Tropical Cyclone Maximum Intensity Research Articles

Footprint of the air-sea momentum transfer saturation observed by ocean wave buoy network in extreme tropical cyclones

Tropical cyclones are one of the most destructive natural phenomena, causing tremendous coastal disasters worldwide. The maximum intensity of tropical cyclones is determined by momentum and heat transfer at the air-sea interface. Momentum transfer corresponds to the momentum loss of tropical cyclones and, consequently, to the underlying ocean's momentum gain causing extreme ocean waves and storm surges. Air-side observations of wind profiles and ocean-side observations of ocean subsurface currents show a slowdown of momentum transfer under high tropical cyclone wind speeds in the previous studies. However, there is still disagreement regarding the slowdown owing to lack of data. Here, we show momentum transfer under high wind speed conditions by observing ocean waves. Although ocean wave observations are highly spatially limited, we deployed a fleet of drifting ocean wave buoys covering the active area of tropical cyclones in the Western North Pacific. The buoy fleet captured extreme waves near the eye of the strongest category 5 tropical cyclone, indicating an ocean wave footprint of the momentum transfer saturation above surface wind speeds of 25 m/s. Our approach from ocean surface wave observation is a unique contribution to determination on air-sea momentum transfer slowdown under extreme wind speeds, which can compensate for the knowledge from conventional air and ocean-side observations. This finding advances tropical cyclones, extreme ocean waves, and storm surge modeling.

Decadal Prediction of Location of Tropical Cyclone Maximum Intensity Over the Western North Pacific

AbstractAttaining skillful decadal predictions for the western North Pacific tropical cyclone (TC) emerges as a formidable challenge, mainly stemming from the limited prediction skills of Pacific Decadal Variability (PDV) within the state‐of‐the‐art models. Assessing sixth Coupled Model Intercomparison Project models' retrospective predictions finds that the predictability of PDV transcends the expectations set by raw forecasts featured by constrained temporal skills and low signal‐to‐noise ratio. Employing a refined approach, we selectively identify the models that capture the diverse phases of PDV and subsequently adjust their variances. This tailored approach yields a compelling concordance between the predicted PDV and observation in phases and variances. Anchored in the heightened prediction skill of PDV, we establish a sophisticated statistical model adept at predicting the latitude of TC's lifetime maximum intensity (LMI). The near‐term prediction indicates a sustained poleward migration of LMI latitude by 1.53° during 2020–2027, increasing subtropical East Asia's TC‐related disaster vulnerability in the coming decade.

The Projected Poleward Shift of Tropical Cyclogenesis at a Global Scale Under Climate Change in MRI‐AGCM3.2H

AbstractFuture climate projections suggest a poleward shift of the maximum intensity of tropical cyclones (TCs) over the western North Pacific. However, the global nature of the latitudinal change in TC genesis under global warming remains poorly understood. We show, using large‐ensemble high‐resolution atmospheric model simulations (d4PDF) with four warming scenarios, that the poleward shift is a robust change over the globe, attributable to the weakening of the Hadley circulation. The weakened ascent driven by the upper‐tropospheric warming suppresses the TC genesis within 5°–20° latitudes, whereas the weakened descent enhances the TC genesis in the poleward latitudes. We further estimate the poleward shift of TC genesis to emerge at the 2 K global warming over the Arabian Sea, South Atlantic and Pacific Oceans and at the 4 K warming over the North Pacific. The present results underscore the potential for increasing social and economic risks associated with TCs at higher latitudes.

Characterization and process understanding of tropical cyclone-induced floods derived from observations in Shenzhen, China

Coastal cities like Shenzhen are confronting escalating flood risks under the combined impact of climate change and rapid urbanization, especially the tropical cyclones (TC)-induced flood. Incorporating the impact of climate change and urbanization on the flood, this study constructed a new TC-induced flood model on western Shenzhen embedded with a unique statistical approach. Based on extensive historical data and machine learning techniques, the temporal characteristics and changes of flooding were revealed. The results reveal an increase in the frequency of TC-induced floods between 1964–2022, especially after the 1990s, which is attributed to a decrease in the distance of the location of the maximum intensity of TCs (observed within an 800 km range of the study area) relative to the land, averaging a reduction of 11.4 km per decade. This shift towards land is likely due to changes in the locations of TC genesis. Furthermore, the ‘rainfall sea level’ threshold for western Shenzhen was accordingly derived from the results of modelling, which would enable decision-makers to quickly assess TC-induced flood risks. The study’s proposed methods offer alternative approaches for predicting TC-induced floods in regions where the gathering of hydro-meteorological data is challenging or where economic and technological resources are limited.

