台湾地形对台风Meranti(1010)经过海峡地区时迅速增强的影响研究

台风在趋近大陆过程中强度一般衰减, 但Meranti(1010)北上进入台湾海峡过程中却迅速加强, 且在登陆福建时达到最强。采用中国气象局台风资料、NCEP GFS 0.5°×0.5°再分析资料及台湾雷达资料, 结合中尺度数值模式WRF(The Weather Research and Forecasting Model)开展台湾地形敏感性试验, 研究Meranti进入台湾海峡过程中的结构变化及迅速加强机理。结果表明:台湾地形是Meranti迅速加强的一个重要影响因子。Meranti北上过程中, 一方面通过台湾岛地形分流作用及其背风坡效应在台湾海峡内诱生中尺度涡旋, 形成正负相间的涡度分布, 激发出与台风相关的扰动波列。地形强迫抬升及扰动波列可加强垂直运动和积云对流, 有利于台风对流发展。另一方面, 台湾地形还通过改变环境气流使台风高空辐散场加强, 环境风垂直切变减小, 形成有利于台风发展的环流背景。比较不同高度台湾地形试验中台风动能收支发现, 台湾地形激发的扰动波列和积云对流增强了次网格尺度系统与台风间能量的交换, 成为Meranti登陆前迅速加强的主要动能源。     

Impacts of Horizontal and Vertical Resolutions on the Microphysical Structure and Boundary Layer Fluxes of Typhoon Hato(2017)

We set four sets of simulation experiments to explore the impacts of horizontal resolution(HR) and vertical resolution(VR) on the microphysical structure and boundary layer fluxes of tropical cyclone(TC) Hato(2017). The study shows that higher HR tends to strengthen TC. Increasing VR in the upper layers tends to weaken TC, while increasing VR in the lower layers tends to strengthen TC. Simulated amounts of all hydrometeors were larger with higher HR. Increasing VR at the upper level enhanced the mixing ratios of cloud ice and cloud snow,while increasing VR at the lower level elevated the mixing ratios of graupel and rainwater. HR has greater impact on the distributions of hydrometeors. Higher HR has a more complete ring structure of the eyewall and more concentrated hydrometeors along the cloud wall. Increasing VR at the lower level has little impact on the distribution of TC hydrometeors, while increasing VR at the upper level enhances the cloud thickness of the eyewall area.Surface latent heat flux(SLHF) is influenced greatly by resolution. Higher HR leads to larger water vapor fluxes and larger latent heat, which would result in a stronger TC. A large amount of false latent heat was generated when HR was too high, leading to an extremely strong TC, VR has a smaller impact on SLHF than HR. But increasing VR at the upper-level reduces the SLHF and weakens TC, and elevating VR at the lower-level increases the SLHF and strengthens TC. The changes in surface water vapor flux and SLHF were practically identical and the simulation results were improved when HR and VR were more coordinated. The friction velocity was greater with higher VR.Enhancing VR at the lower level increased the friction velocity, while increasing VR at the upper level reduced it.     

Impacts of Horizontal and Vertical Resolutions on the Microphysical Structure and Boundary Layer Fluxes of Typhoon Hato

We set four sets of simulation experiments to explore the impacts of horizontal resolution (HR) and vertical resolution (VR) on the microphysical structure and boundary layer fluxes of tropical cyclone (TC) Hato (2017). The study shows that higher HR tends to strengthen TC. Increasing VR in the upper layers tends to weaken TC, while increasing VR in the lower layers tends to strengthen TC. Simulated amounts of all hydrometeors were larger with higher HR. Increasing VR at the upper level enhanced the mixing ratios of cloud ice and cloud snow, while increasing VR at the lower level elevated the mixing ratios of graupel and rainwater. HR has greater impact on the distributions of hydrometeors. Higher HR has a more complete ring structure of the eyewall and more concentrated hydrometeors along the cloud wall. Increasing VR at the lower level has little impact on the distribution of TC hydrometeors, while increasing VR at the upper level enhances the cloud thickness of the eyewall area. Surface latent heat flux (SLHF) is influenced greatly by resolution. Higher HR leads to larger water vapor fluxes and larger latent heat, which would result in a stronger TC. A large amount of false latent heat was generated when HR was too high, leading to an extremely strong TC, VR has a smaller impact on SLHF than HR. But increasing VR at the upper-level reduces the SLHF and weakens TC, and elevating VR at the lower-level increases the SLHF and strengthens TC. The changes in surface water vapor flux and SLHF were practically identical and the simulation results were improved when HR and VR were more coordinated. The friction velocity was greater with higher VR. Enhancing VR at the lower level increased the friction velocity, while increasing VR at the upper level reduced it.     

