东亚沿海气旋生成的数值个例研究

本文用ECMWF(欧洲中期天气预报中心)有限区域模式,对潜热释放和地面感热通量在一个东亚沿海气旋生成个例中的相对重要性,进行了研究。在包括所有物理过程的对照试验中,气旋的迅速发展同观测的结果类似。在无潜热反馈的试验中,当高空短波槽接近位于沿海的地面倒槽时,仅出现一个浅低区,但未出现进一步发展。这样我们可认为,这些物理过程的反馈促使斜压的强迫作用加强。在对照试验中,很好地模拟了大发展以前的一支越来越不平衡的副热带急流、一支非地转的低空急流和伴生的垂直逆环流,但在“干”试验(无潜热加热)中,它们未被模拟出来。潜热加热对加强急流环流和诸系统内的垂直耦合具有深刻影响。这看来促进了沿海迅速的气旋生成。感热加热对地面发展的影响约占18％。它有助于在沿海900百帕以下建立一个位温对比。当无感热加热时,模式潜热释放减少。无感热和潜热加热的试验结果表明,感热加热的影响部分地是通过湿过程而不是直接加热。     

一次东海气旋爆发性发展数值模拟

在2006年6月1日发生在东海的爆发性气旋的天气学分析基础上,进行数值模拟,以进一步认识其爆发性发展的物理机制。结果表明:这次东海气旋的地面系统来自台湾岛附近的海上倒槽,在浙北沿海遇到高空比较深厚的低槽,大尺度高空槽通过槽前正涡度输送和槽前后冷暖平流对斜压不稳定的加强作用为气旋发展和维持提供有利的环境。同时,高空急流也通过其出口区的辐散和暖平流为气旋发展和维持提供有利的环境。中低层在舟山海域的强涡度平流的抬升作用产生降水,并通过水汽凝结引起潜热释放,进一步造成地面气旋系统的发展,此时气旋和降水形成正反馈机制。潜热释放被高空槽前斜压不稳定和急流出口区右侧辐散共同引起的上升运动激发后,对气旋的发展起着重要作用。     

登陆热带气旋与夏季风相互作用对暴雨的影响

利用《热带气旋年鉴》资料、NCEP/NCAR再分析资料采用动态合成分析方法,研究了登陆热带气旋降水与夏季风急流之间的关系,同时对登陆热带气旋与夏季风急流发生相互作用的典型个例强热带风暴Bilis (0604) 利用数值模拟方法研究了二者之间的相互作用对暴雨的影响。结果表明:登陆后造成大范围强降水的热带气旋往往与低层急流长时间相连,其水汽通量和潜热能显著大于弱降水热带气旋。数值试验结果表明:夏季风低空急流向热带气旋输送水汽对热带气旋结构维持有利,当水汽输送被截断后,热带气旋气旋性结构被破坏,强降水减弱、范围明显缩小;季风急流风速增强时可增加水汽通量输送,使得强降水范围增加、强度增强;在夏季风影响背景下,热带气旋在陆上的移动改变水汽和不稳定能量的分布,而热带气旋本身独特的动力结构使得强降水强度增加。     

春季江淮气旋及影响的统计分析

以Lu[1]改进的温带气旋识别方法为基础,结合江苏省73个人工气象观测站的降水资料,统计分析了近35年来春季江淮气旋及其与江苏春季暴雨的关系。结果表明,近35年来,春季江淮气旋发生的次数呈现趋势性递减的变化,其源地主要集中在安徽西南部的大别山东侧和西北部的淮河上游平原。江淮气旋对沿江苏南的春季暴雨有重要影响,而对淮北地区暴雨的影响最弱,给江苏春季带来区域性暴雨的江淮气旋主要是介于中尺度和天气尺度之间的次天气尺度系统。引起淮北和江淮之间两个区域暴雨的江淮气旋源于皖、豫、鲁三省交界处的比例较高。春季江淮气旋造成的暴雨区主要位于气旋中心附近和气旋的南部。其中,淮北地区雨区主要位于气旋中心附近,沿江苏南的雨区在气旋中心和南部均有分布。气旋中心涡度和风速大小、低空西南急流的位置和水汽通量辐合的位置是暴雨落区差异的主要原因。     