Analysis of Large-Scale Environmental Features during Maximum Intensity of Tropical Cyclones Using Reanalysis Data

The present study investigates the variation in large-scale environments during the maximum intensity of tropical cyclones (TCs) formed in the Bay of Bengal. TC tracks are classified into four groups based on their direction of movement using the k-means clustering technique. Results from the pressure level and azimuthal-averaged radial-height wind fields near the vortex centre show weak deep layer wind shear (WS) and abundant moisture in all clusters. However, large-scale environmental differences in the northwest quadrant are identified with a contrasting combination of WS and humid environment between clusters. The composites of OLR are also analyzed during maximum intensities of TCs. Results show that anomalous high OLR in the west–northwest direction from the vortex centre, along with the low OLR around the vortex centre, signify the formation of a strong OLR dipole during TC peak intensity. Furthermore, OLR dipole metrics, such as magnitude, orientation, and distance, are observed by having mean of 235 Wm−2, 147, and 1782 km along with standard deviation of 14 Wm−2, 34°, and 492 km, respectively. The identified large-scale environmental fields from this study could provide valuable insights for predicting the intensity and movement of TCs.

Environmental Conditions Determining the Timing of the Lifetime Maximum Intensity of Tropical Cyclones over the Western North Pacific and Their Frequency of Occurrence

Environmental conditions determining the timing of the lifetime maximum intensities of tropical cyclones (TCs) are investigated for the TCs over the western North Pacific during the period 2008-2017. The results show that the land controls the timings of the lifetime maximum intensities in 42% of the TCs over this basin, indicating that accurate track forecasts are beneficial for TC intensity forecasts. With respect to other TCs that are not affected by the land (i.e., Ocean-TCs), the timings of their lifetime maximum intensities are determined by multiple oceanic factors. In particular, interactions between TCs and cold-core eddies occur in a large proportion (nearly 60%) of Ocean-TCs at or shortly after the times of their lifetime maximum intensities, especially in strong TCs (categories 4 and 5), suggesting that a consideration of the above interactions is necessary for improving TC intensity forecasting skills. In addition, unfavorable oceanic heat content conditions become common as the latitude increases over 25°N, influencing half of the Ocean-TCs. Strong vertical wind shear contributes detrimentally to the atmospheric environment in 17% of the TCs over this basin, especially in moderate and weak TCs. In contrast, neither the maximum potential intensity nor the humidity in the middle level of the atmosphere plays dominant roles when TCs turn from their peak intensities to weakening.

Surface Gravity Wave Effect on Hurricane Energetics

Theoretical researches have established that the energy dynamics of a mature tropical cyclone may be idealized to be very similar to a theoretical Carnot heat engine. Assuming the dissipative heating of the atmospheric boundary layer and the net production of mechanical energy in the cyclone dominate the energy budget of the storm, the potential maximum wind speed of the cyclone can be approximated as a function of the air–sea temperature difference (Ts − T0) and specific enthalpy (k0*−k) difference: |Vmax|2≈CkCDTs−T0T0(k0*−k). Although this theory gives a straighforward estimate of maximum tropical cyclone intensity, studies found that few real storms achieve this theoretical maximum estimated using climatological atmospheric conditions and sea surface temperatures. The discrepancies were attributed to a lack of knowledge of the values of the drag coefficient (CD) and surface exchange coefficient for enthalpy (Ck), and on insufficient upper ocean thermal measurements under hurricanes. Recent observational and numerical studies have unearthed another possible factor for these discrepancies by showing that the energy flux into surface gravity waves under extreme wind conditions can be an order of magnitude greater than formerly believed, and thus may play an important role in the energy budget of tropical cyclones. In this study, numerical experiments are performed to investigate the effect of surface gravity waves under a range of idealized tropical cyclone winds. The wave fields are simulated using the WAVEWATCH III model. Our results demonstrate that by considering the energy flux to surface gravity waves, the potential maximum wind speed can be reduced by up to 12% and this ratio varies with the storm size, intensity, and translation speed.

