Tropical Cyclone Eyewall Research Articles

Reducing a Tropical Cyclone Weak-Intensity Bias in a Global Numerical Weather Prediction System

Abstract The operational Canadian Global Deterministic Prediction System suffers from a weak-intensity bias for simulated tropical cyclones. The presence of this bias is confirmed in progressively simplified experiments using a hierarchical system development technique. Within a semi-idealized, simplified-physics framework, an unexpected insensitivity to the representation of relevant physical processes leads to investigation of the model’s semi-Lagrangian dynamical core. The root cause of the weak-intensity bias is identified as excessive numerical dissipation caused by substantial off-centering in the two time-level time integration scheme used to solve the governing equations. Any (semi)implicit semi-Lagrangian model that employs such off-centering to enhance numerical stability will be afflicted by a misalignment of the pressure gradient force in strong vortices. Although the associated drag is maximized in the tropical cyclone eyewall, the impact on storm intensity can be mitigated through an intercomparison-constrained adjustment of the model’s temporal discretization. The revised configuration is more sensitive to changes in physical parameterizations and simulated tropical cyclone intensities are improved at each step of increasing experimental complexity. Although some rebalancing of the operational system may be required to adapt to the increased effective resolution, significant reduction of the weak-intensity bias will improve the quality of Canadian guidance for global tropical cyclone forecasting. Significance Statement Global numerical weather prediction systems provide important guidance to forecasters about tropical cyclone development, motion, and intensity. Despite recent improvements in the Canadian operational model’s ability to predict tropical cyclone formation, the system systematically underpredicts the intensity of these storms. In this study, we use a set of increasingly simplified experiments to identify the source of this error, which lies in the numerical time-stepping scheme used to solve the model equations. By decreasing numerical drag on the tropical cyclone circulation, intensity predictions that resemble those of other global modeling systems are achieved. This will improve the quality of Canadian tropical cyclone guidance for forecasters around the world.

Microphysics of Heavy Rain Associated With the Eyewall and Inner Rainbands of Typhoon Meranti (2016)

AbstractHeavy rain in a tropical cyclone (TC) tends to occur in its eyewall and inner spiral rainbands. The distinct dynamical and thermodynamical structures between TC eyewall and inner rainbands were argued to be responsible for different rain microphysics by previous studies. Thus, this study investigated the microphysical characteristics of heavy rain (≥10 mm hr−1) associated with the eyewall and inner rainbands of Typhoon Meranti (2016), based on the joint observations of two disdrometers and a dual‐polarization radar (DPR) as well as automatic weather stations in Fujian province of China. The surface disdrometer observation showed that the eyewall rain has a larger mean raindrop diameter and a smaller mean concentration than the inner‐rainband rain. In general agreement with the disdrometer observation, the DPR observation confirmed larger sizes of raindrops near the ground in the eyewall. However, the vertical profiles of polarimetric variables showed larger values almost throughout the atmosphere in the eyewall, namely that more efficient ice‐ and warm‐cloud processes are evident for the production of ice and rain particles. This result looks contradictory to the lower concentration of the eyewall rain than the inner‐rainband rain as measured by surface disdrometers. This is because stronger updrafts observed in the eyewall not only facilitate the production and growth of hydrometeors, but also prevent small raindrops from falling to the ground. Consequently, the surface rain in the eyewall has larger raindrop diameter and lower concentration than that in the inner rainbands, despite a larger number of ice and rain particles in the atmosphere.

M-PERC: A New Satellite Microwave-Based Model to Diagnose the Onset of Tropical Cyclone Eyewall Replacement Cycles

Abstract Eyewall replacement cycles (ERCs) in tropical cyclones (TCs) are generally associated with rapid changes in TC wind intensity and broadening of the TC wind field, both of which can create unique forecasting challenges. As part of the NOAA Joint Hurricane Testbed Project, a new model was developed to provide operational probabilistic guidance on ERC onset. The model is based on the time evolution of TC wind intensity and passive satellite microwave imagery and is named “M-PERC” for Microwave-Based Probability of Eyewall Replacement Cycle. The model was initially developed in the Atlantic basin but is found to be globally applicable and skillful. The development of M-PERC and its performance characteristics are described here, as well as a new intensity prediction model that extends previous work. Application of these models is expected to contribute to a reduction of TC intensity forecast error.

