Tropical Cyclone Size Research Articles

Revisiting the Relationship Between Tropical Cyclone Size and Intensity Over the Western North Pacific

AbstractThis study revisits the nonlinear relationship between tropical cyclone (TC) size and intensity over the western North Pacific (WNP). Using best track data from the Japan Meteorological Agency from 2007–2019, we find that the statistical turning point on a fitted size‐intensity curve only exists for TCs with greater lifetime peak intensities (e.g., typhoons). The track and evolution type are two factors determining the statistical size‐intensity relationship. The nonlinear size‐intensity relationship is only observed for recurving typhoons reaching their peak intensities and sizes at different times, due to a nonmonotonic size change during the weakening stage. The slight size decrease in the initial weakening stage can be linked to a TC weakening‐induced shrinkage balanced by a baroclinic expansion when recurving typhoons move into higher latitudes. Finally, different evolution types for WNP typhoons can be determined by the ratio of intensity/size to its theoretical maximum.

Size and Structure of Dry and Moist Reversible Tropical Cyclones

AbstractThe size and structure of tropical cyclones (TCs) are investigated using idealized numerical simulations. Three simulations are conducted: a pure dry TC (DRY), a moist reversible TC (REV) with fallout of hydrometeors in the atmosphere disallowed, and a typical TC (CTL). It was found that the width of the eyewall ascent region and the radius of maximum windrmare much larger in DRY and REV than those in CTL. This is closely related to the deep inflow layer (~4 km) in DRY and REV associated with a different entropy restoration mechanism under the subsidence region. With the wide ascents, the close link betweenrmand the outer radius in DRY and REV can be well predicted by the Emanuel and Rotunno (ER11) model. The magnitude of subsidence, mainly controlled by the vertical gradient of entropy in the mid- and upper troposphere, is nearly one order greater in DRY and REV than that in CTL. This study demonstrates that the falling nature of hydrometeors poses a strong constraint on the size and structure of real world TCs via the entropy distribution in the subsidence region. The wide ascent, self-stratification in the outflow, and decently reproduced wind profile in DRY and REV suggest that DRY and REV behave like a prototype of the ER11 model with CTL being an extreme type.

Ensemble Based Diagnosis of the Track Errors of Super Typhoon Mangkhut (2018)

Super Typhoon Mangkhut (2018) was the most high-impact typhoon in 2018 because of its long lifespan and significant intensity. The operational track forecasts in the short-to-medium range (deterministic and probabilistic forecast) showed a great uncertainty and the forecast landing points varied with different lead times. This study applied ensembles of high-resolution ECMWF forecasts to investigate the major factors and mechanisms of the bias production of the Mangkhut forecast track. The ensembles with the largest track bias were analyzed to examine the possible bias associated factors. The results suggested that environmental steering flows were the main cause for the erroneous southward track error with a variance contribution of 72%. The tropical cyclone (TC) size difference and the interaction of the TC with the subtropical high (SH) were other two key factors that contributed to the track error. Particularly, larger TCs may have led to a stronger erosion of the southern part of the SH, and thus induced significant changes in the large-scale environment and eventually resulted in an additional northward movement of TC.

Sea surface current response patterns to tropical cyclones

Abstract Tropical cyclones (TCs) are strong synoptic systems which induce strong sea surface currents. This paper first cross-checks a detailed surface current response to specific TCs Rammasun (2014), Kalmaegi (2014) and Sarika (2016) based on buoy/mooring observations as well as a three-dimensional numerical model (three-dimensional version of the Price-Weller-Pinkel model, 3DPWP) and a one-dimensional semi-analytical model which simplifies the driving forcing into wind stress and Coriolis force, then estimates the impact of all possible tropical cyclones using the semi-analytical model. The results show that the sea surface current response to the kinetic energy input of TCs, which is dependent on the TC configurations (translation speed, size and intensity) and the environmental configurations (Coriolis frequency and upper ocean stratification), can be represented by two simple parameters, namely the TC nondimensional translation speed (S) and the TC wind force parameter ( V ~ c ) or TC wind energy parameter ( E ~ ). S represents a combined effect of TC translation speed, size and Coriolis frequency, determines the structure of surface current response. V ~ c or E ~ represents a combined effect of TC intensity, mixed layer depth and Coriolis frequency, determines the intensity of surface current response or wind energy input into surface currents. Ekman-like divergence dominates the sea surface current response when S is small (0 5). The response pattern with S > 5 was rarely studied before. The three values range of S take up ~30.36%, ~69.36% and ~0.28% of all recorded TCs during 2001–2017, which explains why the second response pattern with 0.4 5 has been rarely studied before. Besides, the surface rightward bias is greatest at S = 2.5 (S = 1.45) for current speed (kinetic energy input rate). V ~ c ( E ~ ) determine the amplitude of current speed (wind energy input into currents). This work provides a simple and easy-to-use method to estimate the surface current response pattern to TCs when TCs and associated environmental configurations are given, which may help to improve the parameterization of TCs in regional and climate modeling. It also suggests that S is a better index than TC translation speed to classify TCs when studying the oceanic response.