1979—2018年江淮气旋时空分布特征

利用MICAPS地面观测资料和中央气象台历史天气图资料,对1979—2018年的585个江淮气旋进行了时空分布特征的研究,发现江淮气旋的发生次数年变化有长、中、短多周期共存,月分布在4—6月（暖季）最多,10—12月（冷季）最少;江淮气旋发生的最集中区域为鄱阳湖盆地和洞庭湖盆地,其次在淮河上游和大别山东北侧,空间分布大值区随环流的季节变化出现一定的南北偏移。利用ERA-Interim月平均再分析资料分析冷、暖季天气背景发现,暖季孟加拉湾存在深厚南支槽,低空西南急流输送暖湿气流,使水汽通量在江淮流域及上游辐合;江淮流域处于暖季锋生正值中心,且位于高空急流入口区右侧,有利于暖季江淮气旋生成。冷季仅在700 hPa的孟加拉湾北侧有浅槽,850 hPa为高压脊,江淮地区为冷平流控制,低空急流不明显,对水汽的输送弱;江淮流域锋生正值区不连续且上游为锋消区,高空急流轴偏南,气象条件不利于江淮气旋生成。从动力和水汽条件上,暖季均比冷季更有利于江淮气旋的发生。     

非绝热加热对江淮气旋影响的数值模拟

本文采用数值模拟方法考察了潜热、感热和水汽蒸发等诸因素对江淮气旋的影响。对各种试验进行了涡度平衡与能量平衡的诊断分析,结果表明:江淮气旋初生时,扰动从基本气流中获得能量,正压不稳定起着重要作用;在江淮气旋发展后期,斜压性比较明显。潜热释放有意义地修正了系统的相速,一定程度上加强了系统的强度。海上感热和水汽蒸发促进了深对流发展,加大降水,同时加强了有效位能的释放,从而加强了系统发展,但它们的作用是有阶段性的。     

湿地边界层通量影响热带气旋登陆维持和降水的数值试验

用MM5v3模式及其TC Bogus方案对台风Nina(7503)在中国大陆上的维持加强过程进行模拟,并通过各种敏感性试验,研究了饱和湿地对热带气旋在大陆上维持和降水的影响。结果表明:湿地边界层内各通量对登陆热带气旋的强度、结构及其降水具有明显影响。潜热通量和感热通量均有利于热带气旋的维持和加强,其中潜热输送的作用显著,感热输送的作用较小。潜热和感热通量有利于热带气旋雨带螺旋结构的维持,对台风降水分布有明显影响,动量通量对热带气旋风场有显著的削弱作用,是其能量耗散、低压填塞的主要原因,但对台风降水具有局地增幅作用。     

安徽一次大范围暴雨和大风过程的成因分析

对2010年6月8日08时—9日08时发生在安徽麦收区的一次大范围暴雨和大风过程进行天气学分析,分析结果表明:在低层辐合、高层辐散的有利环流配置条件下,高压的阻挡、低空西南急流和低空东风急流是形成这次暴雨的最主要原因。高压阻挡使降雨维持的时间较长,两支低空急流不仅为暴雨区提供了充沛的水汽,而且两者之间的相互作用使得降雨进一步增强。通过对低空东风急流的作用做深入分析发现:低空东风急流的存在不仅使低空西南急流输送的水汽在其左侧累积,而且低空东风急流上的正涡度平流也迫使江淮气旋向麦收区方向缓慢移动,并使其不断发展,进一步加大了江淮气旋移动前方的气压梯度,使风力加大,东风急流进一步加强,形成正反馈机制。另外东风急流的动量下传又使得地面出现偏东大风。     

一次历史罕见的雨转暴雪天气过程分析

受强冷空气和低空切变共同影响,2009年2月12日17时至13日08时,集安市出现了历史同期最强的冬季大到暴雨天气,其它市县出现了历史同期最强的雨转暴雪天气过程,本文以常规气象资料及数值预报资料为基础,从大尺度环流特征、影响天气系统、雨转暴雪的前期气温分析、温度场结构特征、各气压层大气温度结构特征,动力条件及高低空急流配置、水汽条件、卫星云图等方面对此次天气过程进行分析。结果表明：本次强降水是产生在欧亚中高纬度呈-槽-脊经向环流形势下,500hPa北涡南槽、地面江淮气旋、850hPa切变线是主要影响天气系统：地面江淮气旋东移加强北上对雨转暴雪天气的形成和维持起到重要作用,2月12日最高气温上升到6～8℃,如此高温为通化地区降水积累了强大动力和能量来源,也是本次降水开始是雨原因。降水开始时我省的东南部受暖锋控制,降水以雨的性质为主,随着冷锋东移南下,我区自北到向南依次转为降雪。高低空急流的动力耦合作用、低空的西南急流水汽输送带从孟加拉湾、南海、东海、黄海带来异常充沛的水汽和强烈的辐合所产生的垂直上升运动是本次强降水的重要原因;低层北方冷空气与南方暖湿气流交汇使低层形成强锋区,为雨转暴雪的产生提供了动力。     

