A Modeling Study on Tropical Cyclone Structural Changes in Response to Ambient Moisture Variations
A Modeling Study on Tropical Cyclone Structural Changes in Response to Ambient Moisture Variations
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DOI：
10.2151/jmsj.2012-512
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发表时间：
2012
影响因子：
3.1
通讯作者：
Y. Ying;Qinghong Zhang
中科院分区：
文献类型：
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作者：
Y. Ying;Qinghong Zhang
近年来的研究强调了水汽在改变热带气旋涡结构中的重要作用。外围雨带释放的潜热引起了次级环流的变化，对TC内核强度有负向影响。本研究旨在进一步探讨垂直风切变存在时的TC结构特性。利用WRF （Weather Research and Forecasting）模式对台风塔利姆（2005）进行了模拟，并通过人为改变TC涡周围的水汽量和分布进行了敏感性试验。由于东风垂直风切变的存在，模拟的台风塔利姆发展出准静止的外围雨带，集中在西南（左下切变）扇区。北风（外雨带上游）进入TC核的速度比其他方向快，因此台风“塔利姆”对北方的湿度变化比南方更敏感。随着外部雨带的水分供应增加，模拟的tc尺寸变大。然而，由于外部雨带释放的潜热引起上升气流，减少了中低层径向流入，使绝对角动量平流到内核，因此它们的内核强度和强度减弱。相反，由于干燥的环境抑制了外核区的对流，在减少水分供应的情况下，模拟的tc尺寸变小。相对没有对流的外核区域有利于形成强径向流入，加速了内核的自旋过程。这导致tc收缩，而其内部核心的强度和强度增加。虽然外核区的水分对内核的强化有负面影响，但它通过在外核区诱导更多的对流，有助于维持外核的强度和TC大小。因此，TC外核心区丰富的水汽供应对TC一次环流水平范围的增长至关重要。Et al. 2008)。最近，Hendricks等人（2010）在他们的综合分析中发现，在西太平洋，减弱的tc（定义为24小时最大风速减少超过6.5 m s-1）比增强的tc表现出更低的环境相对湿度。结果表明，低空较干燥的空气从西北外围包裹入TC核心。在大西洋，来自撒哈拉空气层的干燥空气侵入TC核心被认为对TC强度有负面影响（Braun 2010）。然而，也有研究表明，由于动态调整TC涡结构可以延缓环境影响，TC强度对环境RH变化的瞬态响应与上述结果不同(Kimball 2006；通讯作者：张庆红，北京大学物理学院大气与海洋科学系，北京100871 E-mail: qzhang@pku.edu.cn©2012，日本气象学会学报第90卷第5期756 Hill and Lackmann 2009)。TC的瞬态行为给天气预报员带来了困难，他们希望在登陆前几小时准确预测TC的强度变化。Kimball（2006）在其模拟研究中发现，较高环境相对湿度的TC会产生更多的雨带对流，从而将中层低等效位温（theta-e）空气带入入流层，从而对TC的增强产生负面影响。由此产生的TC在短期内的强化率会降低。但是，由于雨带也起到了阻止干燥空气侵入核心区域的屏障作用，因此具有更多雨带对流的TC可能会在更长的时间内加强。为了进一步解释TC在不同热力学条件下的瞬态响应，Wang（2009）进行了一组针对不同雨带绝热加热和冷却速率下TC结构变化的数值实验。他提出，外螺旋雨带上非绝热升温速率较大的TC会扩大和减弱。结果表明，雨带内侧的压力下降更大，因为那里的惯性稳定性更强。因此，增加的绝热加热速率产生降低的压力梯度，并导致减弱的TC。Hill和Lackmann（2009）的实验在位涡（PV）框架下验证了这些结果。他们推断，风场的横向范围（TC大小）与由于外部雨带释放的潜热而导致的PV增长有关。Holland和Merrill（1984）提出，TC涡旋不仅可以改变最大风速（强度），还可以改变风场的范围（大小和强度）。在图1中，我们绘制了2003~2010年西太平洋tc的强度-大小图。很明显，对于大型tc（面积超过200公里），强度和大小之间的相关性并不好。有些tc在尺寸减小的同时会增强，有些tc在尺寸不断增大而强度不变。因此，短期强度预测需要更好地预测TC结构变化。实际大气中TC结构变化的预测更为复杂，因为TC往往会形成不对称的雨带对流和涡旋结构（Lonfat et al. 2004）。这种不对称的主要原因是垂直变化的风暴相对流，即垂直风切变取代了每一层的涡旋（Wang and Holland 1996）。结果表明，在TC中心左侧的下切变处对流增强。Frank和Ritchie（2001）在模型模拟和Heymsfield等人（2006）在观测中也发现了这种垂直剪切引起的不对称性。最近，Riemer和Montgomery（2011）用他们简单的TC-环境相互作用的运动学模型表明，垂直风切变改变了环境空气到达TC核心的路径。结果表明，随着剪切方向的不同，TC与不对称干湿环境空气的相互作用会导致不同强度的变化。本研究的目的是探讨在垂直风切变引起的不对称存在下，TC涡的行为。除了之前理想化的实验结果（Wang 2009; Hill and Lackmann 2009），我们相信我们的实验更真实的TC环流可以更详细地描绘TC与环境湿度的相互作用。选取西北太平洋台风“塔利姆”（2005）为实例进行数值试验，并对其周围湿度场进行了人工改造。分析了在这种环境湿度扰动下产生的TC结构变化。在第二节中，我们简要地描述了数据、数值模型的配置和灵敏度实验的设计。第3节将提供模型模拟结果，第4节将给出这些结果的解释。第5节将得出结论，并提出有关可能进一步研究的几个问题。图1所示。2003~2010年西北太平洋TCs的大小和强度的发生次数（阴影）。强度定义为最大持续低层风速，大小定义为大风半径（17 m s-1）。强度和粒径均采用JTWC最佳轨迹数据计算。台风塔利姆（2005）在其发展期间的结构演化路径以黑线表示。台风“塔利姆”经历了一个规模增长期（8月29日00~12UTC点a ~ b），随后是一个增强期（8月00~12UTC点c ~ d）
Recent studies have emphasized the important role of moisture in altering tropical cyclone (TC) vortex structure. Latent heat released in outer rainbands induces change in the secondary circulation, and exerts negative impact on TC inner core intensity. This study is to further explore the TC structural behavior with the presence of vertical wind shear. Typhoon Talim (2005) was simulated using the Weather Research and Forecasting (WRF) model, and sensitivity experiments were conducted by artificially modifying the amount and distribution of moisture around TC vortex. With the presence of an easterly vertical wind shear, the simulated Typhoon Talim developed