Tropical Cyclone Rainbands Research Articles

Revisiting the Contributions of Surface Sensible and Latent Heat Fluxes to the Outer Spiral Rainbands of Tropical Cyclones

AbstractThe separate effects of the sensible heat flux (SHF) and latent heat flux (LHF) on the outer spiral rainbands of tropical cyclones (TCs) have not received sufficient attention. This study examines the separate contributions of the SHF and LHF to the outer spiral rainbands of TCs via a series of sensitivity experiments using the three‐dimensional cloud‐resolving Weather Research and Forecasting model. The results indicate that removing the outer SHF suppresses the activity of the outer spiral rainbands and results in a stronger and smaller‐scale TC, whereas decreasing the outer LHF by the same amount has only a slight impact on the outer spiral rainbands. Further investigations indicate that the positive radial gradient of the potential temperature at the cold‐pool outer edge is crucial for the strength of outer spiral rainbands. The strong positive radial gradient of the potential temperature gives rise to a negative radial gradient of the horizontal pressure at the cold‐pool outer edge, and thus an outward radial pressure gradient force. As a result, strong outflow exists near the cold‐pool outer edge, producing horizontal convergence and eventually lifting air to generate new convective cells. Distinct from the LHF, the SHF directly modulates the potential temperature immediately outside the cold pool and thus the radial gradient of the potential temperature at the cold‐pool outer edge. Removing the outer SHF leads to a significantly lower potential temperature outside the cold pool, which is averse to the formation of a larger radial gradient of the potential temperature at the cold‐pool outer edge.

Origin of outer tropical cyclone rainbands

Outer tropical cyclone rainbands (TCRs) are a concentrated region of heavy precipitation and hazardous weather within tropical cyclones (TCs). Outer TCRs pose considerable risk to human societies, but their origin remains unresolved. Here, we identify a total of 1029 outer TCRs at their formative stage from 95 TCs and present a large collection of radar observations in order to establish a robust foundation of the natural diversity of rainband origin. The results show the dominance of outer origin for the observed outer TCRs, in distinct contrast to theoretical modeling works of outer TCRs, which propose inner-origin scenarios. Our analysis also suggests that squall-line dynamics are a common, but not the sole, mechanism responsible for outer TCR formation. The nature of preexisting outer precipitation is found to be an important factor to influence the squall-line and non-squall-line outer TCR initiation.

Tropical cyclone detection by combining wavelet transform with deep learning using infrared satellite images

ABSTRACT A tropical cyclone (TC) is a meteorological disaster that occurs over tropical or subtropical oceans. Automatic TC identification from a satellite image is essential to the subsequent automatic intelligent determination of TC positioning and intensity. Few recent studies have examined the automatic detection of TCs from satellite images. In this paper, wavelet (W) transformation, efficient (E) channel attention mechanism and YOLOX (Y) are combined to construct a deep learning model for TC detection, referred to as WEY-TCNet. WEY-TCNet automatically detects TCs from infrared satellite images. To improve the detection accuracy, a discrete wavelet transform is first used to decompose the infrared satellite image, after which the horizontal high-frequency components, including the TC rainband structure and inner core area, are extracted and fused with the original image. This step enhances the TC structure and brightness temperature gradient characterization. The fused image is then used as the input to WEY-TCNet. The ECANet channel attention mechanism was added to the original YOLOX-S network structure to focus on the key features of TC, such as structure and brightness temperature while suppressing invalid features and noise. This study constructed a satellite image dataset based on the infrared images of the Chinese Fengyun2D geostationary satellite. The dataset contains infrared satellite images of TCs in various life cycle stages, from generation to extinction. Our experiments showed that the WEY-TCNet model proposed in this paper achieves 91.49% detection mAP on the TC dataset, compared with the 86.69% detection mAP of the original YOLOX-S model, significantly improving it.

