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.

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.

In this study, data from 794 GPS dropsondes deployed by research aircraft in 13 hurricanes are analyzed to study the characteristic height scales of the hurricane boundary layer. The height scales are defined in a variety of ways: the height of the maximum total wind speed, the inflow layer depth, and the mixed layer depth. The height of the maximum wind speed and the inflow layer depth are referred to as the dynamical boundary layer heights, while the mixed layer depth is referred to as the thermodynamical boundary layer height. The data analyses show that there is a clear separation of the thermodynamical and dynamical boundary layer heights. Consistent with previous studies on the boundary layer structure in individual storms, the dynamical boundary layer height is found to decrease with decreasing radius to the storm center. The thermodynamic boundary layer height, which is much shallower than the dynamical boundary layer height, is also found to decrease with decreasing radius to the storm center. The results also suggest that using the traditional critical Richardson number method to determine the boundary layer height may not accurately reproduce the height scale of the hurricane boundary layer. These different height scales reveal the complexity of the hurricane boundary layer structure that should be captured in hurricane model simulations.

Despite the significant impacts of torrential rainfall from tropical cyclones at landfall, quantitative precipitation forecasting (QPF) remains an unsolved problem. A key task in improving tropical cyclone QPF is understanding the factors that affect the intensity and distribution of rainfall around the storm. These include the storm motion, topography, and orientation of the coast, and interactions with the environmental flow. The combination of these effects can produce rainfall distributions that may be nearly axisymmetric or highly asymmetric and rainfall amounts that range from 1 or 2 cm to >30 cm. This study investigates the interactions between a storm and its environmental flow through a numerical simulation of Hurricane Bonnie (1998) that focuses on the role of vertical wind shear in governing azimuthal variations of rainfall. The simulation uses the high-resolution nonhydrostatic fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to simulate the storm between 0000 UTC 22 August and 0000 UTC 27 August 1998. During this period significant changes in the vertical shear occurred in the simulation. It changed from strong west-southwesterly, and across track, to much weaker south-southwesterly, and along track. Nearly concurrently, the azimuthal distribution of convection changed from a distinct wavenumber-1 pattern to almost azimuthally symmetric by the end of the time period. The strongest convection in the core was generally located on the downshear left side of the shear vector when the shear was strong. The azimuthal distributions and magnitudes of low-level radial inflow, reflectivity, boundary layer divergence, and low-level vertical motion all varied consistently with the evolution of the vertical shear. Additionally, the vortex showed a generally downshear tilt from the vertical. The magnitude of the tilt correlated well with changes in magnitude of the environmental shear. The accumulated rainfall was distributed symmetrically across the track of the storm when the shear was strong and across track, and it was distributed asymmetrically across the track of the storm when the shear was weak and along track.

The role of convective-scale processes in a 1.67-km mesoscale model simulation of the rapid intensification (RI) of Hurricane Dennis (2005) is presented. The structure and evolution of inner-core precipitating areas during RI, the statistical properties of precipitation during times experiencing vigorous convection (termed convective bursts here) and how they differ from nonburst times, possible differences in convective bursts associated with RI and those not associated with RI, and the impacts of precipitation morphology on the vortex-scale structure and evolution during RI are all examined. The onset of RI is linked to an increase in the areal extent of convective precipitation in the inner core, while the inner-core stratiform precipitating area remains unchanged and the intensity increases only after RI has begun. RI is not tied to a dramatic increase in the number of convective bursts nor in the characteristics of the bursts, such as burst intensity. Rather, the immediate cause of RI is a significant increase in updraft mass flux, particularly in the lowest 1.5 km. This increase in updraft mass flux is accomplished primarily by updrafts on the order of 1-2 m s 21 , representing the bulk of the vertical motion distribution. However, a period of enhanced updraft mass flux in the midlevels by moderate to strong (.5 m s 21 ) updrafts located inside the radius of maximum winds occurs ;6 h prior to RI, indicating a synergistic relationship between convective bursts and the background secondary circulation prior to RI. This result supports the assertion that both buoyantly driven updrafts and slantwise near-neutral ascent are important features in eyewall structure, evolution, and intensification, including RI.