Dynamically Derived Tropical Cyclone Intensity Changes over the Western North Pacific

This article explores the relationship between vertical wind shear (VWS) and tropical cyclone (TC) genesis in the Mozambique Channel (MC) for the period 1979–2019. Additionally, SST, low-level relative vorticity, 700 hPa relative humidity and upper-level divergence were also analyzed for the peak cyclogenesis months to explore their relative contributions. The analyses were done using NCEP/NCAR Reanalysis-1 for the atmospheric fields, monthly Optimum Interpolation SST V2, and for the cyclogenesis the TC best track data from the La Reunion–Regional Specialized Meteorological and Joint Typhoon Warning Centre. A total of 43 TCs generated in the MC were observed for the analysed period. The maximum frequency of cyclogenesis in the MC was observed during January and February and the spatial location of maximum TC genesis was coincident with the minimum values of the VWS. The VWS showed significant correlations with TC intensity, particularly when considering the upper atmosphere (200–500 hPa) or the bulk (200–850 hPa) VWS. The mean composites of the cyclogenesis months over the MC of SST, relative humidity at 700 hPa, divergence at upper atmosphere, showed significant values. However, linear correlations between these factors vs. TC genesis frequency and intensity were not significant. Analyses of interannual correlations between VWS and Niño-3.4 (subtropical southwest Indian Ocean dipole-SIOD) showed statistically significant positive (negative) correlations at different lags, suggesting that La Niña and the positive phase of SIOD conditions are favorable to weaker VWS and thus to intensification of TCs in the Mozambique Channel. Thirteen landfall cases were observed with seven over the Madagascar west coast and six over the Mozambique coast. The landfall over the Madagascar (Mozambique) coast was associated with strengthened (weakened) VWS.

Impacts of Climate Change on Tropical Cyclones in the Western North Pacific Basin. Part II: Late Twenty-First Century Projections

The development and performance of a statistical - dynamical tropical cyclone intensity forecast model, which was developed for the United States of America’s Joint Typhoon Warning Center (JTWC), is described. This model, called the Southern Hemisphere Statistical Typhoon Intensity Prediction Scheme (SH STIPS), mirrors similar capabilities created for use in the western North Pacific and North Indian Ocean tropical cyclone basins. The model is created by fitting an optimal combination of factors related to climatology and persistence, intensification potential, vertical wind shear, dynamic size/intensity forecasts and atmospheric stability. All of these factors except the climatology and persistence information are derived from global forecast model analyses and forecasts. In July 2005 the SH STIPS model began a real-time evaluation period. The forecasts from the SH STIPS model have outperformed the combined climatology and persistence based forecast and thus are skillful in independent testing since that time. Since October 2006, SH STIPS has been the primary member in an operational consensus forecast of tropical cyclone intensity change provided to the JTWC. Documentation is provided for potential users of forecasts based on this methodology and for researchers interested in developing similar capabilities in the future.

Tropical cyclone (TC) intensity is controlled by both environmental factors and its internal structure. This paper investigates the influence of inner core features derived from satellite data on TC intensity changes and explores effective methods to improve TC intensity forecast accuracy. We construct models for forecasting TC intensity over the next 5 days using the light gradient boosting machine (LGBM) algorithm in the northwestern Pacific. The models integrate climatological and persistence predictors, large-scale environmental variables at the initial time, inner core features extracted from satellite data, and variables related to the forecasted TC location. The models are trained on TC data from 2005 to 2019, tested using best track data from 2020 to 2022 and real-time data in 2023. Results show that the inner core features derived from satellite data are helpful for intensity forecasts within 36 h, and variables related to the forecasted location are helpful for improving TC intensity forecast accuracy in most cases, especially for the forecasts beyond 48 h. Compared with the National Centers for Environmental Prediction (NCEP) and European Centre for Medium-Range Weather Forecasts (ECMWF) models, our models demonstrate 4%–48% and 19%–70% decreases in mean absolute errors (MAEs) in real-time intensity forecasts. The most important predictors identified include potential future intensity change, intensity changes during the previous 12 h, vertical wind shear, and sea–land ratio. The inner core feature extracted from satellite data is also among the top 10 important variables for short-time intensity forecasts. Significance Statement This study advances tropical cyclone (TC) intensity forecasting by integrating climatological and persistence predictors, large-scale environmental variables at the initial time, satellite-derived inner core features, and variables related to the forecasted TC location into the light gradient boosting machine (LGBM) models. Our models significantly improve forecast accuracy, reducing mean absolute errors by 4%–48% compared with the National Centers for Environmental Prediction (NCEP) and 19%–70% compared with the European Centre for Medium-Range Weather Forecasts (ECMWF). Results denote that the inner core features improve the intensity prediction in the short time, and variables related to the forecasted TC location improve the intensity forecasts in most cases, especially for the forecasts beyond 48 h. This study provides valuable insights for enhancing operational TC intensity forecasting.

