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1. The Mean Kinematic Structure of the Tropical Cyclone Boundary Layer and Its Relationship to Intensity Change

   https://doi.org/10.1175/MWR-D-21-0335.1  

   Zhang, Jun A. ; Rogers, Robert F. ; Reasor, Paul D. ; Gamache, John ( January 2023 , Monthly Weather Review)

   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 inflowmore »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.« less
2. An Observational Study of the Symmetric Boundary Layer Structure and Tropical Cyclone Intensity

   https://doi.org/10.3390/atmos11020158  

   Ren, Yifang ; Zhang, Jun A. ; Vigh, Jonathan L. ; Zhu, Ping ; Liu, Hailong ; Wang, Xiang ; Wadler, Joshua B. ( February 2020 , Atmosphere)

   This study analyses Global Positioning System dropsondes to document the axisymmetric tropical cyclone (TC) boundary-layer structure, based on storm intensity. A total of 2608 dropsondes from 42 named TCs in the Atlantic basin from 1998 to 2017 are used in the composite analyses. The results show that the axisymmetric inflow layer depth, the height of maximum tangential wind speed, and the thermodynamic mixed layer depth are all shallower in more intense TCs. The results also show that more intense TCs tend to have a deep layer of the near-saturated air inside the radius of maximum wind speed (RMW). The magnitude of the radial gradient of equivalent potential temperature (θe) near the RMW correlates positively with storm intensity. Above the inflow layer, composite structures of TCs with different intensities all possess a ring of anomalously cool temperatures surrounding the warm-core, with the magnitude of the warm-core anomaly proportional to TC intensity. The boundary layer composites presented here provide a climatology of how axisymmetric TC boundary layer structure changes with intensity.
3. Why does rapid contraction of the radius of maximum wind precede rapid intensification in tropical cyclones?

   https://doi.org/10.1175/JAS-D-21-0129.1  

   Li, Yuanlong ; Wang, Yuqing ; Lin, Yanluan ; Wang, Xin ( August 2021 , Journal of the Atmospheric Sciences)

   Abstract The radius of maximum wind (RMW) has been found to contract rapidly well preceding rapid intensification in tropical cyclones (TCs) in recent literature but the understanding of the involved dynamics is incomplete. In this study, this phenomenon is revisited based on ensemble axisymmetric numerical simulations. Consistent with previous studies, because the absolute angular momentum (AAM) is not conserved following the RMW, the phenomenon can not be understood based on the AAM-based dynamics. Both budgets of tangential wind and the rate of change in the RMW are shown to provide dynamical insights into the simulated relationship between the rapid intensification and rapid RMW contraction. During the rapid RMW contraction stage, due to the weak TC intensity and large RMW, the moderate negative radial gradient of radial vorticity flux and small curvature of the radial distribution of tangential wind near the RMW favor rapid RMW contraction but weak diabatic heating far inside the RMW leads to weak low-level inflow and small radial absolute vorticity flux near the RMW and thus a relatively small intensification rate. As RMW contraction continues and TC intensity increases, diabatic heating inside the RMW and radial inflow near the RMW increase, leading to a substantial increase inmore »radial absolute vorticity flux near the RMW and thus the rapid TC intensification. However, the RMW contraction rate decreases rapidly due to the rapid increase in the curvature of the radial distribution of tangential wind near the RMW as the TC intensifies rapidly and RMW decreases.« less
4. Impact of the Low Wavenumber Structure in the Initial Vortex Wind Analyses on the Prediction of the Intensification of Hurricane Patricia (2015)

   https://doi.org/10.1029/2022JD037082  

   Zhong, Quanjia ; Wang, Xuguang ; Ding, Ruiqiang ; Lu, Xu ; Huang, Yongjie ; Duan, Wansuo ; Liu, Lei ( January 2023 , Journal of Geophysical Research: Atmospheres)

   Ensemble clustering analysis was performed to explore the role of the initial hurricane vortex‐scale wind structure in the prediction of the intensification of Hurricane Patricia (2015). Convection‐allowing ensemble forecasts were classified into spin‐down (SPD) and spin‐up (SPU) groups. Specifically, 10 members with an intensification rate >0 m/s and 10 members with an intensification rate <0 m/s for the first 6 hr were defined as the SPD and SPU members. The result showed that the tangential winds outside the inner‐core region in the SPD members were weaker compared to the SPU members. Additionally, the SPD members had a weaker inflow near the surface and a weaker outflow between the heights of 8 and 12 km than the SPU members. The SPU members showed more significant azimuthal asymmetry than the SPD members in the surface, tangential and radial winds. Wavenumber analysis showed that the low wavenumber components dominated the differences between the SPD and SPU members. Numerical experiments were conducted to test the hypothesis generated by the clustering analysis. It was found that the storm's maximum wind speed (MWS) intensified during the first 6 hr of the model forecast if only the low wavenumber structure in the SPU members was included in the initial conditions, whereasmore »it decayed during the first 6 hr if only the low wavenumber structure in the SPD members was included. This result confirms that the low wavenumber structure of the initial wind analyses was important in predicting the intensity changes of Hurricane Patricia (2015).

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5. Contribution of mean and eddy momentum processes to tropical cyclone intensification

   https://doi.org/10.1002/qj.3837  

   Montgomery, Michael T. ; Kilroy, Gerard ; Smith, Roger K. ; Črnivec, Nina ( August 2020 , Quarterly Journal of the Royal Meteorological Society)

   An idealized, three‐dimensional, 1 km horizontal grid spacing numerical simulation of a rapidly intensifying tropical cyclone is used to extend basic knowledge on the role of mean and eddy momentum transfer on the dynamics of the intensification process. Examination of terms in the tangential and radial velocity tendency equations provides an improved quantitative understanding of the dynamics of the spin‐up process within the inner‐core boundary layer and eyewall regions of the system‐scale vortex. Unbalanced and non‐axisymmetric processes are prominent features of the rapid spin‐up process. In particular, the wind asymmetries, associated in part with the asymmetric deep convection, make a substantive contribution (∼30%) to the maximum wind speed inside the radius of this maximum. The analysis provides a novel explanation for inflow jets sandwiching the upper‐tropospheric outflow layer which are frequently found in numerical model simulations. In addition, it provides an opportunity to assess the applicability of generalized Ekman balance during rapid vortex spin‐up. The maximum tangential wind occurs within and near the top of the frictional inflow layer and as much as 10 km inside the maximum gradient wind. Spin‐up in the friction layer is accompanied by supergradient winds that exceed the gradient wind by up to 20%. Overall, themore »results affirm prior work pointing to significant limitations of a purely axisymmetric balance description, for example, gradient balance/Ekman balance, when applied to a rapidly intensifying tropical cyclone.

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