Analysis of the inner rainbands of tropical cyclones over the South China Sea during the landfall process

Tropical cyclones (TCs) can undergo offshore rapid intensification (RI) shortly before making landfall over the South China Sea (SCS). In this study, the inner rainbands distribution of both RI and non-RI landfall TCs (LTCs) in the SCS during 2015–2020 is examined based on a multi-source merged precipitation dataset. It is found that those RI LTCs exhibit a relatively higher averaged rain rate in the inner core region than that of non-RI LTCs. Both offshore RI cases and non-RI LTCs appear to have an increasing tendency of averaged rain rate after landfall, with the rain rate peak of the RI cases a few hours earlier than that of non-RI cases. By defining an axisymmetric index, the inner rainband evolution of both offshore RI cases and non-RI ones are further discussed. For both categories, most of the axisymmetric rainfall is concentric around the center and over 70% axisymmetric rainfall dominates the inner core region within three times of radius of maximum wind speed (RMW). It is shown that there is an obvious inwards shrinkage of axisymmetric rainfall for both offshore RI and non-RI cases. Analysis of typical RI and non-RI LTCs (1713 Hato and 1714 Pakhar) also shows an increasing rain rate of the inner rainbands soon after landfall, with larger amplitude for RI example than non-RI case. The inner rainbands of 1713 Hato show that a clockwise propagation with maximum enhancement happened at the down-shear left-hand side a few hours after landfall.

Determining Tropical Cyclone Center and Rainband Size in Geostationary Satellite Imagery

Brightness temperature (TB) observations at an infrared channel (10.3 μm) of the Advanced Baseline Imager (ABI) on board the U. S. 16th Geostationary Operational Environmental Satellite (GOES-16) are used for determining tropical cyclone (TC) center positions and rainband sizes. Firstly, an azimuthal spectral analysis method is employed to obtain an azimuthally symmetric center of a TC. Then, inner and outer rainbands radii, denoted as RIR and ROR, respectively, are estimated based on radial gradients of TB observations at different azimuthal angles. The radius RIR describes the size of the TC inner-core region, and the radius ROR reflects the maximum radial extent of TC rainbands. Compared with the best track centers, the root mean square differences of ABI-determined centers for tropical storms and hurricanes, which totals 108 samples, are 45.35 and 29.06 km, respectively. The larger the average wavenumber-0 amplitude, the smaller the difference between the ABI-determined center and the best track center. The TB-determined RIR is close but not identical to the radius of the outermost closed isobar and usually coincides with the radius where the strongest wavenumber 1 asymmetry is located. The annulus defined by the two circles with radii of ROR and RIR is the asymmetric area of rainbands described by azimuthal wavenumbers 1–3. In general, amplitudes of wavenumber 0 component centered on the ABI-determined center are greater than or equal to those from the best track. For a case of a 60 km distance between the ABI-determined and the best track TC center, the innermost azimuthal waves of wavenumbers 1–3 are nicely distributed along or within the radial distance RIR that is determined based on the ABI-determined TC center. If RIR is determined based on the best track, the azimuthal waves of wavenumbers 1–3 are found at several radial distances that are smaller than RIR. The TC center positions, and rainband size radii are important for many applications, including specification of a bogus vortex for hurricane initialization and verification of propagation mechanism of vortex Rossby waves.

Statistical Analysis of Convective Updrafts in Tropical Cyclone Rainbands Observed by Airborne Doppler Radar

AbstractTen years of airborne Doppler radar observations are used to study convective updrafts' kinematic and reflectivity structures in tropical cyclone (TC) rainbands. An automated algorithm is developed to identify the strongest rainband updrafts across 12 hurricane‐strength TCs. The selected updrafts are then collectively analyzed by their frequency, radius, azimuthal location (relative to the 200–850 hPa environmental wind shear), structural characteristics, and secondary circulation (radial/vertical) flow pattern. Rainband updrafts become deeper and stronger with increasing radius. A wavenumber‐1 asymmetry arises, showing that in the downshear (upshear) quadrants of the TC, updrafts are more (less) frequent and deeper (shallower). In the downshear quadrants, updrafts primarily have in‐up‐out or in‐up‐in secondary circulation patterns. The in‐up‐out circulation is the most frequent pattern and has the deepest updraft and reflectivity tower. Upshear, the updrafts generally have out‐up‐in or in‐up‐in patterns. The radial flow of the updraft circulations largely follows the vortex‐scale radial flow shear‐induced asymmetry, being increased low‐level inflow (outflow) and midlevel outflow (inflow) in the downshear (upshear) quadrants. It is hypothesized that the convective‐scale circulations are significantly influenced by the vortex‐scale radial flow at the updraft base and top altitudes. Other processes of the convective life cycle, such as bottom‐up decay of aging convective updrafts due to increased low‐level downdrafts, can influence the base altitude and, thus, the base radial flow of the updraft circulation. The findings presented in this study support previous literature regarding convective‐scale patterns of organized rainband convection in a mature, sheared TC.

