Differences in the inner-core structure of intensifying [IN; intensity increase of at least 20 kt (24 h) 21 , where 1 kt 5 0.51 m s 21 ] and steady-state [SS; intensity remaining between 610 kt (24 h) 21 ] tropical cyclones (TCs) are examined using composites of airborne Doppler observations collected from NOAA P-3 aircraft missions. The IN dataset contains 40 eyewall passes from 14 separate missions, while the SS dataset contains 53 eyewall passes from 14 separate missions. Intensifying TCs have a ringlike vorticity structure inside the radius of maximum wind (RMW); lower vorticity in the outer core; a deeper, stronger inflow layer; and stronger axisymmetric eyewall upward motion compared with steady-state TCs. There is little difference in the vortex tilt between 2 and 7 km, and both IN and SS TCs show an eyewall precipitation and updraft asymmetry whose maxima are located in the downshear and downshear-left region. The azimuthal coverage of eyewall and outer-core precipitation is greater for IN TCs. There is little difference in the distribution of downdrafts and weak to moderate updrafts in the eyewall. The primary difference is seen at the high end of the vertical velocity spectrum, where IN TCs have a larger number of convective bursts. These bursts accomplish more vertical mass flux, but they compose such a small portion of the total vertical velocity distribution that there is little difference in the shape of the net mass flux profile. The radial location of convective bursts for IN TCs is preferentially located inside the RMW, where the axisymmetric vorticity is generally higher, whereas for SS TCs the bursts are located outside the RMW.

The asymmetric dynamics of the hurricane inner-core region is examined through a novel analysis of high temporal resolution, three-dimensional wind fields derived from airborne dual-Doppler radar. Seven consecutive composites of Hurricane Olivia's (1994) wind field with 30-min time resolution depict a weakening storm undergoing substantial structural changes. The symmetric and asymmetric mechanisms involved in this transformation are considered separately. To zeroth order the weakening of the primary circulation is consistent with the axisymmetric vortex spindown theory of Eliassen and Lystad for a neutrally stratified atmosphere. Vertical shear, however, increased dramatically during the observation period, leading to a strong projection of the convection onto an azimuthal wavenumber 1 pattern oriented along the maximum vertical shear vector. Recent theoretical ideas elucidating the dynamics of vortices in vertical shear are used to help explain this asymmetry. The role of asymmetric vorticity dynamics in explaining some of the physics of hurricane intensity change motivates a special focus on Olivia's vorticity structure. It is found that an azimuthal wavenumber 2 feature dominates the asymmetry in relative vorticity below 3-km height. The characteristics of this asymmetry deduced from reflectivity and wind composites during a portion of the observation period show some consistency with a wavenumber 2 discrete vortex Rossby edge wave. Barotropic instability is suggested as a source for the wavenumber 2 asymmetry through a series of barotropic numerical simulations. Trailing bands of vorticity with radial wavelengths of 5-10 km are observed in the inner core approximately 20 km from the storm center, and may be symmetrizing vortex Rossby waves. Elevated reflectivity bands with radial scales comparable to those of the vorticity bands, also near 20-25-km radius, may be associated with these vorticity features.

Following a recent demonstration of multicase compositing of axisymmetric tropical cyclone (TC) structure derived from airborne Doppler radar measurements, the authors extend the analysis to the asymmetric structure using an unprecedented database from 75 TC flights. In particular, they examine the precipitation and kinematic asymmetry forced by the TC's motion and interaction with vertical wind shear. For the first time they quantify the average magnitude and phase of the three-dimensional shear-relative kinematic asymmetry of observed TCs through a composite approach. The composite analysis confirms principal features of the shear-relative TC asymmetry documented in prior numerical and observational studies (e.g., downshear tilt, downshear-right convective initiation, and a downshear-left precipitation maximum). The statistical significance of the composite shear-relative structure is demonstrated through a stratification of cases by shear magnitude. The impact of storm motion on eyewall convective asymmetry appears to be secondary to the much greater constraint placed by vertical wind shear on the organization of convection, in agreement with prior studies using lightning and precipitation data.

A new paradigm for the resiliency of tropical cyclone (TC) vortices in vertical shear flow is presented. To elucidate the basic dynamics, the authors follow previous work and consider initially barotropic vortices on an f plane. It is argued that the diabatically driven secondary circulation of the TC is not directly responsible for maintaining the vertical alignment of the vortex. Rather, an inviscid damping mechanism intrinsic to the dry adiabatic dynamics of the TC vortex suppresses departures from the upright state. Recent work has demonstrated that tilted quasigeostrophic vortices consisting of a core of positive vorticity surrounded by a skirt of lesser positive vorticity align through projection of the tilt asymmetry onto vortex Rossby waves (VRWs) and their subsequent damping (VRW damping). This work is extended here to the finite Rossby number (Ro) regime characteristic of real TCs. It is shown that the VRW damping mechanism provides a direct means of reducing the tilt of intense cyclonic vortices (Ro Ͼ 1) in unidirectional vertical shear. Moreover, intense TC-like, but initially barotropic, vortices are shown to be much more resilient to vertical shearing than previously believed. For initially upright, observationally based TC-like vortices in vertical shear, the existence of a ''downshear-left'' tilt equilibrium is demonstrated when the VRW damping is nonnegligible. On the basis of these findings, the axisymmetric component of the diabatically driven secondary circulation is argued to contribute indirectly to vortex resiliency against shear by increasing Ro and enhancing the radial gradient of azimuthal-mean potential vorticity. This, in addition to the reduction of static stability in moist ascent regions, increases the efficiency of the VRW damping mechanism.

The structure and evolution of Hurricane Earl (2010) during its rapid intensification as sampled by aircraft is studied here. Rapid intensification occurs in two stages. During the early stage, covering ;24 h, Earl was a tropical storm experiencing moderate northeasterly shear with an asymmetric distribution of convection, and the symmetric structure was shallow, broad, and diffuse. The upper-level circulation center was significantly displaced from the lower-level circulation at the beginning of this stage. Deep, vigorous convection-termed convective bursts-was located on the east side of the storm and appeared to play a role in positioning the upper-level cyclonic circulation center above the low-level center. By the end of this stage the vortex was aligned and extended over a deep layer, and rapid intensification began. During the late stage, rapid intensification continued as Earl intensified ;20 m s 21 during the next 24 h. The vortex remained aligned in the presence of weaker vertical shear, although azimuthal asymmetries persisted that were characteristic of vortices in shear. Convective bursts concentrated near the radius of maximum winds, with the majority located inside the radius of maximum winds. Each of the two stages described here raises questions about the role of convective-and vortex-scale processes in rapid intensification. During the early stage, the focus is on the role of convective bursts and their associated mesoscale convective system on vortex alignment and the onset of rapid intensification. During the late stage, the focus is on the processes that explain the observed radial distribution of convective bursts that peak inside the radius of maximum winds.