台湾地形诱生中尺度系统对台风Meranti(1010)迅速加强影响的数值研究

台风Meranti（1010）北上进入台湾海峡过程中迅速加强,登陆时达到其最大强度。利用中国气象局上海台风研究所最佳路径资料、NCEP GFS 0.5°×0.5°资料及中尺度数值模式WRF,诊断分析台湾地形诱生的中尺度系统对台风Meranti迅速加强的影响。研究发现,Meranti在进入海峡过程中,台湾地形在台湾海峡内诱生出中尺度涡旋,激发中尺度扰动波列,加强台风环流内的垂直运动。台风水汽、热量的收支诊断表明,强烈的上升运动使热量和水汽向上输送,加强台风内的积云对流和潜热释放,使其强度增强。计算台湾地形诱生中尺度系统与台风间的动能交换发现,中尺度系统通过加强垂直运动向台风中高层输送涡动动能,使中尺度系统动能向台风动能转换,为Meranti的迅速加强提供能源。敏感性试验表明,如果台湾地形不存在,中尺度系统消失,台风的水汽、热量的向上输送和积云对流明显减弱,Meranti则不能达到迅速加强标准。     

Latitudinal dependence of the dry air effect on tropical cyclone development

The impacts of dry air on tropical cyclone (TC) development at different latitudes with no mean flows are investigated with idealized simulations. It is found that the effective radius of the dry air is sensitive to its vertical distribution and the background earth rotation. The effect of low-level dry-air layer in inhibiting TC development decreases with increasing latitude. At lower latitudes, the greater boundary layer gradient wind imbalance results in a strong low-level inflow, and the dry air can easily penetrate into the TC inner-core region. The intruding dry air inhibits the inner-core deep convection and leads to marked asymmetric convective structure, which significantly suppresses TC development. In contrast, at higher latitudes, the dry air gets moistened before reaching the TC inner-core region due to a weaker radial inflow but can suppress the development of the outer spiral rainbands. The suppressed outer spiral rainbands lead to a weaker barrier effect to the boundary layer inflow and help TC development. Furthermore, the lower the altitude of dry-air layer resides, the greater the impact on TC intensification. The low-level pathway associated with the boundary layer inflow plays an important role on how dry-air layer acts on a TC without considering the mean flow effects. Through examining the climatological distribution of the moisture field, we expect that the intrusion of dry air can be more frequent in the North Atlantic area and therefore has more effects on TC development than in the western North Pacific.     

大涡模式分辨率对海洋信风区大气边界层结构和演变模拟的影响

利用BOMEX（巴巴多斯海洋与气象学试验）的探空资料和LEM（大涡模式）,通过改变LEM水平分辨率的敏感性数值试验,对比分析不同尺度的湍涡对信风积云边界层中混合层和云层的结构、演变以及对流形式和强度的影响。结果表明,水平分辨率较高时模拟的湍涡尺度较小、混合层顶的夹卷作用较强,模拟的混合层较暖、较干,而且模拟的对流泡尺度较小、强度较大,能够模拟出较精细的边界层结构;而水平分辨率较低时则相反。模拟的湍涡尺度对海洋信风区边界层积云中液态水混合比的模拟结果影响较大:LEM模拟的湍涡尺度较小时模拟的信风积云形成的时间较早、云顶高度较高,单个云块的体积较小但数目较多,液态水含量较高;而模拟的湍涡尺度较大时则相反。虽然水平分辨率为50 m和125 m的试验都能模拟出较精细的信风边界层中混合层、云层的结构和演变特征,但是,考虑到提高分辨率在模拟过程中产生的噪音信号对结果的影响以及计算时间等问题,LEM采用125 m的水平网格距是对海洋信风边界层积云对流模拟较为理想的选择。      