Tropical Cyclone Size Change under Ocean Warming and Associated Responses of Tropical Cyclone Destructiveness: Idealized Experiments

The power dissipation index (PDI), which is defined as the sum of the cube of tropical cyclone (TC) maximum wind speed during TC lifetime, is widely used to estimate the TC destructive potential. However, due to the lack of high-resolution observations, little attention has been paid to the contribution of TC size change to TC destructive potential in response to ocean warming. In this study, sensitivity experiments are performed by using the high-resolution Weather Research and Forecasting (WRF) model to investigate the responses of TC size and TC destructive potential to prescribed sea surface temperature (SST) increase under the present climate condition. The results show that TC size increases with the ocean warming. Possible reasons for TC size change are investigated with a focus on the outer air–sea moisture difference (ASMD). As SST increases, ASMD in the outer zone of the TC is larger than that in the inner zone, which increases the surface entropy flux (SEF) of the outer zone. This change in the radial distribution of SEF causes the increase of tangential wind in the outer zone, which further increases SEF, resulting in a positive feedback between outer-zone SEF and outer-zone tangential wind. This feedback leads to the increase of the ra-dius of gale-force wind, leading to the expansion of TC size. Moreover, to estimate the contribution of TC size change to TC destructiveness, we calculate TC size-dependent destructive potential (PDS) as the storm size information is available in the model outputs, as well as PDI that does not consider the effect of TC size change. We find that PDS increases exponentially as SST increases from 1 to 4°C, while PDI increases linearly; hence the former is soon much greater than the latter. This suggests that the growth effect of TC size cannot be ignored in estimating destruc-tiveness under ocean warming.

Autumn Tropical Cyclones over the Western North Pacific during 1949-2016: A Statistical Study

We used tropical cyclone (TC) best track data for 1949–2016, provided by the Shanghai Typhoon Institute, China Meteorological Administration (CMA-STI), and a TC size dataset (1980-2016) derived from geostationary satellite infrared images to analyze the statistical characteristics of autumn TCs over the western North Pacific (WNP). We investigated TC genesis frequency, location, track density, intensity, outer size, and landfalling features, as well as their temporal and spatial evolution characteristics. On average, the number of autumn TCs accounted for 42.1% of the annual total, slightly less than that of summer TCs (42.7%). However, TCs classified as strong typhoons or super typhoons were more frequent in autumn than in summer. In most years of the 68-yr study period, there was an inverse relationship between the number of autumn TCs and that of summer TCs. The genesis of autumn TCs was concentrated at three centers over the WNP: the first is located near (14°N, 115°E) over the northeastern South China Sea and the other two are located in the vast oceanic area east of the Philippines around (14°N, 135°E) and (14°N, 145°E), respectively. In terms of intensity, the eight strongest TCs during the study period all occurred in autumn. It is revealed that autumn TCs were featured with strong typhoons and super typhoons, with the latter accounting for 28.1% of the total number of autumn TCs. Statistically, the average 34-knot radius (R34) of autumn TCs increased with TC intensity. From 1949 to 2016, 164 autumn TCs made landfall in China, with an average annual number of 2.4. Autumn TCs were most likely to make landfall in Guangdong Province, followed by Hainan Province and Taiwan Island.

