Core Of Tropical Cyclones Research Articles

Parameterization of Vertical Turbulent Transport in the Inner Core of Tropical Cyclones and Its Impact on Storm Intensification. Part I: Sensitivity to Turbulent Mixing Length

Abstract In the inner core of a tropical cyclone, turbulence not only exists in the boundary layer (BL) but can also be generated above the BL by eyewall and rainband clouds. Thus, the treatment of vertical turbulent mixing must go beyond the conventional scope of the BL. The turbulence schemes formulated based on the turbulent kinetic energy (TKE) are attractive as they are applicable to both deep and shallow convection regimes in the tropical cyclone (TC) inner core provided that the TKE production and dissipation can be appropriately determined. However, TKE schemes are not self-closed. They must be closed by an empirically prescribed vertical profile of mixing length. This motivates this study to investigate the sensitivity of the simulated TC intensification to the sloping curvature and asymptotic length scale of mixing length, the two parameters that determine the vertical distribution of a prescribed mixing length. To tackle the problem, both idealized and real-case TC simulations are performed. The results show that the simulated TC intensification is sensitive to the sloping curvature of mixing length but only exhibits marginal sensitivity to the asymptotic length scale. The underlying reasons for such sensitivities are explored analytically based on the Mellor and Yamada level-2 turbulence model and the analyses of azimuthal-mean tangential wind budget. The results highlight the uncertainty and importance of mixing length in the numerical prediction of TCs and suggest that future research should focus on searching for physical constraints on mixing length, particularly in the low- to midtroposphere, using observations and large-eddy simulations. Significance Statement The parametric representation of subgrid-scale turbulent mixing is one of the major sources of uncertainty in numerical predictions of tropical cyclones (TCs). This study investigates how the numerical prediction of TC intensification is affected by the turbulent mixing length, a length scale that is required to close a turbulence scheme formulated based on the turbulent kinetic energy (TKE). The research highlights the uncertainty and importance of mixing length in numerical prediction of TCs and suggests that future research should focus on searching for physical constraints on the mixing length, particularly in the low- to midtroposphere, using observations and large-eddy simulations.

Parameterization of Vertical Turbulent Transport in the Inner Core of Tropical Cyclones and Its Impact on Storm Intensification. Part II: Understanding TC Intensification in a Generalized Sawyer–Eliassen Diagnostic Framework

Abstract An analytical method for diagnosing the interaction between the primary and secondary circulations of a tropical cyclone (TC) and vortex intensification is developed. It includes a diagnostic equation describing the mean secondary circulation of a TC in an unbalanced framework by including the radial eddy forcing in the analytical system. It is an extension of the Sawyer–Eliassen equation (SEE) developed from the strict gradient-wind balance. This generalized SEE (GSEE) remediates some of the limitations of SEE and can be used to diagnose both balanced and unbalanced dynamical processes during the TC evolution. Using GSEE, this study investigates how the tangential and radial eddy forcing affects the TC intensification simulated by the Hurricane Weather Research and Forecasting Model (HWRF) with differently parameterized turbulent mixing. The diagnostic results show that the supergradient component of radial eddy forcing contributes positively to the acceleration of the peak tangential wind, whereas the subgradient component of the radial eddy forcing tends to lower the height of peak tangential wind. The relative importance of negative and positive effects of tangential eddy forcing on TC intensification varies depending on the details of turbulence parameterization. For a turbulent kinetic energy (TKE) scheme used in this study, a large sloping curvature of mixing length in the low troposphere causes the tangential eddy forcing to produce a net positive tangential wind tendency near the location of the peak tangential wind. In contrast, a small sloping curvature of mixing length generates a net negative tangential wind tendency at the peak tangential wind. Significance Statement The interaction between the primary and secondary circulations of a tropical cyclone (TC) plays a key role in TC evolution. Historically, the secondary circulation induced by turbulence and convection is often described by a so-called Sawyer–Eliassen equation (SEE). While SEE has provided much insight into the TC dynamics in the past, the assumption of gradient-wind balance used by SEE prevents it from understanding TC unbalanced dynamics. To remediate the limitation, we extended the analytical framework into the unbalanced regime by including radial eddy forcing in the analytical system and derived a generalized SEE (GSEE). Using GSEE, this study investigates how tangential and radial eddy forcing affects TC intensification. The result highlights the importance of multiple roles that turbulence plays in the intensification of TCs.

