Review of Studies on Mesoscale Waves: Relationships with the Fine-Scale Structure and Intensity Changes of Typhoons

  • Xin HUANG , 1, 2, 3, 4 ,
  • Huaning DAI 1, 2 ,
  • Yuchun ZHAO , 1, 2 ,
  • Mengxue CUI 1, 2 ,
  • Aiping XUN 1, 5 ,
  • Ning PAN 6 ,
  • Yushu ZHOU 3, 4
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  • 1. Xiamen Key Laboratory of Straits Meteorology, Xiamen Fujian 361012, China
  • 2. Xiamen Meteorological Observatory, Xiamen Fujian 361012, China
  • 3. Key Laboratory of Cloud-Precipitation Physics and Severe Storms, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 4. University of Chinese Academy of Sciences, Beijing 100049, China
  • 5. Xiamen Jimei Meteorological Bureau, Xiamen Fujian 361021, China
  • 6. Fujian Meteorological Observatory, Fuzhou 350008, China
ZHAO Yuchun, research areas include heavy rainfall mechanisms, forecasting methods, and mesoscale dynamics. E-mail:

Received date: 2025-04-30

  Revised date: 2025-07-23

  Online published: 2025-07-15

Supported by

the Natural Science Foundation of Fujian Province(2023J05061)

The National Natural Science Foundation of China(42205011)

Abstract

Current limitations in typhoon forecasting are primarily attributed to insufficient understanding of mesoscale processes. To address this gap, this review synthesizes the current understanding of mesoscale waves in typhoons, including Vortex Rossby Waves (VRWs) and Typhoon-induced Gravity Waves (TGWs). It investigates their generation mechanisms and characteristics, and systematically examines the linkages between these waves and key typhoon structural features, including the eyewall, spiral rainbands, convective intensity, and (a) symmetric structure. Furthermore, the impact of these structural modifications on typhoon intensity is investigated, along with the statistical correlations between wave characteristics and typhoon intensity changes. The results show that: The theoretical frameworks for polygonal eyewall and inner spiral rainband formation have evolved from the TGW approach to that of VRWs. VRWs provide partial explanations for typhoon asymmetric structures and double-eyewall formation while representing one plausible mechanism for outer spiral rainbands. The changes in intensity induced by VRWs manifest through complex processes characterized by differing dynamical responses depending on (i) wave propagation directionality (tangential/radial), (ii) spatial domain (inner-core/outer region) and (iii) levels (mid-lower/upper) at (iv) different periods during the typhoon lifecycle phase (intensification/decay). The wave characteristics of TGWs (including amplitude, wavelength, period, and occurrence frequency) exhibit correlation with changes in typhoon intensity. TGWs, primarily excited by convection in the eyewall and spiral rainbands and rapidly propagating vertically, may serve as precursor signals for typhoon (rapid) intensification. Both VRWs and TGWs can drive the outward radial transport of momentum and heat within typhoons. Through wave-mean flow interactions, they modify local circulation and enhance typhoon symmetry, ultimately contributing to typhoon intensification (including rapid intensification). Some scientific challenges remain in applying VRWs and TGWs to improve fine-scale wind/precipitation distributions and advance the forecasting of changes in typhoon intensity. Current research underscores the necessity of integrating high-resolution numerical simulations with multi-platform coordinated observations to quantitatively analyze mesoscale wave-typhoon interactions, thereby identifying precursor signals for typhoon intensification, including rapid intensification. Tools such as wave spectrum analysis and wave energy flux diagnostics are instrumental in extracting early-warning indicators from both wave characteristics and energy transport perspectives. Advances in satellite and radar detection technologies will enable the validation of theoretical frameworks through multi-platform observational data, ultimately enhancing monitoring and forecasting capabilities for typhoon structural and intensity changes.

Cite this article

Xin HUANG , Huaning DAI , Yuchun ZHAO , Mengxue CUI , Aiping XUN , Ning PAN , Yushu ZHOU . Review of Studies on Mesoscale Waves: Relationships with the Fine-Scale Structure and Intensity Changes of Typhoons[J]. Advances in Earth Science, 2025 , 40(8) : 794 -808 . DOI: 10.11867/j.issn.1001-8166.2025.055