High Resolution Real-data WRF Modeling and Verification of Tropical Cyclone Tornadoes Associated with Hurricane Ivan 2004

This study used high-resolution real-data WRF simulations of Hurricane Ivan (2004) to document the structure of potentially tornadic supercells embedded in tropical cyclone (TC) rainbands. The simulated TC track and intensity matched well with observations post landfall, and the simulated TC structure closely replicated the observed shape and rainbands, as evident by an assessment of observed composite and simulated radar reflectivity. TC tornado surrogates (TCT-Ss) were identified and calibrated using thresholds based on percentile values of maximum updraft helicity (UHmax) and simulated radar reflectivity. Although the magnitude of UHmax generally decreased as the simulated TC moved inland, sensitivity testing revealed that a threshold based on the 99.95% percentile value of UHmax achieved optimal TCT-S coverage and agreement with observed TC tornado (TCT) tracks on domains with 3-km and 1-km grid spacings. Three cells were identified at three different stages of the TC inland evolution and were found to have hook appendages resembling supercells found in midlatitudes. The cells produced storm tops of 13 km, with rotating cores with depths of 4 km above ground level. Although results in this study are unique to Hurricane Ivan, the authors believe the use of convection-allowing models and UH-based surrogate methodologies can be applied to other TC cases.

Meteotsunamis Accompanying Tropical Cyclone Rainbands During Hurricane Harvey

AbstractMeteotsunami waves can be triggered by atmospheric disturbances accompanying tropical cyclone rainbands (TCRs) before, during, and long after a tropical cyclone (TC) makes landfall. Due to a paucity of high‐resolution field data along open coasts during TCs, relatively little is known about the atmospheric forcing that generate and resonantly amplify these ocean waves, nor their coastal impact. This study links high‐resolution field measurements of sea level and air pressure from Hurricane Harvey (2017) with a numerical model to assess the potential for meteotsunami generation by sudden changes in air pressure accompanying TCRs. Previous studies, through the use of idealized models, have suggested that wind is the dominant forcing mechanism for TCR‐induced meteotsunami with negligible contributions from air pressure. Our model simulations show that large air pressure perturbations (∼1–3 mbar) can generate meteotsunamis that are similar in period (∼20 min) and amplitude (∼0.2 m) to surf zone observations. The measured air pressure disturbances were often short in wavelength, which necessitates a numerical model with high temporal and spatial resolution to simulate meteotsunami triggered by this mechanism. Sensitivity analysis indicates that air pressure forcing can produce meteotsunami with amplitudes O(0.5 m) and large spatial extents, but model results are sensitive to atmospheric factors, including model uncertainties (length, forward translation speed, and trajectory of the air pressure disturbance), as well as oceanographic factors (storm surge). The present study provides observational and numerical evidence that suggest that air pressure perturbations likely play a larger role in meteotsunami generation by TCRs than previously identified.

Assimilation of Satellite Microwave Observations over the Rainbands of Tropical Cyclones

AbstractA novel Bayesian Monte Carlo integration (BMCI) technique was developed to retrieve geophysical variables from satellite microwave radiometer data in the presence of tropical cyclones. The BMCI technique includes three steps: generating a stochastic database, simulating satellite brightness temperatures using a radiative transfer model, and retrieving geophysical variables such as profiles of temperature, relative humidity, and cloud liquid and ice water content from real observations. The technique also provides uncertainty estimates for each retrieval and can output the error covariance matrix of selected parameters. The measurements from the Advanced Technology Microwave Sounder (ATMS) on board Suomi National Polar-Orbiting Partnership (Suomi NPP) and the Global Precipitation Measurement (GPM) Microwave Imager (GMI) were used as input. A new technique was developed to correct the ATMS and GMI observations for the beam-filling effect, which is due to small-scale variability of precipitation and clouds when compared with the instrument footprint and also the nonlinear relation between the brightness temperature and precipitation. In addition, the assimilation of the BMCI retrievals into the NASA GEOS model is discussed for Hurricane Maria. The results show that assimilating the BMCI retrievals can influence the dynamical features of the cyclone, including a stronger warm core, a symmetric eye, and vertically aligned wind columns. Two possible factors that may limit the impact of the BMCI retrievals include 1) the resolution of the model (about 25 km), which was too coarse to show the potential of the BMCI data in improving the representation of tropical storms in the model forecast, and 2) the data assimilation system not being able to consider vertically correlated observation errors.

