Tropical Cyclone Boundary Layer Research Articles

Analyzing Coherent Structures in the Tropical Cyclone Boundary Layer Using Large Eddy Simulations

Turbulence within the tropical cyclone boundary layer plays a crucial role in the exchange of heat, moisture, and momentum between the surface and the atmosphere. This study investigates the characteristics of coherent structures, specifically streaks and rolls, using large eddy simulations. Our results highlight significant differences across the three radius cases, with smaller radius exhibiting more intense and organized turbulence and streak/roll structures. Our analyses reveal that thermodynamic conditions significantly impact the timing of initial streak/roll development but do not affect their intensity in the steady state. Wind structures closer to the tropical cyclone center lead to stronger and more rapidly developing streaks/rolls, indicating their critical role in determining the intensity and formation of these features. Sensitivity tests on the Coriolis parameter (f) and radial decay parameter of tangential wind (n) show minimal impact on the development of streaks/rolls, suggesting these factors are less influential compared to wind and thermodynamic conditions.

Tropical Cyclone Boundary Layer Asymmetries in a Tilt-Following Perspective

Abstract Previous observations and modeling studies showed that tropical cyclones (TCs) in a sheared environment develop an asymmetric boundary layer (BL). While the relationship between the BL asymmetries and environmental shear has been demonstrated, the exact cause of these BL asymmetries and the phase relationship between them are less well understood. In this study, we examine the dynamical processes leading to the asymmetric structure of the TC BL in a sheared environment using idealized, convection-permitting model simulations. Our results show that the emergence of the BL asymmetries is closely linked to the TC vortex tilt and rainband processes. Specifically, stratiform diabatic processes in the downtilt-left region result in midlevel descending inflow, which brings midtropospheric, low-θE air toward the BL and forms a surface cold pool in the downtilt-left quadrant. This descending inflow also advects high absolute angular momentum inward, redistributing the vertical vorticity and causing a storm-scale tangential wind acceleration within the downtilt-left quadrant. As the BL low-θE air advances inward, it becomes supergradient and decelerates radially, forming BL outflow in the uptilt-left quadrant. The outflow advects positive relative vorticity uptilt, forming an elliptic BL vorticity and circulation structure. As the tilted TC vortex and the accompanying rainband precess cyclonically over time, the above sequence of events and the resultant BL asymmetries also precess cyclonically, maintaining a quasi-stationary configuration relative to the vortex tilt. These results suggest that the primary organizing factor of the boundary layer asymmetries is the tilted vortex structure and not strictly the environmental shear direction.

Study of boundary layer characteristics during the landfalling of a Nisarga cyclone

One of the most important parameters in meteorology is the mean wind profile in the tropical cyclone boundary layer. The vertical profile of wind speed and wind direction were measured during the period of the Nisarga cyclone from May 31st, 2020, to June 5th, 2020, using the newly installed Phased Array Doppler Sodar system at the Center for Space and Atmospheric Science (CSAS), Sanjay Ghodawat University, Kolhapur (16.74° N, 74.37° E; near India's western coast). Our analysis revealed that the maximum mean wind speed was 17 m/s on June 3, 2020, at 10:00 IST. It also shows the change in wind direction from southwest to southeast on June 2 and 3, 2020. Daily high-resolution reanalysis data in the domain, 0–25°N, 65–110°E, during the period from May 31st to June 5th, 2020, revealed the variation of the atmospheric pressure of the Nisarga cyclone from 1000 to 1008 hPa, sea surface temperature (SST) between 30 °C and 31 °C, outgoing longwave radiation (OLR) between 100 and 240 Wm-2, wind speed between 3 and 15 m/s, and low values of vertical wind shear (VWS) were observed to the north of Nisarga track. These observations may provide more insights for the study of boundary layer turbulence during cyclonic activities.

A comprehensive review on the modeling of tropical cyclone boundary layer wind field

Tropical cyclone (TC) wind field models are becoming increasingly sophisticated and complex. This review systematically discusses a range of models capable of simulating TCs in terms of modifications or simplifications of the governing equation, the Navier–Stokes equations, as a starting point. The discussion focuses on linear models, which include slab models, height-resolving models, and numerical simulation methods, respectively. The linear model offers quick calculations and insights into physical mechanisms, while slab models have limitations in capturing important processes and site conditions. The height-resolving model is widely used for Monte Carlo simulations, providing realistic three-dimensional wind structures. Nonlinear simulations yield reliable results for typhoon trajectory prediction, although they require specific boundary and initial conditions. Integration of nonlinear simulation with artificial intelligence and machine learning shows promise for faster typhoon prediction. However, challenges remain in terms of data training for machine learning models. Future advancements in these areas have the potential to enhance hazard assessment and weather forecasting.