1 引 言

台风是发生在西北太平洋和南海海域的强热带气旋,在北大西洋及东太平洋称为飓风1,其带来的强风暴雨、风暴潮、洪水、滑坡和泥石流等灾害常造成重大人员伤亡和财产损失。我国是全球受台风影响最严重的国家之一2。受全球气候变暖影响,台风生成频率下降,但台风强度变得更强3-6,近年来登陆我国的高强度(台风及以上级别)热带气旋数量呈显著增加趋势7-8,且通常伴随着快速增强过程9-11。但因台风内不同尺度系统非线性相互作用复杂12,台风强度预报还面临许多瓶颈,尤其是在台风快速增强的趋势下,其预报准确性提升相对较慢13-16。事实上,关于大尺度环境场对台风强度变化的影响机制,已开展了较多深入研究并取得了广泛共识17;而对于台风内部影响更为直接的过程18,例如台风云系对流和不对称性等精细结构及其相关的中尺度过程,目前的认识仍十分有限,这限制了对台风强度变化的预报能力。
台风眼墙和螺旋雨带的强度特征及其动态演变一直是台风研究领域的重点问题,其中活跃的强对流情况揭示了台风内部更为精细的动力学特征。20世纪80年代初开始,有研究将台风眼墙与中尺度波动联系起来19,但一直到近20多年,相关研究才逐渐深入。虽然这类研究起步较晚,还不够充分和深入,但现有研究结果表明,台风眼墙的许多特征均受到中尺度波动调控20-30,也是中尺度波动的重要来源31-34。螺旋雨带作为台风的重要结构之一,对于其形成与维持的理论解释尚不统一;自20世纪60年代起,气象学者就开始将台风螺旋雨带的形成、分布、发展演变以及移动(传播)特征等与台风中尺度波动联系在一起,在众多学者的共同努力探索下,相关解释理论也从最初的重力波学说35-37逐渐发展为现在盛行的涡旋罗斯贝波(Votex Rossby Waves,VRWs)学说38-41
提升台风预报能力一直是我国气象事业发展的迫切需求和重点工作,过去几十年台风路径预报取得了显著进步42,相比之下,台风强度预报准确性的提升相对较慢13-16,处于进一步提升的瓶颈期。近年来,随着观测和数值模拟水平的不断提升,台风中尺度波动的影响作用逐渐引起气象学者的关注,他们发现台风中尺度波动发展变化能影响台风精细结构的演变,进而影响台风强度变化,并可作为一种重要的先兆信号,对后续台风强度变化起到一定的指示意义2343-47
台风的风雨分布和强度变化,与台风精细结构及其演变密切相关,而这些精细结构又与台风内的中尺度波动存在紧密联系。因此台风强度预报能力提升的突破口之一,极有可能是台风环流内中尺度波动的相关物理过程。台风内中尺度波动主要包括涡旋罗斯贝波、惯性重力波及二者的混合波(又称混合涡旋罗斯贝—惯性重力波)48-51。陆汉城等51提出,在热带气旋内罗斯贝数较大、非梯度风平衡且存在很强的超梯度流的区域(如眼墙),会形成中尺度混合波,混合波的发展由切向基流的涡度梯度引起。在准平衡运动过程中,以中尺度涡散运动共存为主要特征(散度达到与涡度同量级),则波动以混合波为主;而非平衡运动过程中,以中尺度散度运动为主要特征(散度的量级达到甚至超过涡度值),则波动以惯性重力波为主。类似的,Zhong等33发现,热带气旋的眼墙区域往往存在具有不可分特征的混合波,而在眼区或外部区域,低频的涡旋罗斯贝波和高频的惯性重力波可以共存。此外,也有研究提出,当基本流场的风速仅存在线性切变时,台风中的波动仅为惯性重力波;而当基本流场的风速存在二次径向水平切变或者垂直切变时,波动可能是混合波49。此类混合波的不稳定性可以用来解释多边形眼墙和多涡旋形成的原因33。鉴于混合涡旋罗斯贝—惯性重力波的研究仍相对薄弱,本文主要针对台风内涡旋罗斯贝波、惯性重力波(下文统称为台风重力波,Typhoon-induced Gravity Waves, TGWs)与眼墙、螺旋雨带及台风强度变化的关系展开综述。

2 涡旋罗斯贝波

涡旋罗斯贝波的研究开始于20世纪60年代,MacDonald38发现了台风螺旋雨带与经典罗斯贝波(即行星罗斯贝波)存在相似性,提出“类—罗斯贝波”的概念,随着对该波动认识的不断深入,目前学术界普遍采用“涡旋罗斯贝波”52这一名称。其成波机理为涡旋基本流的涡度梯度( d η ¯/dr),这与经典罗斯贝波的成波机理β项(β=df/dy)等价53,具体又可分为正压涡旋罗斯贝波和斜压涡旋罗斯贝波两类49。台风眼墙附近存在显著的位涡径向梯度,导致波数1和波数2的涡旋罗斯贝波主要分布在该区域32-33,该波动的传播速度比切向风更快25,其中波数2涡旋罗斯贝波的切向传播与眼墙的旋转密切相关54。涡旋罗斯贝波不仅能够解释台风内部的动力过程,还对对流位置和发展演化具有显著影响。例如,Corbosiero等26与Moon等55的研究均指出,波数1和波数2涡旋罗斯贝波的移动发展,与台风内核附近强对流的位置及其变化存在联系,其动力配置不仅调控对流系统的发展40,还能促进螺旋雨带中强对流的形成56