The Role of Turbulence in an Intense Tropical Cyclone: Momentum Diffusion, Eddy Viscosities, and Mixing Lengths

Abstract Improved representation of turbulent processes in numerical models of tropical cyclones (TCs) is expected to improve intensity forecasts. To this end, the authors use a large-eddy simulation (with 31-m horizontal grid spacing) of an idealized category 5 TC to understand the role of turbulent processes in the inner core of TCs and their role on the mean intensity. Azimuthally and temporally averaged budgets of the momentum fields show that TC turbulence acts to weaken the maximum tangential velocity, diminish the strength of radial inflow into the eye, and suppress the magnitude of the mean eyewall updraft. Turbulent flux divergences in both the vertical and radial directions are shown to influence the TC mean wind field, with the vertical being dominant in most of the inflowing boundary layer and the eyewall (analogous to traditional atmospheric boundary layer flows), while the radial becomes important only in the eyewall. The validity of the downgradient eddy viscosity hypothesis is largely confirmed for mean velocity fields, except in narrow regions which generally correspond to weak gradients of the mean fields, as well as a narrow region in the eye. This study also provides guidance for values of effective eddy viscosities and vertical mixing length in the most turbulent regions of intense TCs, which have rarely been measured observationally. A generalized formulation of effective eddy viscosity (including the Reynolds normal stresses) is presented. Significance Statement This study uses a turbulence-resolving simulation of a category 5 tropical cyclone to understand the role of turbulence in intense storms. Results show that turbulence clearly modulates storm structure and intensity. This study provides guidance for the values of turbulent quantities (which are usually parameterized in comparatively coarse operational TC forecast models) in scarcely observed regions of intense storms. Furthermore, a complete formulation of the effective eddy viscosities is proposed, incorporating contributions from typically ignored Reynolds normal stress terms.

Influence of region-dependent error growth on the predictability of track and intensity of Typhoon Chan-hom (2020) in high-resolution HWRF ensembles

Enhancing the predictive capabilities for tropical cyclones (TCs) necessitates comprehensive investigations into their predictability. This study employs convection-allowing ensemble forecasts using the Hurricane Weather Research and Forecasting model, integrating perturbed initial conditions to examine error growth and initial condition error-related predictability (IE-predictability) of TCs. Distinctively, our research concentrates on how the predictability of TC track and intensity is influenced by initial errors in various zones: (1) the inner core and outer rainbands (0–350 km), (2) the near environment (350–1300 km), and (3) the far environment (1300–3500 km).Contrary to many prior studies, our findings suggest that the most critical region for initial errors affecting TC track forecasts is within the TC inner structures. For Typhoon Chan-hom (2020), characterized by significant track forecast discrepancies, the initial inaccuracies in the inner core and outer rainbands substantially affect the TC's proximate environment and track, more so than errors in the near and far environments. Regarding TC intensity, the inner core emerges as the most sensitive area. The surface wind configuration of the TC inner core at larger scales (wavenumbers 0–2) maintains predictability for over three days, whereas its structure at finer scales is only predictable for a few hours. Plain language summaryThe forecasting of tropical cyclones (TCs) has seen significant advancements over recent decades. A critical question arises: to what extent can we further enhance the accuracy of TC predictions? Addressing this requires in-depth exploration of TC predictability. In our study, we utilized convection-allowing ensemble forecasts based on the Hurricane Weather Research and Forecasting (HWRF) model, incorporating perturbed initial conditions to analyze the development of forecast errors and the predictability related to these initial errors in TCs. A novel aspect of this research is its emphasis on the impact of initial errors in different regions on the predictability of TC track and intensity. Our key findings are twofold: (1) The region most impacted by initial errors for TC track predictions may not always be the surrounding environment. Instead, errors within the TC's inner core and outer rainbands, owing to their pronounced interplay, can lead to significantly greater track forecast errors compared to those in the near and far environments. (2) As for TC intensity, the inner core emerges as the most susceptible region. The large-scale components (wavenumbers 0–2) of the TC's inherent vortex flow can be forecasted accurately for over four days, largely due to the predictability of the synoptic-scale environment. In contrast, the smaller-scale components (wavenumbers >2) demonstrate predictability for only a few hours.