Monthly variation and spatial distribution of quadrant tropical cyclone size in the Western North Pacific

AbstractWith the tropical cyclone (TC) size parameter defined as the radius of 17‐m·s−1 oceanic surface wind, 225 TC cases were recorded in the western North Pacific during 2000–2009 based on the QuikSCAT near‐surface wind vector database and the best‐track dataset. In accordance with the symmetry index (the ratio of minimum and maximum quadrant sizes), the TCs were classified into symmetric and asymmetric structures. The asymmetric TCs were divided into four types: the northeast, southeast, southwest, and northwest. The spatio‐temporal characteristics of these TC types were further investigated. The monthly variation and the spatial distribution of maximum quadrant TC size exhibited significant differences among the four types. By contrast, the quadrant size of the symmetric TCs from June to November showed few changes (2.6°–2.7° latitude). It was found that the TC lifetime is an important factor affecting the quadrant TC size because of its close relationship with the activities of the western Pacific subtropical high. In addition, the climatological mean circulation also has a notable influence on the quadrant TC size through superposition of prominent background wind. Symmetric TCs are more likely to occur in the oceanic region where the background low‐level wind is the weakest.

Effects of the outer size on tropical cyclone track forecasts

AbstractThe track forecast of a tropical cyclone (TC) located near the neutral point of a split subtropical high pressure system under the influence of a nearby trough is difficult and can lead to a large forecast spread among different global numerical weather prediction models. Three TCs with large forecast spread under such an environmental flow pattern have been identified: Hagupit (2008), Lupit (2009) and Nida (2009). Simulations using a bogus vortex of different radii of 15 m·s−1 (R15) with the same environmental flow for each TC case are conducted to study the effect of TC size on the forecast track of the TC. With a nearby strong monsoon trough, a TC with a smaller R15 merges with the cyclonic circulation within the trough leading to a larger outer circulation. After removing the symmetric component of the TC from the total wind field, an extensive cyclonic asymmetric gyre is found and its circulation steers the TC to move more westward. The TC thus shifts northward within the neutral point. With a weak monsoon trough further away from the TC, a TC with larger R15 with a larger outer circulation interacts with the monsoon trough, causing the TC to move more westward. With the presence of a westerly trough, a TC with larger R15 and outer circulation interacts with the trough such that the TC moves more northeastward. The importance of a correct representation of TC size in a numerical weather prediction model in predicting the TC track is discussed.

Influence of Tropical Cyclone Intensity and Size on Storm Surge in the Northern East China Sea

Typhoon storm surge research has always been very important and worthy of attention. Less is studied about the impact of tropical cyclone size (TC size) on storm surge, especially in semi-enclosed areas such as the northern East China Sea (NECS). Observational data for Typhoon Winnie (TY9711) and Typhoon Damrey (TY1210) from satellite and tide stations, as well as simulation results from a finite-volume coastal ocean model (FVCOM), were developed to study the effect of TC size on storm surge. Using the maximum wind speed (MXW) to represent the intensity of the tropical cyclone and seven-level wind circle range (R7) to represent the size of the tropical cyclone, an ideal simulation test was conducted. The results indicate that the highest storm surge occurs when the MXW is 40–45 m/s, that storm surge does not undergo significant change with the RWM except for the area near the center of typhoon and that the peak surge values are approximately a linear function of R7. Therefore, the TC size should be considered when estimating storm surge, particularly when predicting marine-economic effects and assessing the risk.

Reexamination of the Tropical Cyclone Wind–Pressure Relationship Based on Pre-1987 Aircraft Data in the Western North Pacific

Abstract The wind–pressure relationship (WPR) for tropical cyclones (TCs) in the western North Pacific is reexamined based on aircraft data, TC best track data, and daily reanalysis data during 1957–87. Minimum sea level pressure (MSLP) was estimated from aircraft reconnaissance, and maximum surface wind speeds (MSWs) were adjusted from the maximum wind speed at flight level. The mean MSLP was found to be higher during 1957–64 than during 1965–87, presumably due to the change in reconnaissance instrumentation and technology, which results in a systematic MSW bias (too high) before 1965 in the China Meteorological Administration (CMA) dataset. Further analyses found that the WPR used in the CMA dataset is more accurate for strong TCs, while the WPR in the Tokyo Regional Specialized Meteorological Center (RSMC) dataset is better for weak TCs after the MSW-RSMC converted by the Dvorak conversion table (1984) and when using the aircraft datasets as a baseline. Several prevailing operational WPRs used in the western North Pacific are reexamined. Results show that the WPR of Knaff and Zehr explains 71% of the variance with a MAE of 9.22 hPa, which represents a significant improvement over other WPRs. Utilizing data after 1965 (a total of 1874 samples), the effects of TC center latitude, size, translation speed, intensification trend, and environmental pressure on the WPRs were examined. Results show that faster-traveling TCs, smaller in size, and located in a higher environmental pressure at lower latitudes, exhibited a higher MSLP for a given MSW. Meanwhile, the latitude, translational speed, and the environmental pressure produces additional improvement, but the TC size and intensity change added only a little skill to the WPR equation.

