Tropical Cyclone Structure Research Articles

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.

Effect of Unidirectional Vertical Wind Shear on Tropical Cyclone Intensity Change—Lower‐Layer Shear Versus Upper‐Layer Shear

AbstractIn this study, a quadruply nested, nonhydrostatic tropical cyclone (TC) model is used to investigate how the structure and intensity of a mature TC respond differently to imposed lower‐layer and upper‐layer unidirectional environmental vertical wind shears (VWSs). Results show that TC intensity in both cases decrease shortly after the VWS is imposed but with quite different subsequent evolutions. The TC weakens much more rapidly for a relatively long period in the upper‐layer shear than in the lower‐layer shear, which is found to be related to the stronger storm‐relative asymmetric flow in the middle‐upper troposphere and the larger vertical vortex tilt in the former than in the latter. The stronger storm‐relative flow in the former imposes a greater ventilation of the warm core in the middle‐upper troposphere, leading to a more significant weakening of the storm. The storm in the lower‐layer shear only weakens initially after the VWS is imposed but then experiences a quasi periodic intensity oscillation with a period of about 24 hr. This quasi periodic behavior is found to be closely related to the boundary layer thermodynamic “discharge/recharge” mechanism associated with the activity of shear‐induced outer spiral rainbands. There is no significant intensity oscillation for the storm embedded in the upper‐layer shear, even though outer spiral rainbands develop quasi periodically also. The boundary layer inflow is very weak in that case and the low equivalent potential temperature air induced by downdrafts in outer spiral rainbands therefore cannot penetrate into the inner core but remains in the outer region.

Recent Advances in Research and Forecasting of Tropical Cyclone Track, Intensity, and Structure at Landfall

This review prepared for the fourth International Workshop on Tropical Cyclone Landfall Processes (IWTCLP-4) summarizes the most recent (2015-2017) theoretical and practical knowledge in the field of tropical cyclone (TC) track, intensity, and structure rapid changes at or near landfall. Although the focus of IWTCLP-IV was on landfall, this summary necessarily embraces the characteristics of storms during their course over the ocean prior to and leading up to landfall. In the past few years, extremely valuable observational datasets have been collected using both aircraft reconnaissance and new geostationary and low-earth orbiting satellites at high temporal and spatial resolution for TC forecasting guidance and research studies. Track deflections for systems near complex topography such as that of Taiwan and La Reunion have been further investigated, and advanced numerical models with high spatial resolution necessary to predict the interaction of the TC circulation with steep island topography have been developed. An analog technique has been designed to meet the need for longer range landfall intensity forecast guidance that will provide more time for emergency preparedness. Probabilistic track and intensity forecasts were also developed to better communicate on forecast uncertainty. Operational practices of several TC forecast centers are described herein and some challenges regarding forecasts and warnings for TCs making landfall are identified. This review concludes with insights from both researchers and forecasters regarding future directions to improve predictions of TC track, intensity, and structure at landfall.

Recent Advances in Research and Forecasting of Tropical Cyclone Rainfall

ABSTRACT In preparation for the Fourth International Workshop on Tropical Cyclone Landfall Processes (IWTCLP-IV), a summary of recent research studies and the forecasting challenges of tropical cyclone (TC) rainfall has been prepared. The extreme rainfall accumulations in Hurricane Harvey (2017) near Houston, Texas and Typhoon Damrey (2017) in southern Vietnam are examples of the TC rainfall forecasting challenges. Some progress is being made in understanding the internal rainfall dynamics via case studies. Environmental effects such as vertical wind shear and terrain-induced rainfall have been studied, as well as the rainfall relationships with TC intensity and structure. Numerical model predictions of TC-related rainfall have been improved via data assimilation, microphysics representation, improved resolution, and ensemble quantitative precipitation forecast techniques. Some attempts have been made to improve the verification techniques as well. A basic forecast challenge for TC-related rainfall is monitoring the existing rainfall distribution via satellite or coastal radars, or from over-land rain gauges. Forecasters also need assistance in understanding how seemingly similar landfall locations relative to the TC experience different rainfall distributions. In addition, forecasters must cope with anomalous TC activity and landfall distributions in response to various environmental effects.