Convection-permitting simulations reveal expanded rainfall extremes of tropical cyclones affecting South Korea due to anthropogenic warming

AbstractUnderstanding how global warming affects tropical cyclone (TC) intensity and precipitation for target regions is essential to preparing for associated damages but detailed processes remain uncertain. This study provides the first quantification of anthropogenic influences on TC characteristics affecting South Korea using convection-permitting model (CPM) simulations (3 km resolution). For the observed four recent TCs that strongly affected South Korea, CPM simulations were performed under current (ALL) and counterfactual conditions without human influences (NAT). The observed sea surface temperature and lateral boundary conditions were used for ALL while changes attributable to human influences (estimated using CMIP6 multimodel simulations) were removed from observed boundary conditions for NAT runs. ALL experiments captured the observed TC intensity and precipitation reasonably. After removing human influences, TC intensity and precipitation were reduced in NAT experiments. Importantly, areas with extreme precipitation (i.e., having precipitation larger than 150 mm) were found to expand by 16–37% in ALL compared to NAT, which was induced by an enhanced upward motion near the TC core and an increase of background water vapor in line with warming. Further, the role of increased moisture was found to become important as TC moves to mid-latitudes. This study provides valuable insights into how greenhouse warming can intensify TC-induced extreme precipitation over East Asia.

A self-similar description of the wave fields generated by tropical cyclones

Today, advanced operational wave models, e.g. WAM, SWAN or WAVEWATCH-III, provide very accurate solutions. Nevertheless, under extreme weather conditions, surface wave predictions can remain challenging. Indeed, for relatively small-scale tropical cyclones (TCs), rapidly evolving in time and space, and possibly not always well sampled with observing systems, extreme winds may not be properly described, and generated wave systems correctly predicted. In that context, Kudryavtsev et al. (2021b) recently proposed a simplified framework to rapidly assess evolving wave fields under typical TC conditions. Using self-similar functions, termed Tropical Cyclone-Wave Geophysical Model Function (TCW GMF), the proposed methodology and initial results demonstrate robustness and efficiency : 2D functions, assimilating a small number of parameters (maximum wind speed, cyclone radius and translation velocity), provide first-guess estimates of surface wave heights, wave lengths and directions within the intense TC core region. Following this strategy, an improved TCW GMF version is proposed to also cover the TC far zone, providing both wind wave information and outrunning swell conditions. This new version more particularly accounts for the wave field sensitivity to the shape of the wind profile. The procedure follows three main steps: (1) estimation of the characteristics of pure wind waves using self-similar matrices; (2) determination of the contour limiting the transition between wind waves to swell regime using empirically-derived universal functions; (3) derivation of analytical functions to describe the swell parameters using initial parameters estimated at this transition contour. Wind waves and swell systems are further superposed to describe the wave parameters for mixed-sea conditions. In this study, IBTrACS are used to initialize the TC’s wind profiles, coordinates and translation velocities. The proposed methodology is then tested using a large altimeter database. More than 700 altimeter measurements crossing different TCs during 2020–2022 years are used, demonstrating overall convincing agreements between first-guess estimates and satellite data.

Estimating the tropical cyclone wind structure using physics-incorporated networks

Satellite-based remote sensing technology plays a significant role in identifying tropical cyclones (TCs), and most of the current research focuses on intensity estimation. However, analyzing the wind structure of TCs, which is directly related to the danger they bring, remains a challenge. By adding prior knowledge of TCs into the model, we propose a physics-incorporated network based on multi-task learning to estimate wind radii and intensity, whose layers can automatically extract rotation-invariant features related to the TC core from multichannel satellite imageries. In addition, we build a more comprehensive dataset, including global Statistical Hurricane Intensity Prediction Scheme (SHIPS) predictors, to tackle the structure task. We compare our model with existing methods, and it shows that our model gets better results in estimating 50-knot and 64-knot wind radii and achieves a 4.87-knot root-mean-squared error (RMSE) of intensity. By predicting probability density functions, our model quantifies the uncertainty of the result. The experimental results show that the incorporation of rotation equivariance into the layers can enhance TC structure estimation. By considering the feature importance of multi-source predictors, we find that our model pays attention to key predictors related to the TC structure. Specifically, the tangential wind speed at 500 km from the TC center and the radius of the 5-knot wind both greatly reduce the error of the estimated parameters. Finally, two case studies show that the proposed model performs well most of the time during TCs’ rapid intensification. However, when TCs’ system is not well organized, estimating the wind structure is challenging.

