Konstantinos Menelaou

Although intense tropical cyclones (TCs) are considered to be axisymmetric vortices, observations reveal that they are often highly asymmetric. Better understanding of the underlying asymmetric dynamics is a critical step toward advancing TC intensity forecasting. In this paper, we revisit the mechanisms behind one of the most frequent asymmetric patterns: the deformation of the core into an elliptical shape. Previously, elliptical eyewalls were primarily thought to be an outcome of barotropic instability, a mechanism that involves the coupling and mutual growth of counterpropagating vortex Rossby (VR) waves. These results were largely based on simplified numerical models that filter out inertia–gravity (IG) waves. Consideration of IG waves introduces the possibility of an additional instability mechanism, one that involves a VR wave that spontaneously emits a spiral IG wave into the environment. We provide evidence that elliptical eyewalls, which may form within a three-dimensional primitive-equation nonlinear model that supports both instability types, can solely originate by the mechanism of spontaneous radiative imbalance. These evidences are supported by a number of nonlinear simulations, supplemental linear eigenmode analysis, and a linear simulation. The potential role of a multimechanistic instability is also briefly addressed.

It is known that concentric eyewalls can influence tropical cyclone (TC) intensity. However, they can also influence TC track. Observations indicate that TCs with concentric eyewalls are often accompanied by wobbling of the inner eyewall, a motion that gives rise to cycloidal tracks. Currently, there is no general consensus of what might constitute the dominant triggering mechanism of these wobbles. In this paper we revisit the fundamentals. The control case constitutes a TC with symmetric concentric eyewalls embedded in a quiescent environment. Two sets of experiments are conducted: one using a two-dimensional nondivergent nonlinear model and the other using a three-dimensional nonlinear model. It is found that when the system is two-dimensional, no wobbling of the inner eyewall is triggered. On the other hand, when the third dimension is introduced, an amplifying wobble is evident. This result contradicts the previous suggestion that wobbles occur only in asymmetric concentric eyewalls. It also contradicts the suggestion that environmental wind shear can be the main trigger. Examination of the dynamics along with complementary linear eigenmode analysis revealed the triggering mechanism to be the excitation of a three-dimensional exponentially growing azimuthal wavenumber-1 instability. This instability is induced by the coupling of two baroclinic vortex Rossby waves across the moat region. Additional sensitivity analyses involving reasonable modifications to vortex shape parameters, perturbation vertical length scale, and Rossby number reveal that this instability can be systematically the most excited. The growth rates are shown to peak for perturbations characterized by realistic vertical length scales, suggesting that this mechanism can be potentially relevant to actual TCs.

This study examines the influences of an axisymmetric heat source on the tangential wind structure of a tropical cyclone (TC). Specifically, the response of a TC due to the effect of convection located in varying inertial stability profiles was calculated. Using an idealized heat source, the thermodynamic efficiency hypothesis and the dynamic hypothesis for lower-level tangential wind acceleration are studied with the use of a balanced 2D model. These two frameworks for calculating the lower-level tangential wind acceleration are then compared to an idealized but thermally forced version of a nonlinear 3D model (WRF). It is found that using either of the 2D balanced model approaches to calculate the tangential wind acceleration results in an underestimation when compared to the full nonlinear simulation. In addition, the thermodynamic efficiency approach also shows a radial shift in the location of the maximum lower-level tangential wind acceleration. Sensitivity experiments in the context of the WRF Model in varying background inertial instabilities were investigated. It is shown that as the eyewall-like heating is shifted to larger values of inertial stability, there is a decrease in the induced secondary circulation in tandem with a spinup of the lower-level tangential winds. This intensification appears to be modulated by the low-level radial advection of absolute vorticity.

A method is outlined for quantitatively assessing the impact of inertia–gravity wave radiation on the multimechanistic instability modes of a columnar stratified vortex that resembles an intense tropical cyclone. The method begins by decomposing the velocity field into one part that is formally associated with sources inside the vortex and another part that is attributed to radiation. The relative importance of radiation is assessed by comparing the rates at which the two partial velocity fields act to amplify the perturbation of an arbitrary tracer field—such as potential vorticity—inside the vortex. Further insight is gained by decomposing the formal vortex contribution to the amplification rate into subparts that are primarily associated with distinct vortex Rossby waves and critical-layer perturbations.

Intense atmospheric vortices such as tropical cyclones experience various asymmetric instabilities during their life cycles. This study investigates how vortex properties and ambient conditions determine the relative importance of different mechanisms that can simultaneously influence the growth of an asymmetric perturbation. The focus is on three-dimensional disturbances of barotropic vortices with nonmonotonic radial distributions of potential vorticity. The primary modes of instability are examined for Rossby numbers between 10 and 100 and Froude numbers in the broad neighborhood of unity. This parameter regime is deemed appropriate for tropical cyclone perturbations with vertical length scales ranging from the depth of the vortex to moderately smaller scales. At relatively small Froude numbers, the main cause of instability inferred from analysis typically involves the interaction of vortex Rossby waves with each other and/or critical-layer potential vorticity perturbations. As the Froude number increases from its lower bound, the main cause of instability transitions to inertia–gravity wave radiation. In some cases, the transition occurs abruptly at a critical point where a mode whose growth is driven almost entirely by radiation suddenly becomes dominant. In other cases, the transition is gradual and less direct as the fastest-growing mode continuously changes its structure. Examination of the angular pseudomomentum budget helps quantify the impact of radiation. The radiation-driven instabilities examined herein are shown to be quite fast and potentially relevant to real-world tropical cyclones. Their sensitivities to parameterized moisture and outer vorticity skirts are briefly addressed.