Aerosol Indirect Effects on Water Vapor in the UTLS of Typhoon Saomai (2006)

AbstractThe evolution of landfalling Typhoon Saomai (2006) is simulated using the Weather Research and Forecasting model with spectral bin microphysics scheme, showing tropical cyclone (TC) could vertically transport water vapor from troposphere to upper troposphere and lower stratosphere (UTLS). Three aerosol sensitivity experiments are conducted to explore the indirect effects of aerosol served as cloud condensation nuclei (CCN) on overshooting convection (OTC) and water vapor content in UTLS, namely MAR (maritime), MIX (semi‐continental) and CON (continental). Results revealed that CON simulation with high initial aerosol concentration showing invigorated convection at the periphery of the TC, which interfered with the reformation of TC eyewall, leading to TC weakening finally. In addition, CON simulated the least water content in UTLS through suppressing the number of convection overshooting the tropopause level. Most overshooting events take place during TC developing stage in the eyewall region before landfall. Higher levels of CCN concentration could lead to decreased number of OTC in TC, consistently reduced the water vapor content in UTLS. CON and MIX experiments simulated respectively 21.9% (16.2%) and 9.96% (1.5%) less OTC (water vapor content in UTLS) than MAR simulation. Water vapor content in UTLS is affected by the interaction of OTC and ice‐phase microphysics. The interesting point is that the CCN sensitivity experiments show that water vapor content in UTLS of a TC is sensitive to the CCN concentration in the boundary layer.

Modifications to Three‐Dimensional Turbulence Parameterization for Tropical Cyclone Simulation at Convection‐Permitting Resolution

AbstractAdequate representation of the subgrid‐scale (SGS) turbulent fluxes associated with convective clouds in the eyewall and rainbands above the boundary layer is important for simulating the formation of tropical cyclone (TC) dynamic and thermal structure, as well as the evolution and intensification of the TC. Two sets of benchmark large‐eddy simulations (LESs) for an idealized TC during the rapid intensification and mature stages were conducted. The turbulent transport above the boundary layer in the TC eyewall and rainbands exhibits a remarkable countergradient characteristic, which is poorly represented by the traditional eddy‐diffusivity closure. In contrast, the H‐gradient closure based on the horizontal gradients of the resolved variables is capable of accurately capturing the countergradient features and exhibiting a spatial distribution of SGS fluxes that mimics much better the coarse‐grained fluxes from the LES benchmarks. Moreover, the H‐gradient closure allows for the backscatter transfer of energy. By implementing the H‐gradient closure into a three‐dimensional turbulence parameterization, the TC simulated using the modified parameterization bears closer resemblance to the LES benchmarks in terms of the spatial distribution of SGS fluxes, TC intensity, primary and secondary circulations, and cloud morphology.

Numerical Study of the Response of Typhoon Hato (2017) to Grouped Mesoscale Eddies in the Northern South China Sea

AbstractThe influences of grouped anticyclonic eddies (AEs) in the ocean on the intensity of tropical cyclone (TC) in the atmosphere are investigated in the present study. The research was carried out using numerical methods taking the development of TC Hato (2017) in the northern South China Sea (SCS) as an example. Three AEs were observed, one at the TC track (defined as the inner area) and the other two in the south of the TC center (defined as the outer area), at about three times the radius of maximum wind speed. The results show that the outer AEs suppress TC development, but the inner AEs greatly favor TC development. This opposite influence primarily results from the different responses of TC secondary circulation to the AE‐induced local sea surface warm anomaly in the inner and outer areas. The outer AE triggers a low sea level pressure anomaly and convergent wind toward the AE, which weakens TC inflow at the lower layers and further weakens TC secondary circulation. Consequently, the energy conversion from ocean heat energy to TC kinetic energy is decreased by the weakened TC secondary circulation. The AE in the outer area also extends the TC eyewall, weakens the TC warm core, and increases the outer precipitation. As a result, the TC intensity is weakened. The inner AE causes opposite changes in TC secondary circulation, eyewall, warm core, and outer precipitation, so that favors TC development. These results provide evidence for the suppression effect of the outer AEs on TC development, providing a new perspective toward improving TC intensity forecasts.

