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The axisymmetric structure of the inner-core hurricane boundary layer (BL) during intensification [IN; intensity tendency ≥20 kt (24 h) −1 , where 1 kt ≈ 0.5144 m s −1 ], weakening [WE; intensity tendency <−10 kt (24 h) −1 ], and steady-state [SS; the remainder] periods are analyzed using composites of GPS dropwindsondes from reconnaissance missions between 1998 and 2015. A total of 3091 dropsondes were composited for analysis below 2.5-km elevation—1086 during IN, 1042 during WE, and 963 during SS. In nonintensifying hurricanes, the low-level tangential wind is greater outside the radius of maximum wind (RMW) than for intensifying hurricanes, implying higher inertial stability ( I 2 ) at those radii for nonintensifying hurricanes. Differences in tangential wind structure (and I 2 ) between the groups also imply differences in secondary circulation. The IN radial inflow layer is of nearly equal or greater thickness than nonintensifying groups, and all groups show an inflow maximum just outside the RMW. Nonintensifying hurricanes have stronger inflow outside the eyewall region, likely associated with frictionally forced ascent out of the BL and enhanced subsidence into the BL at radii outside the RMW. Equivalent potential temperatures ( θ e ) and conditional stability are highest inside the RMW of nonintensifying storms, which is potentially related to TC intensity. At greater radii, inflow layer θ e is lowest in WE hurricanes, suggesting greater subsidence or more convective downdrafts at those radii compared to IN and SS hurricanes. Comparisons of prior observational and theoretical studies are highlighted, especially those relating BL structure to large-scale vortex structure, convection, and intensity.
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The Mean Kinematic Structure of the Tropical Cyclone Boundary Layer and Its Relationship to Intensity Change
Abstract This study investigates the relationship between the azimuthally averaged kinematic structure of the tropical cyclone boundary layer (TCBL) and storm intensity, intensity change, and vortex structure above the BL. These relationships are explored using composites of airborne Doppler radar vertical profiles, which have a higher vertical resolution than typically used three-dimensional analyses and, therefore, better capture TCBL structure. Results show that the BL height, defined by the depth of the inflow layer, is greater in weak storms than in strong storms. The inflow layer outside the radius of maximum tangential wind speed (RMW) is deeper in intensifying storms than in nonintensifying storms at an early stage. The peak BL convergence inside the RMW is larger in intensifying storms than in nonintensifying storms. Updrafts originating from the TCBL are concentrated near the RMW for intensifying TCs, while updrafts span a large radial range outside the RMW for nonintensifying TCs. In terms of vortex structure above the BL, storms with a quickly decaying radial profile of tangential wind outside the RMW (“narrow” vortices) tend to have a deeper inflow layer outside the RMW, stronger inflow near the RMW, deeper and more concentrated strong updrafts close to the RMW, and weaker inflow in the outer core region than those with a slowly decaying tangential wind profile (“broad” vortices). The narrow TCs also tend to intensify faster than broad TCs, suggesting that a key relationship exists among vortex shape, the BL kinematic structure, and TC intensity change. This relationship is further explored by comparisons of absolute angular momentum budget terms for each vortex shape.
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- PAR ID:
- 10451813
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Monthly Weather Review
- Volume:
- 151
- Issue:
- 1
- ISSN:
- 0027-0644
- Page Range / eLocation ID:
- 63 to 84
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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