A tropical cyclone intensity prediction model using conditional generative adversarial network

The relationship between sea surface temperature (SST) and tropical cyclone (TC) intensity change exhibits a strong dependence on the current TC intensity. Using western North Pacific TC observations from 1982 to 2018, a threshold SST (TSST) is identified as the SST required to maintain TC intensity. TSST increases with TC intensity, with TCs intensifying and weakening when SST is higher and lower than TSST, respectively. Across the dataset, mean TC intensity change is proportional to the difference between SST and TSST. This study also formulates an equation to quantify TC intensity change using SST and current TC intensity, which replicates 99.46% of the mean observed TC intensity changes. This equation could serve as an alternative to the linear regression‐based relationship between SST and TC intensity change that is widely used in statistical‐dynamical intensity models, thereby improving intensity forecasts.

This study investigates interagency discrepancies among best‐track estimates of tropical cyclone (TC) intensity in the western North Pacific, provided by the Joint Typhoon Warning Center (JTWC), the China Meteorological Administration (CMA), and the Regional Specialized Meteorological Center (RSMC) Tokyo during 2013 to 2019. The results reveal evident differences in maximum wind speed (MSW) estimates, where linear systematic differences are significant. However, the Dvorak parameter (CI) numbers derived from the MSWs reported by the three agencies are internally consistent. Further analysis suggests that the remained CI discrepancies are related to differences in the estimation of intensity trends, initial intensities, and TC positions among these datasets. In addition, the CI estimates provided by the JTWC for TCs over the open ocean are generally higher than those reported by the CMA and RSMC. However, the estimates from CMA and RSMC tend to give higher TC intensities for the TCs in the mainland and coastal areas of China and Japan, respectively, than those over the open ocean with the same intensity in JTWC dataset. This pattern potentially reflects the extensive use of surface observations by these two agencies for landfalling and offshore TCs. These results may help the research community to get more accurate details about the TCs in WNP from the best track datasets of different agencies.

The hot debate over the influence of global warming on tropical cyclone (TC) activity in the western North Pacific over the past several decades is partly due to the diversity of TC data sets used in recent publications. This study investigates differences of track, intensity, frequency, and the associated long‐term trends for those TCs that were simultaneously recorded by the best track data sets of the Joint Typhoon Warning Center (JTWC), the Regional Specialized Meteorological Center (RSMC) Tokyo, and the Shanghai Typhoon Institute (STI). Though the differences in TC tracks among these data sets are negligibly small, the JTWC data set tends to classify TCs of category 2–3 as category 4–5, leading to an upward trend in the annual frequency of category 4–5 TCs and the annual accumulated power dissipation index, as reported by Webster et al. (2005) and Emanuel (2005). This trend and potential destructiveness over the period 1977–2007 are found only with the JTWC data set, but downward trends are apparent in the RSMC and STI data sets. It is concluded that the different algorithms used in determining TC intensity may cause the trend discrepancies of TC activity in the western North Pacific.

Using primitive equations with adiabatic, non‐friction non‐environmental steering, the dynamical equilibrium features of tropical cyclones (TC) are investigated. The parameter, dI/dt, was obtained and its dynamical balance formulation describing TC intensity change. The results of the we discussed the characteristics of TC motion and intensity change in barotropic and baroclinic model showed that the stationary motion of TC possess “equivalent‐geostrophic equilibrium” and isolines of potential vorticity and E(E = φ + (u2 + v2)/2) coincide with streamlines. The TC intensity will be weakened by “ventilation flow” in “beta (β) gyres” and “shear gyres” from the positive (negative) wind vertical shear at high‐ (low‐) level. The positive (negative) vorticity vertical convection of high‐ (low‐) level strengthens TC intensity. Generally, the TC center has the tendency to drift poleward and there is a gradient (ascendent) direction of vorticity vertical convection at high‐ (low‐) level. The TC motion is connected with the asymmetry structure of vertical shear and vertical motion, the “γ effect” (γ = ∂β/∂y) that is one of the dynamical mechanisms of “ventilation flow” steering TC center motion.