An Evaluation of Tropical Cyclone Rainfall Structures in the HighResMIP Simulations against Satellite Observations

Abstract This study evaluates tropical cyclone (TC) rainfall structures in the CMIP6 HighResMIP global climate model (GCM) simulations against satellite rainfall retrievals. We specifically focus on TCs within the deep tropics (25°S–25°N). Analysis of TC rain rate composites indicates that in comparison to the satellite observations at the same intensity, many HighResMIP simulations tend to overproduce rain rates around TCs, in terms of both maximum rain rate magnitude and area-averaged rain rates. In addition, as model horizontal resolution increases, the magnitude of the peak rain rate appears to increase. However, the area-averaged rain rates decrease with increasing horizontal resolution, partly due to the TC eyewall being located closer to the TC center, thus occupying a smaller area and contributing less to the area-averaged rain rates. The effect of ocean coupling is to lower the TC rain rates, bringing them closer to the satellite observations, due to reduced horizontal moisture flux convergence and surface latent heat flux beneath TCs. Examination of horizontal rain rate distributions indicates that vertical wind shear–induced rainfall asymmetries in HighResMIP-simulated TCs are qualitatively consistent with the observations. In addition, a positive relationship is observed between the area-averaged inner-core rainfall and TC intensification likelihoods across the HighResMIP simulations, as GCM simulations producing stronger TCs more frequently have the greater rainfall close to the center, in agreement with previous theoretical and GCM simulation results.

Wind Gusts Associated with Tornado-Scale Vortices in the Tropical Cyclone Boundary Layer: A Numerical Simulation

It has been demonstrated that the tornado-scale vortex (TSV) is one of the fine-scale structures associated with extreme updrafts in the tropical cyclone boundary layer (TCBL), but the relationship between surface wind gusts and TSVs is still unclear. In this study, the one-second model output simulated in the Weather Research and Forecast (WRF) model with the large eddy simulation (WRF-LES) is used to investigate the relationships between TSVs and surface wind gusts. Results show that surface wind gust factors in the regions where TSVs are prevalent are significantly larger than those in other regions. 88% of the maximum gust factors associated with TSVs (vertical velocity larger than 20 m s−1) are larger than 1.25 (gust factors larger than 1.25 account for only 1% of the 1-min gust factors in the TC inner core), and the mean maximum 1-min gust factor associated with a TSV is larger than 1.3, while the mean 1-min gust factor in the TC inner core is only 1.1. The surface gust factors associated with TSVs in tropical cyclone eyewall can reach about 1.8, which can cause severe surface wind hazards. This study suggests that potential risk will increase in the regions where TSVs are prevalent because of the large wind gusts and gust factors. Finer real-time observations are needed to monitor the evolution of TSVs for improving the operational TC-related surface gust warnings.

A SAR-Based Parametric Model for Tropical Cyclone Tangential Wind Speed Estimation

The tangential wind speed increases from the center to the eyewall of tropical cyclones (TC) along the radial direction and begins to decay when it extends outward. The tangential wind profile model is one of the most effective and widely used methods to reconstruct the TC radial wind speed. This paper proposes a parametric tangential wind profile (TWP) model based on high-spatial-resolution SAR imagery. The new model functions are piecewise with maximum tangential wind speed as a threshold, and all of them are designed as nonlinear. Notably, the derivative at the segmentation threshold is zero to ensure a smooth transition of the estimated wind speed profile. With the SAR-derived azimuth-averaged wind speed, we can determine the model parameters and get the tangential wind speed. The TWP model outperforms the commonly used SMRV model, as it better resolves the tangential wind profile shape as depicted by both SAR-derived winds and hurricane hunter Stepped-Frequency Microwave Radiometer (SFMR) derived winds. A comprehensive analysis of the TWP model parameters is carried out by fitting tangential winds for 620 hurricane hunter flights. Interestingly, the tangential wind profiles for major hurricanes show a similar shape. The proposed TWP model can be used for improved TC characterization and forecasting purposes.

