Moderate Vertical Wind Shear Research Articles

Investigation of the Synoptic and Dynamical Characteristics of Cyclone Shaheen (2021) and Its Influence on the Omani Coastal Region

Tropical Cyclone Shaheen (TCS), originating in the Arabian Sea on 30 September 2021, followed an east-to-west trajectory and made landfall as a category-1 cyclone in northern Oman on 3 October 2021, causing severe floods and damages before dissipating in the United Arab Emirates. This study aims to analyze the synoptic and dynamical conditions influencing Shaheen’s genesis and evolution. Utilizing ERA5 reanalysis data, SEVIRI-EUMETSAT imagery, and Sorbonne University Atmospheric Forecasting System (SUAFS) outputs, it was found that Shaheen manifested as a warm-core cyclone with moderate vertical wind shear within the eyewall. Distinctive features included a trajectory aligned with rising sea surface temperatures and increased specific humidity levels at 700 hPa in the Arabian Sea. As Shaheen approached the Gulf of Oman, a significant increase in rainfall rates occurred, correlated with variations in sea surface temperatures and vertical wind shear. Comparative analysis between SUAFS and ERA5 data revealed a slight northward shift in the SUAFS track and landfall. Advance warnings highlighted heavy rainfall, rough seas, and strong winds. This study provides valuable insights into the meteorological factors contributing to Shaheen’s formation and impact.

Diverging Behaviors of Simulated Tropical Cyclones in Moderate Vertical Wind Shear

Abstract As a follow-on to a previous study that examined the tilt and precession evolution of tropical cyclones (TCs) in a critical shear regime, this study examines the processes leading to the subsequent divergent evolutions in tilt and intensity. The control experiment fails to resume its precession and reintensify, while the perturbed experiments with enhanced upper-level inner-core vorticity resume the precession after a precession hiatus period. In the control experiment, a mesoscale negative absolute vorticity region forms at the upper levels due to tilting in strong downtilt convection. This upper-level, negative-vorticity region is inertially unstable, causing the inward acceleration of upper-level radial inflow. This upper-level inflow subsequently becomes negatively buoyant due to diabatic cooling and descends, bringing midlevel, low equivalent potential temperature (θE) air into the inner-core TC boundary layer, significantly disrupting the low-level TC circulation. Consequently, the disrupted TC vortex in the control is unable to recover. The upper-level negative vorticity region is absent in the perturbed experiments due to weaker downtilt convection, preventing the emergence of the disruptive inner-core downdraft. The weaker downtilt convection is caused by several factors. First, a stronger circulation aloft advects hydrometeors farther downwind, resulting in greater separation of the cooling-driven downdraft from the convective updraft region, and thus weaker dynamically forced lifting at low levels. Second, the mean θE of the low-level air feeding downtilt convection is smaller. Third, there is stronger and deeper adiabatic descent uptilt, causing more low-θE air diluting the downtilt updraft region. These results show how the full vortex structure is important to diverging TC evolutions in moderately sheared environments.

Factors Associated with the Downshear Reformation of Tropical Cyclones

Abstract The conditions associated with tropical cyclones undergoing downshear reformation are explored for the North Atlantic basin from 1998 to 2020. These storms were compared to analog tropical cyclones with similar intensity, vertical wind shear, and maximum potential intensity, but did not undergo downshear reformation. Storm-centered, shear-relative composites were generated using ERA5 and GridSat-B1 data. Downshear reformation predominately occurs for tropical cyclones of tropical storm intensity embedded in moderate vertical wind shear. A comparison between composites suggests that reformed storms are characterized by greater low-level and midtropospheric relative humidity downshear, larger surface latent heat fluxes downshear and left of shear, and larger low-level equivalent potential temperatures and CAPE right of shear. These factors increase thermodynamic favorability, building a reservoir of potential energy and decreasing dry air entrainment, promoting sustained convection downshear, and favoring the development of a new center. Significance Statement The development of a new low-level circulation center in tropical cyclones that replaces the original center, called downshear reformation, can affect the structure and intensity of storms, representing a challenge in forecasting tropical cyclones. While there have been a handful of case studies on downshear reformation, this study aims to more comprehensively understand the conditions that favor downshear reformation by comparing a large set of North Atlantic tropical cyclones that underwent reformation with a similar set of tropical cyclones that did not undergo reformation. Tropical cyclones that undergo reformation have a moister environment, larger surface evaporation, and higher low-level instability in specific regions that help sustain deep, downshear convection that favors the development of a new center.