A Tale of Two Vortex Evolutions: Using a High-Resolution Ensemble to Assess the Impacts of Ventilation on a Tropical Cyclone Rapid Intensification Event

Abstract The multiscale nature of tropical cyclone (TC) intensity change under moderate vertical wind shear was explored through an ensemble of high-resolution simulations of Hurricane Gonzalo (2014). Ensemble intensity forecasts were characterized by large short-term (36-h) uncertainty, with a forecast intensity spread of over 20 m s−1, due to differences in the timing of rapid intensification (RI) onset. Two subsets of ensemble members were examined, referred to as early-RI and late-RI members. The two ensemble groups displayed significantly different vortex evolutions under the influence of a nearby upper-tropospheric trough and an associated dry-air intrusion. Mid-to-upper-tropospheric ventilation in late-RI members was linked to a disruption of inner-core diabatic heating, a more tilted vortex, and vortex breakdown, as the simulated TCs transitioned from a vorticity annulus toward a monopole structure. A column-integrated moist static energy (MSE) budget revealed the important role of horizontal advection in depleting MSE from the TC core, while mesoscale subsidence beneath the dry-air intrusion acted to dry a deep layer of the troposphere. Eventually, the dry-air intrusion retreated from late-RI members as vertical wind shear weakened, the magnitude of vortex tilt decreased, and late-RI members began to rapidly intensify, ultimately reaching a similar intensity as early-RI members. Conversely, the vortex structures of early-RI members were shown to exhibit greater intrinsic resilience to tilting from vertical wind shear, and early-RI members were able to fend off the dry-air intrusion relatively unscathed. The different TC intensity evolutions can be traced back to differences in the initial TC vortex structure and intensity. Significance Statement Despite recent advances, tropical cyclone intensity forecasts struggle to accurately predict episodes of rapid intensification. Such forecasts become increasingly challenging when a storm is embedded within an environment of moderate vertical wind shear. This study uses an ensemble of high-resolution simulations to examine how environmental influences can affect the tropical cyclone vortex and precipitation structure, which, in turn, modulate the intensity of the storm and the onset of rapid intensification. We propose a feedback that exists where slightly weaker and less resilient vortices are more susceptible to ventilation from dry, environmental air, aided in part by differential advection from the tilted circulation, resulting in a degradation of vortex organization and a delayed onset of rapid intensification.

On the dynamics of the eradication of a warm core mesoscale eddy after the passage of Hurricane Irma (2017)

Mesoscale oceanic eddies are known to influence air-sea interactions during tropical cyclone (TC) passage by either hindering or sustaining air-sea fluxes into the TC core. However, little is known as to how TCs alter the dynamical structure of such eddies during and after TC passage. Through simulations of Hurricane Irma (2017) using the Navy’s Coupled Ocean Atmospheric Mesoscale Prediction Scheme for Tropical Cyclones, it is found that a simulated mesoscale warm core eddy (WCE) located to the right of the model track disappeared shortly after the TC had passed over it. To further explore this phenomena, time series of ocean kinetic energy, available potential energy (APE), a vorticity budget, and a potential vorticity (PV) budget are examined to understand how TC-induced upper ocean currents influenced the eddy disappearance. Time series of APE within the model-simulated eddy show APE reaching a minimum as the circulation of the eddy disappeared in the model field and positive vorticity, dominated by vertical vorticity advection, became a maximum approximately 6 h after TC passage. These results were confirmed by the PV budget. The succession of subsurface vorticity changes suggest that the WCE was eradicated by a near-inertial wave wake that occurred earlier than expected. As found in a prior study, the early-onset of a near-inertial wake occurs when the model simulated TC with a small radius of maximum winds interacts with the subsurface ocean current field. The findings are important with regards to TC forecasting because the accurate representation and placement of eddies in the model field is necessary to correctly predict the coupled air-sea response during TC passage and for subsequent TCs with similar tracks. Such feedbacks from TCs can also have near-term and long-term influence on the prediction of air-sea interaction in the local and regional marine environment.

