APPLICATION OF BDA SCHEME IN TYPHOON TRACK PREDICTION

The MM5, which is the PSU/NCAR mesoscale nonhydrostatic limited-area model, and its adjoining modeling system are used in this paper. Taking T106 analysis data as background field the authors generate an optimal initial condition of a typhoon by using two bogus data assimilation schemes, and conduct some numerical simulating experiments. The results of No.9608 typhoon (Gloria) show that the optimal initial field have some dramatic improvements, such as inaccurate position of typhoon center, weaker typhoon circulation and incomplete inner structure of the typhoon, which are caused by shortage of data over the sea. Some improvements have been made in the track forecast. Through several comparing experiments, the initialization optimized by BDA scheme is found to be more reasonable than GFDL scheme and its typhoon track forecast is better.     

A Bogus Typhoon Scheme and Its Application to a Movable Nested Mesh Model

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适应性非对称人造台风方案对热带气旋路径的模拟预报

设计了一种新的适应性非对称人造台风方案。该方案中，初始场中植入的非对称人造台风不但与初始场之间更为协调，而且还包括较多的非对称信息。应用此方案对两个台风个例的路径进行了数值模拟试验，模拟结果与实况较为接近。     

2011年“洛克”和“桑卡”双台风预报效果的初值试验研究

采用南海台风模式,对2011年业务预报误差较大的台风“洛克”（1115）和“桑卡”（1116）双台风的初值方案进行试验和研究,目的在于寻求改进预报的线索,从而提升台风模式性能。针对“洛克”和“桑卡”台风设计了几组初值方案对比试验,结果表明,仅对弱台风“桑卡”进行重定位和bogus的初值处理,与对双台风都进行初值处理相比较,两台风的路径预报误差减小。分析认为仅对弱台风作初值处理以改善其涡旋环流的影响是该双台风路径预报得以改进的原因。对2011—2012年所有弱台风进行批量预报试验,结果表明对弱台风采用重定位和bogus的初值处理,台风路径预报和强度预报的误差减小。对弱台风进行重定位和bogus初值处理,可改善模式对弱台风的预报效果。此外,目前南海台风模式中现有的bogus方法构造的涡旋相对于弱台风而言云顶偏高,可考虑发展针对弱台风的涡旋模型。      

UPGRADING OF TROPICAL LIMITED-AREA NUMERICAL MODEL AND APPLICATION IN TYPHOON FORECAST

A new tropical limited-area numerical model system developed in recent years is described in detail,which includes model formulation, physical processes and data initialization. Results of the application in numerical forecast from 1993 to 1995 are presented. In the forecast of typhoon, the new system shows a distinct improvement over a previous one.     

热带有限区数值预报模式的升级及在台风预报中的应用

较详细地描述了作者近年发展的一个新的热带有限区数值预报系统，包括模式的框架、物理过程以及初值的形成。给出了自１９９３－１９９５年应用于中风数值预报的结果。并与过去的热带有限区数值预报模式系统作了比较，新的数值预报系统对台风预报的改进是十分明显的。     

采用预报涡旋的初始化方案对2015年台风“莲花”、“灿鸿”的试验研究

提出了一种采用预报涡旋的初始化方案,用预报涡旋代替bogus模型参与构建模式初始场,采用权重形式合成预报涡旋和分析涡旋获取台风初始涡旋。针对2015年“莲花”和“灿鸿”台风,基于该初始化方案设计了一系列对比试验进行数值模拟,并对结果进行分析。结果表明:(1)该方案得到的台风初始涡旋结构比bogus模型合理;(2)预报涡旋权重不宜取太大;(3)从长时效预报效果看,采用24 h内预报涡旋比采用长时效预报涡旋台风的路径和强度误差减小;(4)采用同一权重方案对“莲花”、“灿鸿”预报的改进效果不同,其原因与预报涡旋和分析涡旋的协调程度有关。多台风情形下可在初步评估的基础上采用不同时效的预报涡旋和不同权重方案。      