Bifurcation Points for Tropical Cyclone Genesis and Intensification in Sheared and Dry Environments

Abstract The combination of moderate vertical wind shear (VWS) and dry environments can produce the most uncertain scenarios for tropical cyclone (TC) genesis and intensification. We investigated the sources of increased uncertainty of TC development under moderate VWS and dry environments using a set of Weather Research and Forecasting (WRF) ensemble simulations. Statistical analysis of ensemble members for precursor events and time-lagged correlations indicates that successful TC development is dependent on a specific set of precursor events. A deficiency in any of these precursor events leads to a failure of TC intensification. The uncertainty of TC intensification can be largely attributed to the probabilistic characteristics of precursor events lining up together before TC intensification. The critical bifurcation point between successful and failed trials in these idealized simulations is the sustained vortex alignment process. Even for the failed intensification cases, most simulations showed deep organized convection, which reformed a midlevel vortex. However, for the failed cycles, the new midlevel vortex could not sustain vertical alignment with the low-level center and was carried away by VWS shortly. Under the most uncertain setup (VWS = 7.5 m s−1 and 50% moisture), the latest-developing ensemble member had seven events of tilt decreasing and increasing again that occurred during the 8 days before genesis. Some unsuccessful precursor events looked very close to the successful ones, implying limits on the intrinsic predictability for TC genesis and intensification in moderately sheared and dry environments. Significance Statement The aim of this study is to identify a critical bifurcation point that determines whether tropical disturbances in moderately sheared and dry environments will develop into intense storms or dissipate. When it comes to predicting the formation and strength of tropical cyclones, vertical wind shear, where the environmental wind changes with height, presents a challenging scenario. When the shear is neither too weak nor too strong, some systems manage to develop into cyclones, while others get torn apart under similar shear conditions. Understanding the differences between these outcomes remains a puzzle. Through extensive computer simulations, we have discovered a key factor that contributes to the uncertainty surrounding the alignment of the midlevel vortex with the center of the low-level vortex. These results reveal the complexity and multiple sources of uncertainty involved in forecasting tropical cyclone intensification, providing valuable insights into why moderate shear is a particularly challenging regime to predict tropical genesis and intensification.

Evaluating the Role of Land Surface Moisture in Generating Asymmetrical Precipitation during the Landfall of Hurricane Florence (2018)

This study focuses on the role of land surface moisture in generating asymmetrical precipitation surrounding a nearly stationary Hurricane Florence (2018) during landfall. Previous idealized modeling studies have suggested that atmospheric stability varies surrounding a tropical cyclone (TC) during landfall, with the atmosphere destabilizing off-shore and stabilizing on-shore. However, this finding has not been studied using a real modeling framework. Here, we produce high-resolution numerical simulations to examine the variations in precipitation and atmospheric stability surrounding Hurricane Florence. In addition to a control simulation (CTRL), two additional simulations are performed by altering the land surface cover to be moister (WETX) or drier (DRYX) compared with the CTRL. In the experiment, the altered land surface affects the equivalent potential temperature within the boundary layer. Due to changes in moisture, there are consistent but minor impacts on the spatial patterns of moist static instability. This study found that rainbands in the inner core and distant rainband regions responded differently to changes in land surface moisture. Within the inner core region of the TC, WETX produced more precipitation that was more symmetrical compared with DRYX. In DRYX, there was increased moist static instability in the outer rainband region over water and decreased moist static instability in the outer rainband region over land, which may have contributed to the enhanced precipitation asymmetries. Still, both experiments produced asymmetrical precipitation distributions, suggesting that alterations to land surface moisture had a minor impact on the precipitation asymmetries in Hurricane Florence. We conclude that precipitation asymmetries are primarily dynamically driven by weak to moderate vertical wind shear and asymmetries in moisture flux convergence.

Effects of vertical shear on intensification of tropical cyclones of different initial sizes