Airborne Doppler Radar Observations of Tropical Cyclone Boundary Layer Kinematic Structure and Evolution During Landfall

AbstractAirborne Doppler radar observations of the wind field in the tropical cyclone boundary layer (TCBL) during the landfall of Hurricane Ida (2021) are examined here. Asymmetries in tangential and radial flow are governed by tropical cyclone (TC) motion and vertical wind shear prior to landfall, while frictional effects dominate the asymmetry location during landfall. Strong TCBL inflow on the offshore‐flow side of Ida occurs during landfall, while the location of the peak tangential wind at the top of the TCBL during this period is located on the onshore‐flow side. A comparison of these observations with a numerical simulation of TC landfall shows many consistencies with the modeling study, though there are some notable differences that may be related to differences in the characteristics of the land surface between the simulation and the observations here.

Large-Eddy Simulations of the Tropical Cyclone Boundary Layer at Landfall in an Idealized Urban Environment

Abstract Populated urban areas along many coastal regions are vulnerable to landfalling tropical cyclones (TCs). To the detriment of surface parameterizations in mesoscale models, the complexities of turbulence at high TC wind speeds in urban canopies are presently poorly understood. Thus, this study explores the impacts of urban morphology on TC-strength winds and boundary layer turbulence in landfalling TCs. To better quantify how urban structures interact with TC winds, large-eddy simulations (LESs) are conducted with the Cloud Model 1 (CM1). This implementation of CM1 includes immersed boundary conditions (IBCs) to represent buildings and eddy recycling to maintain realistic turbulent flow perturbations. Within the IBCs, an idealized coastal city with varying scales is introduced. TC winds impinge perpendicularly to the urbanized coastline. Numerical experiments show that buildings generate distinct, intricate flow patterns that vary significantly as the city structure is varied. Urban IBCs produce much stronger turbulent kinetic energy than is produced by conventional surface parameterizations. Strong effective eddy viscosity due to resolved eddy mixing is displayed in the wake of buildings within the urban canopy, while deep and enhanced effective eddy viscosity is present downstream. Such effects are not seen in a comparison LES using a simple surface parameterization with high roughness values. Wind tunneling effects in streamwise canyons enhance pedestrian-level winds well beyond what is possible without buildings. In the arena of regional mesoscale modeling, this type of LES framework with IBCs can be used to improve parameters in surface and boundary layer schemes to more accurately represent the drag coefficient and the eddy viscosity in landfalling TC boundary layers. Significance Statement This is among the first large-eddy simulation model studies to examine the impacts of tropical cyclone–like winds around explicitly resolved buildings. This work is a step forward in bridging the gap between engineering studies that use computational fluid dynamics models or laboratory experiments for flow through cities and mesoscale model simulations of landfalling tropical cyclones that use surface parameterizations specialized for urban land use.

A Parametric Model of Tropical Cyclone Surface Winds for Sea and Land

Abstract Parametric models of tropical cyclone winds are widely used for risk assessment. Although tropical cyclones often present their worst wind risk to humanity during landfall, parametric models that represent land–sea differences are rare. This paper presents a parametric model with explicit representation of land–sea differences. Statistical models were developed over each surface of the frictional wind speed reduction from gradient level to 10 m, and of the surface inflow angle, based on about 1200 simulations with a three-dimensional dynamical boundary layer model. The wind profile of Willoughby et al. is used to represent the gradient flow, and a maximum likelihood scheme used to fit this profile to best track data. The mean RMS difference between the statistical and dynamical surface winds within 100 km of the storm center is 0.78 m s−1 and 4.26° over sea, and 1.04 m s−1 and 4.59° over land. During landfall, the use of a common gradient-level structure, but different surface roughnesses, provides dynamical consistency between the estimated winds over sea and land. A simple representation of internal boundary layers is applied near the coast. Analysis of the dynamical simulations revealed substantial consistency with observational studies of the tropical cyclone boundary layer, including that the azimuth of the surface wind maximum is on average 65° from the front of the storm, in the left-forward quadrant in the Southern Hemisphere. There was, however, substantial variability around this figure, with the maximum occurring in the opposite forward quadrant in storms that were intense, and/or had a relatively rapid decrease in wind speed outside of the radius of maximum winds.