Observed sub-hectometer-scale low level jets in surface-layer velocity profiles of landfalling typhoons

This paper extends previous studies concerning the structure of wind in tropical cyclones (TCs) approaching land mass over coastal waters. New sets of field measurements from wind observation towers and Doppler SODARS at levels between the land surface and 100 m height are presented. The measured mean (over 10-min periods) of the horizontal velocity U(r) shows the usual eye, eyewall and outer vortex structure. But very significantly they also exhibit that the mean vertical profile, U(z), does not increase monotonically as in a usual surface boundary layer in the backside eyewall regions. Rather, it is in the form of a jet, with a maximum velocity, ULLJ, at a height zLLJ, lying between 40 and 60 m. The data also show that the mean velocity gradient, defined by the ratio ULLJ/U(10), is typically smaller in the front side of a tropical cyclone in the direction of its motion as compared to the backside. This asymmetric distribution does not vary significantly with the radius. The mechanisms that result in the asymmetric distribution of the low-level jet are complex. One possible source is the downward transportation of convective turbulence. In the backside of a TC, the downflow is dominated, and it transports high velocity flow downward and enhances the mean momentum of wind flow at lower heights. Another factor may be the influence of the swell translation in different azimuth of a TC structure. These mechanisms need further investigation based on high resolution observations through a collaborative multiplatform measurement involving, e.g., Doppler SODAR, conventional sensors and numerical simulations.

Measuring Radial and Tangential Changes in Tropical Cyclone Rain Fields Using Metrics of Dispersion and Closure

Although tropical cyclone (TC) rain fields assume varying spatial configurations, many studies only use areal coverage to compare TCs. To provide additional spatial information, this study calculates metrics of closure, or the tangential completeness of reflectivity regions surrounding the circulation center, and dispersion, or the spread of reflectivity outwards from the storm center. Two hurricanes that encountered different conditions after landfall are compared. Humberto (2007) experienced rapid intensification (RI), stronger vertical wind shear, and more moisture than Jeanne (2004), which was more intense, weakened gradually, and became extratropical. A GIS framework was used to convert radar reflectivity regions into polygons and measure their area, closure, and dispersion. Closure corresponded most closely to storm intensity, as the eye became exposed when both TCs weakened to tropical storm intensity. Dispersion increased by 10 km·hr−1 as both TCs developed precipitation along frontal boundaries. As closure tended to change earlier than dispersion and area, closure may be most sensitive to subtle changes in environmental conditions, particularly as the storm’s core experiences the entrainment of dry air and erodes. Displacement provided a combined radial and tangential component to the location of the rainfall regions to confirm placement along the frontal boundaries. Examining area alone cannot reveal these patterns. The spatial metrics reveal changes in TC structure, such as the lag between onset of RI and maximum closure, which should be generalizable to TCs experiencing similar conditions. Future work will calculate these metrics for additional TCs to quantify structural changes in response to their surrounding environment.

A 13-Year Global Climatology of Tropical Cyclone Warm-Core Structures from AIRS Data

Abstract There is uncertainty as to whether the typical warm-core structure of tropical cyclones (TCs) is featured as an upper-level warm core or not. It has been hypothesized that data from the satellite-borne Advanced Microwave Sounding Unit (AMSU) are inadequate to resolve a realistic TC warm-core structure. This study first evaluates 13 years of Atmospheric Infrared Sounder (AIRS) temperature retrieval against recent dropsonde measurements in TCs. AIRS can resolve the TC warm-core structure well, comparable to the dropsonde observations, although the AMSU-A retrievals fail to do so. Using 13-yr AIRS data in global TCs, a global climatology of the TC warm-core structure is generated in this study. The typical warm-core height is at the upper level around 300–400 hPa for all TCs and increases with TC intensity: 400 hPa (~8 km) for tropical storms, 300 hPa (~10 km) for category 1–3 hurricanes, 250–300 hPa (~10–11 km) for category 4 hurricanes, and 150 hPa (~14 km) for category 5 hurricanes. The range of warm-core height varies with TC intensity as well. A strong correlation between TC intensity and warm-core strength is found. A weaker but still significant correlation between TC intensity and warm-core height is also found.

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.

Modeling of the Inner Structure of Tropical Cyclones: Equation of Flow on Wind Trajectories

We consider a system of equations that describes the air motion inside tropical cyclones, including consequences of the condensation of water vapor. We examine the system of nonlinear ordinary differential equations on wind trajectories in the plane of radial and vertical variables. We prove the unique solvability of the system of differential equations on a segment suitable for modeling of tropical cyclones.