Simulated dynamics and thermodynamics processes leading to the rapid intensification of rare tropical cyclones over the North Indian Oceans

The life cycle dynamics and intensification processes of three long-duration tropical cyclones (TCs), viz., Fani (2019), Luban (2018), and Ockhi (2017) formed over the North Indian Ocean (NIO) have been investigated by developing a high-resolution (6 km × 6 km) mesoscale analysis using WRF and En3DVAR data assimilation system. The release of CAPE in nearly saturated middle-level relative humidity caused intense diabatic heating, leading to an increase in low-level convergence triggering rapid intensification (RI). The strengthening of the relative vorticity tendency terms was due to vertical stretching (TC Fani) and middle tropospheric advection (TCs Luban and Ockhi). The increase or decrease in upper-tropospheric divergence led to RI through two different mechanisms. The increase in upper divergence strengthens the vortical convection (in TC Luban and Fani) by enhancing the moisture and heat transport, whereas its decrease caused a reduction in the upper-level ventilation flow at 200 hPa followed by moisture accumulation, enhanced diabatic heating, and strengthened the warm core (TC Ockhi). The RI caused the vortex of three cyclones to extend up to the upper troposphere. The well organised wind during RI led the unorganised, weak, discontinuous vertical vortex columns to become organised with intense vertical velocity throughout the column. Spatial distributions of Okubo–Wiess (OW) parameter showed TC core dominated by vorticity than strain, since deep depression (DD) stages.

An Analysis of Tropical Cyclone Vortex and Convective Characteristics in Relation to Storm Intensity Using a Novel Airborne Doppler Radar Database

Abstract This analysis introduces a novel airborne Doppler radar database, referred to as the Tropical Cyclone Radar Archive of Doppler Analyses with Re-centering (TC-RADAR). TC-RADAR comprises over 900 analyses from 273 flights into TCs in the North Atlantic, eastern North Pacific, and central North Pacific basins between 1997 and 2020. This database contains abundant sampling across a wide range of TC intensities, which facilitated a comprehensive observational analysis on how the three-dimensional, kinematic TC inner-core structure is related to TC intensity. To examine the storm-relative TC structure, we implemented a novel TC center-finding algorithm. Here, we show that TCs below hurricane intensity tend to have monopolar radial profiles of vorticity and a wide range of vortex tilt magnitudes. As TC intensity increases, vorticity becomes maximized within an annulus inward of the peak wind, the vortex decays more slowly with height, and the vortex tends to be more aligned in the vertical. The TC secondary circulation is also strongly linked to TC intensity, as more intense storms have shallower and stronger lower-tropospheric inflow as well as larger azimuthally averaged ascent. The distribution of vertical velocity is found to vary with TC intensity, height, and radial domain. These results—and the capabilities of TC-RADAR—motivate multiple avenues for future work, which are discussed. Significance Statement Acquiring observations of the inner core of tropical cyclones (TCs) is a challenge due to the hazardous conditions inherent to the storm. A proven method of sampling the TC core region is the use of airborne radar. This study presents a novel database comprising over 900 airborne radar analyses collected in storms between 1997 and 2020, which is freely available to the research community. Here we demonstrate the utility of the database by examining how the three-dimensional structure of the TC core region changes depending upon the intensity of the storm. By identifying how the baseline TC vortex structure varies with TC intensity, this work provides the foundation for multiple future research avenues and model evaluation efforts.