Negative Pressure Perturbations Associated With Tornado‐Scale Vortices in the Tropical Cyclone Boundary Layer

AbstractDangerous turbulences were observed in the eyewall of category 5 Hurricane Hugo (1989) and the surface pressure associated with the turbulence was 8 hPa lower than the mean pressure in the eye. In this study, it is demonstrated that small scale, negative pressure perturbations in the tropical cyclone eyewall are associated with the tornado‐scale vortex (TSV) simulated with the large‐eddy simulation technique in a semi‐idealized numerical experiment. The negative pressure perturbation associated with TSVs is nonhydrostatic, dynamically resulting from the shearing effect. As a result, the negative pressure perturbation is linearly correlated with the vertical component of relative vorticity and maximum wind perturbation. Since the TSV is prevalent near the inner edge of the tropical cyclone eyewall, it is suggested that caution should be taken to estimate the central pressure of tropical cyclones with the extremely low pressure observed in the eyewall.

Improved Climatology of Tropical Cyclone Precipitation from Satellite Passive Microwave Measurements

AbstractAccurate precipitation climatology is presented for tropical depression (TD), tropical storm (TS), and tropical cyclone (TC) over oceans using the recently-released, consistent and high quality precipitation datasets from all passive microwave sensors covering 1998-2012 along with the Automated Rotational Center Hurricane Eye Retrieval (ARCHER)-based TC center positions. Impacts with respect to the direction of both TC movement and the 200-850 hPa wind shear on the spatial distributions of TC precipitation are analyzed. The TC eyewall contraction process during its intensification is noted by a decrease in the radius of maximum rainrate with an increase in TC intensity. For global TCs, the maximum rainrate with respect to the direction of TC movement is located in the down-motion quadrants for TD, TS, and Cat 1-3 TCs, and in a concentric pattern for Cat 4-5 TCs. A consistent maximum TC precipitation with respect to the direction of the 200-850 hPa wind shear is shown in the down shear left quadrant (DSLQ). With respect to direction of TC movement, spatial patterns of TC precipitation vary with basins and show different features for weak and strong storms. The maximum rainrate is always located in DSLQ for all TC categories and basins, except the Southern Hemisphere basin where it is in the down shear right quadrant (DSRQ). This study not only confirms previously published results on TC precipitation distributions relative to vertical wind shear direction, but also provides a detailed distribution for each TC category and TS, while TD storms display an enhanced rainfall rate ahead of the down shear quadrants.

The Impact of Storm-Induced SST Cooling on Storm Size and Destructiveness: Results from Atmosphere-Ocean Coupled Simulations

In this study, both an atmospheric model [Weather Research and Forecasting (WRF) model] and an atmosphere (WRF)-ocean (Princeton Ocean Model; POM) coupled model are used to simulate the tropical cyclone (TC) Kaemi (2006). By comparing the simulation results of the models, effects of oceanic elements, especially the TC-induced sea surface temperature (SST) cooling, on the simulated TC size and destructiveness are identified and analyzed. The results show that there are no notable differences in the simulated TC track and its intensity between the uncoupled and coupled experiments; however, there are large differences in the TC size (i.e., the radius of gale-force wind) between the two experiments, and it is the TC-induced SST cooling that decreases the TC size. The SST cooling contributes to the decrease of air-sea moisture difference (ASMD) outside the TC eyewall, which subsequently leads to the decreases in surface enthalpy flux (SEF), radial sea-level pressure gradient, absolute vorticity advection, and wind speed outside the TC eyewall. As a result, the TC size and size-dependent TC destructive potential all decrease remarkably.

Simulation of Extreme Updrafts in the Tropical Cyclone Eyewall

Strong vertical motion (>10 m s−1) has profound implications for tropical cyclone (TC) structure changes and intensity. While extreme updrafts in the TC are occasionally observed in real TCs, the associated small-scale features remain unclear. Based on an analysis of the extreme eyewall updrafts in two numerical experiments conducted with the Advanced Research version of the Weather Research and Forecasting (WRF) model, in which the large-eddy simulation (LES) technique was used with the finest grid spacings of 37 and 111 m, for the first time this study demonstrates that the simulated extreme updrafts that occur mainly in the enhanced eyewall convection on the down-shear left side are comparable to available observations. The simulated extreme updraft exhibits relatively high frequencies in the lower (750 m), middle (6.5 km) and upper (13 km) troposphere, which are associated with different types of small-scale structures. While the lower-level extreme updraft is mainly related to the tornado-scale vortex, the extreme updraft at upper levels is closely associated with a pair of counter-rotating horizontal rolls oriented generally along the TC tangential flow, which are closely associated with the enhanced eyewall convection. The extreme updraft at middle levels is related to relatively complicated small-scale structures. The study suggests that extreme updrafts can be simulated when the grid spacing is about 100 m or less in the WRF-LES framework, although the simulated small-scale features need further verification in both observation and simulation.