2.1 涡旋罗斯贝波与眼墙的关系

最早关于台风眼墙与中尺度波动进行的关联研究,聚焦于台风椭圆形或多边形眼墙,相关研究结果表明,台风眼墙的形态特征与不同波数的涡旋罗斯贝波密切相关。多项研究发现,椭圆形眼墙主要由波数2的涡旋罗斯贝波主导,该波动在眼墙附近呈现逆方位角传播特征,显著影响眼墙的旋转20222657。而多边形眼墙的形成机制更为复杂,Schubert等21通过理论分析和数值模拟证明,多边形眼墙以及其中的中尺度涡是由眼墙两侧波数4的涡旋罗斯贝波指数增长导致的;Kossin等58进一步研究发现,台风眼墙周围的中尺度涡发生合并,导致眼墙出现各种多边形形状。虽然他们的研究结论是建立在高度理想化的二维框架上的,但这些结论后来也通过三维模拟得到了证实59-60。Mashiko61通过数值模拟研究发现,台风“Rusa”(2002)眼墙的主导波模态从近地面到高层呈现由波数2(中尺度涡主导模态)向波数1涡旋罗斯贝波模态转变的特征,这种波模态的垂直结构不仅与观测结果一致,也与Schubert等21和Kossin等58的数值理想试验结果具有相似之处。Wang22进一步提出多边形眼墙的形成源于波数1和波数2涡旋罗斯贝波的组合。以上研究结果表明,台风眼墙的形态差异本质上反映了不同波数涡旋罗斯贝波的动力作用机制,其中椭圆形眼墙多与波数2的涡旋罗斯贝波相关,而多边形眼墙则涉及高波数或者不同波数涡旋罗斯贝波叠加所产生的复杂相互作用。
现有研究揭示,涡旋罗斯贝波是影响台风非对称对流的重要内动力强迫因子。Wang22-24的研究表明,距离台风中心70 km内的非对称结构由波数1和波数2的涡旋罗斯贝波主导。位涡环内侧与外侧的位涡随半径呈相反的变化,这种变化特征激发了相对切向气流逆时针和顺时针传播的涡旋罗斯贝波,当二者传播角速度相同时,即发生锁相现象;锁相后不稳定能量增强,随后或者逐渐轴对称化融入台风涡旋主体2143,或者围绕台风中心保持稳定的旋转形成多边形眼墙58。区别于上述单眼墙结构台风中2列涡旋罗斯贝波的I型相互作用,双眼墙结构台风中内眼墙外侧与外眼墙内侧之间,存在2列涡旋罗斯贝波通过moat区的II型相互作用62。内眼墙波数2涡旋罗斯贝波径向外传,至外眼墙内边缘后与此处的波数1涡旋罗斯贝波发生锁相,进而导致后者增强;随后其又与外眼墙外边缘的波数2涡旋罗斯贝波发生第二次锁相,进而造成外眼墙呈现异常非对称特征(图1)。涡旋罗斯贝波对台风非对称结构的作用也已在观测中被证实。Reasor等54对飓风“Olivia”(1994)的机载雷达观测分析表明,涡旋罗斯贝波主导了台风整体波数1特征,以及3 km高度以下的波数2特征。不仅如此,雷达观测还发现双眼墙结构的台风非对称结构情况更为复杂,内、外眼墙可能受到不同主导强迫因子的调控,例如,台风“Lekima”(2019)外眼墙受涡旋罗斯贝波影响(通过不同波数能量的外传和锁相发生)在逆风切一侧产生强回波,但其内眼墙的非对称性仍由环境垂直风切主导(图162。值得注意的是,台风临近登陆或登陆后,其眼墙非对称结构还可能受到海陆边界及地形的影响。罗哲贤等63提出了一种涡旋罗斯贝波的新概念——地形涡旋罗斯贝波,其研究揭示:该波的传播速度对台风中心位置及台风的非对称结构均产生影响。Fernández-Cabán等64利用沿岸雷达对飓风“Harvey”(2017)登陆过程的分析表明,海陆粗糙度差异导致的边界层辐合也会加强眼墙中因涡度扰动激发出的涡旋罗斯贝波,从而维持登陆过程中眼墙强降水的非对称分布。
图1 基于温州S波段业务雷达水平反射率因子波数分解的台风“利奇马”外眼墙受涡旋罗斯贝波影响过程分析62

(a)、(c)分别为波数2、波数1径向变化随时间演变(填色),白色虚线指示内眼墙外边缘,黑色实线指示多次外传过程;(b)外眼墙外侧波数2正位相的时间演变(填色)、外眼墙内侧波数1正位相的时间演变(等值线),红(蓝)色实线指示涡旋罗斯贝波传播过程,空(实)心表示波动外传起(止),UL、UR、DR、DL分别表示逆风切左侧、逆风切右侧、顺风切右侧、顺风切左侧;(d)外眼墙波数1能量时间演变。

Fig. 1 The process of the outer eyewall of typhoon Lekima being affected by Vortex Rossby Waves: spectral analysis of horizontal reflectivity using S-band operational radar data in Wenzhou62

The radius-time Hovmöller diagram of (a) wavenumber-2 and (c) wavenumber-1 (shading), the white dashed line indicates the outer edge of the inner eyewal, the thick black solid lines track the outward propagation of the wavenumber-2 asymmetries. (b) The azimuth-time Hovmöller diagram of wavenumber-1 reflectivity (contour) outside the outer eyewall and wavenumber-2 reflectivity inside the outer eyewall (shading). The thick blue and red solid lines track the rotation of the wavenumber-1 and -2 asymmetries, respectively. Hollow circles and solid dots indicate the start and end of the azimuthal propagation of the VRWs, respectively. UL, UR, DR, and DL respectively denote the upper shear left side, upper shear right side, lower shear right side, and lower shear left side. (d) The time series of wavenumber-1 reflectivity power radially averaged in the outer eyewall.