Characterization of Tropical Cyclone Intensity Using the HY-2B Scatterometer Wind Data

The estimation of tropical cyclone (TC) intensity using Ku-band scatterometer data is challenging due to rain perturbation and signal saturation in the radar backscatter measurements. In this paper, an alternative approach to directly taking the maximum scatterometer-derived wind speed is proposed to assess the TC intensity. First, the TC center location is identified based on the unique characteristics of wind stress divergence/curl near the TC core. Then the radial extent of 17-m/s winds (i.e., R17) is calculated using the wind field data from the Haiyang-2B (HY-2B) scatterometer (HSCAT). The feasibility of HSCAT wind radii in determining TC intensity is evaluated using the maximum sustained wind speed (MSW) in the China Meteorological Administration best-track database. It shows that the HSCAT R17 value generally better correlates with the best-track MSW than the HSCAT maximum wind speed, therefore indicating the potential of using the HSCAT data to improve the TC nowcasting capabilities.

Simulating the Effects of Land Surface Characteristics on Planetary Boundary Layer Parameters for a Modeled Landfalling Tropical Cyclone

This study examined whether varying moisture availability and roughness length for the land surface under a simulated Tropical Cyclone (TC) could affect its production of precipitation. The TC moved over the heterogeneous land surface of the southeastern U.S. in the control simulation, while the other simulations featured homogeneous land surfaces that were wet rough, wet smooth, dry rough, and dry smooth. Results suggest that the near-surface atmosphere was modified by the changes to the land surface, where the wet cases have higher latent and lower sensible heat flux values, and rough cases exhibit higher values of friction velocity. The analysis of areal-averaged rain rates and the area receiving low and high rain rates shows that simulations having a moist land surface produce higher rain rates and larger areas of low rain rates in the TC’s inner core. The dry and rough land surfaces produced a higher coverage of high rain rates in the outer regions. Key differences among the simulations happened as the TC core moved over land, while the outer rainbands produced more rain when moving over the coastline. These findings support the assertion that the modifications of the land surface can influence precipitation production within a landfalling TC.

The Asymmetric Precipitation Evolution in Weak Landfalling Tropical Cyclone Rumbia (2018) Over East China

The rainfall in landfalling TC is not always correlated with the storm intensity. Some weak landfalling TCs could bring extremely heavy rainfall during and after landfall. Such extreme events are very challenging to operational forecasts and often lead to disasters in the affected regions. Tropical storm Rumbia (2018) made its landfall in Shanghai with weak intensity but led to long-lasting and increasing rainfall to East China. The asymmetric rainfall evolution of Rumbia during and after its landfall was diagnosed based on the fifth generation European Centre for Medium-Range Weather Forecasting (ECMWF) reanalysis (ERA5) data, the tropical cyclone (TC) best-track data, and rainfall observations from China Meteorological Administration (CMA). Results showed that Rumbia was embedded in an environment with a deep-layer (300–850 hPa) southwesterly vertical wind shear (VWS). The maximum rainfall mostly occurred downshear-left in its inner-core region and downshear-right in the outer-core region. The translation of Rumbia also contributed to the rainfall distribution to some extent, especially prior to and just after its landfall. The strong southwesterly-southeasterly summer monsoon flow transported water vapor from the tropical ocean and the East China Sea to the TC core region, providing moisture and convective instability conditions in the mid-lower troposphere for the sustained rainfall even after Rumbia moved well inland. The results also showed that the low-level convective instability and the deep-layer environmental VWS played an important role in deepening the inflow boundary layer and the redevelopment of the secondary circulation, thus contributing to the heavy rainfall in the northeast quadrant of Rumbia after its landfall. However, further in-depth studies are recommended in regard of the rainfall evolution in the weak TCs. This study further calls for a continuous understanding of the involved physical processes/mechanisms that are responsible for the extreme rainfall induced by landfalling TCs, which can help improve the rainfall forecast skills and support damage mitigation in the future.