Surface Chlorophyll Enhancement in Mesoscale Eddies by Submesoscale Spiral Bands

AbstractOceanic submesoscale processes (with scales ~1–10 km and approximately days) have been progressively recognized as an important upwelling mechanism to close the upper ocean nutrient budget and sustain the primary production in euphotic layer. Spiral band is one of the most typical submesoscale structures of mesoscale eddies, characterized with enhancement of surface chlorophyll concentration. By combining satellite ocean‐color, satellite‐altimetry, and surface drifter data, we find that the oceanic spiral chlorophyll bands emerge globally and share a series of structural and kinematic features with atmospheric spiral rain bands of tropical cyclones, which indicate that they are footprints of the vertical motions induced by vortex Rossby waves embedded in eddies. As vortex Rossby waves are closely related to eddy evolution under background deformation, further observational analysis indicates that an intense eddy energy variation and a strong background deformation field constitute the favorable conditions for the emergence of the spiral chlorophyll bands.

Transformation of Infragravity Waves during Hurricane Overwash

Infragravity (IG) waves are expected to contribute significantly to coastal flooding and sediment transport during hurricane overwash, yet the dynamics of these low-frequency waves during hurricane impact remain poorly documented and understood. This paper utilizes hydrodynamic measurements collected during Hurricane Harvey (2017) across a low-lying barrier-island cut (Texas, U.S.A.) during sea-to-bay directed flow (i.e., overwash). IG waves were observed to propagate across the island for a period of five hours, superimposed on and depth modulated by very-low frequency storm-driven variability in water level (5.6 min to 2.8 h periods). These sea-level anomalies are hypothesized to be meteotsunami initiated by tropical cyclone rainbands. Estimates of IG energy flux show that IG energy was largely reduced across the island (79–86%) and the magnitude of energy loss was greatest for the lowest-frequency IG waves (<0.01 Hz). Using multitaper bispectral analysis, it is shown that, during overwash, nonlinear triad interactions on the sea-side of the barrier island result in energy transfer from the low-frequency IG peak to bound harmonics at high IG frequencies (>0.01 Hz). Assuming this pattern of nonlinear energy exchange persists across the wide and downward sloping barrier-island cut, it likely contributes to the observed frequency-dependence of cross-barrier IG energy losses during this relatively low surge event (<1 m).

Outer Tropical Cyclone Rainbands Associated with Typhoon Matmo (2014)

Abstract On 23 July 2014, a commercial aircraft (GE222) crashed near the Ma-Gong Airport on Penghu Island off the southwestern coast of Taiwan as it struggled to land in the stormy weather that was caused by the outer tropical cyclone rainbands (OTCRs) of Typhoon Matmo. This study aims to document the detailed aspects of airflow and precipitation of OTCRs through high-resolution radar and surface observations and to identify how these observed structures contribute to aviation weather hazards. Analyses indicate that the weather at the airport was significantly influenced by the passage of three OTCRs (R1, R2, and R3), and these rainbands share common characteristics of squall-line-like airflow and precipitation structures. As GE222 descended to approach the runway and flew immediately behind and roughly parallel to the leading edge of R3, the aircraft encountered the heaviest precipitation of the rainband and the prominent crosswind that was a manifestation of the rear-to-front flow generated locally by the rainband. The heavy rain–induced poor visibility and the occurrence of strong crosswinds were primary weather hazards affecting this flight event. Momentum budget analyses suggest that the frontward pressure gradient force provided by the near-surface, convectively generated mesohigh played a major role in driving the low-level rear-to-front flow inside the band. The results from the present study imply that closely monitoring convective activities in the outer regions of tropical cyclones and their potential transformation into squall-line-like storms is crucial to complement the routine aviation alert of severe weather under the influence of tropical cyclones.