Relationship between Early-Stage Features and Lifetime Maximum Intensity of Tropical Cyclones over the Western North Pacific

The relationship between early-stage features and lifetime maximum intensity (LMI) of tropical cyclones (TCs) over the Western North Pacific (WNP) was investigated by ensemble machine learning methods and composite analysis in this study. By selecting key features of TCs’ vortex attributes and environmental conditions, a two-step AdaBoost model demonstrated accuracy of about 75% in distinguishing weak and strong TCs at genesis and a coefficient of determination (R2) of 0.30 for LMI estimation from the early stage of strong TCs, suggesting an underlying relationship between LMI and early-stage features. The composite analysis reveals that TCs with higher LMI are characterized by lower latitude embedded in a continuous band of high low-troposphere vorticity, more compact circulation at both the upper and lower levels of the troposphere, stronger circulation at the mid-troposphere, a higher outflow layer with stronger convection, a more symmetrical structure of high-level moisture distribution, a slower translation speed, and a greater intensification rate around genesis. Specifically, TCs with greater “tightness” at genesis may have a better chance of strengthening to major TCs (LMI ≥ 96 kt), since it represents a combination of the inner and outer-core wind structure related to TCs’ rapid intensification and eyewall replacement cycle.

Decision-Tree-Based Classification of Lifetime Maximum Intensity of Tropical Cyclones in the Tropical Western North Pacific

The National Typhoon Center of the Korea Meteorological Administration developed a statistical–dynamical typhoon intensity prediction model for the western North Pacific, the CSTIPS-DAT, using a track-pattern clustering technique. The model led to significant improvements in the prediction of the intensity of tropical cyclones (TCs). However, relatively large errors have been found in a cluster located in the tropical western North Pacific (TWNP), mainly because of the large predictand variance. In this study, a decision-tree algorithm was employed to reduce the predictand variance for TCs in the TWNP. The tree predicts the likelihood of a TC reaching a maximum lifetime intensity greater than 70 knots at its genesis. The developed four rules suggest that the pre-existing ocean thermal structures along the track and the latitude of a TC’s position play significant roles in the determination of its intensity. The developed decision-tree classification exhibited 90.0% and 80.5% accuracy in the training and test periods, respectively. These results suggest that intensity prediction with the CSTIPS-DAT can be further improved by developing independent statistical models for TC groups classified by the present algorithm.

A Thermodynamic Perspective on Steady-State Tropical Cyclones

AbstractTheories for the maximum intensity of tropical cyclones (TCs) assume steady state. However, many TCs in simulations that run for tens of days tend to decay considerably from an early steady state in the core (CS), before stabilizing at a final equilibrium steady state (ES). This decay raises the question of whether CS or ES should be used as a comparison to the maximum intensity theories. To understand the differences between CS and ES, we investigate why TCs decay and attempt to simulate a TC with steady intensity over a 100-day period. Using the axisymmetric Cloud Model 1, we find that the CS TC decay is due to a large-scale drying of the subsidence region. Such a drying is very pronounced in axisymmetric models because shallow-to-midlevel convection is not represented accurately enough to moisten air in the subsidence region. Simulations with an added moisture relaxation term in the subsidence region and dry cyclones without any moisture both remain in a steady state for over 100 days, without decaying appreciably after the spinup period. These simulations indicate that the decay in TC simulations is due to the irreversible removal of precipitation combined with the lack of a moistening mechanism in the subsidence region. Once either of these conditions is removed, the decay disappears and the CS and ES intensities become essentially equivalent.

Recent migration of tropical cyclones toward coasts.

Poleward migrations of tropical cyclones have been observed globally, but their impact on coastal areas remains unclear. We investigated the change in global tropical cyclone activity in coastal regions over the period 1982-2018. We found that the distance of tropical cyclone maximum intensity to land has decreased by about 30 kilometers per decade, and that the annual frequency of global tropical cyclones increases with proximity to land by about two additional cyclones per decade. Trend analysis reveals a robust migration of tropical cyclone activity toward coasts, concurrent with poleward migration of cyclone locations as well as a statistically significant westward shift. This zonal shift of tropical cyclone tracks may be mainly driven by global zonal changes in environmental steering flow.