quasi-stationary outer rainbands that concentrate in the southwestern (downshear left) sector. Air from the north (upstream side of the outer rainbands) traveled faster into TC core than air from the other directions, thus Typhoon Talim was more sensitive to moisture variations in the north than in the south. With enhanced moisture supply into outer rainbands, simulated TCs grow larger in size. However, their inner core intensity and strength are weakened because latent heat released in outer rainbands induces updrafts and reduces midto low-level radial inflow that advects absolute angular momentum into inner core. On the contrary, TCs simulated with reduced moisture supply become smaller in size since drier environment inhibits convection in outer core region. The relatively convection-free outer core region favors the formation of strong radial inflow that accelerates the inner core spinup process. This causes TCs to contract while their inner core strength and intensity increase. Although moisture in outer core region imposes a negative effect on inner core intensification, it contributes to the maintenance of outer core strength and TC size by inducing more convection in the outer core region. Thus, abundant moisture supply in TC outer core region is critical to the growth of horizontal extent of TC primary circulation. et al. 2008). Recently, Hendricks et al. (2010) showed in their composite analysis that, in the West Pacific Ocean, weakening TCs (defined as 24-hour decrease in maximum wind speed more than 6.5 m s–1) displayed lower ambient RH than intensifying TCs. They showed that drier air in the low level wraps into TC core from the northwest periphery. In the Atlantic Ocean, dry air from the Saharan Air Layer intruding into TC core is considered a negative impact on TC intensity (Braun 2010). However, some researchers showed that the transient response of TC intensity to environmental RH change is different from the aforementioned results because dynamic adjustment of the TC vortex structure could delay the environmental influences (Kimball 2006; Corresponding author: Qinghong Zhang, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, China E-mail: qzhang@pku.edu.cn ©2012, Meteorological Society of Japan Journal of the Meteorological Society of Japan Vol. 90, No. 5 756 Hill and Lackmann 2009). The transient TC behavior brings difficulty to forecasters, who would like to accurately predict TC intensity change just hours before landfall. Kimball (2006) revealed in her modeling study that TC with higher ambient RH develops more rainband convection, which exerts negative impacts on TC intensification by bringing down mid-level low equivalent potential temperature (theta-e) air into the inflow layer. The resulting TC will have a reduced intensification rate in the short-term perspective. But, since the rainband also act as barriers preventing dry air from intruding into the core region, TC with more rainband convection might instead intensify for a longer period of time. To further explain TC’s transient response in different thermodynamic conditions, Wang (2009) performed a set of numerical experiments concerning TC structural changes in response to different rainband diabatic heating and cooling rates. He proposed that TC with larger diabatic heating rate in the outer spiral rainbands will expand and weaken. The results showed that pressure dropped more on the inward side of rainbands