Impacts of the diurnal cycle of radiation on tropical cyclone intensification and structure

To investigate the impacts of the diurnal cycle on tropical cyclones （TCs）,a set of idealized simulations were conducted by specifying different radiation （i.e.,nighttime-only,daytime-only,full diurnal cycle）.It was found that,for an initially weak storm,it developed faster during nighttime than daytime.The impacts of radiation were not only on TC intensification,but also on TC structure and size.The nighttime storm tended to have a larger size than its daytime counterparts.During nighttime,the radiative cooling steepened the lapse rate and thus reduced the static stability in cloudy regions,enhancing convection.Diabatic heating associated with outer convection induced boundary layer inflows,which led to outward expansion of tangential winds and thus increased the storm size.     

登陆台风“凡亚比”(1011)合力散度特征诊断研究——垂直分布特征

本文利用2010年1011号台风“凡亚比”登陆过程高分辨率数值模拟资料,诊断分析了“凡亚比”台风环流合力散度的垂直分布及其演变特征。结果指出,合力散度的显著区一直与台风系统相伴随,可以有效地示踪热带气旋（Tropical Cyclone,简称TC）的移动,并能较好地识别TC强度、结构的发展演变。台风中心偏东一侧流入层的合力散度异常信号首先出现并发展,反映出环流的非对称特征。随着台风趋于成熟,合力散度逐渐增强,高度扩展,对称性也逐渐增加;台风中心上空为合力辐合,外围为合力辐散,垂直方向上合力辐合与辐散相间的结构对应上升运动极值区及强降水,即对应台风眼墙位置。合力散度面积指数和强度指数的分析指出,垂直方向上辐合与辐散面积指数负相关;各层的合力辐合强度指数普遍大于辐散强度指数,垂直方向上两强度指数呈显著的正相关关系;结合面积指数与强度指数,可知垂直方向上合力辐合与辐散此消彼长。运用合力散度方程对该垂直分布特征的成因展开分析,发现风速u分量平流随经度变化项和风速v分量平流随纬度的变化项是TC眼区合力辐合部分的主要贡献项,垂直运动项决定了TC眼墙的合力辐合与辐散相间的垂直分布特征。     

Numerical Study on Impact of the Boundary Layer Fluxes over Wetland on Sustention and Rainfall of Landfalling Tropical Cyclones

Numerical studies have been carried out to investigate the sustention and intensification of Typhoon Nina (7503), and the impacts of saturated wetland on the sustention and rainfall of tropical cyclone (TC) over land through sensitivity experiments, using the PSU/NCAR non-hydrostatic mesoscale model MM5v3 and its TC bogus scheme. The results show that the vertical transfer of fluxes in the boundary layer over saturated wetland has significant influence on the intensity, structure, and rainfall of a landfalling TC. The latent heating flux and the sensible heating flux are both favourable for TC sustaining and intensification on which the latent heating transfer is more favourable than the sensible heating transfer. They are also favourable for the maintenance of the spiral structure, and have an evident effect on the distribution of TC rainfall. The momentum flux weakens the TC vortex wind fields significantly, and is the dominant factor to dissipate and fill in a low pressure system, while it increases the local precipitation induced by a typhoon.     

Sensitivity of the warm core of tropical cyclones to solar radiation

To investigate the impacts of solar radiation on tropical cyclone (TC) warm-core structure (i.e., the magnitude and height), a pair of idealized simulations are conducted by specifying different strengths of solar shortwave radiation. It is found that the TC warm core is highly sensitive to the shortwave radiative effect. For the nighttime storm, a tendency for a more intense warm core is found, with an elevated height compared to its daytime counterpart. As pointed out by previous studies, the radiative cooling during nighttime destabilizes the local and large-scale environment and thus promotes deep moist convection, which enhances the TC's intensity. Due to the different inertial stabilities, the diabatic heating in the eyewall will force different secondary circulations. For a strong TC with a deeper vertical structure, this promotes a thin upper-level inflow layer. This inflow carries the lower stratospheric air with high potential temperature and descends adiabatically in the eye, resulting in significant upper-level warming. The Sawyer-Eliassen diagnosis further confirms that the height of the maximum temperature anomaly is likely attributable to the balance among the forced secondary circulations.     