Consistent Impacts of Surface Enthalpy and Drag Coefficient Uncertainty between an Analytical Model and Simulated Tropical Cyclone Maximum Intensity and Storm Structure

Abstract Several previous studies have demonstrated the significant sensitivity of simulated tropical cyclone structure and intensity to variations in surface-exchange coefficients for enthalpy (Ck) and momentum (Cd), respectively. In this study we investigate the consistency of the estimated peak intensity, intensification rate, and steady-state structure between an analytical model and idealized axisymmetric numerical simulations for both constant Ck and Cd values and various wind speed–dependent representations of Ck and Cd. The present analysis with constant Ck and Cd values demonstrates that the maximum wind speed is similar for identical Ck/Cd values less than 1, regardless of whether changes were made to Ck or Cd. However, for a given Ck/Cd greater than 1, the simulated and theoretical maximum wind speed are both greater if Cd is decreased compared to Ck increased. This behavior results because of a smaller enthalpy disequilibrium at the radius of maximum winds for larger Ck. Additionally, the intensification rate is shown to increase with Ck and Cd and the steady-state normalized wind speed beyond the radius of maximum winds is shown to increase with increasing Cd. Experiments with wind speed–dependent Ck and Cd were found to be generally consistent, in terms of the intensification rate and the simulated and analytical-model-estimated maximum wind speed, with the experiments with constant Ck and Cd.

Tropical Cyclones and Climate Change Assessment: Part II: Projected Response to Anthropogenic Warming

AbstractModel projections of tropical cyclone (TC) activity response to anthropogenic warming in climate models are assessed. Observations, theory, and models, with increasing robustness, indicate rising global TC risk for some metrics that are projected to impact multiple regions. A 2°C anthropogenic global warming is projected to impact TC activity as follows. 1) The most confident TC-related projection is that sea level rise accompanying the warming will lead to higher storm inundation levels, assuming all other factors are unchanged. 2) For TC precipitation rates, there is at least medium-to-high confidence in an increase globally, with a median projected increase of 14%, or close to the rate of tropical water vapor increase with warming, at constant relative humidity. 3) For TC intensity, 10 of 11 authors had at least medium-to-high confidence that the global average will increase. The median projected increase in lifetime maximum surface wind speeds is about 5% (range: 1%–10%) in available higher-resolution studies. 4) For the global proportion (as opposed to frequency) of TCs that reach very intense (category 4–5) levels, there is at least medium-to-high confidence in an increase, with a median projected change of +13%. Author opinion was more mixed and confidence levels lower for the following projections: 5) a further poleward expansion of the latitude of maximum TC intensity in the western North Pacific; 6) a decrease of global TC frequency, as projected in most studies; 7) an increase in global very intense TC frequency (category 4–5), seen most prominently in higher-resolution models; and 8) a slowdown in TC translation speed.

A Look at the Relationship between the Large-Scale Tropospheric Static Stability and the Tropical Cyclone Maximum Intensity

AbstractVarious modeling and observational studies have suggested that tropical cyclone (TC) intensity tends to increase in the future due to projected warmer sea surface temperature (SST). This study examines the effects of the tropospheric stratification that could potentially offset the direct increase of TC intensity associated with the warmer SST. Using reanalysis datasets and TC records in the northwestern Pacific and the North Atlantic basins, it is shown that there exists a consistently negative correlation between the annually averaged TC intensity and the basinwide average of the tropospheric static stability. This negative correlation is more robust in the northwestern Pacific basin when using the TC lifetime maximum intensity but is somewhat less significant in the North Atlantic basin. Further separation of the troposphere into a lower (1000–500 hPa) and an upper layer (500–200 hPa) reveals that it is the upper-tropospheric static stability that plays a more dominant role in governing the TC intensity variability. The negating effects of a stable troposphere on TC intensity as found in this study suggest a partial offset of the projected increase in the TC potential intensity due to the future warmer SST. Thus, the tropospheric static stability is one of the key large-scale factors that need to be properly taken into account in studies of long-term TC intensity change.

Relationship between sea surface temperature and the maximum intensity of tropical cyclones affecting Vietnam's coastline

AbstractEstimating an upper bound for tropical cyclone (TC) intensity in a given environment is important for TC intensity forecasts and risk management. While theoretical limits for the TC maximum potential intensity have been well examined in previous studies, the actual maximum intensity ( Vmax) that a TC attains in real atmospheric conditions varies from basin to basin and is subject to much more uncertainties. This study examines different empirical relationships between sea surface temperature (SST) and Vmax in the South China Sea that affect Vietnam's coastal region. Using different SST datasets and the TC intensity record from 1982 to 2016, it is shown that Vmax in this area increases slower than the typical linear or exponential function form found in previous studies. Such different dependence of Vmax on SST in the South China Sea is due to the narrow range of SST variation in this region, which is between 24 and 30°C as compared to the range of 15 and 30°C used in previous studies. An alternative functional form based on the natural logarithm of SST is then proposed, which captures better the characteristics of the actual TC maximum intensity in Vietnam's coastal region. Examination of the variability of Vmax also indicates an uptrend of the TC maximum intensity over last 30 years in the South China Sea, which has a potentially large effect on Vietnam's coastal region. This increase of Vmax is consistent with the warmer trend of the SST as projected in the Fifth Assessment by the Intergovernmental Panel on Climate Change (IPCC).