Parameters of lightning activity in the tropical cyclones at the stage of maximum development in August 2016

This article discusses the parameters of thunderstorm activity in the structure of tropical cyclones that had an impact on the Russian Far East in August 2016. The study shows in detail the change in the cloud mass and describes its structure during the lifetime of the most powerful tropical cyclone Lionrock (2016). It is shown that the density distribution of lightning discharges in the range of tropical cyclones (up to 1000 km), reaching the typhoon stage, is characterized by two local maxima. The distribution density of lightning discharges in the range of tropical cyclones that have reached a strong tropical storm is characterized by one maximum. The lifetime of thunderstorm cells that stand out is 40-50 minutes in average. Before the typhoon stage, the area of thunderstorm cells increased, and then, the number of cells as well as the intensity of discharges in the cell increased, but their lifetime and area decreased.

Azimuthally averaged structure of Hurricane Edouard (2014) just after peak intensity

Analyses of dropsonde data collected in Hurricane Edouard (2014) just after its mature stage are presented. These data have unprecedentedly high spatial resolution, based on 87 dropsondes released by the unmanned NASA Global Hawk from an altitude of 18 km during the Hurricane and Severe Storm Sentinel (HS3) field campaign. Attempts are made to relate the analyses of the data to theories of tropical cyclone structure and behaviour. The tangential wind and thermal fields show the classical structure of a warm‐core vortex, in this case with a secondary eyewall feature. Additionally, the equivalent potential temperature field (θe) shows the expected structure with a mid‐tropospheric minimum at outer radii and contours of θe flaring upwards and outwards at inner radii. With some imagination, these contours are roughly congruent to the surfaces of absolute angular momentum. However, details of the analysed radial velocity field are quite sensitive to the way in which the sonde data are partitioned to produce an azimuthal average. This sensitivity is compounded by an apparent limitation of the assumed steadiness of the storm over the period of data collection.

Tropical Cyclone Diurnal Cycle Signals in a Hurricane Nature Run

The diurnal cycle of tropical convection and tropical cyclones (TCs) has been previously described in observational-, satellite-, and modeling-based studies. The main objective of this work is to expand on these earlier studies by identifying signals of the TC diurnal cycle (TCDC) in a hurricane nature run, characterize their evolution in time and space, and better understand the processes that cause them. Based on previous studies that identified optimal conditions for the TCDC, a select period of the hurricane nature run is examined when the simulated storm was intense, in a low shear environment, and sufficiently far from land. When analyses are constrained by these conditions, marked radially propagating diurnal signals in radiation, thermodynamics, winds, and precipitation that affect a deep layer of the troposphere become evident in the model. These propagating diurnal signals, or TC diurnal pulses, are a distinguishing characteristic of the TCDC and manifest as a surge in upper-level outflow with underlying radially propagating tropical squall-line-like features. The results of this work support previous studies that examined the TCDC using satellite data and have implications for numerical modeling of TCs and furthering our understanding of how the TCDC forms, evolves, and possibly impacts TC structure and intensity.

Impacts of Radiation and Upper-Tropospheric Temperatures on Tropical Cyclone Structure and Intensity

Abstract Potential intensity theory predicts that the upper-tropospheric temperature acts as an important constraint on tropical cyclone (TC) intensity. The physical mechanisms through which the upper troposphere impacts TC intensity and structure have not been fully explored, however, due in part to limited observations and the complex interactions between clouds, radiation, and TC dynamics. In this study, idealized Weather Research and Forecasting Model ensembles initialized with a combination of three different tropopause temperatures and with no radiation, longwave radiation only, and full diurnal radiation are used to examine the physical mechanisms in the TC–upper-tropospheric temperature relationship on weather time scales. Simulated TC intensity and structure are strongly sensitive to colder tropopause temperatures using only longwave radiation, but are less sensitive using full radiation and no radiation. Colder tropopause temperatures result in deeper convection and increased ice mass aloft in all cases, but are more intense only when radiation was included. Deeper convection leads to increased local longwave cooling rates but reduced top-of-the-atmosphere outgoing longwave radiation, such that the total radiative heat sink is reduced from a Carnot engine perspective in stronger storms. We hypothesize that a balanced response in the secondary circulation described by the Eliassen equation arises from upper-troposphere radiative cooling anomalies that lead to stronger tangential winds. The results of this study further suggest that radiation and cloud–radiative feedbacks have important impacts on weather time scales.