高分辨率模式中积云参数化方案对模拟台风“凡亚比”的影响

在高水平分辨率模式(3～6 km)中,对于是否应该再使用积云参数化方案,仍存在着争论.为此,利用WRF模式,在5 km水平分辨率下,研究了不同云降水方案对一次台风过程模拟的影响,并对影响原因进行了初步探索.结果表明,即使在5 km高水平分辨率下,使用积云参数化方案仍能有效改善对台风路径的模拟,同时,成熟的混合冰相微物理方案对模拟台风路径也非常重要;对台风强度模拟,对积云参数化方案的选择较为敏感和复杂;在48 h预报时效内,只使用微物理方案模拟的降水较好,使用积云参数化方案容易产生较多的虚假降水,但能改善第3天24 h累积降水模拟.这些研究结果为利用高水平分辨率模式模拟台风和改进积云参数化方案提供一定借鉴.     

EFFECTS OF VERTICAL WIND SHEAR ON INTENSITY AND STRUCTURE OF TROPICAL CYCLONE

In this study,the effect of vertical wind shear(VWS)on the intensification of tropical cyclone(TC)is investigated via the numerical simulations.Results indicate that weak shear tends to facilitate the development of TC while strong shear appears to inhibit the intensification of TC.As the VWS is imposed on the TC,the vortex of the cyclone tends to tilt vertically and significantly in the upper troposphere.Consequently,the upward motion is considerably enhanced in the downshear side of the storm center and correspondingly,the low-to mid-level potential temperature decreases under the effect of adiabatic cooling,which leads to the increase of the low-to mid-level static instability and relative humidity and then facilitates the burst of convection.In the case of weak shear,the vertical tilting of the vortex is weak and the increase of ascent,static instability and relative humidity occur in the area close to the TC center.Therefore,active convection happens in the TC center region and facilitates the enhancement of vorticity in the inner core region and then the intensification of TC.In contrast,due to strong VWS,the increase of the ascent,static instability and relative humidity induced by the vertical tilting mainly appear in the outer region of TC in the case with stronger shear,and the convection in the inner-core area of TC is rather weak and convective activity mainly happens in the outer-region of the TC.Therefore,the development of a warm core is inhibited and then the intensification of TC is delayed.Different from previous numerical results obtained by imposing VWS suddenly to a strong TC,the simulation performed in this work shows that,even when the VWS is as strong as 12 m s-1,the tropical storm can still experience rapid intensification and finally develop into a strong tropical cyclone after a relatively long period of adjustment.It is found that the convection plays an important role in the adjusting period.On one hand,the convection leads to the horizontal convergence of the low-level vorticity flux and therefore leads to the enhancement of the low-level vorticity in the inner-core area of the cyclone.On the other hand,the active ascent accompanying the convection tends to transport the low-level vorticity to the middle levels.The enhanced vorticity in the lower to middle troposphere strengths the interaction between the low-and mid-level cyclonical circulation and the upper-level circulation deviated from the storm center under the effect of VWS.As a result,the vertical tilting of the vortex is considerably decreased,and then the cyclone starts to develop rapidly.     

数值模式垂直分辨率对台风大涡模拟的影响

随着计算能力的提升,台风数值模拟大量采用了大涡尺度模拟,其水平分辨率已达到数10 m的量级,而垂直分辨率提升不大,其问题是数值模式的垂直分辨率对台风大涡模拟的影响如何?因此,本文利用WRF(Weather Research and Forecasting)模式开展理想台风模拟,在不同的模式垂直层次(42,69和90层)...     