在台风业务系统中使用卫星云导风资料的试验

在国家气象中心台隐数值预报业务系统中引入国家卫星气象中心提供的卫星云导风资料,进行两项预报试验。第一项是把卫星云导风资料作为观测资料进行最优插值客观分析;第二项是将云导风资料加入到轴对称的人造台风模型风场上,使之产生非对称风场。试验结果表明：卫星云导风资料作为观测资料进入模式后对减小台风路径预报误差效果明显;用云导风形成人造台风模型非对称风场可进一步提高台风路径预报精度。     

全球数值模式中的台风初始化Ⅱ: 业务应用

由于缺少大量有效的观测资料,台风初始化对数值天气预报业务模式而言,仍然是一个悬而末决的难题.中国国家气象中心自从1996年将台风数值预报系统投入业务运行以来,一直使用经验的人造bogus涡旋台风初始化技术.实际上,不同时期的台风有着不同的环流结构,即使同一个台风在不同的生命期也具有不同的结构特征,而这些结构特征的差异并不能依靠现有的bogus涡旋技术体现出来,这种主观方法的统一性与台风在时空上的差异性形成了强烈的反差.最近,基于国家气象中心全球资料分析同化-预报循环系统,设计和发展了一套新的台风初始化业务方案,它主要由初始涡旋形成、涡旋重定位和涡旋调整3部分过程组成.相比于业务中使用的人造bogus涡旋台风初始化方案,新方案在很大程度上减少了人为因素对台风涡旋结构的影响,而更多地是依靠数值模式自身的动力和物理过程来协调约束产生三维空间的涡旋结构.应用新方案,文中对生成于西北太平洋的2006年0605号台风格美(Kaemi)进行了数值试验,初步分析表明,新方案在实现台风涡旋环流结构的初始化方面效果较好,同时,对台风格美多个时次的预报结果也显示,相比于业务使用的bogus方案而言,新方案对台风路径平均预报误差有了大幅度的降低.     

台风路径实时数值预报的初步试验

根据国家气象中心建立的台风路径数值预报试验系统，引入了模型台风。模型台风是利用获得的场面参数构成的，然后再叠加到客观分析场上作为台风路径预报的初始场。引入模型台风后，无论是台风中心位置还是其流场、质量场与实际观测比较接近，且预报效果有一定提高。利用模型台风对1992年Eli（9205号）和Janis（9210号）台风进行了实时的预报试验。结果表明对这二个台风的移动趋势基本上能预报出来，预报的台风路径与实况进行了对比，并与气候持续预报作了比较，效果较好。     

几种台风初始化方法及其对比实验

利用中尺度MM5V3模式及其伴随系统，运用四维变分同化方法，将Bogus台风作为观测资料进而优化台风初始场，对0422号台风个例进行了数值模拟和试验，并与传统Bogus方法和Nudging方法进行比较。结果表明，经过BDA方案优化的台风初始场更加接近实际台风的结构，有效地提高了台风路径和强度预报的准确率。     

台风登陆前后异常变化的研究进展

国家重点基础研究发展计划（973计划）项目——“台风登陆前后异常变化及机理研究”经过5年（2009—2013年）的研究完成研究任务。项目系统性地开展了16个台风的外场观测试验,在浅水区的海气拖曳系数、边界层高度变化等方面获得新的观测结果;发展了针对登陆台风的多源资料融合方法,例如台风雷达风场反演和同化技术取得进展;揭示了不同尺度系统活动导致台风路径突变、登陆台风复苏、双眼壁的形成等的物理机制;改进了涡旋初始化和积云对流参数化技术,提出了新的云辐射参数化方案,建立了台风海-陆-气耦合模式,台风数值预报关键技术的研发取得明显进展,文中介绍该项目所取得的代表性成果,对台风登陆前后异常变化的科学问题及研究进展进行综述。     

Initialization and simulation of a landfalling typhoon using a variational bogus mapped data assimilation (BMDA)