近年来,涡旋罗斯贝波成为台风双眼墙形成机制的主要理论之一65。作为台风研究中的重点和难点问题,双眼墙的形成及其演变机制还未形成统一认识,有多项研究探讨了涡旋罗斯贝波与台风外眼墙形成的关系25-2866-69。最开始,研究人员基于简单模型提出,在波流相互作用下,涡旋罗斯贝波的停滞半径附近存在能量积累,形成切向风的次大值区,有助于外雨带的形成,进一步导致外眼墙形成25-28。但后续基于高分辨率数值模式的模拟研究认为,涡旋罗斯贝波主要通过间接机制影响外眼墙的形成。例如,Qiu等66通过数值理想试验验证了β-skirt轴对称机制70,并指出持续活跃的涡旋罗斯贝波在内眼墙产生并向外传播,使得β-skirt向外拓展从而为外雨带中对流的轴对称化提供了足够的径向空间,其涡度扰动的轴对称化是导致外眼墙的主要原因。后续研究进一步支持了这一观点。涡旋罗斯贝波向外传播时能够调整位涡的径向分布,从而导致水汽在停滞半径处累积,促进对流的发展,但其对外眼墙形成的直接贡献有限2667。Judt等68基于飓风“Rita”和“Katrina”的第五代宾夕法尼亚州立大学/国家大气研究中心中尺度模式(Fifth-Generation Penn State/NCAR Mesoscale Model,MM5)数值模拟结果提出,在外眼墙形成之前该区域的位涡梯度接近于零,同时该区域存在的下沉运动和形变效应均不利于涡旋罗斯贝波的向外传播,而雨带中对流活动产生的局地位涡异常才是外眼墙形成的关键[图2(a)]。然而,Abarca等69基于同一台风个例开展天气研究与预报模型(Weather Research and Forecasting,WRF)数值模拟研究,却支持了涡旋罗斯贝波在外眼墙形成过程中发挥重要作用的观点。这可能与台风内对流呈现位涡偶极子特征有关,后者采用了更适合对此分析的位涡拟能,结果表明,涡旋罗斯贝波外传导致停滞半径处的位涡拟能与外眼墙的形成存在密切关联[图2(b)和图2(c)]。两个研究所采用的数值模式、参数化方案和模拟分辨率等均不同,这可能导致模拟结果出现差异。此外,现有数值模拟的空间分辨率不足,对停滞半径有关过程的刻画能力有限,故现阶段仍然无法排除涡旋罗斯贝波直接影响外眼墙形成的可能性66。从现有研究来看,涡旋罗斯贝波的外传能够影响外眼墙的形成,但学界对其直接或间接作用还未达成共识,仍需要有更多高分辨率数值模拟研究对比或者观测分析验证。
图2 涡旋罗斯贝波用于解释双眼墙形成

(a)飓风“Rita”和“Katrina”中对流产生位涡(PV)的径向分布示意图,在“Rita”的内眼墙和外眼墙之间的区域没有涡旋罗斯贝波传播,这表明波不是双眼墙形成的贡献因素68;飓风(b)“Katrina”和(c)“Rita”的850 hPa位涡拟能69

Fig. 2 The role of vortex Rossby waves in double eyewall formation

(a) Schematics of radial distributions of convection, vortex Rossby waves, azimuthally averaged tangential wind speed, Potential Vorticity (PV), and PV generation by convection in Hurricane “Rita” and “Katrina”68. The absence of vortex Rossby wave propagation in the moat region between the primary and secondary eyewalls in Rita, which indicates that the waves are not a contributing factor; PV enstrophy at 850 hPa for Hurricane (b) “Rita” and (c) “Katrina”69.

2.2 涡旋罗斯贝波与螺旋雨带的关系

螺旋雨带是台风最重要的结构之一。对于螺旋雨带形成与维持的理论解释存在多种学说。在众多台风螺旋雨带相关的观测和模拟研究中,涡旋罗斯贝波学说逐渐被气象学者们重视。MacDonald38与Guinn等39提出台风螺旋雨带是由涡旋罗斯贝波造成的,这一结论得到了后续许多研究的支持和验证265571-72。螺旋雨带的波列结构具有涡旋罗斯贝波特征,其与涡旋罗斯贝波在时间和空间的位置上均保持一致,径向移速也与涡旋罗斯贝波的波速量级一致40-41。在台风成熟阶段,扰动均从台风涡旋中心呈螺旋带状径向流出,而在台风强度变化期间,强烈的切向气流可以将涡度拉伸成向台风中心盘旋的细丝7173,这类涡丝被视为涡旋罗斯贝波。此外,涡旋罗斯贝波也能影响螺旋雨带中强对流的发展,对台风强度以及降水强度和分布有一定的诊断作用4056
事实上,螺旋雨带可细分为内雨带和外雨带39,或者主雨带、次雨带和远距离雨带74,不同类型的螺旋雨带动热力特征不同,涡旋罗斯贝波主要可用于解释内(次)螺旋雨带的形成和传播特征等2272。Li等72通过数值模拟研究指出,内雨带和外雨带可能是2种不同的状态,二者频繁相互作用于涡旋罗斯贝波外传的停滞半径处,涡旋罗斯贝波主要导致内雨带的外传。Corbosiero等26基于地基雷达观测资料很好地验证了这一结论,他们研究发现与内雨带相关的雷达反射率特征明显随时间径向向外传播,其径向速度与涡旋罗斯贝波理论一致。Chen等25通过数值模拟试验,验证内雨带位涡异常的方位角、垂直相速度与涡旋罗斯贝波理论一致。次雨带与内雨带的定义非常相似,同样有研究表明波数1和波数2的涡旋罗斯贝波能够影响台风次雨带的位置和传播71-72,次雨带具有与涡旋罗斯贝波一致的径向和切向传播72。上述涡旋罗斯贝波驱动内雨带机制已被广泛认可。
Qiu等66进一步提出,螺旋罗斯贝波对外雨带影响较大,他们认为持续活跃的涡旋罗斯贝波为外雨带中对流活动的轴对称化提供了足够的径向空间。Li等75提出,外雨带其中一种形成机制与顺风切内雨带产生的切变涡旋罗斯贝波有关。不过他们还提出外雨带另外2种形成机制:其一,单一形变引起的内雨带直接形成外雨带;其二,局地形变的逆风切右侧对流促使内雨带混合合并,并组织成顺风切内雨带,继而发展为外雨带。Yu等76也提出,外雨带更可能是在台风外部区域局地生成的。可见,涡旋罗斯贝波可能只是外雨带形成的其中一种机制。