热带气旋的体积及与强度关系的研究

热带气旋(TC)的结构(含形态)与强度及其变化关系密切,著名的Dvorak定强技术即为TC形态(水平)变化与强度关系的生动描述,近年来水平尺度与强度变化的关系也渐受关注。然而,至今未涉及整体形态(即体积)与TC强度变化的关系。利用欧洲中期数值预报中心(ECMWF)0.25 °的ERA-Interim再分析资料,统计并初步分析了2006—2015年西北太平洋TC的外围水平尺度和“体积”的特征及其与强度的可能关系:水平尺度与TC强度的相关性总体较弱;而TC“体积”与强度的相关性更显著,且TC“体积”随强度增强而增大的关系适用于所有强度级别;此外,TC垂直尺度(正涡度区伸展高度)与强度也有一定的正相关,且在TC较弱时(台风强度以下)更显著。伴随较弱TC增强的主要是垂直尺度的增大,当TC达到台风强度后,与TC强度继续增强相伴随的主要是水平尺度的增大。TC“体积”能较好地综合表征水平尺度和垂直尺度与TC强度变化的关系,借助TC“体积”对TC强度预报有一定的参考价值。      

热带气旋眼墙非对称结构的研究综述

热带气旋的眼墙非对称结构与其发展过程密切相关。在热带气旋移动过程中,非对称风场伴随着边界层内非对称摩擦而引起的辐合,影响着热带气旋眼墙内的对流分布。此外,风垂直切变作为影响热带气旋强度的重要因子,将上层暖心吹离表层环流,引起眼墙垂直运动的非对称,导致云、降水在方位角方向的非均匀分布。当存在平均涡度的径向梯度时,罗斯贝类型的波动可以存在于涡旋内核区域,影响眼墙非对称结构。海洋为热带气旋提供潜热和感热形式的能量,是热带气旋发展的重要能量来源,关于海洋如何影响热带气旋眼墙非对称结构的相关研究较少。文中着重回顾了热带气旋与海洋相互作用的研究成果,并提出海洋影响热带气旋眼墙非对称结构的机制。海洋对热带气旋最显著的响应特征是冷尾效应,该效应通过降低海表温度,减少海洋向大气输送的潜热和感热,从而影响热带气旋眼墙非对称结构。此外,海浪改变海表粗糙度,通过边界层影响移动热带气旋的眼墙结构。     

2006年超级台风“桑美”强度与结构变化的数值模拟研究

使用一个高分辨率、非静力数值模式WRF模式对2006年超级台风Saomei强度和结构进行了数值模拟研究.首先,评估了Makin的粗糙度长度公式对台风Saomei强度和结构变化的影响,结果表明,采用新参数后,使得模拟的台风强度变化与实况最佳路径资料的强度变化更一致,对超级台风Saomei强度预报有改进;但对台风路径的影响不大.通过QuikSCAT、雷达和TRMM非常规资料的验证,进一步表明模拟的台风Saomei的结构与实况很接近,可以再现台风内核区域的部分"双眼墙"和"Annular"结构.其次,通过对台风Saomei边界层过程模拟的改进,表明在平均风速大于40 m/s时边界层各物理量明显改善,使得模式最大强度比传统的简单外推插值方案有显著改进,特别是在台风最强阶段,当台风Saomei眼墙区域的海表面拖曳系数C_d的相对变小,使得其眼墙区域的平均切向风速、径向风速、垂直风速、温度距平、涡旋动能和绝对角动量等物理量均有增强.表明台风Saomei眼墙氏域(20-40 km)各物理量的贡献对其强度和结构变化的影响十分重要.最后,在此基础上进一步分析模式海温和大尺度环境垂直风切变对台风Saomei强度和结构变化的可能影响,讨论了台风Saomei在其增强和消弱阶段中,大尺度环境垂直风切变对其强度变化的负反馈作用.     