Seasonal Cycles of Along-Track Tropical Cyclone Maximum Intensity

Abstract This study investigates relationships between observed tropical cyclone (TC) maximum intensities and potential intensity (PI) over the seasonal cycle. To directly compare observed and potential intensities, one must account for month-to-month variability in TC tracks and frequencies. Historical TC best track data and reanalysis PI calculations are combined to develop an along-track record of observed maximum and potential intensities for each storm in the satellite-era (1980–2015) across four ocean basins. Overall, observed maximum intensity seasonal cycles agree well with those of along-track PI. An extreme value theory application shows that at least 25 storms must be observed in a given month to have high confidence that the most intense wind speeds of historical TCs follow along-track PI seasonality. In the North Atlantic and Southern Hemisphere regions, there are too few observed storms outside their traditional TC seasons, limiting PI applicability across the seasonal cycle. Small intraseasonal along-track PI variabilities in these regions are driven by TC thermodynamic disequilibrium and sea surface temperatures. Thermodynamic disequilibrium drives seasonal cycles of eastern North Pacific along-track PI and observed maximum intensity, which minimize in August and maximize in June and October. Western North Pacific along-track PI and observed maximum intensity seasonal cycles are relatively flat, and have a local minimum in August because of reduced thermodynamic efficiency, which is linked to anomalously warm near-tropopause outflow temperatures. Powerful (>65 m s−1) western Pacific TCs historically occur in every month except January, due to a combination of tropopause region and SST seasonal influences.

Recent Research Progress on Tropical Cyclone Structure and Intensity

ABSTRACT This article provides a balanced, brief review on the research progress in the area of tropical cyclone (TC) structure and intensity achieved in the past three decade. Efforts have been made to introduce basic concepts and new findings relevant to the understanding of TC structure and intensity in ways as simple and appreciate as possible. After a brief discussion on the axisymmetric and asymmetric structure of mature TCs, progress in our understanding of spiral rainbands, concentric eyewall cycle, annular hurricane structure, and the inner-core size of TCs is highlighted. This is followed by discussions on the maximum potential intensity (MPI) of TCs and factors that limit TC maximum intensity. Some important remaining issues that need to be studied and addressed in the near future by the research community are identified and briefly discussed as well.

Comment on “Spatial and Temporal Trends in the Location of the Lifetime Maximum Intensity of Tropical Cyclones” by Tennille and Ellis

The latitude where tropical cyclones (TCs) reach their peak intensity has migrated poleward in some regions [...]

Reply to “Comments on ‘Spatial and Temporal Trends in the Location of the Lifetime Maximum Intensity of Tropical Cyclones’”

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Simulation of Wave‐Current Interactions Under Hurricane Conditions Using an Unstructured‐Grid Model: Impacts on Ocean Waves

AbstractThe effect of wave‐current interactions on ocean waves under hurricane conditions is investigated through application of the unstructured‐grid finite‐volume community ocean model (FVCOM) coupled to the unstructured‐grid surface wave model (SWAVE) in the North Atlantic Ocean. We study wave‐current interactions during the life cycles of extratropical hurricanes Juan (2003) and Bill (2009) as they propagate from subtropical waters to midlatitudes in the Northwest Atlantic. Simulations of wave parameters in each hurricane are shown to compare well with buoy and satellite altimeter observations, in terms of winds, significant wave heights, wave energy spectra, and wave directions. This is partially achieved by restricting the drag coefficient. It is well known that the maximum intensity of tropical cyclones depends on the ratio of the enthalpy coefficient to the drag coefficient. The latter increases with wind speed, levels off with category one hurricanes and may drop for even higher winds. In our study, we find that setting a limiting value on the drag coefficient improves the simulations of waves at peak storm intensities. Simulation of wave‐current interactions is also shown to improve the simulation of the wave heights and wave energy spectra. This is notable at the peak of the storms, in comparisons with observations from buoys in areas of both deep water and relatively shallow water. The effect of currents on significant wave heights is shown to reach 0.4 m for hurricane Juan and 1.0 m for hurricane Bill, or as much as about ±10% for wave heights distributed along the storm track.