Characteristics of the Outer Rainband Stratiform Sector in Numerically Simulated Tropical Cyclones: Lower-Layer Shear versus Upper-Layer Shear

Idealized numerical simulations are conducted in this study to comparatively investigate the characteristics of the stratiform sector in the outer rainbands of tropical cyclones (TCs) in lower- and upper-layer vertical wind shear (VWS) with moderate magnitude. Consistent with the results in previous studies, the outer rainband stratiform sector of the TCs simulated in both experiments is generally located downshear left. Upper-layer VWS tends to produce stronger asymmetric outflow at upper levels in the downshear-left quadrant than lower-layer shear. This stronger asymmetric outflow transports more water vapor radially outward from the inner core to the outer core at upper levels in the downshear-left quadrant in the upper-layer shear experiment. More depositional growth of both graupel and cloud ice thus occurs downshear left in upper layers in the outer core, yielding more diabatic heating and stronger upward motions, particularly in the stratiformdominated part of the stratiform sector in the upper-layer shear experiment. Resultingly, a better-organized stratiform sector in the outer rainbands is found in the upper-layer VWS experiment than in the lower-layer VWS experiment. The diabatic heating associated with the stratiform sector produces strong midlevel outflow on the radially inward side of, and weak midlevel inflow on the radially outward side of, the heating core, with lower-level inflow beneath the midlevel outflow and upper-level inflow above. The upper-layer VWS tends to produce a deeper asymmetric inflow layer in the outer rainband stratiform sector, with more significant lower-level inflow and tangential jets in the upper-layer VWS experiment.

Tropical cyclone rainbands can trigger meteotsunamis

Tropical cyclones are one of the most destructive natural hazards and much of the damage and casualties they cause are flood-related. Accurate characterization and prediction of total water levels during extreme storms is necessary to minimize coastal impacts. While meteotsunamis are known to influence water levels and to produce severe consequences, their impacts during tropical cyclones are underappreciated. This study demonstrates that meteotsunami waves commonly occur during tropical cyclones, and that they can contribute significantly to total water levels. We use an idealized coupled ocean–atmosphere–wave numerical model to analyze tropical cyclone-induced meteotsunami generation and propagation mechanisms. We show that the most extreme meteotsunami events are triggered by inherent features of the structure of tropical cyclones: inner and outer spiral rainbands. While outer distant spiral rainbands produce single-peak meteotsunami waves, inner spiral rainbands trigger longer lasting wave trains on the front side of the tropical cyclones.

Variable Raindrop Size Distributions in Different Rainbands Associated With Typhoon Fitow (2013)

AbstractThe microphysical characteristics of rain may vary in different rain regions of a tropical cyclone (TC), but few studies have demonstrated the differences in raindrop size distributions (RSDs) of convective rain in different rainbands of a specific TC. This study examines the RSD characteristics and evolution of convective rain within outer rainbands and a coastal‐front‐like rainband associated with Typhoon Fitow, based on observational data from a disdrometer at Shibo station in Shanghai, China, during 6–7 October 2013. Considering the fast passage of convective TC rainbands over the disdrometer and the low rain rate of stratiform rain in the outer area, this study proposes a modified rain‐type classification method based on the disdrometer data. This study indicates that convective outer‐rainband rain (ORR) and coastal‐front rain (CFR) have different rain parameters, three parameters of the gamma model, radar reflectivity–rain rate (Z‐R), and shape–slope (μ‐Λ) relationships. The convective ORR has higher concentrations at all drop sizes than the convective CFR as well as larger spectral width, leading to the greater rainfall rate. The different Z‐R relationships suggest the necessity of a variable relationship for quantitative precipitation estimation (QPE) in different rain regions of the TC. This study also demonstrates for the first time that the RSD evolution with increasing rain rate is different in various convective rainbands associated with Fitow, suggesting that different microphysical parameterization schemes may be required for different rainbands in TC models.

Tracking a Long-Lasting Outer Tropical Cyclone Rainband: Origin and Convective Transformation