The role of asymmetric convection to the intensity change of a weak vortex is investigated with the aid of a “dry” thermally forced model. Numerical experiments are conducted, starting with a weak vortex forced by a localized thermal anomaly. The concept of wave activity, the Eliassen–Palm flux, and eddy kinetic energy are then applied to identify the nature of the dominant generated waves and to diagnose their kinematics, structure, and impact on the primary vortex. The physical reasons for which disagreements with previous studies exist are also investigated utilizing the governing equation for potential vorticity (PV) perturbations and a number of sensitivity experiments. From the control experiment, it is found that the response of the vortex is dominated by the radiation of a damped sheared vortex Rossby wave (VRW) that acts to accelerate the symmetric flow through the transport of angular momentum. An increase of the kinetic energy of the symmetric flow by the VRW is shown also from the eddy kinetic energy budget. Additional tests performed on the structure and the magnitude of the initial thermal forcing confirm the robustness of the results and emphasize the significance of the wave–mean flow interaction to the intensification process. From the sensitivity experiments, it is found that for a localized thermal anomaly, regardless of the baroclinicity of the vortex and the radial and vertical gradients of the thermal forcing, the resultant PV perturbation follows a damping behavior, thus suggesting that deceleration of the vortex should not be expected.

Some aspects of the problem of secondary eyewall formation (SEF) are investigated with the aid of an idealized model. A series of experiments are conducted, starting with a strong annular vortex embedded in a quiescent background flow and forced by the sustained heating associated with a spiral rainband (control experiment). Following this, two experiments are configured to assess the impact of vertical wind shear (VWS) in the SEF process. The importance of the boundary layer force imbalance is finally investigated in a number of simulations in which surface and boundary layer physics are included. From the control experiment, it is found that in the absence of background environmental flow, the sustained latent heating associated with a spiral rainband can form a secondary eyewall even in the absence of a frictional boundary layer. The presence of VWS acts negatively in the SEF process by disrupting the organization of the potential vorticity induced by the rainband. When boundary layer physics is included, some similarities with previous studies are seen, but there is no SEF. These results suggest that the boundary layer most likely contributes to, rather than initiate, a secondary eyewall.

An analysis of a high-resolution dataset from a realistic simulation of Hurricane Wilma (2005) was performed to understand the mechanism for the formation of a prominent polygonal eyewall and mesovortices during the rapid intensifying stage of the hurricane. The impact of these asymmetries on the intensity change of the hurricane vortex was assessed using the empirical normal mode (ENM) method and Eliassen–Palm (EP) flux calculations. The results indicated that the eyewall of Wilma exhibited an early azimuthal wavenumber-4 (m = 4) asymmetry followed by a transition to lower-wavenumber asymmetries. The simulated reflectivity and the spatial structure of potential vorticity (PV) anomalies strongly suggest that barotropic instability is the most likely driving mechanism for these asymmetries. From the ENM analysis, it was found that the dominant modes for m = 4 and m = 3 asymmetries are vortex Rossby waves (VRWs) that possess characteristics of unstable modes, supporting the importance of barotropic instability. The EP flux calculations associated with these modes indicate that the VRWs act to decelerate the flow at the initial radius of maximum wind while they act to accelerate the flow radially inside and outside of this location, suggesting that VRWs may provide a positive impact to the intensification of the vortex. The present results complement previous findings of theoretical and highly idealized numerical studies that a polygonal eyewall and mesovortices are the result of barotropic instability, thereby furnishing a bridge between idealized studies and observations. The work also provides new insight on the role of asymmetries and VRWs in the intensification of hurricanes.

In this study, the multiple nested state‐of‐the‐art Weather Research and Forecasting (WRF) model is used to investigate the impact of the subgrid‐scale (SGS) vertical turbulent mixing parametrization on hurricane eyewall asymmetric structures and the formation of eyewall mesovortices. Hurricane Isabel (2003) was simulated by a series of numerical experiments with different SGS vertical turbulent mixing parametrizations including the Yonsei University, Mellor–Yamada–Janjic, Mellor–Yamada–Nakanishi–Niino 2.5 level and Mellor–Yamada–Nakanishi–Niino 3 level schemes. The simulations show that the vertical turbulent mixing scheme not only substantially affects the SGS vertical transport of heat and moisture but also has an important bearing on the storm axisymmetric structure, eyewall mesovortices and other resolved asymmetric features in the vicinity of the hurricane eyewall. The analyses show that the vertical turbulent mixing processes provide a mechanism to affect the barotropic instability for generation of eyewall mesovortices through changing the vortex basic state potential vorticity (PV) field and generating eyewall disturbances with different frequencies. Our numerical experiments show for given external conditions that the magnitude and vertical distribution of the eddy exchange coefficients are the key factors that regulate the characteristics of eyewall disturbances. Such a modulation of eyewall structure by the eddy exchange coefficients is realized through the complicated interaction among SGS vertical turbulent mixing, mesoscale structures, diabatic heating and barotropic instability.