Variability in tropical cyclone (TC) days in the western North Pacific (WNP) since the late 1970s is investigated based on two datasets. As an overall behavior, the intense TC days have increased for the last 30 years from both the Japan Meteorological Agency (JMA) dataset and the Joint Typhoon Warning Center (JTWC) dataset. Both datasets show that TC days with an intensity of Saffir-Simpson category 2 or higher have increased by 15-30% over the past 30 years. In terms of the detailed behavior of this increase in intense TC days, the contribution obtained from the JMA dataset is different from that of the JTWC. The JMA dataset reveals that the increase in moderately intense TC days contributes to the overall increase in intense TC days, while the JTWC dataset shows that the increase in extremely intense TC days has the dominant contribution. The difference between the two datasets becomes significant after 1987 when aircraft reconnaissance by the US Air Force was deactivated. The difference between the assessed contributions is due to different implementations of the Dvorak technique, the basis for TC intensity estimation at the JMA and the JTWC after the deactivation of aircraft reconnaissance.

The effects of two environmental dynamical factors, namely, the transitional speed and vertical wind shear, on tropical cyclone (TC) intensification, intensity, and lifetime peak intensity were analyzed based on observations in the western North Pacific during 1981–2003. In general, both the fast translation and strong vertical shear are negative to both TC intensification and the lifetime peak intensity. Both the very intense TCs and the TCs with rapid intensification rate are found only to occur in a narrow range of translational speeds between 3 and 8 m s−1, and in relatively weak vertical shear. The overwhelming majority of western North Pacific TCs reach their lifetime peak intensity just prior to recurvature where their environmental steering flow and vertical shear are both weak. The results show that few TCs intensified when they moved faster than 15 m s−1, or when their large-scale environmental vertical shear is larger than 20 m s−1. The intensification rate of TCs is found to increase with decreasing vertical shear while the majority of the weakening storms experience relatively strong vertical shear. Overall, strong vertical shear prohibits rapid intensification and most likely results in the weakening of TCs, similar to the fast storm translation. Based on the statistical analysis, a new empirical maximum potential intensity (MPI) has been developed, which includes the combined negative effect of translational speed and vertical shear as the environmental dynamical control in addition to the positive contribution of SST and the outflow temperature as the thermodynamic control. The new empirical MPI can not only provide more accurate estimation of TC maximum intensity but also better explain the observed behavior of the TC maximum intensity and help explain the thermodynamic and environmental dynamical controls of TC intensity. Implications of the new empirical MPI are discussed.

An updated climatology of Atlantic basin tropical cyclone (TC) intensity change in the presence of upper tropospheric trough forcing is presented. To control for changes in the background thermodynamic environment, a methodology that normalizes intensity change by the potential intensity of the TC is used to more narrowly focus on the effect of troughs compared to previous studies. Relative to the full sample of Atlantic TCs, troughs are a negative influence on intensification: trough interaction cases are 4% less likely to intensify and 5% more likely to weaken. Troughs are especially detrimental compared to TCs without trough forcing: trough interaction cases are 14% less likely to intensify and 13% more likely to weaken. Additionally, eddy flux convergence of angular momentum, previously shown to positively affect TC intensity change, is shown to be a weak predictor of intensity change compared to vertical wind shear, which is enhanced during a trough interaction.