边界层过程对暴雨影响的敏感性试验

用MM5v3模式对江淮暴雨进行了计入与不计入边界层和计入与不计入地面通量的敏感性试验,结果指出二者的影响是一致的,地面通量是边界层影响的主要部分。一般说,不计边界层影响,则湿位涡、水汽通量散度、涡度、低空急流等分布发生变化,其中心位置改变,强度也有改变(大部分减弱),造成总的降水减弱和降水位置的改变。边界层正是通过湿位涡、水汽通量散度、涡度、低空急流等因素的综合影响来影响暴雨,并影响暴雨的落区和强度。     

Simulation of formation of a near-equatorial typhoon Vamei (2001)

Summary A community mesoscale model is used to simulate and understand processes that led to the formation and intensification of the near-equatorial typhoon Vamei that formed in the South China Sea in December, 2001. The simulated typhoon resembles the observed in that it had a short lifetime and a small size, formed near the equator (south of 2° N), and reached category-one intensity. The formation involved the interactions between the scales of the background cyclonic circulation (the Borneo Vortex of order ∼100 km) and of mesoscale convective vortices (MCVs, in the order ∼10 km). Before tropical cyclone formation MCVs formed along a convergent, horizontal shear vorticity line on the eastern edge of an exceptionally strong monsoonal northerly wind surge. The typhoon genesis is marked by three rapid intensification periods, which are associated with the rapid growth of potential vorticity (PV). A vorticity budget analysis reveals that the increases in low-level vorticity during the rapid intensification periods are attributed to enhanced horizontal vorticity fluxes into the storm core. The increase of the horizontal vorticity flux is associated with the merging of areas of high PV associated with MCVs into the storm core as they are advected by background cyclonic flows. The increases in PV at upper levels are associated with the evaporation of upper level stratiform precipitation and increases of vertical potential temperature gradient below the maximum stratiform cloud layer. It appears that two key sources of PV at upper and lower levels are crucial for the build up of high PV and a deepening of a cyclonic layer throughout the troposphere.     

ANALYSIS OF A CASE OF EAST ASIA COASTAL CYCLOGENESIS*

The evolution of an explosive cyclone off the East Asia coast in March 1979 is described.A shortwave trough in the southern branch of upper-level westerlies initiated the incipient cyclone.Later,a polar trough in the north amplified and became in phase with the southern shortwave to form a major trough.This major trough was responsible for the rapid intensification of the surface cyclone.In the early development stage,warm and moist air was transported northward to the developing area by a strong low-level jet.The ageostrophic wind associated with the low-level jet contributed to the frontogenesis,creating a favorable low-level environment for the rapid deepening.A low-level positive potential vorticity anomaly was created prior to the onset of rapid deepening.It was a result of frontal cloud condensation.The cyclone intensified rapidly when stratospheric air with high potential vorticity penetrated to the mid-troposphere.The rapid deepening took place at a location under the left-exit region of an amplifying jet streak behind the major trough and the right-entrance region of another anticyclonically-curved subtropical jet streak in a quasi-stationary ridge overJapan.Due to the blocking effect of the Tibetan Plateau,two shortwave disturbances were observed in the upper-level westerlies on the north and south sides of the Plateau.The southern disturbance initiated the incipient surface cyclone,while the amplifying northern disturbance was responsible for the rapid deepening.Thus,the evolution of the explosive cyclone in this case can be regarded as consecutive Petterssen's "type-B" cyclogenesis in two separate stages.     

大连地区暖锋暴雨个例成因与雷达回波特征

利用常规观测资料、自动站资料和多普勒雷达资料等,对2009年7月14日大连暴雨局部大暴雨过程进行详细分析。结果表明：这是一次暖锋大暴雨过程,高空河套槽北抬和北支槽尾段相叠加,中低层在渤海北部到大连地区形成涡旋环流,700 hPa气旋式较大曲率处在地面暖锋上空,大连地区位于地面气旋顶部即暖锋顶部,造成强降水的产生。强湿区,配合暖锋前低层辐合中心、高层辐散中心,为暖锋大暴雨天气提供水汽和动力条件。从雷达回波分析可以看出,暖锋前部45dBz的β中尺度反射率及速度场上“单牛眼”特征,是造成此次暴雨过程及短时暴雨的直接原因。VWP资料分析表明,低层东南急流与高层西南急流形成切变层的高度以及两支急流的强度变化,与暖锋对应并决定降水的强弱。     

