet to be established. This study employs a self-organizing map clustering method to identify four distinct shear-relative convective patterns at RI onset: one relatively symmetric pattern and three asymmetric patterns with the strongest convection in different shear-relative quadrants. Each cluster is linked to unique dynamic and thermodynamic environmental configurations that drive diversities in convective patterns. These findings deepen our understanding of the interplay between convection, environmental factors, and RI, highlighting the need to account for intercase variations in convective and environmental properties in future statistical models for RI forecasting.

This study investigated the kinetic and potential energy budgets during the slow (SI) and rapid intensification (RI) periods of Typhoon Mujigae, which occurred in October 2015, by conducting a Weather Research and Forecasting model simulation. We found that the kinetic energy conversion between asymmetric and symmetric rotational circulations in the lower troposphere plays a pivotal role in initiating the difference in the tendency of symmetric rotational kinetic energy between the RI and SI via the horizontal advection process of asymmetric circulations. Southerly and easterly environmental flows transported asymmetric anticyclonic flows in SI and cyclonic flows in RI to the radius of maximum wind, respectively, where asymmetric rotational circulation was developed by vertical shear of environmental flow. Such transports initiate the growth of symmetric cyclonic rotational circulation from SI to RI. The kinetic energy conversion of symmetric divergent circulations supported by symmetric potential energy accelerates the growth of symmetric rotational kinetic energy from SI to RI. These findings can shed light on the fundamental dynamics of the RI of tropical cyclones.

Evolution of Thermodynamic Structures During Rapid Growth and Decay of Extremely Severe Cyclonic Storm Chapala (2015)

An assessment of model capability on rapid intensification prediction of tropical cyclones in the South China Sea

With increasing air and sea temperatures, the thermodynamic environments over the oceans are becoming more favourable for the development of intense tropical cyclones (TCs) with rapid intensification (RI). The South China coastal region consists of highly densely populated cities, especially over the Pearl River Delta (PRD) region. Intense TCs maintaining their strength or the RI of TCs close to the coastal region can present substantial forecasting challenges and have significant potential impacts on the coastal population. This study investigates the effect of sea-surface and sub-surface temperatures and salinity on the intensification of five TCs, namely Super Typhoon Hato in 2017, Super Typhoon Mangkhut in 2018, and Typhoon Talim, Super Typhoon Saola, and Severe Typhoon Koinu in 2023, which have significantly affected the South China coastal region and triggered high TC warning signals in Hong Kong in the past few years. This analysis utilised the Hong Kong Observatory’s TC best-track and intensity data, along with sea temperature and salinity profiles generated using the China Ocean ReAnalysis version 2 (CORA2) product from the National Marine Data and Information Service of China. It was found that high sea-surface temperatures (SST) of 30 °C or above for a depth of about 20 m, low sea-surface salinity (SSS) levels of 33.8 psu or below for a depth of at least 20 m, and strong salinity stratification of at least 0.6 psu per 100 m depth might offer useful hints for predicting the RI of TCs over the western North Pacific and the South China Sea (SCS) in operational forecasting, while noting other contributing environmental factors and synoptic flow patterns conducive to RI. This study represents the first documentation of sub-surface salinity’s impact on some intense TCs traversing the SCS during 2017–2023 based on an observational study. Our aim is to supplement operational techniques for forecasting RI with some quantitative guidance based on upper-level ocean observations of temperatures and salinity, on top of well-known but more rapidly changing dynamical factors like low-level convergence, weak vertical wind shear, and upper-level divergent outflow, as forecasted with numerical weather prediction models. This study will also encourage further research to refine the analysis of quantitative contributions from different RI factors and the identification of essential features for developing AI models as one way to improve the forecasting of TC RI before the TC makes landfall near the PRD, with due consideration given to the effect of freshwater river discharge from the Pearl River.

Tropical storm intensity prediction remains a challenge in tropical meteorology. Some tropical storms undergo dramatic rapid intensification and rapid decline. Hurricane researchers have considered particular ambient environmental conditions including the ocean thermal and salinity structure and internal vortex dynamics (e.g., eyewall replacement cycle, hot towers) as factors creating favorable conditions for rapid intensification. At this point, however, it is not exactly known to what extent the state of the sea surface controls tropical cyclone dynamics. Theoretical considerations, laboratory experiments, and numerical simulations suggest that the air-sea interface under tropical cyclones is subject to the Kelvin-Helmholtz type instability. Ejection of large quantities of spray particles due to this instability can produce a two-phase environment, which can attenuate gravity-capillary waves and alter the air-sea coupling. The unified parameterization of waveform and two-phase drag based on the physics of the air-sea interface shows the increase of the aerodynamic drag coefficient with wind speed up to hurricane force ( m s-1). Remarkably, there is a local minimum-"an aerodynamic drag well"-at around m s-1. The negative slope of the dependence on wind-speed between approximately 35 and 60 m s-1 favors rapid storm intensification. In contrast, the positive slope of wind-speed dependence above 60 m s-1 is favorable for a rapid storm decline of the most powerful storms. In fact, the storms that intensify to Category 5 usually rapidly weaken afterward.

In this study, a sensitivity experiment based on a numerical model is conducted to study the influence of the diurnal variation of radiation on the rapid intensification process of Super Typhoon Rammasun (1409) in the South China Sea. The result shows that changing the radiation process can change the onset time of the rapid intensification process of the typhoon and its maximum intensity. Vertical wind shear is a key factor in the TC rapid intensification influenced by diurnal radiation cycle, and the diurnal variation of radiation process causes obvious diurnal variation in vertical wind shear by changing the thermal difference between sea and land. The oscillation of vertical wind shear promotes the diurnal variation of the typhoon rainband, with an opposite variation trend in the control and sensitivity experiments, affecting the local convective available potential energy and inward transport of entropy and thus influencing the onset time of deep convective in the typhoon inner core region. By changing the establishment of the typhoon warm core (especially the upper-level warm core), deep convections finally affect the rapid intensification of the typhoon.