A 30 m scale modeling of extreme gusts during Hurricane Irma (2017) landfall on very small mountainous islands in the Lesser Antilles
Raphaël CécéDidier BernardYann KrienFrédéric LeoneThomas CandelaMatthieu PérocheEmmanuel BiabianyGaël ArnaudAli BelmadaniPhilippe PalanyNarcisse Zahibo
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Abstract. In view of the high vulnerability of the small islands of the Lesser Antilles to cyclonic hazards, realistic very fine scale numerical simulation of hurricane-induced winds is essential to prevent and manage risks. The present innovative modeling aims at combining the most realistically simulated strongest gusts driven by tornado-scale vortices within the eyewall and the most realistic complex terrain effects. The Weather Research and Forecasting (WRF) model with the nonlinear backscatter and anisotropy (NBA) large eddy simulation (LES) configuration was used to reconstruct the devastating landfall of category 5 Hurricane Irma (2017) on Saint Barthélemy and Saint Martin. The results pointed out that the 30 m scale seems necessary to simulate structures of multiple subtornadic-scale vortices leading to extreme peak gusts of 132 m s−1 over the sea. Based on the literature, such extreme gust values have already been observed and are expected for category 5 hurricanes like Irma. Risk areas associated with terrain gust speed-up factors greater than 1 have been identified for the two islands. The comparison between the simulated gusts and the remote sensing building damage highlighted the major role of structure strength linked with the socio-economic development of the territory. The present modeling method could be easily extended to other small mountainous islands to improve the understanding of observed past damage and to develop safer urban management and appropriate building standards.Keywords:
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"Reply to “Comments on ‘Nonlinear Response of a Tropical Cyclone Vortex to Prescribed Eyewall Heating with and without Surface Friction in TCM4: Implications for Tropical Cyclone Intensification’”" published on Dec 2016 by American Meteorological Society.
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Abstract This study investigates the background rotational influences on the secondary eyewall formation (SEF) in tropical cyclones (TCs) in quiescent f ‐plane environments. For given initial structures, simulated vortices tend to experience earlier SEF at lower latitudes. Yet the size of the secondary eyewall does not change monotonically with the latitudes. Specifically, ∼20°N provides the optimal amount of background rotation for the largest secondary eyewall size without considering other environmental forcings. Different background rotation rates affect SEF mainly by modulating the outer‐core convection as well as the wind structures. Specifically, the lower rotation rate causes more outer‐core surface fluxes, thus facilitating the outer rainbands (ORBs) at larger radii. Yet the secondary eyewall does not necessarily form at larger radii at lower latitudes since the transition from the ORBs to secondary eyewall is localized in a region of boundary layer (BL) convergence preceded by accelerated tangential winds. Budget analysis reveals that the differences in the acceleration of outer‐core tangential winds among vortices at different latitudes are dominated by the radial flux of absolute vorticity. Due to the non‐uniform influences of background rotation on the BL inflow and absolute vorticity, the most efficient spin‐up of outer‐core tangential winds is achieved at a medium latitude of 20°N, which leads up to SEF at the largest radii. By comparison, for TCs at lower (higher) latitudes, the lower outer‐core absolute vorticity (radial inflow) limits the acceleration of outer‐core tangential winds, thus placing SEF at smaller radii.
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Abstract The impact of initial structure on storm evolution is examined for the case of a tropical storm entering rapid intensification. At the onset of rapid intensification, satellite cloud signatures suggest that the structural organization of Typhoon Sinlaku (2008) was dominated by a primary band of convection present at outer radii. The development of the eyewall subsequently occurred within this band of deep convection. Numerical forecasts of Sinlaku are initialized at 15- and 5-km resolution using a broad range of vortex scales, at a time when the storm was still weak and its structure not clearly defined. Evidence is presented that beta propagation played a key role in changing the storm’s motion under weak environmental steering. It is found that the track forecast improves over the period when beta propagation is prominent if the vortex is initialized with a large radius of maximum wind (RMW), corresponding with the primary outer cloud band. The initial vortex structure is also suggested to play a critical role in the pathway to rapid intensification, and in the formation of the eyewall for the defined environmental forcing. With an initially large RMW, the forecast captures the evolution of structure and intensity more skillfully. Eyewall formation inside the primary outer convective band for the weak storm is illustrated and some possible dynamical interpretations are discussed.