Idealized experiments were conducted to examine the impacts of moderate vertical wind shear (VWS, i.e., magnitude of 12 m s−1) on the intensification of tropical cyclones (TCs) of different initial sizes. The results showed that the intensity of all the simulated TCs decreased at the first 12-hour integration after imposing the shear. Larger TCs resumed intensification earlier, whereas smaller ones re-intensified more slowly or even failed to intensify. The differences in the intensification rate are likely due to the tilting magnitude. Under VWS, the upper-level TC center exhibited a horizontal displacement relative to the low-level circulation. In general, this displacement was smaller in larger TCs, indicating that larger TCs are less susceptible to VWS. Thermodynamically, the intrusion of mid-level low moist entropy also played a role in suppressing upshear convection. This negative impact was pronounced in smaller TCs. By using the PV-ω equation, the resilience of TC to VWS was compared. The second circulation forced by both dry dynamic and diabatic heating acted to restore the system to a vertically upright position. For larger TCs, more extensive convection was prolific on the downshear side, and its corresponding forced second circulation offset a larger part of the low-to mid-level environmental shear, which made larger TCs more resilient to VWS; i.e., larger TCs produced stronger low to middle counter shear circulation and facilitated vertical realignment. In contrast, for smaller TCs, the updrafts forced by dry dynamic and diabatic heating were deeper but narrower at the initial time, which did not efficiently reduce the mid-level VWS, resulting in greater tilting of the TC, thus making the smaller TCs more vulnerable to VWS.

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.

Structural Changes Preceding the Rapid Intensification of Typhoon Lekima (2019) Under Moderate Vertical Wind Shear

AbstractThe early development of Super Typhoon Lekima (2019) under moderate vertical wind shear (VWS) is examined on the basis of a convection‐permitting simulation with the Weather Research and Forecast model. The tropical cyclone (TC) evolves from a tilted vortex to an upright vortex with a meso‐γ‐scale high‐vorticity core and finally to a vortex with a well‐defined eyewall preceding rapid intensification. The shift of the upper‐middle‐tropospheric vortex to the upshear‐left quadrant is critical to the establishment of an upright TC vortex with high vorticity concentrated in the center. As the upper‐middle‐tropospheric vortex reaches the upshear‐left quadrant, it drifts toward the TC center under the horizontal advective effect of VWS, leading to a reduction of TC vortex tilt. The low stability and high humidity below the upper‐middle‐tropospheric vortex together with the fast increase of upper‐middle‐tropospheric vorticity promote the convection around the TC center, which is important to the maintenance of the nearly upright TC vortex and the development of the high‐vorticity core in TC vortex. The evolution of the TC from an upright vortex with a high‐vorticity core to a vortex with a well‐defined eyewall is bound with the convection near the radius of maximum azimuthal‐mean tangential velocity. The convection enhances the boundary‐layer inflow and then the supergradient force above the radially inward side of the strong boundary‐layer inflow. In this way, the convergence within and above the boundary layer is strengthened, favoring the formation of a ring of deep convection and high vorticity. Such processes are similar to those leading up to secondary eyewall formation.

Structural changes preceding the rapid intensification of Typhoon Lekima (2019) under moderate vertical wind shear

Structural changes preceding the rapid intensification of Typhoon Lekima (2019) under moderate vertical wind shear

How Many Types of Severe Hailstorm Environments Are There Globally?

AbstractUnderstanding how severe hailstorms will respond to climate change remains challenging partially due to an incomplete understanding of how different environments produce hail. Leveraging a record of 14,297 global potential severe hailstorms detected by spaceborne precipitation radar, here for the first time, we explore global differences in the five distinct environmental types producing these storms. Two are found over tropical plains and hills with high convective instability, high‐moderate moisture, and low vertical wind shear (VWS). The third type are supercell environments characterized by strong VWS, with moderate instability and moisture, commonly occurring over mid‐latitude plains. Higher latitude plains and elevated terrain reflect the final two, with moderate VWS and low melting height, instability, and moisture. The variety of hailstorm environment types illustrates distinctions in the associated convective mode and embryo type, highlighting that multiple environment types pose challenges for modeling present frequency and anticipating the response of hail to climate change.

GOES-16 observations of rapidly-intensifying tropical cyclones: Hurricanes Harvey (2017), Maria (2017), and Michael (2018)

AbstractThis study utilizes brightness temperatures (Tb’s) observed by the infrared longwave window band (Ch 14; 11.2 μm) from the Geostationary Operational Environmental Satellite-16 (GOES-16) to examine the structure of Hurricanes Harvey, Maria, and Michael throughout their lifetimes. During the times leading up to their rapid intensifications (RI), two-dimensional inner-core structures are examined to analyze the strength and location of the developing convection. Moderate vertical wind shear in the environments of Harvey and Michael induced a pronounced convective asymmetry prior to RI, followed by a rapid axisymmetrization that occurred essentially in conjunction with RI. The evolutions of the tropical cyclones’ (TCs’) coldest Tb’s indicate that the inner-core convective activity began to increase in the 12 h prior to RI onset, primarily in 2–4-h substantial “bursts”, while substantial convection dominated essentially the entirety of the region within 100 km of the surface center within 12 h of the onset of intensification.Azimuthally averaged Tb evolutions illustrate the development of each TCs’ eye and eyewall, the variability of the radial extent of the central dense overcast associated with the diurnal cycle, as well as details of the evolving convective structures throughout intensification. Hövmoller diagrams of data at constant radii reveal areas of cold Tb’s propagating around the TCs on timescales of 2–3-h. The examination of these features in a deep-layer shear-relative sense reveals that they initiate primarily downshear of the TCs’ surface centers. As RI is reached, these areas of convection are able to propagate into the upshear quadrants, which helps facilitate the onset of more substantial intensification.