In-Orbit Performance of the Constellation of CYGNSS Hurricane Satellites

AbstractThe NASA Cyclone Global Navigation Satellite System (CYGNSS) constellation of eight satellites was successfully launched into low Earth orbit on 15 December 2016. Each satellite carries a radar receiver that measures GPS signals scattered from the surface. Wind speed over the ocean is determined from distortions in the signal caused by wind-driven surface roughness. GPS operates at a sufficiently low frequency to allow for propagation through all precipitation, including the extreme rain rates present in the eyewall of tropical cyclones. The spacing and orbit of the satellites were chosen to optimize frequent sampling of tropical cyclones. In this study, we characterize the CYGNSS ocean surface wind speed measurements by their uncertainty, dynamic range, sensitivity to precipitation, spatial resolution, spatial and temporal sampling, and data latency. The current status of each of these properties is examined and potential future improvements are discussed. In addition, examples are given of current science investigations that make use of the data.

Effects of Sea Spray on Microphysics and Intensity of Deep Convective Clouds Under Strong Winds

AbstractDeep convective clouds similar to those arising in the tropical cyclone eyewall are simulated using a parcel model and 2‐D slab symmetric cloud model with spectral bin microphysics (the Hebrew University Cloud Model). The size distribution of sea spray particles (SSPs) at cloud base is calculated using the Lagrangian‐Eulerian bin microphysics model. The model describes the SSP production, advection, and formation of the size distribution of SSP in the hurricane atmospheric boundary layer at different strong wind speeds. The SSP distributions calculated by the Lagrangian‐Eulerian bin microphysics model are used in the parcel model and the Hebrew University Cloud Model to investigate the microphysical and dynamical effects of SSP on clouds. The SSPs ascending in cloud updrafts dramatically increase the number concentration of cloud drops within a wide range of drop sizes. As a result, sea spray creates clouds with unique property combinations of both maritime and continental types. These clouds have droplet size distributions characterized by a high drop concentration and a low effective radius, as in continental clouds. At the same time, the presence of SSP of a few hundred microns in radii triggers intense rain just above the cloud base, which is typical of extreme maritime clouds. In the presence of large sea spray drops, the smallest cloud condensational nuclei, including the smallest SSP, are activated, giving rise to the permanent in‐cloud nucleation of small droplets, which produce a high concentration of small ice crystals above the level of homogeneous freezing. We showed that the SSP substantially increased the maximum vertical velocity, cloud water content, and mass contents of ice particles. The results are compared with available observed data.

Effect of the Eyewall Cold Pool on the Inner Rainband of a Tropical Cyclone

AbstractThe low‐level cold pool just under a tropical cyclone eyewall and its effect on the formation of inner rainbands are studied, through idealized convection‐permitting simulations. Analysis of the potential temperature budget indicates that the formation of the eyewall cold pool (ECP) results from the evaporation of eyewall rainfall. The ECP is a barrier to boundary layer inflow, which results in clear convergence and vertical updraft at its outer edge. A budget analysis of the vertical motion tendency also indicates that the sum of the buoyancy and vertical pressure gradient terms makes a large contribution to the vertical updraft at the outer edge of the ECP. This then promotes the formation of convection cells and an inner rainband. Two sensitivity experiments further verify that the intensity of the ECP‐triggered inner rainbands is positively correlated with the magnitude of the radial gradient of potential temperature at the outer edge of the ECP.

