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Abstract The widely accepted view of the secondary circulation of a mature tropical cyclone (TC) consists of boundary layer inflow that turns upward through the eyewall and then turns outward to form the outflow layer and the cirrus shield. This view can be traced to schematics shown in several foundational studies of TCs and persists in both the peer-reviewed and popular literature in numerous diagrams and cartoons. Updrafts in rainbands are nearly always depicted as not supplying the primary outflow. However, examination of the mass and moisture budgets of the cirrus outflow shield—i.e., the outflow layer from about 100- to 300-km radius—in mesoscale model simulations of hurricanes reveals a different picture. A significant fraction of the dry airmass flux (varying widely but around 50%) and even larger fraction of the condensate in the outflow comes from rainbands. The mass flux from the eyewall is limited by its small size, and condensate is falling out rapidly. Instead, the condensate shield and outflow mass flux are significantly supplied by deep convection in the surrounding rainbands. These findings are consistent with the recently developed appreciation of the diurnally forced rainband complexes that have been shown to expand the cirrus shield. The simulations show that moist air and condensate can be lifted into the outflow in either narrow convective towers or in mesoscale ascending updrafts, and these features can be found in airborne Doppler radar observations. These findings update our understanding of the physical significance of changes in size and thickness of the cirrus shield. Significance StatementTropical cyclones are recognized from satellite images of their high clouds that spiral outward from the storm center. The size and evolution of this outflow are used by experts and algorithms to estimate the intensity and future behavior of these storms. Conventional wisdom holds that the overwhelming source of these high-altitude clouds is the upward transport of moisture in thunderstorms around the calm center. Computer simulations of tropical cyclones and radar observations taken by aircraft show that in fact most of these clouds come from thunderstorms in the surrounding rainbands. These findings highlight the importance of the rainband convection in controlling the size and thickness of the outflow clouds, which in turn inform our estimates of storm intensity. 

Abstract While recent observational studies of intensifying (IN) versus steady-state (SS) hurricanes have noted several differences in their axisymmetric and asymmetric structures, there remain gaps in the characterization of these differences in a fully three-dimensional framework. To address these limitations, this study investigates differences in the shear-relative asymmetric structure between IN and SS hurricanes using airborne Doppler radar data from a dataset covering an extended period of time. Statistics from individual cases show that IN cases are characterized by peak wavenumber-1 ascent concentrated in the upshear-left (USL) quadrant at ∼12-km height, consistent with previous studies. Moderate updrafts (2–6 m s⁻¹) occur more frequently in the downshear eyewall for IN cases than for SS cases, likely leading to a higher frequency of moderate to strong updrafts USL above 9-km height. Composites of IN cases show that low-level outflow from the eye region associated with maximum wavenumber-1 vorticity inside the radius of maximum wind (RMW) in the downshear-left quadrant converges with low-level inflow outside the RMW, forming a stronger local secondary circulation in the downshear eyewall than SS cases. The vigorous eyewall convection of IN cases produces a net vertical mass flux increasing with height up to ∼5 km and then is almost constant up to 10 km, whereas the net vertical mass flux of SS cases decreases with height above 4 km. Strong USL upper-level ascent provides greater potential for the vertical development of the hurricane vortex, which is argued to be favorable for continued intensification in shear environments. 

Abstract This study uses a recently developed airborne Doppler radar database to explore how vortex misalignment is related to tropical cyclone (TC) precipitation structure and intensity change. It is found that for relatively weak TCs, defined here as storms with a peak 10-m wind of 65 kt (1 kt = 0.51 m s⁻¹) or less, the magnitude of vortex tilt is closely linked to the rate of subsequent TC intensity change, especially over the next 12–36 h. In strong TCs, defined as storms with a peak 10-m wind greater than 65 kt, vortex tilt magnitude is only weakly correlated with TC intensity change. Based on these findings, this study focuses on how vortex tilt is related to TC precipitation structure and intensity change in weak TCs. To illustrate how the TC precipitation structure is related to the magnitude of vortex misalignment, weak TCs are divided into two groups: small-tilt and large-tilt TCs. In large-tilt TCs, storms display a relatively large radius of maximum wind, the precipitation structure is asymmetric, and convection occurs more frequently near the midtropospheric TC center than the lower-tropospheric TC center. Alternatively, small-tilt TCs exhibit a greater areal coverage of precipitation inward of a relatively small radius of maximum wind. Greater rates of TC intensification, including rapid intensification, are shown to occur preferentially for TCs with greater vertical alignment and storms in relatively favorable environments. Significance StatementAccurately predicting tropical cyclone (TC) intensity change is challenging. This is particularly true for storms that undergo rapid intensity changes. Recent numerical modeling studies have suggested that vortex vertical alignment commonly precedes the onset of rapid intensification; however, this consensus is not unanimous. Until now, there has not been a systematic observational analysis of the relationship between vortex misalignment and TC intensity change. This study addresses this gap using a recently developed airborne radar database. We show that the degree of vortex misalignment is a useful predictor for TC intensity change, but primarily for weak storms. In these cases, more aligned TCs exhibit precipitation patterns that favor greater intensification rates. Future work should explore the causes of changes in vortex alignment.