2.3 涡旋罗斯贝波与台风强度变化的关系

涡旋罗斯贝波对台风强度影响的机制复杂,其在台风不同的空间位置和生命史阶段,对台风强度的影响可能完全相反,这侧面反映了波动与涡旋之间复杂的相互作用。具体来看:①不同区域的涡旋罗斯贝波对台风强度的影响不同,眼墙及内核的涡旋罗斯贝波可导致台风强度增强,而内核区域外的涡旋罗斯贝波传播可使台风强度减弱77。②涡旋罗斯贝波在台风高低层也具有相反作用,其能够使对流层中低层切向风加速与高空切向风减速78。③在台风快速增强阶段,涡旋罗斯贝波使最大风圈半径处的切向风减速;而在台风减弱期间,涡旋罗斯贝波反而会加速最大切向风43
涡旋罗斯贝波影响台风强度的机制也非常复杂。涡旋罗斯贝波调制台风非对称结构22-24,而非对称对流可能导致台风强度减弱7179-82,这源于非对称风场环流从对称台风涡旋中获得的能量大于向对称涡旋传递的能量,从而减弱台风对称环流的强度83。这与涡旋罗斯贝波的切向传播效应一致,已有研究表明,其切向传播可导致台风强度减弱:Wang22-23通过数值模式模拟发现,涡旋罗斯贝波的切向传播首先会破坏眼墙结构(最大风圈半径处的切向风减弱),导致台风减弱。不过,当涡旋罗斯贝波与螺旋雨带强扰动同位相时,眼墙可能会经历轴对称重建,导致台风再次增强。
而涡旋罗斯贝波的径向传播使得台风增强52甚至快速增强6684-88。以往研究认为不对称程度很大的台风往往强度较低89,但实际上部分台风的快速增强阶段也可以表现为非对称结构,例如“Guillermo”(1997)、“Gabrielle”(2001)、“Earl”(2010)和“Edouard”(2014),但这类台风的快速增强过程伴随强对流的气旋性旋转,从而使台风结构在后期趋于对称的特征84-88。这可以从Montgomery等52的研究中得到解释,他们的研究发现,不对称对流导致的不对称涡度的轴对称化过程会增强台风,湿对流强迫的涡旋罗斯贝波(涡丝)在径向向外传播的过程中可以将不对称强迫的能量传输到眼墙外,从而导致非对称的涡旋轴对称化,最终使得台风增强,这一观点也在后续众多研究中得到了进一步验证和拓展3190-93。还有研究发现,双眼墙的形成往往早于台风的快速增强时期6681,Qiu等66基于数值理想试验提出,这源于涡旋罗斯贝波持续从眼墙径向外传播,并在停滞半径处形成次级环流,次级环流导致的低层辐合和波流相互作用共同驱动,使得平均切向风加速。上述研究表明,涡旋罗斯贝波出现显著径向外传,预示着波动能量外传以及台风结构的对称化调整,将伴随台风强度增强甚至快速增强。

3 台风重力波

过去几十年里,台风重力波已在包括飞机、探空和卫星在内的多种观测资料中得到证实,其中,Aqua卫星搭载的高光谱大气红外探测仪(Atmospheric Infrared Sounder,AIRS)可有效捕捉台风中心向外辐射的圆形/半圆形温度异常带(图3),该特征已成为识别平流层台风重力波的重要遥感信号94-96。现有研究发现,台风重力波具有较强的能量谱和宽广的波谱分布97,其宽谱域特征造成系统性观测困难,中尺度数值模式就成为了研究台风重力波的有力工具2998
图3 Aqua卫星高光谱大气红外探测仪AIRS 4.3 μm CO2 波段探测到的平流层台风重力波94

(a)飓风“Fabian”(2003);(b)飓风“Paloma”(2008)。

Fig. 3 Stratospheric Typhoon-induced Gravity WavesTGWsfrom the Atmospheric Infrared SounderAIRS/Aqua 4.3 μm CO2 band 94

(a) Hurricane “Fabian” (2003);(b) Hurricane “Paloma” (2008).