Tropical Cyclone Unusual Intensity and Structure Change in the Western North Pacific Observed by Reconnaissance Aircraft During TPARC/TCS08 and ITOP/TCS10

ABSTRACT The combined Tropical Cyclone Structure (TCS-08) and THORPEX Pacific-Asia Regional Campaign (T-PARC) field programs during August and September 2008 and the combined Impact of Typhoons on the Ocean in the Pacific (ITOP) and TCS-10 field programs during August, September, and October 2010 have provided unique simultaneous observations of typhoon structure and intensity together with underlying oceanic conditions during rapid intensification and rapid decay events in the northwest Pacific Ocean basin during the tropical cyclone life cycle. A new observational strategy from aircraft reconnaissance, involving combination (‘COMBO’) deployments of Global Positioning System (GPS) dropwindsondes and Airborne expendable Bathythermographs (AXBTs) has allowed for simultaneous observations of TC structure and intensity while also observing oceanic eddy structure beneath the TC. This strategy, together with deployment of drifting buoys and floats ahead of the TC, has led to observations in Super-TYphoon (STY) Jangmi (2008) and STY Megi (2010) that show the ocean control over TC Rapid Intensification (RI) and Rapid Decay (RD) due to pre-existing warm and cold ocean eddy features, as well as the intensity-limiting effects of oceanic ‘cold wakes’ generated by Typhoons (TYs) Fanapi (2010) and Malakas (2010). Observations made during ‘unusual’ track changes in TY Fanapi illustrate how the TC passes over different ocean eddy features than would have occurred with a ‘normal’ straight track. These track changes led to unusual intensity changes due to subsequent movement over a warm ocean eddy feature that led to unexpected TC intensification, and this was followed by movement over shallow ocean mixed layers that led to strong ‘cold wake’ generation. This feature of ocean control then limited Fanapi’s intensification to values well below their Maximum Potential Intensity (MPI). In addition, ‘COMBO’ observations within pre-TC disturbances show contrasting impacts on typhoon development/non-development for cases of strong ocean control and weak environmental shear (pre-Fanapi, 2010) versus cases of weak ocean control and strong environmental shear (disturbance TCS025, 2008).

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

ABSTRACT Rapid changes in Tropical Cyclone (TC) structure and intensity are the most difficult aspects of TC forecasting. The intensity and structural changes associated with rapid intensification (RI) and rapid weakening (RW) instances of TCs of North Indian Ocean (NIO) are analysed based on 5 cases each for RI and RW. The vertical profiles of heat (Q1) and moisture (Q2) based on heat and moisture budget equations are analysed to bring out the role of diabatic heating during the RI and RW processes. The associated structural changes are analysed based on the Fourier first order wave number-1 asymmetry in the precipitation patterns associated with RI and RW instances. During RI process, there are 2 predominant Q1 maxima associated with latent heat release near the TC centre, one in the upper troposphere and the other in the lower troposphere. During RW process, there is a single dominant Q1 and Q2 maxima in the mid-troposphere. Regarding the structural changes, there is a pronounced front-back asymmetry in both RI and RW cases. The asymmetry amplitude in the inner core is lower in the case of RI events but, is quite high for RW events. Thus, RW events are characterised by large convective asymmetry.

Comparison of Simulated Tropical Cyclone Intensity and Structures Using the WRF with Hydrostatic and Nonhydrostatic Dynamical Cores

This study explored the influence of choosing a nonhydrostatic dynamical core or a hydrostatic dynamical core in the weather research and forecasting (WRF) model on the intensity and structure of simulated tropical cyclones (TCs). A comparison of cloud-resolving simulations using each core revealed significant differences in the TC simulations. In comparison with the nonhydrostatic simulation, the hydrostatic simulation produced a stronger and larger TC, associated with stronger convective activity. A budget analysis of the vertical momentum equation was conducted to investigate the underlying mechanisms. Although the hydrostatic dynamical core was used, the vertical motion was not in strict hydrostatic balance because of the existence of the vertical perturbation pressure gradient force, local buoyancy force, water loading, and sum of the Coriolis and diffusion effects. The contribution of the enhanced vertical perturbation pressure gradient force was found to be more important for stronger upward acceleration in the eyewall in the hydrostatic simulation than in the nonhydrostatic simulation. This is because it leads to intensified convection in the eyewall that releases more latent heat, which induces a larger low-level radial pressure gradient and inflow motion, and eventually leads to a stronger storm.

Can We Improve Parametric Cyclonic Wind Fields Using Recent Satellite Remote Sensing Data?