Hurricane Gustav was one of two major hurricanes that occurred in the Atlantic basin during the 2008 hurricane season. The storm caused more than $6 billion U. S. dollars of damage in Haiti, Jamaica, Cuba, and along the coast of the Gulf of Mexico from 25 August 2008 to 5 September 2008. The greatest amount of damage and highest number of casualties produced by the storm can be attributed to the rapid intensification of the hurricane. Rapid intensification (RI) of a tropical cyclone is defined as an increase in maximum sustained winds by 15.4 m s⁻¹ (30 kts) in a continuous 24 h period. Between 1200 UTC 29 August and 2200 UTC 30 August, the minimum pressure of Gustav decreased by 48 hPa and the maximum sustained winds increased by 80 kts from a tropical storm to a category four hurricane on the Saffir-Simpson scale. This increase in intensity exceeds the definition of RI. Traditionally, the parameters of ocean heat content, vertical wind shear, and atmospheric moisture content have been evaluated to assess changes in tropical cyclone intensity. Kaplan and DeMaria (2003) have shown that sea surface temperatures (SSTs) in excess of 28.4°C and low vertical wind shear values (approximately 5 m s⁻¹) are conducive for RI. A moist atmosphere (i.e., high values of relative humidity) also signals an atmospheric environment conducive to tropical cyclone intensification. The SST field from the National Oceanic and Atmospheric Administration Optimum Interpolation 1/4° Daily SST Analysis (NOAA-OISST) dataset was used to assess the ocean heat content in the vicinity of the storm during its lifespan. Data from the initialization of the Global Forecasting System (GFS) was used to evaluate 200-850 hPa vertical wind shear, 850-700 hPa layer averaged relative humidity, and 200 hPa divergence. Lightning flash data from the World Wide Lightning Location Network (WWLLN) is used to investigate the role of convection in the RI of the storm. The SST and atmospheric relative humidity fields in the vicinity of Hurricane Gustav were conducive to the storm's intensification, but values of vertical wind shear in the vicinity of the storm were greater than 10 m s⁻¹, a magnitude typical of tropical cyclone weakening. A convective band formed 12 h prior to the onset of the storm's RI, which led to the broadening of a 200-hPa divergent area in the vicinity of the storm. It is hypothesized that the broadening of this area allowed for

El Nino has distinctive oceanic and atmospheric signatures that have different influences on tropical cyclone (TC) activity over the western North Pacific (WNP). This study compares TC activity over the WNP basin among strong Central-Pacific (CP), mixed CP, and strong Eastern-Pacific (EP) El Nino. Results suggest that TC activity with strong intensity and long lifespan occurred more frequently during strong (i.e. strong EP and strong CP) El Nino years than during mixed-CP El Nino years. This is attributed primarily to a combined modulation of the amplitude and duration of warm sea-surface temperature anomalies (SSTAs) over the tropical Pacific, and variations in large-scale environmental conditions; i.e. eddy kinetic energy, omega, relative humidity, and vertical wind shear (VWS). During CP (i.e. strong CP and mixed CP) El Nino events, enhanced TC genesis is observed over a large part of the WNP. During strong-CP El Nino events, more TCs are generated east of 140°E in the southwestern WNP, whereas more TCs are generated west of 140°E during mixed-CP El Nino events. This is due to a reduced magnitude of VWS and a westward shift in warm SSTAs over the central equatorial Pacific. When an anomalous anticyclonic circulation over the Indo-China Peninsula is combined with a westward extension of the western North Pacific subtropical high, TCs heading northward are associated with strong mean winds during strong El Nino events, resulting primarily from enhanced eddy kinetic energy, omega, and relative humidity, and weak VWS. Overall, TC activity during the three types of El Nino shows distinct generation locations, evolution patterns, and intensities, and the evolving spatial patterns of SSTA play an important role in modulating TC activity.

Climatological characteristics of simulated intense tropical cyclones (TCs) in the western North Pacific were explored with a 20-km-mesh atmospheric general circulation model (AGCM20) and a 5-km-mesh regional atmospheric nonhydrostatic model (ANHM5). From the AGCM20 climate runs, 34 intense TCs with a minimum central pressure (MCP) less than or equal to 900 hPa were sampled. Downscaling experiments were conducted with the ANHM5 for each intense TC simulated by the AGCM20. Only 23 developed into TCs with MCP ≤ 900 hPa. Most of the best-track TCs with an MCP ≤ 900 hPa underwent rapid intensification (RI) and attained maximum intensities south of 25°N. The AGCM20 simulated a similar number of intense TCs as the best-track datasets. However, the intense AGCM20 TCs tended to intensify longer and more gradually; only half of them underwent RI. The prolonged gradual intensification resulted in significant northward shifts of the location of maximum intensity compared with the location derived from two best-track datasets. The inner-core structure of AGCM20 TCs exhibited weak and shallow eyewall updrafts with maxima below an altitude of 6 km, while downscaling experiments revealed that most of the intense ANHM5 TCs underwent RI with deep and intense eyewall updrafts and attained their maximum intensity at lower latitudes. The altitudes of updraft maxima simulated by the AGCM20 descended rapidly during the phase of greatest intensification as midlevel warming markedly developed. The change in major processes responsible for precipitation in AGCM20 TCs before and after maximum intensification suggests close relationships between the large-scale cloud scheme and midlevel warming and prolonged gradual intensification.