台风内对流被广泛认为是台风重力波产生的原因,而台风内对流产生的大量潜热释放被当作台风重力波激发的主要能量来源99-102,进而有学者提出涡旋罗斯贝波(或者说是其导致的局地、非对称对流)是台风重力波的来源47。还有一些学者从动力学角度提出台风重力波是由内核区域涡度扰动103或位涡扰动104激发的,其中内核区域涡度扰动产生带状结构重力波的过程又被称为Lighthill辐射机制;或是由内核区域不稳定涡旋运动所激发,并且这些涡旋能够限制波动的频率和径向波数105
台风重力波与台风内对流的密切关系导致其与眼墙和螺旋雨带之间存在明显的动力联系,具体表现为:①台风重力波主要由眼墙及螺旋雨带的对流活动所激发3447100106,其中对流爆发经常伴随着重力波的激发4595-96107;②对流的变化影响台风重力波的产生与特征变化106,反过来,台风重力波的传播和反馈作用也会影响对流的强度和结构98,二者存在复杂的相互作用过程。台风重力波激发后快速垂直传播,可能不到1小时就影响到平流层环流95-96107,进而驱动高层出现动量和热量外传,如动量通量108、角动量109和对流凝结潜热45,随后反馈影响台风结构2959110、物理量分布108-109111-113和对流发展45等,从而影响台风强度。

3.1 台风重力波与眼墙的关系

最早将台风眼墙与重力波建立关联的研究,是关于台风多边形眼墙的相关研究。Lewis等19基于Kurihara36和Willoughby37的研究工作,将多边形眼墙视为具有不同波数和周期的重力波的叠加,但后续研究更多地将多边形眼墙与涡旋罗斯贝波相联系(2.1节)。
最近的研究更多地将台风重力波的产生与眼墙内强烈的对流相联系,已有相当一部分研究提出台风重力波是由眼墙内对流激发的。具体来看,一些研究提出,台风重力波是由台风眼墙内垂直速度为10~20 m/s、水平尺度仅为数公里的最激烈的上升气流所激发的114-115;还有一些研究提出其是由台风眼墙内深对流产生的非绝热加热所激发的34。不少学者提出,波动的主要来源是围绕眼墙旋转的对流不对称性,即由与中尺度涡旋相关的低波数不对称性所引起的5460116-117。眼墙的不对称结构会进一步导致对流活动分布不均,进而激发出不同特征的重力波。例如,Nolan106通过观测和数值模拟试验发现,台风重力波的周期受到对流不对称的数量及其相对切向流的旋转速度控制,对流不对称数量越多、旋转速度越快则周期越短。
台风重力波又会通过波—流相互作用反馈到台风环流,通过改变背景风场影响台风眼墙。具体来看,台风重力波驱动台风内高层出现动量和热量外传3045108-109111,通常表现为台风高层出流,这些重力波驱动的过程可以进一步通过改变背景风切变、水平散度以及诱发对流层次级环流间接影响台风眼墙的强度、结构和对流发展30112118。例如,Shibagaki等30根据雷达观测发现,台风重力波导致的倾斜外流区伴随着包含中尺度上升气流的波状结构运动,这也成为了眼墙垂直环流的一部分。此外,Chow等103研究提出,该区域的重力波可能导致眼墙不完整及其上异常强烈的下沉气流,这可以作为台风增强的第二类条件不稳定过程的补充。但总体而言,对于台风重力波直接影响眼墙结构、强度的深入研究还并不多见,也未见相关研究定量评估其影响的程度。

3.2 台风重力波与螺旋雨带的关系

早期的研究发现,螺旋雨带中扰动垂直速度与扰动气压相差π/2相位,符合重力波的极化理论特征,因此将径向向外传播的螺旋雨带的发展归因于台风重力波的外传35-37119,其中,许秦119利用台风重力波解释了内、外螺旋雨带形成的不同机制。然而,台风重力波具有大的径向波长和快速向外的传播速度,在移速上与螺旋雨带存在明显差异,其波速比雨带的移速快了近1个量级,这与早期天气雷达中观察到的雨带并不一致120-121;与此同时,在频散关系推导中,为了研究台风重力波的特性而略去了涡旋波解50,这也是台风重力波理论研究的问题之一。Willoughby122提出,重力波不能为台风内雨带形成一个合理的模型,随着观测、计算机能力和数值模型的改进,越来越多的研究证实螺旋雨带和台风重力波显然是不同的过程,开始探索螺旋雨带与其他对流和动力过程的相关性22395255123。螺旋雨带可根据快速涡丝化带分为内雨带和外雨带,目前,关于内雨带的产生机制的研究主要基于涡旋罗斯贝波理论,外雨带的产生原因还存在一定争议。早先也有不少研究认为,外雨带可能是向外传播的台风重力波的表征36-37103,从这个角度来看,外雨带中发生的湿对流可能是台风重力波引起的向上运动触发的103124。不过近年来许多研究对外雨带的形成提出了新的看法,例如,Li等75研究认为,外雨带的产生并非归因于台风重力波,而是与内雨带的传播密切相关;Yu等76基于多年多普勒雷达观测数据也指出,外雨带更可能是在台风外部区域局地产生的,飑线动力学是一种常见的外雨带形成机制。基于以上理论研究和观测事实发现,无论是螺旋内雨带还是外雨带似乎都不是台风重力波的直接产物。不过,Wang等98则发现向下传播的平流层台风重力波可以在几十分钟内影响对流层上部的对流不稳定区域,促进对流不稳定能量的释放,导致雨带对流增强,这说明台风重力波仍可以间接影响雨带。
更多的研究提出台风重力波是由螺旋雨带中的对流激发的2947。与眼墙的不对称对流类似,螺旋雨带中的不对称对流同样能激发台风重力波,其强度由环境风切矢量左侧的主要对流区调制106。更为深入的研究发现,螺旋雨带中对流的不对称分布情况能够调控重力波的波动特征,例如,Ming等47研究发现,波数1涡旋罗斯贝波是主导台风重力波(水平波长400~800 km)的来源,波数2的涡旋罗斯贝波产生的螺旋雨带中的局地强对流导致了更小尺度的台风重力波(水平波长100~200 km)。