A Study of the Intensity of Tropical Cyclone Idai Using Dual-Polarization Sentinel-1 Data

Monitoring the intensity and size of a tropical cyclone (TC) is a challenging task, and is important for reducing losses of lives and property. In this study, we use Idai, one of the deadliest TCs on record in the Southern Hemisphere, as an example. Dual-polarization synthetic aperture radar (SAR) measurements from the Copernicus Sentinel-1 mission are used to examine the TC structure and intensity. The wind speed is estimated and compared using well known C-band model functions based on calibrated cross-polarization SAR images. Because of the relatively high noise floor of the Sentinel-1 data, wind speeds under 20 m/s from cross-polarization models are ignored and replaced by low to moderate wind speeds retrieved from co-polarization radar signals. Wind fields retrieved from the co- and cross-polarization model results are then merged together to estimate the TC size and the TC fullness scale, a concept related to the wind structure of a storm. Idai has a very strong wind speed and fullness structure, indicating that it was indeed a very intense storm. The approach demonstrates that open and freely available Sentinel-1 SAR data is a unique dataset to estimate the potential destructiveness of similar natural disasters like Idai.

Impacts of Assimilating High-Resolution Atmospheric Motion Vectors Derived from Himawari-8 on Tropical Cyclone Forecast in HWRF

AbstractThe impact of the assimilation of high spatial and temporal resolution atmospheric motion vectors (AMVs) on tropical cyclone (TC) forecasts has been investigated. The high-resolution AMVs are derived from the full disk scan of the new generation geostationary satellite Himawari-8. Forecast experiments for three TCs in 2016 in a western North Pacific basin are performed using the National Centers for Environmental Prediction (NCEP) operational Hurricane Weather Research and Forecasting Model (HWRF). Two different ensemble–variational hybrid data assimilation configurations (using background error covariance created by global ensemble forecast and HWRF ensemble forecast), based on the Gridpoint Statistical Interpolation (GSI), are used for the sensitivity experiments. The results show that the inclusion of high-resolution Himawari-8 AMVs (H8AMV) can benefit the track forecast skill, especially for long-range lead times. The diagnosis of optimal steering flow indicates that the improved track forecast seems to be attributed to the improvement of initial steering flow surrounding the TC. However, the assimilation of H8AMV increases the negative intensity bias and error, especially for short-range forecast lead times. The investigation of the structural change from the assimilation of H8AMV revealed that the following two factors are likely related to this degradation: 1) an increase of inertial stability outside the radius of maximum wind (RMW), which weakens the boundary layer inflow; and 2) a drying around and outside the RMW. Assimilating H8AMV using background error covariance created from HWRF ensemble forecast contributes to a significant reduction in negative intensity bias and error, and there is a significant benefit to TC size forecast.

Relationship Between the Change in Size of Tropical Cyclones and Spatial Patterns of Precipitation

AbstractThe relationship between the size change of tropical cyclones (TCs) and radial distribution of precipitation was investigated for TCs occuring over the western North Pacific Ocean with satellite observation data. The size of a TC is defined as the radial extent of 15 m s−1 winds (R15). The spatial distribution of precipitation was investigated by extracting two subperiods, which are the size‐increasing period and the size‐stationary period, as determined from R15 evolution. The results show a contrast that during the size‐increasing period, precipitation falls both inside and outside the R15, whereas during the size‐stationary period, it is concentrated near the center of the TC and falls less around and outside R15. This contrast is also confirmed statistically by composite analysis using 64 TC cases. Further analysis reveals that this contrast does not depend on the intensity or the change in intensity of the TC, which indicates that the area of precipitation related to the intensity or intensity change of TCs is different from that related to the size change of TCs. Finally, we examined the radial distribution of inertial stability in TCs and found that precipitation during the size‐stationary period occurs in a region with a higher inertial stability than that which occurs during the size‐increasing period.