Impacts of multi‐timescale circulations on meridional heat transport

AbstractRelative contributions to the zonal mean meridional heat transport by the climatological annual mean, climatological annual variation, synoptic, intra‐seasonal and lower‐frequency motions were examined based on the ERA‐Interim reanalysis data for the period of 1981–2015. The meridional heat transport analysed in this study only includes the component related to meridional wind and temperature. In the tropics, the climatological annual mean circulations dominate the long‐term mean meridional heat transport, while the interaction between the climatological annual mean temperature and the seasonal anomalous flow largely contributes to the seasonal variation of the meridional heat transport. In the middle latitudes, the climatological annual mean circulations and transient eddies (mostly synoptic and intra‐seasonal eddies) are of roughly equal importance in the poleward heat transport, leading to the maximum poleward heat transport around 50°N/S. The upper‐ and lower‐tropospheric heat transports by the climatological annual mean circulations appear opposite, with the magnitude of the lower‐tropospheric transport being greater. The preferred maximum zonal mean heat transport at 50°N by the climatological mean flow is attributed to the maximum zonal mean low‐level southerly in situ. The preferred peak latitude of the mid‐latitude poleward heat transport by synoptic eddies near 50°N arises from the combined effect of the strong synoptic‐scale meridional wind and temperature variabilities in situ and their in‐phase relationship. The heat transport by tropical cyclones (TCs) was estimated by applying a statistical relationship between TC intensity and the vertically integrated temperature averaged over the TC core region derived from high‐resolution Weather Research and Forecasting (WRF) model simulations. For northern hemisphere summer, TCs contribute about 35% of the total heat transport in the active TC regions, suggesting that TCs play a critical role in the regional meridional heat transport.

Rainfall asymmetries of the western North Pacific tropical cyclones as inferred from GPM

AbstractIt has been revealed that the vertical wind shear (VWS) can result in the rainfall asymmetry of tropical cyclone (TC), where the VWS is conventionally defined as the environmental horizontal winds between 200‐ and 850‐hPa levels within a certain area from the TC centre. The rainfall maximum is generally displaced downshear to downshear‐left side in the Northern Hemisphere or downshear to downshear‐right side in the Southern Hemisphere. Here, we revisit the rainfall asymmetries of TCs over the western North Pacific using the state‐of‐art Global Precipitation Measurement data. Observations pose that using the conventional VWS to interpret the TC rainfall asymmetry is inopportune. The conventional VWS can only largely grasp the rainfall asymmetry in the inner core of TC, but not in the outer core. We, therefore, explicitly propose a new VWS, namely the effective VWS, to fill this gap. The effective shear layer responsible for the rainfall asymmetry is a function of TC intensity and core region. In addition, we suggest that the angle difference between the effective shears for the inner‐core and outer‐core rainfall asymmetries can help to comprehend the spirality of the rainfall asymmetries that are, the fashions, or patterns of the rainbands. Meanwhile, the moisture flux convergence and storm motion show little effect on the rainfall asymmetry. This study advances the understanding of TC rainfall asymmetry and provides scientific support for disaster prediction, prevention, and mitigation.

Effect of a Low-Frequency Vortex on the Size of Typhoon Lan (2017)

AbstractTyphoon Lan (2017) was one of the largest tropical cyclones (TC) in the western North Pacific Ocean (WNP), and it was developed in a low-frequency (10–90-day filtered) large-scale cyclonic vortex environment. The physical mechanism responsible for the TC’s unusual size was investigated through idealized numerical experiments with the Weather Research and Forecasting Model. Sensitivity experiments showed that the low-frequency cyclonic circulation played an important role in modulating the TC size through the following three processes. First, it weakened the background vertical wind shear and provided a favorable condition for a more rapid growth of Lan. Second, it strengthened a vorticity aggregation process through enhanced background vorticity. As a result, a stronger and more organized TC core was quickly set up, which strengthened the TC intensity and expanded its size. Third, it enhanced the total surface wind speed and surface latent heat flux, strengthening convective instability in the outer region through increased moisture. The development of the outer rainband expanded the radial profile of diabatic heating, leading to greater low-level inflow and tangential wind acceleration in the outer region and thus a large TC size.