Parametric cyclonic wind fields are widely used worldwide for insurance risk underwriting, coastal planning, and storm surge forecasts. They support high-stakes financial, development and emergency decisions. Yet, there is still no consensus on a potentially “best” parametric approach, nor guidance to choose among the great variety of published models. The aim of this paper is to demonstrate that recent progress in estimating extreme surface wind speeds from satellite remote sensing now makes it possible to assess the performance of existing parametric models, and select a relevant one with greater objectivity. In particular, we show that the Cyclone Global Navigation Satellite System (CYGNSS) mission of NASA, along with the Advanced Scatterometer (ASCAT), are able to capture a substantial part of the tropical cyclone structure, and to aid in characterizing the strengths and weaknesses of a number of parametric models. Our results suggest that none of the traditional empirical approaches are the best option in all cases. Rather, the choice of a parametric model depends on several criteria, such as cyclone intensity and the availability of wind radii information. The benefit of using satellite remote sensing data to select a relevant parametric model for a specific case study is tested here by simulating hurricane Maria (2017). The significant wave heights computed by a wave-current hydrodynamic coupled model are found to be in good agreement with the predictions given by the remote sensing data. The results and approach presented in this study should shed new light on how to handle parametric cyclonic wind models, and help the scientific community conduct better wind, wave, and surge analyses for tropical cyclones.

Impact of Assimilating Aircraft Reconnaissance Observations on Tropical Cyclone Initialization and Prediction Using Operational HWRF and GSI Ensemble–Variational Hybrid Data Assimilation

Abstract This study evaluates the impact of assimilating high-resolution, inner-core reconnaissance observations on tropical cyclone initialization and prediction in the 2013 version of the operational Hurricane Weather Research and Forecasting (HWRF) Model. The 2013 HWRF data assimilation system is a GSI-based hybrid ensemble–variational system that, in this study, uses the Global Data Assimilation System ensemble to estimate flow-dependent background error covariance. Assimilation of inner-core observations improves track forecasts and reduces intensity error after 18–24 h. The positive impact on the intensity forecast is mainly found in weak storms, where inner-core assimilation produces more accurate tropical cyclone structures and reduces positive intensity bias. Despite such positive benefits, there is degradation in short-term intensity forecasts that is attributable to spindown of strong storms, which has also been seen in other studies. There are several reasons for the degradation of intense storms. First, a newly discovered interaction between model biases and the HWRF vortex initialization procedure causes the first-guess wind speed aloft to be too strong in the inner core. The problem worsens for the strongest storms, leading to a poor first-guess fit to observations. Though assimilation of reconnaissance observations results in analyses that better fit the observations, it also causes a negative intensity bias at the surface. In addition, the covariance provided by the NCEP global model is inaccurate for assimilating inner-core observations, and model physics biases result in a mismatch between simulated and observed structure. The model ultimately cannot maintain the analysis structure during the forecast, leading to spindown.

Analyzing Tropical Cyclone Structures during Secondary Eyewall Formation Using Aircraft In Situ Observations

Abstract The dynamical mechanisms for secondary eyewall formation (SEF) in tropical cyclones (TCs) are not yet fully understood. Most hypotheses for SEF rely on the early presence of persistent and widespread rainband convection outside of the primary eyewall. This convection eventually coalesces into a secondary eyewall through both axisymmetric and asymmetric processes, but the extent and importance of these dynamical processes and their associated convective structures remain unclear. This study examines the evolution of axisymmetric and asymmetric structures in a composite analysis of Atlantic TCs from 1999 to 2015 using aircraft reconnaissance observations from the Extended Flight-Level Dataset for Tropical Cyclones (FLIGHT+). Compared to intensifying TCs that did not experience SEF, TCs undergoing SEF showed axisymmetric broadening of the outer wind field in the tangential wind and angular momentum profiles before SEF. Thermodynamic observations indicated features consistent with strengthening eyewall convection. We also analyzed TCs in shear-relative quadrants to examine the evolution of asymmetric kinematic and thermodynamic structures during SEF. Utilizing a new normalization technique based on the radii of both eyewalls, we isolated the structures surrounding the secondary eyewall before and during SEF. Using this technique, we found that kinematic structures of the developing secondary eyewall were most prominent in the left-of-shear half, and the thermodynamic structures of the secondary eyewall became more axisymmetric during SEF. Asymmetries developed in the primary eyewall thermodynamics as it decayed. Understanding the evolution of these observed structures characteristic to SEF will improve our ability to predict SEF and the resulting changes in TC intensity and structure.

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).