Evaluation of Two Initialization Schemes for Simulating the Rapid Intensification of Typhoon Lekima (2019)

Two different initialization schemes for tropical cyclone (TC) prediction in numerical models are evaluated based on a case study of Typhoon Lekima (2019). The first is a dynamical initialization (DI) scheme where the axisymmetric TC vortex in the initial conditions is spun up through the 6-h cycle runs before the initial forecast time. The second scheme is a bogussing scheme where the analysis TC vortex is replaced by a synthetic Rankine vortex. Results show that although both initialization schemes can help improve the simulated rapid intensification (RI) of Lekima, the simulation employing the DI scheme (DIS) reproduces better the RI onset and intensification rate than that employing the bogussing scheme (BOG). Further analyses show the cycle runs of DI help establish a realistic TC structure with stronger secondary circulation than those in the control run and BOG, leading to fast vortex spinup and contraction of the radius of maximum wind (RMW). The resultant strong inner-core primary circulation favors precession of the midlevel vortex under the moderate vertical wind shear (VWS) and thus helps vortex alignment, contributing to an earlier RI onset. Afterwards, the decreased vertical shear and the stronger convection inside the RMW support the persistent RI of Lekima in DIS. In contrast, the reduced VWS is not well captured and the inner-core convection is weaker and resides farther away from the TC center in BOG, leading to slower intensification. The results imply that the DI effectively improves the prediction of the inner-core process, which is crucial to the RI forecast.

Precipitation Processes and Vortex Alignment during the Intensification of a Weak Tropical Cyclone in Moderate Vertical Shear

AbstractThe mechanisms underlying the development of a deep, aligned vortex, and the role of convection and vertical shear in this process, are explored by examining airborne Doppler radar and deep-layer dropsonde observations of the intensification of Hurricane Hermine (2016), a long-lived tropical depression that intensified to hurricane strength in the presence of moderate vertical wind shear. During Hermine’s intensification the low-level circulation appeared to shift toward locations of deep convection that occurred primarily downshear. Hermine began to steadily intensify once a compact low-level vortex developed within a region of deep convection in close proximity to a midlevel circulation, causing vorticity to amplify in the lower troposphere primarily through stretching and tilting from the deep convection. A notable transition of the vertical mass flux profile downshear of the low-level vortex to a bottom-heavy profile also occurred at this time. The transition in the mass flux profile was associated with more widespread moderate convection and a change in the structure of the deep convection to a bottom-heavy mass flux profile, resulting in greater stretching of vorticity in the lower troposphere of the downshear environment. These structural changes in the convection were related to a moistening in the midtroposphere downshear, a stabilization in the lower troposphere, and the development of a mid- to upper-level warm anomaly associated with the developing midlevel circulation. The evolution of precipitation structure shown here suggests a multiscale cooperative interaction across the convective and mesoscale that facilitates an aligned vortex that persists beyond convective time scales, allowing Hermine to steadily intensify to hurricane strength.

Assessing the Influence of Convective Downdrafts and Surface Enthalpy Fluxes on Tropical Cyclone Intensity Change in Moderate Vertical Wind Shear

AbstractThe thermodynamic impacts of downdraft-induced cooling/drying and downstream recovery via surface enthalpy fluxes within tropical cyclones (TCs) were investigated using dropsonde observations collected from 1996 to 2017. This study focused on relatively weak TCs (tropical depression, tropical storm, category 1 hurricane) that were subjected to moderate (4.5–11.0 m s−1) levels of environmental vertical wind shear. The dropsonde data were analyzed in a shear-relative framework and binned according to TC intensity change in the 24 h following the dropsonde observation time, allowing for comparison between storms that underwent different intensity changes. Moisture and temperature asymmetries in the lower troposphere yielded a relative maximum in lower-tropospheric conditional instability in the downshear quadrants and a relative minimum in instability in the upshear quadrants, regardless of intensity change. However, the instability increased as the intensification rate increased, particularly in the downshear quadrants. This was due to increased boundary layer moist entropy relative to the temperature profile above the boundary layer. Additionally, significantly larger surface enthalpy fluxes were observed as the intensification rate increased, particularly in the upshear quadrants. These results suggest that in intensifying storms, enhanced surface enthalpy fluxes in the upshear quadrants allow downdraft-modified boundary layer air to recover moisture and heat more effectively as it is advected cyclonically around the storm. By the time the air reaches the downshear quadrants, the lower-tropospheric conditional instability is enhanced, which is speculated to be more favorable for updraft growth and deep convection.