Boundary layer profile of decaying and non‐decaying tropical storms near landfall

AbstractThe vertical profile of the wind structure of translating tropical cyclones, including the associated azimuthal asymmetry, has been the subject of existing theoretical and observational studies using dropsondes. Most of these studies are based on data collected from relatively strong cyclones over the Atlantic. Here we explore the tropical cyclone boundary layer wind profile of mainly relatively weak landfalling cyclones near Hong Kong. We find that decaying tropical storms have a much larger mid‐ to low‐level inflow angle than those that are intensifying or in steady‐state. The inflow angles of intensifying, steady‐state and decaying tropical storms converge towards the top of the boundary layer. The wind speed reduces through the boundary layer in a similar way in all three cases. The combination of these factors means that decaying tropical storms have stronger inflow than intensifying and steady‐state ones. We attribute these local effects to remote enhanced surface friction over land when the storms are weakening.

Airstream Association of Large Boundary Layer Rolls during Extratropical Transition of Post-Tropical Cyclone Sandy (2012)

Better understanding of roll vortices that often occur in the tropical cyclone (TC) boundary layer is required to improve forecasts of TC intensification and the granularity of damaging surface winds. It is especially important to characterize rolls over a wide variety of TCs, their environments, and TC development phases. Boundary layer rolls have been observed in TCs since 1998, but only recently in a TC during its extratropical transition phase. The work reported herein is the first to analyze how boundary layer rolls are distributed among the extratropical features of a transitioning TC. To this end, routine and special operational observations recorded during landfalling Post-tropical Cyclone Sandy (2012) were leveraged, including radar, surface, rawinsonde, and aircraft reconnaissance observations. Large rolls occurred in cold airstreams, both in the cold conveyor belt within the northwestern storm quadrant and in the secluding airstream within the northeastern quadrant, but roll presence was much diminished within the intervening warm sector. The large size of the rolls and their confinement to cold airstreams is attributed to an optimum inflow layer depth, which is deep enough below a strong stable layer to accommodate deep and strong positive radial wind shear to promote roll growth, yet not so deep as to limit radial wind shear magnitude, as occurred in the warm sector.

The Sensitivity of Large Eddy Simulations to Grid Resolution in Tropical Cyclone High Wind Area Applications

The question of at what resolution the large eddy simulations (LESs) of a tropical cyclone (TC) high wind area may converge largely remains unanswered. To address this issue, LESs with five resolutions of 300 m, 100 m, 60 m, 33 m, and 20 m are performed in this study to simulate a high wind area near the radius of maximum wind of Typhoon Chanthu (2021) using the Weather Research and Forecasting (WRF) model. The results show that, for a limited area LES, model grid resolution may alter the local turbulence structure to generate significantly different extreme values of temperature, moisture, and winds, but it only has a marginal impact on the median values of these variables throughout the vertical column. All simulations are able to capture the turbulent roll vortices in the TC boundary layer, but the structure and intensity of the rolls vary substantially in different resolution simulations. Local hectometer-scale eddies with vertical velocities exceeding 10 m s−1 are only observed in the 20 m resolution simulation but not in the coarser resolution simulations. The ratio of the resolved turbulent momentum fluxes and turbulent kinetic energies (TKEs) to the total momentum fluxes and TKEs appears to show some convergence of LESs when the grid resolution reaches 100 m or finer, suggesting that it is an acceptable grid resolution for LES applications in TC simulations.

Observed Turbulent Dissipation Rate in a Landfalling Tropical Cyclone Boundary Layer