because of the greater inertial stability there. Thus, an increased diabatic heating rate yields reduced pressure gradient and results in a weakening TC. The experiments by Hill and Lackmann (2009) verified these results in the potential vorticity (PV) framework. They deduced that the lateral extent of wind field (TC size) is related to the PV growth due to latent heat released in outer rainbands. As was proposed by Holland and Merrill (1984), TC vortices could vary not only in maximum wind speed (intensity), but also in the extent of wind field (size and strength). In Fig. 1 we plot the intensity-size diagram for west Pacific TCs during 2003~2010. It is clear that, for large TCs (with size over 200 km), the intensity and size are not well correlated. Some TCs may intensify while their size is decreasing, and some may have continuously increasing size while intensity remains unchanged. Thus, better prediction of TC structural change is needed for the short-term intensity forecast. The prediction of TC structural changes is even more complicated in the real atmosphere because TCs often develop asymmetric rainband convection and vortex structure (Lonfat et al. 2004). The main cause of such asymmetries is the vertically changing storm relative flow, namely vertical wind shear that displaces vortices on each level (Wang and Holland 1996). They showed that convection was enhanced to the downshear left of the TC center. Such vertical shear-induced asymmetry was also found by Frank and Ritchie (2001) in model simulation and by Heymsfield et al. (2006) in observation. Most recently, Riemer and Montgomery (2011) showed with their simple kinematic model of TC-environment interaction that vertical wind shear modifies the pathway of environmental air reaching TC core. They indicate that the TC’s interaction with asymmetric dry or moist environmental air could result in different intensity changes depending on the shear direction. The purpose of this study is to explore the behavior of TC vortices with the presence of asymmetries induced by vertical wind shear. In addition to previous idealized experiment results (Wang 2009; Hill and Lackmann 2009), we believe our experiments with more realistic TC circulation could draw a more detailed picture of TC’s interaction with ambient moisture. Typhoon Talim (2005) in Northwestern Pacific is selected as a real case for numerical experiments, and its ambient moisture field is artificially modified. The resulting TC structural changes in response to such ambient moisture perturbations are analyzed. In Section 2, we briefly describe the data, configuration of numerical model and the design of sensitivity experiments. Section 3 will provide the model simulation results and interpretations of these results will be presented in Section 4. Section 5 will draw conclusion and raise several issues concerning possible further studies. Fig. 1. Number of occurrence (shadings) of size and intensity of Northwest Pacific TCs during 2003~2010. Intensity is defined as maximum sustained low level wind speed, and size as radius of gale-force wind (17 m s–1). Both intensity and size were calculated using JTWC best track data. The structural evolution path of Typhoon Talim (2005) during its developing period is shown as a black line. Typhoon Talim experienced a size-growth period (point a to b as 00~12UTC, August 29), followed by an intensification period (point c to d as 00~12UTC, August