The role of oceanic fluxes and initial data in the numerical prediction of an intense coastal storm

On 18–19 February 1979, an intense cyclone developed along the east coast of the United States and produced heavy snowfall accumulations from Virginia to southeast New York. A series of forecast experiments was conducted to assess the accuracy of the GLA model's prediction of this storm and the importance of oceanic heat and moisture fluxes and initial data to the cyclogenesis. The GLA model forecast from the GLA NOSAT analysis at 0000 GMT 18 February correctly predicted that intense coastal cyclogenesis and heavy precipitation would occur, even though important subsynoptic details of the development were underestimated or not forecast. A repetition of this forecast with surface heat and moisture fluxes eliminated failed to predict any cyclogenesis while a similar forecast with only the surface moisture flux excluded showed only very weak cyclonic development. An extended-range forecast from 0000 GMT 16 February as well as forecasts from the GLA FGGE analysis or the NMC analysis at 0000 GMT 18 February interpolated to the GLA grid predicted weaker coastal low development than the forecast from the NOSAT analysis.Detailed examination of these forecasts shows that diabatic heating resulting from oceanic fluxes increased low-level baroclinicity, decreased static stability and significantly contributed both to the generation of low-level cyclonic vorticity, and to the intensification and slow rate of movement of an upper-level ridge over the western Atlantic. As an upper-level short-wave trough approached this ridge, the diabatic heating associated with the release of latent heat intensified and the gradient of vorticity, vorticity advection and upper-level divergence in advance of the trough were increased, which provided strong forcing for the surface cyclogenesis.An examination of the NMC and GLA analyses indicated that a weaker representation of the upper-level trough in the interpolated NMC analysis was primarily responsible for the resulting forecast differences. Comparison of the GLA FGGE and NOSAT initial analyses showed that the FGGE analysis of cloud-track wind data probably underestimated the maximum wind speeds associated with an upper-level jet streak near the east coast. This diminished the effect of the oceanic fluxes in the forecast from the FGGE analysis and resulted in weaker cyclogenesis.     

春夏季节黄河气旋经渤海发展时影响因子对比研究

利用2008—2012年台站资料、NCEP(National Centers for Environ mental Prediction) FNL(Final Operational Global Analysis)1°×1°再分析资料,将近5年经过渤海持续发展的黄河气旋分为夏季型和春季型,采用动态合成法对两类气旋的结构和黄渤海海域的热力、动力、水汽等影响因子进行对比分析。结果表明：经过渤海时,夏季型气旋主要伴随大范围的强降水,而春季型气旋主要形成强风区。春夏季黄河气旋均为冷暖交汇的斜压性结构,但夏季型有偏暖中心,斜压性弱于春季型。春季高空急流位于气旋南部,其左侧正涡度区维持气旋的深厚,且气旋后部高空动量下传与锋面二级环流及平坦海面配合有利于气旋低层大风迅速增强。夏季高空急流位于气旋北部,高空强辐散区和低层辐合区配置加强了气旋中的上升运动,有利于气旋强降水和凝结潜热释放。气旋发展阶段,扰动位能向动能的转化,支持气旋动能的维持与加强。湿位涡计算显示,夏季气旋中有深厚的干空气下沉,干湿梯度强,尺度大,有利于气旋的强降水,春季气旋中干湿梯度小,分布零散,对应降水强度和范围均小。黄渤海为气旋主要水汽输送通道,夏季海温相对春季高,水汽充沛,春季水汽辐合量仅为夏季三分之一。海洋下垫面作用对春季气旋影响大,在夏季作用不明显。夏季海面潜热加热影响为主,春季感热加热影响明显。     

Sensitivity of Tropical Cyclone Intensity and Structure to Planetary Boundary Layer Parameterization