Abstract Based on turbulence measurements from sonic anemometers instrumented at multiple levels on a 356-m-tall meteorological tower located on the south coast of China, an observation study of the turbulent dissipation rate (ε) in a landfalling tropical cyclone boundary layer (TCBL) is conducted. Three indirect methods (i.e., the power spectra, the second- and the third-order structure functions) are compared for the calculation of ε. The third-order structure function computes the smallest ε among the three methods, but shows the largest uncertainty. The second-order structure function gives similar ε estimates as the power spectra, and is adopted for its reduced uncertainty. The measured ε in the landfalling TCBL is O(10−1) m2 s−3, much greater than typical atmospheric boundary layer values as well as oceanic TCBL values. The value of ε is found to scale with the local friction velocity rather than the surface friction velocity, implying a highly localized nature of turbulence. Conventional parameterizations of ε are evaluated against observations. Process-based ε models assuming a local balance between shear production and dissipation prove inadequate, as shear production merely accounts for half of the dissipation away from the surface. In comparison, scaling-based ε models used by planetary boundary layer (PBL) schemes are more advantageous. With both tuning of the model coefficients and adjustment of the dissipation length scale, the performance of an ε model in a widely used PBL scheme is shown to produce similar values to the observations. Significance Statement Dissipation in a turbulent flow refers to the conversion of kinetic energy to heat through molecular viscosity, and is of key dynamic and thermal–dynamic importance in the tropical cyclone boundary layer (TCBL). Past observations of ε in the TCBL have been rare. Few existing ones are based on surface and flight measurements that are either constrained in height or limited in time. This study utilizes turbulence measurements taken from a 356-m meteorological tower to study dissipation of a landfalling TC. The tall tower platform offers a valuable turbulence dataset that extends beyond the surface layer of the TCBL. The observations are used to improve the physical understanding of dissipation, and to evaluate diagnostic ε models for numerical weather prediction.

A Model of the Sea–Land Transition of the Mean Wind Profile in the Tropical Cyclone Boundary Layer Considering Climate Changes

The tropical cyclone boundary layer (TCBL) connecting the underlying terrain and the upper atmosphere plays a crucial role in the overall dynamics of a tropical cyclone system. When tropical cyclones approach the coastline, the wind field inside the TCBL makes a sea–land transition to impact both onshore and offshore structures. So better understanding of the wind field inside the TCBL in the sea–land transition zone is of great importance. To this end, a semiempirical model that integrates the sea–land transition model from the Engineering Sciences Data Unit (ESDU), Huang’s refined TCBL wind field model, and the climate change scenarios from the Coupled Model Intercomparison Project Phase 6 (CMIP6) is used to investigate the influence of climate changes on the sea–land transition of the TCBL wind flow in Hong Kong. More specifically, such a semiempirical method is employed in a series of Monte-Carlo simulations to predict the wind profiles inside the TCBL across the coastline of Hong Kong under the impact of future climate changes. The wind profiles calculated based on the Monte-Carlo simulation results reveal that, under the influences of the most severe climate change scenario, slightly higher and significantly lower wind speeds are found at altitudes above and below 400 m, respectively, compared to the wind speeds recommended in the Hong Kong Wind Code of Practice. Such findings imply that the wind profile model currently adopted by the Hong Kong authorities in assessing the safety of low- to high-rise buildings may be unnecessarily over-conservative under the influence of climate change. On the other hand, the coded wind loads on super-tall buildings slightly underestimate the typhoon impacts under the severe climate change conditions anticipated for coastal southern China.

Large‐Eddy Simulation of Sea Spray Impacts on Fluxes in the High‐Wind Boundary Layer

AbstractThe exchange of enthalpy and momentum at the air‐sea interface is an important process in tropical cyclone (TC) development and intensification, and the effects of sea spray have long been uncertain, particularly at high wind speeds. Here we use a coupled large‐eddy simulation and Lagrangian cloud model to run high‐resolution simulations of an idealized, spray‐laden TC boundary layer. Simulations are performed with and without spray generation, using a realistic sea spray generation function over wind speeds relevant to TCs. We show that CD is not affected by spray droplets at 10‐m wind speeds up to 48 m s−1. CE and CH are modified during an initial transient adjustment stage, but quickly relax back to their unladen values due to surface layer saturation. This relaxation occurs faster for higher winds, because the increased spray concentration more rapidly saturates the surface layer.

Impact of Model Resolution on Secondary Eyewall Formation and Eyewall Replacement Cycle

AbstractDepicting Secondary Eyewall Formation (SEF) and Eyewall Replacement Cycle (ERC) in a numerical model is important for tropical cyclone (TC) forecasting. However, there is no consensus about what resolutions are appropriate to describe SEF/ERC within a full‐physics mesoscale model. In this study, numerical experiments are conducted to examine the impact of the horizontal and vertical resolutions on SEF/ERC. The mesoscale model is configured through nesting to the horizontal grid spacings of 6, 4, 2, 1.33, 0.67‐km, and with 27‐ and 54‐levels on an f‐plane in a quiescent environment. In addition, there are more levels below 1.5‐km to better describe the TC boundary layer (TCBL). The simulations with 6 and 4‐km grid spacings show no obvious SEF/ERC regardless of the number of vertical levels. When the horizontal grid spacings decrease to 2‐km or smaller, the simulations manifest SEF/ERC. These results are supported by a few simulations with the ARW model using similar configurations. Furthermore, the spectra of kinetic energy and vertical velocity from various resolutions confirm that the grid spacings should be smaller than 4‐km to resolve SEF/ERC. The impact of doubling vertical levels on the SEF/ERC is not as significant as doubling the horizontal resolutions. Finally, we discuss the coupling between the balanced/unbalanced flows (above/in the TCBL), and their effect on SEF. It is proposed that the coupled balanced/unbalanced processes that generate the quasi‐steady cooling zone in the primary eyewall and two warming regions inside and beyond the cooling zone are essential for SEF.