3.3 台风重力波与台风强度变化的关系

许多研究发现平流层台风重力波特征与台风强度具有相关性。台风重力波在台风加强期间表现出更大的振幅4799,一方面是因为随着台风增强,台风中心最大风速发生变化,重力波强度也会发生显著变化99。另一方面则是因为随着台风增强,台风内对流增强并加深,其释放的潜热发生变化,会形成振幅更大的重力波125-127;重力波周期也会发生改变94,径向波长更短106。随着台风的发展和加强,台风重力波特征发生较大改变的原因似乎与对流的持续性及其相对于最大风圈半径的位置和内核的惯性稳定性有关94106。Hoffmann等44基于AIRS观测统计分析了13.5年的平流层重力波活动与热带气旋强度变化的关系,结果表明,热带气旋增强期间出现强重力波的概率是减弱期间的2倍[图4(a)]。同样,Wright102基于卫星搭载的HIRDLS(High Resolution Dynamics Limb Sounder)、MLS(Microwave Limb Sounder)和SABER(Sounding of the Atmosphere using Broadband Emission Radiometry)观测统计了15年平流层重力波与热带气旋的关系,结果表明伴随着更强重力波活动的热带气旋发展为强风暴的概率更大。Wu等128基于数值模拟研究验证了Hoffmann等44和Wright102的统计结论,并且发现台风强度变化系统性滞后于平流层重力波强度变化[图4(b)]。这些研究表明台风内平流层重力波活动可能是提前指示台风强度增强甚至快速增强的重要信号,因此,对其开展监测可作为台风强度变化预报预警的重要指示信号4494
图4 台风强度与重力波特征的相关关系

(a)热带气旋减弱或增强阶段的平流层台风重力波事件数量44,其中MSW表示10分钟最大持续风速;(b)重力波强度(GWI)与最大加热率(MaxHR)、最大地面风速(MSFCW)、最低海平面气压(MSLP)的滞后相关系数 τ 128

Fig. 4 The correlation between typhoon intensity and the characteristics of gravity waves

(a) Number of Gravity Waves (GW) events associated with increasing or decreasing MSW (10 min maximum sustained wind)44; (b) The “best” time lag τ between GW Intensity (GWI) and the Maximum Heating Rate (MaxHR), the Maximum Surface Wind speed (MSFCW), and Minimum Sea Level Pressure (MSLP), respectively128.

现有研究发现,台风重力波在揭示台风内核结构随时间动态演变中发挥作用46129,可用于诊断对流爆发对台风加强的影响45。但相对于其他影响台风强度的因素,目前台风重力波影响台风强度变化的物理机制还较少,主要分为以下2个方面:①台风重力波传播过程中驱动台风内部动量和热量向外传输,通过波流相互作用改变局地环流场,进而影响台风强度,不同的台风发展阶段对应不同的传输特征。例如,Kim等100-101的研究结果表明,夏季向东传播的重力波输送的正动量通量是维持中层大气风场和温度场的主要来源,台风重力波向上传播的过程中会发生波流相互作用。虽然台风引起的重力波是局地现象,但波流相互作用可以影响局地大尺度环流,可通过改变背景风切变、散度场及台风中心的最小海平面气压等,诱发对流层次级环流来影响台风强度变化,从而有利于台风增强或者快速增强发生108-109111-113。进一步研究指出,台风重力波能够导致高层水平辐散,并在台风的发展和成熟阶段起到减小切变的作用,减小台风中心最小海平面气压,从而有利于台风快速增强112118。此外,Horinouchi等45的研究指出,对流爆发的云砧常以台风重力波的形式传播,云砧呈圆形展开,随后台风重力波与云砧边缘分离并进一步传播、继续输送对流潜热。这一过程能够引起台风环流内的对流发展变化,进而有助于台风加强,因此他们还提出台风重力波可应用于诊断对流爆发对台风加强的影响。②台风重力波在各个方向上传播且传播距离很远,促进台风内对称化结构加强从而有利于台风增强2959110。不对称程度很大的台风往往强度较低,反映出风场不对称对台风强度发展的抑制作用89。与此同时,台风周围层状降水增多、分布范围较广且呈现环状轴对称特征,常被认为是快速增强开始的指标110130-132。这意味着台风快速增强很可能与台风重力波的环状向外传播促使结构对称化有关,其作用机理与2.3节中涡旋罗斯贝波的外传过程类似。这一设想也得到了Zhang等113研究的支持,他们认为由于台风重力波驱动冷云信号外传,垂直环流增强并形成更对称的对流和降水导致台风快速增强,但该机制中关于台风重力波的作用还有待更深入的验证。