The Unexpected Rapid Intensification of Tropical Cyclones in Moderate Vertical Wind Shear. Part III: Outflow–Environment Interaction

Abstract Interactions between the upper-level outflow of a sheared, rapidly intensifying tropical cyclone (TC) and the background environmental flow in an idealized model are presented. The most important finding is that the divergent outflow from convection localized by the tilt of the vortex serves to divert the background environmental flow around the TC, thus reducing the local vertical wind shear. We show that this effect can be understood from basic theoretical arguments related to Bernoulli flow around an obstacle. In the simulation discussed, the environmental flow diversion by the outflow is limited to 2 km below the tropopause in the 12–14-km (250–150 hPa) layer. Synthetic water vapor satellite imagery confirms the presence of upshear arcs in the cloud field, matching satellite observations. These arcs, which exist in the same layer as the outflow, are caused by slow-moving wave features and serve as visual markers of the outflow–environment interface. The blocking effect where the outflow and the environmental winds meet creates a dynamic high pressure whose pressure gradient extends nearly 1000 km upwind, thus causing the environmental winds to slow down, to converge, and to sink. We discuss these results with respect to the first part of this three-part study, and apply them to another atypical rapid intensification hurricane: Matthew (2016).

Rapid Intensification of Typhoon Mujigae (2015) under Different Sea Surface Temperatures: Structural Changes Leading to Rapid Intensification

Abstract The notable prelandfall rapid intensification (RI) of Typhoon Mujigae (2015) over abnormally warm water with moderate vertical wind shear (VWS) is investigated by performing a set of full-physics model simulations initialized with different sea surface temperatures (SSTs). While all experiments can reproduce RI, tropical cyclones (TCs) in cooler experiments initiate the RI 13 h later than those in warmer experiments. A comparison of structural changes preceding RI onset in two representative experiments with warmer and cooler SSTs (i.e., CTL and S1) indicates that both TCs undergo similar vertical alignment despite the moderate VWS. RI onset in CTL occurs ~8 h before the full vertical alignment, while that in S1 occurs ~5 h after. In both experiments precipitation becomes more symmetrically distributed around the vortex as vortex tilt decreases. In CTL, precipitation symmetricity is higher in the inner-core region, particularly for stratiform precipitation. All experiments indicate that RI onset occurs when the radius of maximum wind (RMW) contraction reaches a certain degree measured in terms of local Rossby number. The contraction occurs much earlier in CTL, leading to earlier RI. These results suggest that vertical alignment, albeit necessary, is not an effective RI indicator under different SSTs, while a more immediate cause of RI is the formation of a strong/compact inner core with high precipitation symmetry. Diagnoses using the Sawyer–Eliassen equation indicate that in CTL the enhanced microphysical diabatic heating of additional midlevel and deep convection along with surface friction contribute to stronger boundary layer inflow near/inside the RMW, facilitating earlier RMW contraction.

A Hypothesis for the Intensification of Tropical Cyclones under Moderate Vertical Wind Shear

Abstract A major open issue in tropical meteorology is how and why some tropical cyclones intensify under moderate vertical wind shear. This study tackles that issue by diagnosing physical processes of tropical cyclone intensification in a moderately sheared environment using a 20-member ensemble of idealized simulations. Consistent with previous studies, the ensemble shows that the onset of intensification largely depends on the timing of vortex tilt reduction and symmetrization of precipitation. A new contribution of this work is a process-based analysis following a shear-induced midtropospheric vortex with its associated precipitation. This analysis shows that tilt reduction and symmetrization precede intensification because those processes are associated with a substantial increase in near-surface vertical mass fluxes and equivalent potential temperature. A vorticity budget demonstrates that the increased near-surface vertical mass fluxes aid intensification via near-surface stretching of absolute vorticity and free-tropospheric tilting of horizontal vorticity. Importantly, tilt reduction happens because of a vortex merger process—not because of advective vortex alignment—that yields a single closed circulation over a deep layer. Vortex merger only happens after the midtropospheric vortex reaches upshear left, where the flow configuration favors near-surface vortex stretching, deep updrafts, and a substantial reduction of low-entropy fluxes. These results lead to the hypothesis that intensification under moderate shear happens if and when a “restructuring” process is completed, after which a closed circulation favors persistent vorticity spinup and recirculating warm, moist air parcels.