The Mean Kinematic Structure of the Tropical Cyclone Boundary Layer and Its Relationship to Intensity Change

Abstract This study investigates the relationship between the azimuthally averaged kinematic structure of the tropical cyclone boundary layer (TCBL) and storm intensity, intensity change, and vortex structure above the BL. These relationships are explored using composites of airborne Doppler radar vertical profiles, which have a higher vertical resolution than typically used three-dimensional analyses and, therefore, better capture TCBL structure. Results show that the BL height, defined by the depth of the inflow layer, is greater in weak storms than in strong storms. The inflow layer outside the radius of maximum tangential wind speed (RMW) is deeper in intensifying storms than in nonintensifying storms at an early stage. The peak BL convergence inside the RMW is larger in intensifying storms than in nonintensifying storms. Updrafts originating from the TCBL are concentrated near the RMW for intensifying TCs, while updrafts span a large radial range outside the RMW for nonintensifying TCs. In terms of vortex structure above the BL, storms with a quickly decaying radial profile of tangential wind outside the RMW (“narrow” vortices) tend to have a deeper inflow layer outside the RMW, stronger inflow near the RMW, deeper and more concentrated strong updrafts close to the RMW, and weaker inflow in the outer core region than those with a slowly decaying tangential wind profile (“broad” vortices). The narrow TCs also tend to intensify faster than broad TCs, suggesting that a key relationship exists among vortex shape, the BL kinematic structure, and TC intensity change. This relationship is further explored by comparisons of absolute angular momentum budget terms for each vortex shape.

Stationary and Transient Asymmetric Features in Tropical Cyclone Eye with Wavenumber-1 Instability: Case Study for Typhoon Haishen (2020) with Atmospheric Motion Vectors from 30-Second Imaging

Abstract Dynamics of low-level flows in the eye of Typhoon Haishen (2020) in its late phase of intensification are investigated with a special rapid-scan observation of the Himawari-8 geosynchronous satellite conducted every 30 s. This is accomplished by deriving storm-relative atmospheric motion vectors at an unprecedentedly high spatiotemporal resolution by tracking clouds across five consecutive visible-light reflectivity. The overall low-level circulation center was situated several kilometers away from the storm center defined in terms of the inner edge of the lower part of eyewall clouds. The shift direction is rearward of the storm translation, consistently with a numerical study of the tropical cyclone (TC) boundary layer. Over the analysis period of 10 h, azimuthal-mean tangential wind around this center was increased at each radius within the eye, and the rotational angular velocity was nearly homogenized. The instantaneous low-level circulation center is found to orbit around the overall circulation center at distances around 5 km. Its orbital angular speed was close to the maximum angular speed of azimuthal-mean tangential winds. This rotating transient disturbance is found to transport angular momentum inward, which explains the tangential wind increase and the angular velocity homogenization in the eye. These features are consistent with an algebraically growing wavenumber-1 barotropic instability, whose impact on TC structures has not been explored. This instability enhances wavenumber-1 asymmetry in ring-shaped vorticity, which can be induced by various processes such as translation, environmental shear, and exponential barotropic instability. Therefore, it may appear broadly in TCs to affect wind distribution in their eyes. Significance Statement Axially asymmetric transient features in the inner cores of tropical storms have been suggested to profoundly affect the structures and the time evolutions of tropical storms. However, the scarcity of observations has hindered studying such processes observationally. By using a specially conducted high-frequency satellite imaging of Typhoon Haishen (2020), we derived atmospheric motion vectors nearly homogeneously at an unprecedentedly high spatiotemporal resolution. Various kinds of asymmetric motions in low-level flows in the eye were found. Of particular interest is a special type of wavenumber-1 instability whose role has not drawn much attention; the instability was found to provide angular momentum transport consistent with the measured homogenization of the rotation.