4 存在问题与展望

过去,由于观测资料分辨率不足以及数值模式对中尺度过程的刻画能力不足,台风内中尺度过程(尤其是中尺度波动)的相关研究进展缓慢。近年来,随着计算机技术的飞速发展与数值模式的不断改进,模式对中尺度过程的准确刻画能力不断提高;同时,高时空分辨率气象卫星和天气雷达的探测能力不断提升,这些共同推动了对台风中尺度波动影响作用的深入探索,使其逐渐具备可行性。本文综述了台风中尺度波动的产生、特征及其与台风眼墙、螺旋雨带中精细结构的关系,介绍了台风强度变化受到台风中尺度波动影响作用的几种情况。在国内外学者的共同努力下,中尺度波动与台风精细结构以及强度变化关系有了进一步认识,但仍有一些科学问题值得思考和深入研究:
(1)波动与台风精细结构关系方面。现有研究揭示了2类中尺度波动之间存在显著的波—波相互作用,共同调制着眼墙和内(次)雨带的强度及结构特征,其中涡旋罗斯贝波的作用更为直接。非圆形眼墙形成、螺旋内(次)雨带外传的理论机制已由重力波学说发展为涡旋罗斯贝波学说,涡旋罗斯贝波对眼墙和螺旋雨带中非对称对流结构的重要作用也已得到普遍认可。具体来看,涡旋罗斯贝波能够调控眼墙与螺旋雨带的不对称对流结构并驱动内(次)雨带外传,进而激发台风重力波并影响其波动特征,而台风重力波通过波—流相互作用也会进一步调制眼墙和螺旋雨带中对流的强度及结构。
然而,涡旋罗斯贝波在双眼墙和外雨带形成机制中的作用仍有一定争议,主要体现在:外眼墙的形成可能源于涡旋罗斯贝波外传直接影响,或由外雨带对流组织化主导;而外雨带本身则可能也受外传涡旋罗斯贝波调控,或由台风外部局地作用生成。双眼墙的形成机制作为当前台风研究的重点难点问题,基于涡旋罗斯贝波的简单理论模型与数值模拟结果之间存在差异,这可能源于理论假设和实际环境条件的偏差;不同数值模拟研究结果之间也存在差异,这可能源于模式动力框架差异以及模式分辨率和物理参数化方案等限制,同时,分析过程中采用不同的特征物理量表征波动也可能影响其对关键物理过程的刻画。
观测验证有望揭示这些差异的来源,并定性或定量评估主导物理过程的作用。例如,台风“Lekima”(2019)与“Doksuri”(2023)均在我国近海出现双眼墙结构,可基于沿海雷达组网对双眼墙的形成和演变过程开展深入分析,远海台风的双眼墙结构也可利用高分系列卫星或机载雷达等观测资料进行分析。实际上,双眼墙和外雨带的形成都可能涉及多种机制的共同作用,这需要借助更多理论、数值模拟与观测分析研究,将涡旋罗斯贝理论与其他动力学机制联系起来,以完善其形成机理。
(2)波动与台风强度变化关系方面。现有研究揭示2类中尺度波动的径向向外传播均可驱动台风内部动量和热量外传,通过波流相互作用改变局地环流场并增强台风对称性,从而促进台风增强甚至出现快速增强。其中,台风重力波多种特征都与台风强度相关,且其变化超前于强度变化,这可用于探索指示台风强度变化(特别是快速增强)的前兆信号。而涡旋罗斯贝波的切向传播导致台风减弱,其对台风强度的影响还表现出复杂的区域选择性调控与生命史动态响应,具体来看,涡旋罗斯贝波在台风内核(外围)区域导致台风增强(减弱),使台风低层(高层)旋转加速(减速),在台风增强(减弱)时期使其减弱(增强)。
现有研究已反映出中尺度波动与台风涡旋之间复杂的波—流相互作用,因此,想要进一步探索台风增强尤其是快速增强的可靠且具体的预警指标,还需要结合更多高分辨率数值模拟研究与多平台协同观测进行定量分析,为台风强度变化预报提供新的观测视角和理论依据。一方面,波谱分析可有效提取台风生命史各阶段的波动特征,通过关联台风强度变化,可提炼出台风增强包括快速增强的前兆信号;另一方面,可通过波动能量通量或中尺度三维Eliassen-Palm通量等物理量,定量监测台风强度变化前波动能量的输送情况,进一步结合台风对称度指数,从物理机制的角度探寻台风增强及快速增强的前兆信号。
值得期待的是,随着气象卫星(如我国风云三号系列卫星搭载的红外高光谱大气探测仪)和天气雷达等对波动和对流探测能力的持续提升,结合中国气象局部署的多平台协同外场科学试验和野外科学试验基地等对台风内部结构的精细探测,更多数值模式研究中提出的理论可从观测中得到验证,波动相关指示信号也可逐步在观测中验证和应用,这将有助于提高台风精细结构和强度变化的监测预警能力。
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