An Idealized Numerical Study of Shear-Relative Low-Level Mean Flow on Tropical Cyclone Intensity and Size

Abstract Given comparable background vertical wind shear (VWS) magnitudes, the initially imposed shear-relative low-level mean flow (LMF) is hypothesized to modify the structure and convective features of a tropical cyclone (TC). This study uses idealized Weather Research and Forecasting Model simulations to examine TC structure and convection affected by various LMFs directed toward eight shear-relative orientations. The simulated TC affected by an initially imposed LMF directed toward downshear left yields an anomalously high intensification rate, while an upshear-right LMF yields a relatively high expansion rate. These two shear-relative LMF orientations affect the asymmetry of both surface fluxes and frictional inflow in the boundary layer and thus modify the TC convection. During the early development stage, the initially imposed downshear-left LMF promotes inner-core convection because of high boundary layer moisture fluxes into the inner core and is thus favorable for TC intensification because of large radial fluxes of azimuthal mean vorticity near the radius of maximum wind in the boundary layer. However, TCs affected by various LMFs may modify the near-TC VWS differently, making the intensity evolution afterward more complicated. The TC with a fast-established eyewall in response to the downshear-left LMF further reduces the near-TC VWS, maintaining a relatively high intensification rate. For the upshear-right LMF that leads to active and sustained rainbands in the downshear quadrants, TC size expansion is promoted by a positive radial flux of eddy vorticity near the radius of 34-kt wind (1 kt ≈ 0.51 m s−1) because the vorticity associated with the rainbands is in phase with the storm-motion-relative inflow.

Challenges and Future Directions in Ocean Wave Modeling — A Review

An increasing number of instances in extreme weather events over the global oceans have deepened the concerns on the impact of climate change. The frequency of extreme weather events is also seen to increase attributes to climate change across the globe. In the Indian context, there has been about 285 reported flooding events over the period from 1950 to 2017 that affected nearly 850 million people with many causalities. As the global oceans become stormier, the effects are seen in rising sea level and infrastructural facilities. Major flooding events are caused by tropical cyclone-induced storm surge and associated breaking waves. These extreme weather events coupled with sea level rise have serious repercussions on the coastal vulnerability. Also recently, there is an upsurge in the intensity and tropical cyclone size that forms over the North Indian Ocean region that brought attention among the scientific community. The worst possible scenario of extreme water level can occur when the time of storm surge occurrence coincides with the astronomical high water. This review paper provides a comprehensive overview on the research developments and efforts made in ocean wave modeling in particular for the Indian seas. As per the Fifth Assessment Report of Intergovernmental Panel on Climate Change (IPCC), the role and influence of ocean surface gravity wave in the climate system are considered to be very important. At present, numerical models are widely used and that can be used to hindcast and forecast the wave characteristics over both regional and global ocean basin scales. A detailed overview on the observational techniques is listed along with the historical perspective and recent developments in wind-wave modeling for the Indian seas. Recent developments in computing technology and advanced numerical techniques have made it possible to solve the complex problems in coastal science and engineering using state-of-the-art numerical models providing realistic estimates, cost effective and having immense potential in operational weather centers. This review also deals with some of the important issues and future directions in wind-wave modeling studies such as improvements required in momentum transfer, bottom dissipation, and rain–wave interaction effects that require detailed understanding and concentrated efforts.

Impact of Cumulus Parameterization on Model Convergence of Tropical Cyclone Destructive Potential Simulation at Grey-Zone Resolutions: A Numerical Investigation

The Weather Research Forecast model (WRF) is used to examine the destructive potential of tropical cyclone (TC) Shanshan (2006) at various horizontal resolutions (7.5 km–1 km) with different cumulus parameterization (CP) schemes. It is found that the calculated Power Dissipation Index (PDI) increases while the size-dependent destructive potential (PDS) decreases as the grid spacing decreases for all CP-scheme simulations, which indicates a weak model convergence in both PDI and PDS calculations. Moreover, it is change of the storm intensity and inner-core size that lead to the non-convergence of PDI and PDS respectively. At a higher resolution, convection becomes more explicitly resolved, which leads to larger diabatic heating. As a result, the radial pressure gradient force (PGF) increases, and the radius of maximum wind (RMW) decreases. The area of strong diabatic heating subsequently becomes closer to the TC center, which further increases the TC intensity and the PGF near the eyewall. With such a positive feedback loop, the PGF increases and the RMW decreases as the resolution increases. Note that a perfect model should converge well in the simulation of both TC intensity and size, and thus converge in the PDS. For some CP experiments, the calculated PDS convergence is relatively strong, but it is a result of offset between the non-convergent simulations of TC intensity and size. In contrast, the Grell–Freitas scheme exhibits a stronger convergence in the simulations of TC intensity and size, although the convergence in PDS is relatively weak, but is closer to the truth.