The Unexpected Rapid Intensification of Tropical Cyclones in Moderate Vertical Wind Shear. Part I: Overview and Observations

AbstractA satellite-based investigation is performed of a class of tropical cyclones (TCs) that unexpectedly undergo rapid intensification (RI) in moderate vertical wind shear between 5 and 10 m s−1 calculated as 200–850-hPa shear. This study makes use of both infrared (IR; 11 μm) and water vapor (WV; 6.5 μm) geostationary satellite data, the Statistical Hurricane Prediction Intensity System (SHIPS), and model reanalyses to highlight commonalities of the six TCs. The commonalities serve as predictive guides for forecasters and common features that can be used to constrain and verify idealized modeling studies. Each of the TCs exhibits a convective cloud structure that is identified as a tilt-modulated convective asymmetry (TCA). These TCAs share similar shapes, upshear-relative positions, and IR cloud-top temperatures (below −70°C). They pulse over the core of the TC with a periodicity of between 4 and 8 h. Using WV satellite imagery, two additional features identified are asymmetric warming/drying upshear of the TC relative to downshear, as well as radially thin arc-shaped clouds on the upshear side. The WV brightness temperatures of these arcs are between −40° and −60°C. All of the TCs are sheared by upper-level anticyclones, which limits the strongest environmental winds to near the tropopause.

The Unexpected Rapid Intensification of Tropical Cyclones in Moderate Vertical Wind Shear. Part II: Vortex Tilt

AbstractWe investigate a class of tropical cyclones (TCs) that undergo rapid intensification (RI) in moderate vertical wind shear through analysis of a series of idealized model simulations. Two key findings derived from observational analysis are that the average 200–850-hPa shear value is 7.5 m s−1 and that the TCs displayed coherent cloud structures, deemed tilt-modulated convective asymmetries (TCA), which feature pulses of deep convection with periods of between 4 and 8 h. Additionally, all of the TCs are embedded in an environment that is characterized by shear associated with anticyclones, a factor that limits depth of the strongest environmental winds in the vertical. The idealized TC develops in the presence of relatively shallow environmental wind shear of an anticyclone. An analysis of the TC tilt in the vertical demonstrates that the source of the observed 4–8-h periodicity of the TCAs can be explained by smaller-scale nutations of the tilt on the longer, slower upshear precession. When the environmental wind shear occurs over a deeper layer similar to that of a trough, the TC does not develop. The TCAs are characterized as collections of updrafts that are buoyant throughout the depth of the TC since they rise into a cold anomaly caused by the tilting vortex. At 90 h into the simulation, RI occurs, and the tilt nutations (and hence the TCAs) cease to occur.

Climatological Analysis of Tropical Cyclone Intensity Changes under Moderate Vertical Wind Shear

Abstract Although infrequent, tropical cyclones (TCs) can intensify under moderate vertical wind shear (VWS). A potential hypothesis is that other factors—associated with both the TC and its environment—can help offset the effects of VWS and aid intensification. This hypothesis was tested with a large dataset of 6-hourly best tracks and environmental diagnostics for global TCs between 1982 and 2014. Moderate VWS was objectively defined as 4.5–11.0 m s−1, which represents the 25th–75th percentiles of the global distribution of 200–850-hPa VWS magnitude around TCs. Intensifying events (i.e., unique 6-hourly data points) were compared against steady-state events to determine which TC and environmental characteristics favored intensification under moderate VWS. This comparison showed that intensifying events were significantly stronger, closer to the equator, larger, and moving with a more westward motion than steady-state events. Furthermore, intensifying events moved within environments characterized by warmer sea surface temperatures, greater midtropospheric water vapor, and more easterly VWS than steady-state events. Storm-relative, shear-relative composites suggested that the coupling between water vapor, surface latent heat fluxes, and storm-relative flow asymmetries was conducive for less dry air intrusions and more symmetric rainfall in intensifying events. Last, the comparison showed no systematic differences between environmental wind profiles possibly due to the large temporal variability of VWS.