The advanced weather research and forecasting model is used to investigate the influence of planetary boundary layer (PBL) processes on intensity and structure of the storm Phailin (2013). Five simulations are conducted with five PBL schemes; Yonsei University (YSU), Mellor?Yamada?Nakanishi?Niino order2.5 (MYNN2), Assymetric Convective Model2 (ACM2), Medium Range Forecast (MRF), and Bougeault and Lacarrere (BouLac). The simulation duration includes the pre???intensification and rapid intensification phase of Phailin before landfall. Results indicate that during the pre???intensification phase, storm’s track and intensity are not much sensitivity to PBL but structural changes are noted. A significant sensitivity of track and intensity to PBL parameterizations are found during rapid intensification phase. BouLac and MRF produced two extremes with 39 hPa intense and 16 km compact storm for BouLac than MRF. Further analysis reveals an outward movement of air parcel just above the boundary layer which causes spin-down for YSU and MYNN2. BouLac is associated with stronger eddy diffusivity and moisture fluxes within the boundary layer and stronger cyclonic vorticity just above the boundary layer than other experiments. Stronger cyclonic vorticity above the boundary layer in BouLac favors intense updraft, facilitating more moisture transport from the boundary layer to upper layers aiding stronger secondary circulation and robustly intensifying the storm. A relatively deeper and drier inflow layer associated with weaker cyclonic vorticity just above the boundary layer reduces the moisture transport and weaken the secondary circulation for MRF than others.     

春季中国东部海域气旋急流在气旋发展中的作用研究

对2008—2014年中国东部海域春季海上发展气旋进行了统计与诊断分析。结果表明:1)这类气旋属于较浅薄的低值系统,垂直伸展高度多在600 h Pa以下,水平尺度多在1 500 km以内。伴随的强天气为大风、大浪与强降水,落区主要位于气旋东南部。2)气旋环流各层的大风急流区构成了气旋的东南部位,称为气旋急流。从高层到低层,气旋急流轴在垂直方向上呈逆时针旋转,形成气旋上大下小的漏斗形状。3)气旋急流左侧的气旋式切变有利于气旋中心强度的维持,上层气旋急流左侧对应下层气旋急流前部流速辐合区,有利于气旋式动力抽吸及在气旋东南部形成强的垂直上升运动区。各层气旋急流配置导致气旋的非对称结构,以及气旋要素的非对称分布。气旋急流向气旋中输入螺旋度以及充足的水汽,并在东南部强烈抬升,增强了凝结潜热释放,从热力和动力两方面促进气旋发展及强天气落区。4)春季下垫面温度分布(锋区)有利于气旋急流的增强,并通过西北部非绝热冷却和东南部非绝热加热,增强气旋斜压性。高空环境西风急流位于气旋右侧,形成了整层偏差风辐合,有效增强低层气旋急流。同时高空动量下传位于气旋西侧,首先增强气旋西北部的弱流部分(即气旋螺旋结构的下沉支),进而增强整个气旋的螺旋环流,促使气旋急流也从下层开始增强。     

On the sensitivity of predictions of maritime cyclogenesis to convective precipitation and sea temperature

Abstract

The influences of surface fluxes and convective precipitation are investigated for two 36‐h periods of cyclogenesis over the northeastern Pacific Ocean. Three methods are tested of specifying the fraction of moisture supply that produces convective precipitation in a modified form of Kuo's (1974) parametrization scheme using an 8‐level primitive equations model.

When convection is included, precipitation amounts are greater and the cyclone deepening is better predicted than when convection is not included. Predicted cyclogenesis is very sensitive to sea temperature. As the low moves over warmer water, the effect of sensible heating is to increase the moisture convergence in the atmospheric boundary layer. This increases the precipitation rates and accelerates deepening. It is concluded that the CISK mechanism plays an important role in extratropical cyclogenesis.     

Evolution of the Mid-tropospheric Vortex during the Formation of Super Typhoon Megi(2010)

As a follow-up of a previously published article on the contribution of tropical waves, this study explores the evolution of the mid-tropospheric mesoscale cyclonic vortex(MV) during the formation of Typhoon Megi(2010) with a successful cloud-resolving simulation. It is found that the formation and intensification of the MV were related to the deep convection and subsequent stratiform precipitation, while the weakening of the MV was related to the shallow convection. Both the upward transport of vorticity related to the deep convection and the horizontal convergence associated with the stratiform precipitation contributed to the formation and intensification of the MV. Even though the latter was dominant, the former could not be ignored, especially in the early stage of the MV. The MV played dual roles in the formation of Megi. On the one hand, the formation and intensification of MV were primarily associated with the stratiform precipitation, which induced the low-level divergence inhibiting the spin-up of the near-surface cyclonic circulation. On the other hand, the coupled low-level cold core under the MV benefited the accumulation of the convective available potential energy(CAPE),which was favorable for the convective activity. A sensitivity experiment with the evaporative cooling turned off indicated that the development of the MV retarded the genesis process of Megi.