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.

Radar Observation of Precipitation Asymmetries in Tropical Cyclones Making Landfall on East China Coast

ABSTRACT This study explores, for the first time, the asymmetric distribution of precipitation in tropical cyclones (TCs) making landfall along east China coast using reflectivity data collected from coastal Doppler radars at mainland China and Taiwan. Six TCs (Saomai, Khanun, Wipha, Matsa, Rananim and Krosa) from 2004 to 2007 are examined. The temporal and spatial evolution of these TCs’ inner and outer core asymmetric precipitation patterns before and after landfall is investigated. The radius of inner-core region is a function of the size of a TC apart from a fixed radius (100 km) adopted in previous studies. All six TCs possessed distinct asymmetric precipitation patterns between the inner- and outer- core regions. The amplitude of asymmetry decreases with the increasing TC intensity and it displays an ascending (descending) trend in the inner (outer) core. In the inner-core region, the heavy rainfall with reflectivity factor above 40 dBZ tends to locate at the downshear side before landfall. Four cases have precipitation maxima on the downshear left side, in agreement with previous studies. As TCs approaching land (~ 2 hr before landfall), their precipitation maxima generally shift to the front quadrant of the motion partly due to the interaction of TC with the land surface. In the outer-core region, the precipitation maxima occur in the front quadrant of the motion in five of the six cases before landfall. After landfall, the precipitation maxima shift from the right-front quadrant clockwisely to the right-rear quadrant of the motion collocated well with the mountainous areas along the coast, which indicates the impact of topography forcing on the precipitation distribution. This study illustrated how the precipitation asymmetry in the inner- and outer-core at different stages of TC landfall is affected by storm motion, vertical wind shear and topography.

A Multi-Model Consensus Forecast Technique for Tropical Cyclone Intensity Based on Model Output Calibration

ABSTRACT We analyzed the errors associated with forecasts of tropical cyclone (TC) intensity from 2010-2012 in the western North Pacific region made by seven operational numerical weather prediction models. The results show that the forecast error is significantly related to the initial error as well as the initial TC intensity, size, and translation speed. Other factors highly related to the forecast error include the environmental sea surface pressure, vertical wind shear and maximum potential intensity. We used stepwise regression to set up model forecast error estimation equations, which were used to calibrate the model output. Independent experiments showed that the calibrated model forecasts have significant skill compared to the original model output. Finally, a multimodel consensus forecast technique for TC intensity was developed based on the calibrated model output; this technique has 28% (15-20%) skill at 12 h (24-72 h) compared to the climatology and persistence forecasts of TC intensity. This consensus technique has greater skill than the consensus forecast based on the original model output and therefore it has the potential to be applied in operation.

Lifetime Evolution of Outer Tropical Cyclone Size and Structure as Diagnosed from Reanalysis and Climate Model Data

The present study examines the lifetime evolution of outer tropical cyclone (TC) size and structure in the North Atlantic (NA) and western North Pacific (WNP). The metric for outer TC size is the radius at which the azimuthal-mean 10-m azimuthal wind equals 8 m s−1 ( r8) derived from the NCEP Climate Forecast System Reanalysis (CFSR) and GFDL High-Resolution Forecast-Oriented Low Ocean Resolution model (HiFLOR). Radial profiles of the azimuthal-mean 10-m azimuthal wind are also analyzed to demonstrate that the results are robust across a broad range of wind radii. The analysis shows that most TCs in both basins are characterized by 1) minimum lifetime r8 at genesis, 2) subsequent substantial increases in r8 as the TC wind field expands, 3) peak r8 values occurring near or after the midpoint of the TC lifetime, and 4) nontrivial decreases in r8 and outer winds during the latter part of the TC lifetime. Compared to the NA, WNP TCs are systematically larger up until the end of their lifetime, exhibit r8 growth and decay rates that are larger in magnitude, and are characterized by an earlier onset of lifetime maximum r8 near their lifetime midpoint. In both basins, the TCs exhibiting the largest r8 increases are the longest lived, especially those that traverse the longest distances (i.e., recurving TCs). Finally, analysis of TCs undergoing extratropical transition (ET) shows that NA TCs exhibit negligible changes in r8 during ET, while WNP ET cases either show r8 decreases (CFSR) or negligible changes in r8 (HiFLOR).