Impact of climate change on intense Bay of Bengal tropical cyclones of the post-monsoon season: a pseudo global warming approach

Tropical cyclones (TCs) that make landfall over India’s east coast are responsible for significant loss of life along affected coastlines. TCs forming over the Bay of Bengal (BoB) region in October, November, and December have, in the past, intensified significantly at the time of landfall. The effects of climate change on TCs of different strengths and their characteristics such as track, intensity, precipitation, and convective available potential energy over the BoB region have not been well studied. This study explores the effects of climate change on two TCs of very severe cyclonic storm (VSCS) category (TC Vardah and TC Madi), and two other TCs of extremely severe cyclonic storm (ESCS) category (TC Hudhud and TC Phailin) formed over BoB, both in the short term (2035) and long term (2075). The high-resolution Weather Research and Forecasting (WRF) model is used to simulate the TCs under current and future climate conditions. The simulated TC track and intensity in the current climate agree well with the observations. To explore the impacts of climate change on TCs, the mean climate change signal, computed from future projections of the Community Climate System Model (CCSM4) in different representative concentration pathway (RCP) scenarios, is added to current climate conditions by using the pseudo global warming method. Results show a climate change-related reduction in TC translation speed, deepening of TC core, increased maximum surface wind, and increased precipitation over land in future RCP (4.5, 6.0, and 8.5) scenarios. The TCs in future RCP scenarios are seen to be more intensified compared to current climate simulations. Results demonstrate that all VSCS and ESCS category TCs considered in this study are likely to further intensify to the next higher category level with respect to their current classification, particularly in the far future RCP 6.0 and in the far future RCP 8.5 scenarios. The cyclone damage potential index of TC Vardah, TC Hudhud, and TC Phailin is projected to increase in a future warming climate.

Evaluation of a Physics-Based Tropical Cyclone Rainfall Model for Risk Assessment

AbstractHeavy rainfall generated by landfalling tropical cyclones (TCs) can cause extreme flooding. A physics-based TC rainfall model (TCRM) has been developed and coupled with a TC climatology model to study TC rainfall climatology. In this study, we evaluate TCRM with rainfall observations made by satellite (of North Atlantic TCs from 1999 to 2018) and radar (of 36 U.S. landfalling TCs); we also examine the influence on the rainfall estimation of the key input to TCRM—the wind profile. We found that TCRM can simulate relatively well the rainfall from TCs that have a coherent and compact structure and limited interaction with other meteorological systems. The model can simulate the total rainfall from TCs well, although it often overestimates rainfall in the inner core of TCs, slightly underestimates rainfall in the outer regions, and renders a less asymmetric rainfall structure than the observations. It can capture rainfall distribution in coastal areas relatively well but may underestimate rainfall maximums in mountainous regions and has less capability to accurately simulate TC rainfall in higher latitudes. Also, it can capture the interannual variability of TC rainfall and averaged features of the time series of TC rainfall but cannot accurately reproduce the probability distribution of short-term (1 h) rainfall. Among the tested theoretical wind profile inputs to TCRM, a complete wind profile that accurately describes the wind structure in both the inner ascending and outer descending regions of the storm is found to perform the best in accurately generating various rainfall metrics.

A numerical study of convective-scale downdrafts in the outer core of tropical cyclones in vertically varying environmental flows

The characteristics of convective-scale downdrafts in the outer core of tropical cyclones in the lower- and upper-layer vertical wind shear (VWS) are investigated based on two high-resolution idealized numerical experiments. Four types of outer-core downdrafts, originating from the lower troposphere, the midtroposphere, the upper level, and the tropopause, respectively, are found. The downdrafts originating from the lower and midtroposphere can penetrate down near the surface, and those originating from the tropopause in upper-layer VWS tend to penetrate more downward than in lower-layer VWS. Downdrafts tend to be located in the more upwind portion of the downshear-right quadrant in lower-layer VWS than in upper-layer VWS. The frequency of downdrafts outside and upwind of the parent updraft increases with the increasing downdraft top height. Vertical momentum budgets indicate that downward-oriented buoyancy due to the evaporational cooling of rainwater and precipitation drag mainly contribute to the occurrence of low-level downdrafts, and the midlevel and upper-level downdrafts originate due to precipitation drag and are strengthened by the downward-oriented, buoyancy-induced perturbation pressure gradient. The processes governing the downdrafts from the tropopause are different between the two experiments. More icy-type particles are produced and transported outward at upper levels in the lower-layer shear experiment, resulting in larger downward-oriented buoyancy due to the sublimational cooling of icy-type particles and contributing to the development/maintenance of the downdraft from the tropopause in that experiment. However, the downward-oriented perturbation pressure gradient leads to the development/maintenance of the downdraft from the tropopause in the upper-layer shear experiment.