Deep Learning Parameterization of the Tropical Cyclone Boundary Layer

AbstractThe research on tropical cyclone (TC) relieson numerical models and simulations, with the currently widely used boundary layer parameterization posing a significant challenge on accurately predicting turbulent mixing. However, machine learning has opened up new possibilities for boundary layer sub‐grid process parameterization. In this study, a deep‐learning parameterization scheme for the TC boundary layer (DeepBL) on predicting turbulent flux is proposed. DeepBL comprises a one‐dimensional convolutional structure that relies on a small number of learnable parameters that accomplishes an error reduction compared to the standard fully connected neural networks. Furthermore, a nonlinear transformation scheme is introduced to alleviate the training data's skewness and improve the DeepBL performance by affording a smaller prediction error. Specifically, the output data of a large‐eddy simulation of an idealized TC are used to train, validate, and test DeepBL, affording significantly better performance than the YSU scheme in the Weather Research and Forecast model. Interpretability analysis on DeepBL demonstrates that the deep‐learning parameterization scheme is physically reasonable.

Observed Turbulent Dissipation Rate in a Landfalling Tropical Cyclone Boundary Layer

Observed Turbulent Dissipation Rate in a Landfalling Tropical Cyclone Boundary Layer

Improving Analysis and Prediction of Tropical Cyclones by Assimilating Radar and GNSS-R Wind Observations: Ensemble Data Assimilation and Observing System Simulation Experiments Using a Coupled Atmosphere–Ocean Model

Abstract This study investigates the impact of assimilating ground-based radar reflectivity and wind data on tropical cyclone (TC) intensity prediction. The effect on a high-impact TC in the western North Pacific region that penetrated the Bashi Channel is examined. A multiscale correction based on the successive covariance localization (SCL) method is adopted to improve the analysis and forecast performance. In addition, GNSS-R wind speed is assimilated jointly in the rapid update assimilation framework to complement the TC boundary layer where radar data are limited. Model experiments are conducted and evaluated using the observing system simulation experiments (OSSEs) framework with a coupled atmosphere–ocean model nature run. Taking the experiment without data assimilation as the baseline, assimilating the radar data with the standard localization and SCL methods reduces the wind speed analysis error by 12% and 44%, respectively. The SCL method dominates the improvement in TC intensity prediction with a lead time longer than 2 days and the TC’s peak intensity forecast is improved by 18 hPa. The additional assimilation of the GNSS-R wind speed observation further reduces the wind speed error in the low-level analysis by 12% and 5% under the standard and SCL radar assimilation framework, respectively. GNSS-R wind assimilation leads to a further 6-hPa improvement in TC’s peak intensity. However, the sampling error introduced by the SCL method restrains the effect of GNSS-R assimilation. Sensitivity experiments with different GNSS-R data arrangements show that better GNSS-R wind coverage and additional wind direction information can further improve the TC analysis. Significance Statement Tropical cyclone (TC) intensity prediction over the western North Pacific (WNP) region remains a significant challenge due to limited observations. This study aims to improve the TC intensity prediction in WNP by assimilating the ground-based radar data using a multiscale correction framework and incorporating with the satellite ocean surface wind speed observation. We particularly focus on a high-impact TC like Typhoon Hato (2017), which penetrated the Bashi Channel and later made landfall in China, causing great damage. Our results showed that the assimilation strategy improved the TC intensity prediction for a lead time longer than 2 days. These results demonstrate the great potential of these observations and can provide guidance for future applications in operation centers.

Supergradient Winds in Simulated Tropical Cyclones

Abstract In a previous paper a formula was derived for the maximum potential intensity of the tangential wind in a tropical cyclone called PI+. The formula, PI+2 = EPI2 + αrmwmηm, where EPI is the maximum potential intensity of the gradient wind and αrmwmηm represents the supergradient winds. The latter term is the product of the radius rm, the vertical velocity wm, the azimuthal vorticity ηm at the radius and height of the maximum tangential wind (rm, zm), and the (nearly constant) α. Examination of a series of simulations of idealized tropical cyclones indicate an increasing contribution from the supergradient-wind term to PI+ as the radius of maximum wind increases. In the present paper, the physical content of the supergradient-wind term is developed showing how it is directly related to tropical cyclone boundary layer dynamics. It is found that , where −umin is the maximum boundary layer radial inflow velocity and lυ(z) is the vertical mixing length.