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Abstract The secondary eyewall replacement cycle (ERC) is an important aspect for tropical cyclone (TC) intensity and structure forecasts. Both observational studies and idealized simulations are conducted to explore the sensitivity of secondary eyewall formation (SEF) to the initial structure of the tropical cyclone. It is found that a TC with a larger size (i.e. both the radial of maximum wind and outer size) is apt to SEF. Furthermore, a larger TC likely has a potential to form a larger outer eyewall and thus a wider moat region. The different SE structures may lead to different intensity fluctuations. This study motivates further research with respect to how initial structure influences the storm intensity and structure changes.
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"Comments on “Nonlinear Response of a Tropical Cyclone Vortex to Prescribed Eyewall Heating with and without Surface Friction in TCM4: Implications for Tropical Cyclone Intensification”" published on Dec 2016 by American Meteorological Society.
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Abstract Dangerous turbulences were observed in the eyewall of category 5 Hurricane Hugo (1989) and the surface pressure associated with the turbulence was 8 hPa lower than the mean pressure in the eye. In this study, it is demonstrated that small scale, negative pressure perturbations in the tropical cyclone eyewall are associated with the tornado‐scale vortex (TSV) simulated with the large‐eddy simulation technique in a semi‐idealized numerical experiment. The negative pressure perturbation associated with TSVs is nonhydrostatic, dynamically resulting from the shearing effect. As a result, the negative pressure perturbation is linearly correlated with the vertical component of relative vorticity and maximum wind perturbation. Since the TSV is prevalent near the inner edge of the tropical cyclone eyewall, it is suggested that caution should be taken to estimate the central pressure of tropical cyclones with the extremely low pressure observed in the eyewall.
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Powered flights in the form of manned or unmanned aerial vehicles (UAVs) have been flying into tropical cyclones to obtain vital atmospheric measurements with flight duration typically lasting between 12 and 36 hours. Convective vertical motion properties of tropical cyclones have previously been studied. This work investigates the possibility to achieve persistent flight by harnessing the generally pervasive updrafts in the eyewall of tropical cyclones. A sailplane UAV capable of vertical take-off and landing (VTOL) is proposed and its flight characteristics simulated. Results suggest that the concept of persistent flight within the eyewall is promising and may be extendable to the rainband regions.
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ABSTRACT Some recent studies have utilized flight-level (700 mb) winds to document the maximum wind speeds (Vmax) and radius of Vmax (Rmax) of the original and secondary eyewalls during 24 Atlantic hurricane eyewall replacement cycles (ERC). In this study, Hurricane Wind (H*Wind) analyses of Atlantic hurricanes during 2003-2005 are utilized to document changes in the outer vortex surface wind profile beyond the secondary eyewall, with a focus on the radii of gale-force winds (R34) that are often defined operationally as size changes. In Mode 1, complete and partial ERCs in which the pre-, during-, and post-ERC outer wind profiles have approximately the same shape, the outward displacements of Rmax leads to size (R34) increases as much as 100 km. Mode 2 ERCs are characterized by sharpened wind profiles outside the secondary eyewall that offset the larger Rmax radii to produce only small R34 increases. While statistically significant results are not obtained, the differences in size changes for Mode 1 and Mode 2 SEF cases suggest practical significance for forecasts and warnings.
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On Sea Surface Roughness Parameterization and Its Effect on Tropical Cyclone Structure and Intensity
包括高风，为所有风政体的海表面动量粗糙长度的一个新 parameterization 计划在热带气旋(TC ) 下面调节从全球放基于大小被构造系统(GPS ) 下投式探空仪。也就是，它复制观察政体转变有风的增加的 drag 系数的增加加快到 40 m s ? 1，随风的另外的增加由减少列在后面速度。TC 的结构和紧张上的这 parameterization 的效果用一个最新发达的数字模型，被评估 TCM4。处于最大的表面风速度并且由在最小的海表面压力落下与的 8.1 hPa (5.9 hPa ) 的最后的紧张被 10.5% 增加的结果表演(8.9%)( 没有) 消散的加热。这紧张增加被发现对在表面层和很少的减少的摩擦驱散主要到期在 eyewall 传送对流对表面热含量流动或潜伏的热版本做。暴风雨结构上的新 parameterization 的效果被发现不足道并且随 eyewall 的正切的风的增加和眼睛的温度异例的增加仅仅在内部核心区域发生。这是因为差别在拖系数仅仅在一个小区域出现在 eyewall 下面。结果的含意简短被讨论。关键词海表面粗糙 - 热带气旋 - 热带气旋结构和紧张 - 拖系数 - 数字模型
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