AbstractThis study used radar and surface observations to track a long-lasting outer tropical cyclone rainband (TCR) of Typhoon Jangmi (2008) over a considerable period of time (~10 h) from its formative to mature stage. Detailed analyses of these unique observations indicate that the TCR was initiated on the eastern side of the typhoon at a radial distance of ~190 km as it detached from the upwind segment of a stratiform rainband located close to the inner-core boundary. The outer rainband, as it propagated cyclonically outward, underwent a prominent convective transformation from generally stratiform precipitation during the earlier period to highly organized, convective precipitation during its mature stage. The transformation was accompanied by a clear trend of surface kinematics and thermodynamics toward squall-line-like features. The observed intensification of the rainband was not simply related to the spatial variation of the ambient CAPE or potential instability; instead, the dynamical interaction between the prerainband vertical shear and cold pools, with progression toward increasingly optimal conditions over time, provides a reasonable explanation for the temporal alternation of the precipitation intensity. The increasing intensity of cold pools was suggested to play an essential role in the convective transformation for the rainband. The propagation characteristics of the studied TCR were distinctly different from those of wave disturbances frequently documented within the cores of tropical cyclones; however, they were consistent with the theoretically predicted propagation of convectively generated cold pools. The convective transformation, as documented in the present case, is anticipated to be one of the fundamental processes determining the evolving and structural nature of outer TCRs.

Regional Differences in the Spatial Patterns of North Atlantic Tropical Cyclone Rainbands Through Landfall

Regional Differences in the Spatial Patterns of North Atlantic Tropical Cyclone Rainbands Through Landfall

Identification of Tropical Cyclone Centers in SAR Imagery Based on Template Matching and Particle Swarm Optimization Algorithms

Synthetic aperture radar (SAR) has emerged as a new tool for tropical cyclone (TC) monitoring by providing information on the location of TC centers. However, SAR does not usually cover the entire TC domain due to its limited swath width. In this paper, we develop a procedure to identify the location of the center of a TC when an SAR image only covers the rain band portion of the TC but not the eye. The algorithm is based on both an image processing procedure and the available knowledge of the inherent rain-band structure of a TC. The three-step algorithm includes: 1) applying a Canny edge detector to find the curves associated with rain bands; 2) defining two filter criteria to select the spiral curves that resemble the estimation based on a TC rain-band model; 3) searching for the optimal matching solution using the particle swarm optimization algorithm. Numerical experiments with images without TC eye information show that the proposed method can effectively locate the centers of TCs. We compare the experimental results with the best track data to indicate the accuracy. Then, we compare the inflow angle model and the logarithmic spiral model and find that the inflow angle model is more accurate for TC center identification.

An Algorithm to Enhance Nowcast of Rainfall Brought by Tropical Cyclones through Separation of Motions

ABSTRACT The Hong Kong Observatory operates an in-house developed nowcasting system, namely “Short-range Warning of Intense Rainstorms in Localized Systems (SWIRLS)”, to support the operation of rainstorm and severe weather warnings as well as to provide rainfall nowcast services for the public and for special users in Hong Kong. Aiming to enhancing its performance in nowcast of rainfall brought by tropical cyclones, a new radar echo tracking scheme that separates the motion of the spiraling rain bands from the overall movement of tropical cyclone has been developed. Back-testing with historical cases in the past ten years reveals that the new scheme is more capable of preserving tropical cyclone rain band structures and can enhance forecast skills.

The Boundary Layer Dynamics of Tropical Cyclone Rainbands

Abstract Spiral bands are ubiquitous features in tropical cyclones and significantly affect boundary layer thermodynamics, yet knowledge of their boundary layer dynamics is lacking. Prompted by recent work that has shown that relatively weak axisymmetric vorticity perturbations outside of the radius of maximum winds in tropical cyclones can produce remarkably strong frictional convergence, and by the observation that most secondary eyewalls appear to form by the “wrapping up” of a spiral rainband, the effect of asymmetric vorticity features that mimic spiral bands is studied. The mass field corresponding to an axisymmetric vortex with added spiral vorticity band is constructed using the nonlinear balance equation, and supplied to a three-dimensional boundary layer model. The resulting flow has strong low-level convergence and a marked updraft extending along the vorticity band and some distance downwind. There is a marked along-band wind maximum in the upper boundary layer, similar to observations, which is up to about 20% stronger than the balanced flow. A marked gradient in the inflow-layer depth exists across the band and there is an increase in the surface wind factor (the ratio of surface wind speed to nonlinear-balanced wind speed) near the band. The boundary layer dynamics near a rainband therefore form a continuum with the flow near a secondary eyewall. None of these features are due to convective momentum transports, which are absent from the model. The sensitivities of the flow to band length, width, location, crossing angle, and amplitude are examined, and the possible contribution of boundary layer dynamics to the formation of the tropical cyclone rainbands discussed.