Effect of the Initial Vortex Structure on Intensity Change During Eyewall Replacement Cycle of Tropical Cyclones: A Numerical Study

This study investigates the effect of the initial tropical cyclone (TC) vortex structure on the intensity change during the eyewall replacement cycle (ERC) of TCs based on two idealized simulations using the Weather Research and Forecasting (WRF) model. Results show that an initially smaller TC with weaker outer winds experienced a much more drastic intensity change during the ERC than an initially larger TC with stronger outer winds. It is found that an initially larger TC vortex with stronger outer winds favored the development of more active spiral rainbands outside the outer eyewall, which slowed down the contraction and intensification of the outer eyewall and thus prolonged the duration of the concentric eyewall and slow intensity evolution. In contrast, the initially smaller TC with weaker outer winds corresponded to higher inertial stability in the inner core and weaker inertial stability but stronger filamentation outside the outer eyewall. These led to stronger boundary layer inflow, stronger updraft and convection in the outer eyewall, and suppressed convective activity outside the outer eyewall. These resulted in the rapid weakening during the formation of the outer eyewall, followed by a rapid re-intensification of the TC during the ERC. Our study demonstrates that accurate in- itialization of the TC structure in numerical models is crucial for predicting changes in TC intensity during the ERC. Additionally, monitoring the activity of spiral rainbands outside the outer eyewall can help to improve short-term intensity forecasts for TCs experiencing ERCs.     

Numerical Simulation of the Rapid Intensification of Hurricane Katrina(2005):Sensitivity to Boundary Layer Parameterization Schemes

Accurate forecasting of the intensity changes of hurricanes is an important yet challenging problem in numerical weather prediction. The rapid intensification of Hurricane Katrina(2005) before its landfall in the southern US is studied with the Advanced Research version of the WRF(Weather Research and Forecasting) model. The sensitivity of numerical simulations to two popular planetary boundary layer(PBL) schemes, the Mellor–Yamada–Janjic(MYJ) and the Yonsei University(YSU) schemes, is investigated. It is found that, compared with the YSU simulation, the simulation with the MYJ scheme produces better track and intensity evolution, better vortex structure, and more accurate landfall time and location. Large discrepancies(e.g.,over 10 hPa in simulated minimum sea level pressure) are found between the two simulations during the rapid intensification period. Further diagnosis indicates that stronger surface fluxes and vertical mixing in the PBL from the simulation with the MYJ scheme lead to enhanced air–sea interaction, which helps generate more realistic simulations of the rapid intensification process. Overall, the results from this study suggest that improved representation of surface fluxes and vertical mixing in the PBL is essential for accurate prediction of hurricane intensity changes.     

Effects of a Dry-Mass Conserving Dynamical Core on the Simulation of Tropical Cyclones

The accurate forecasting of tropical cyclones(TCs) is a challenging task. The purpose of this study was to investigate the effects of a dry-mass conserving(DMC) hydrostatic global spectral dynamical core on TC simulation. Experiments were conducted with DMC and total(moist) mass conserving(TMC) dynamical cores. The TC forecast performance was first evaluated considering 20 TCs in the West Pacific region observed during the 2020 typhoon season. The impacts of the DMC dynamical core on forecasts o...     

The Uncertainty of Tropical Cyclone Intensity and Structure Based on Different Parameterization Schemes of Planetary Boundary Layer

Based on different parameterization schemes of planetary boundary layer (PBL), the uncertainty of intensity and structure of the Super-strong Typhoon Rammasun (1409) is investigated using the WRF model (v3.4) with six PBL parameterization schemes. Results indicate that PBL uncertainty leads to the uncertainty in tropical cyclone (TC) prediction, which increases with forecast time. The uncertainty in TC prediction is mainly reflected in the uncertainty in TC intensity, with significant differences in the TC intensity forecasts using various PBL schemes. The uncertainty in TC prediction is also reflected in the uncertainty in TC structures. Greater intensity is accompanied by smaller vortex width, tighter vortex structure, stronger wind in the near-surface layer and middle and lower troposphere, stronger inflow (outflow) wind at the lower (upper) levels, stronger vertical upward wind, smaller thickness of the eye wall, smaller outward extension of the eye wall, and warmer warm core at the upper levels of eye. PBL height, surface upward heat flux and water vapor flux are important factors that cause the uncertainty in TC intensity and structure. The more surface upward heat flux and water vapor flux and the lower PBL height, the faster TC development and the stronger TC intensity.