Sea-Salt Aerosol Effects on the Simulated Microphysics and Precipitation in a Tropical Cyclone

We investigate the effects of sea-salt aerosol (SSA) activated as cloud condensation nuclei on the microphysical processes, precipitation, and thermodynamics of a tropical cyclone (TC). The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was used together with a parameterization of SSA production. Three simulations, with different levels of SSA emission (CTL, LOW, HIGH), were conducted. The simulation results show that SSA contributes to the processes of autoconversion of cloud water and accretion of cloud water by rain, thereby promoting rain formation. The latent heat release increases with SSA emission, slightly increasing horizontal wind speeds of the TC. The presence of SSA also regulates the thermodynamic structure and precipitation of the TC. In the HIGH simulation, higher latent heat release gives rise to stronger updrafts in the TC eyewall area, leading to enhanced precipitation. In the LOW simulation, due to decreased latent heat release, the temperature in the TC eye is lower, enhancing the downdrafts in the region; and because of conservation of mass, updrafts in the eyewall also strengthen slightly; as a result, precipitation in the LOW experiment is a little higher than that in the CTL experiment. Overall, the relationship between the precipitation rate and SSA emission is nonlinear.

Optical Properties and Spatial Variation of Tropical Cyclone Cloud Systems From TRMM and MODIS in the East Asia Region: 2010–2014

AbstractThe tropical cyclone (TC) in East Asia varies strongly each year, indicating the different spectral characteristics of the TC cloud systems (TCCSs) with the distance from the center of TC. We report a new perspective of optical properties of TCCS by using the Moderate Resolution Imaging Spectroradiometer and Tropical Rainfall Measuring Mission data over the years from 2010 to 2014 for the 35 TCs in East Asia. We investigate the spatial distribution of the cloud‐top height, radar reflectivity, polarization corrected temperature in 85.5 GHz (PCT85.5), hydrometeor vertical profile, cloud water path, cloud optical thickness, and cloud particle effective radius of TCCS in the development, maturity, and decay stages of TC. The results indicate the significant differences of cloud characteristics in the maturity stage in comparison to other two stages, showing the higher radar reflectivity up to 30 dBZ and the PCT85.5 as low as 232.98 K in the TC eye wall. Meanwhile, the mean hydrometeor profile indicates more ice particles accumulating above the height of 10 km but less liquid droplets near 3 km. The peak of cloud water path and cloud optical thickness in the maturity stage can reach to 1,300 g/m2 and 75, respectively. The mean cloud particle effective radius and precipitation in the three stages could be influenced by aerosols. Based on these quantified results, several ideal fitting equations and the models are constructed to denote the properties of TCCS. Two cases are analyzed to examine the accuracy of the quantified mean state for different stages of TCCS and the relationship between aerosols and clouds.

Prevalence of tornado-scale vortices in the tropical cyclone eyewall

Analyses of datasets from manned research flights that penetrated hurricane eyes and tropical cyclone (TC) damage surveys strongly suggest the existence of tornado-scale vortices in the turbulent boundary layer of the TC eyewall. However, their small horizontal scale, their fast movement, and the associated severe turbulence make the tornado-scale vortex very difficult to observe directly. To understand tornado-scale vortices in the TC eyewall and their influence on the TC vortex, mesoscale rainbands, and convective clouds, a numerical experiment including seven nested domains with the smallest horizontal grid interval of 37 m is conducted to perform a large eddy simulation (LES) with the Advanced Weather Research and Forecast (WRF) model. We show that most of the observed features associated with tornado-scale vortices can be realistically simulated in the WRF-LES framework. The numerical simulation confirms the existence of simulated tornado-scale vortices in the turbulent boundary layer of the TC eyewall. Our numerical experiment suggests that tornado-scale vortices are prevalent at the inner edge of the intense eyewall convection.

Simulated sensitivity of the tropical cyclone eyewall replacement cycle to the ambient temperature profile

In this study, the impacts of the environmental temperature profile on the tropical cyclone eyewall replacement cycle are examined using idealized numerical simulations. It is found that the environmental thermal condition can greatly affect the formation and structure of a secondary eyewall and the intensity change during the eyewall replacement cycle. Simulation with a warmer thermal profile produces a larger moat and a prolonged eyewall replacement cycle. It is revealed that the enhanced static stability greatly suppresses convection, and thus causes slow secondary eyewall formation. The possible processes influencing the decay of inner eyewall convection are investigated. It is revealed that the demise of the inner eyewall is related to a choking effect associated with outer eyewall convection, the radial distribution of moist entropy fluxes within the moat region, the enhanced static stability in the inner-core region, and the interaction between the inner and outer eyewalls due to the barotropic instability. This study motivates further research into how environmental conditions influence tropical cyclone dynamics and thermodynamics.