Summary Recently, a new data assimilation method called “3-dimensional variational data assimilation of mapped observation (3DVM)” has been developed by the authors. We have shown that the new method is very efficient and inexpensive compared with its counterpart 4-dimensional variational data assimilation (4DVar). The new method has been implemented into the Penn State/NCAR mesoscale model MM5V1 (MM5_3DVM). In this study, we apply the new method to the bogus data assimilation (BDA) available in the original MM5 with the 4DVar. By the new approach, a specified sea-level pressure (SLP) field (bogus data) is incorporated into MM5 through the 3DVM (for convenient, we call it variational bogus mapped data assimilation – BMDA) instead of the original 4DVar data assimilation. To demonstrate the effectiveness of the new 3DVM method, initialization and simulation of a landfalling typhoon – typhoon Dan (1999) over the western North Pacific with the new method are compared with that with its counterpart 4DVar in MM5. Results show that the initial structure and the simulated intensity and track are improved more significantly using 3DVM than 4DVar. Sensitivity experiments also show that the simulated typhoon track and intensity are more sensitive to the size of the assimilation window in the 4DVar than that in the 3DVM. Meanwhile, 3DVM takes much less computing cost than its counterpart 4DVar for a given time window.     

一个人造台风方案及其在移动套网格模式中的应用

发展了一个用于台风路径预报的初始场人造台风方案。该方案除包含对称台风环流外,也考虑了非对称凤的作用。使用双向移动套网格模式作的试验预报结果表明,初始场中引入人造台风后能明显提高路径预报的水平。      

四维变分同化方法在台风初始化及其数值模拟中的应用——对9608号(Herb)台风的研究结果(英)

采用MM5数值预报模式,以及以MM5和它的伴随模式为基础的四维变分同化系统,以NCEP/NCAR再分析资料为背景场,运用BDA(bogus data assimilation)方案优化热带气旋的初始结构,从而一定程度上弥补了海洋上资料缺乏所造成的台风中心位置不准确,台风环流偏弱,台风内部结构不完整等问题,在此基础上对台风进行了72小时数值预报。通过对多个台风个例的研究表明,BDA方案对台风的初始化处理,有效性地构造出了原来分析场所缺乏的台风内部结构,并且与模式初始场协调,因而大大改进了台风中心强度的预报,对于台风路径的预报也有改进,但不是十分理想,还有待提高。     

四维变分同化方法在台风初始化及其数值模拟中的应用--对9608号(Herb)台风的研究结果

The MM5 and its four dimensional variational data assimilation (4D-Var) system are used in this paper. Based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data, the authors generate an optimal initial condition for a typhoon by using the bogus data assimilation (BDA) scheme. BDA is able to recover many of the structural features of typhoons including a warm-core vertex, the correct center position, and the strong circulation.As a result of BDA using a bogus surface low, dramatic improvement is achieved in the 72 h prediction of typhoon Herb. Through several cases, the initialization by BDA effectively generates the harmonious inner structure of the typhoon, but which is lacking in the original analysis field. Therefore the intensity forecast is improved greatly. Some improvements are made in the track forecast, but more work still needs to be done.     

越赤道气流ISO特征在台风季节预报中的可行性研究

越赤道气流作为南北半球物质、能量和水汽的输送通道,它的季节内变化引起台风水汽源变化,影响台风的生成频数和路径。该文研究了越赤道气流ISO特征在台风季节预报中的可行性。台风路径在传统的分类基础上细分为7种,对不同路径的台风所对应的低频流场进行超前和滞后合成,发现不同台风路径对应着不同的低频越赤道气流通道,低层20°N附近的低频气旋外围气流总是预示着台风路径未来的走向。低层低频流场的演变表明,越赤道气流的振荡周期为准40 d,它的ISO特征影响低纬地区的低频气旋的位置和强度,进而影响台风路径。高层低频流场刚好与低层反向,越赤道气流不再是以通道出现而是呈现区域特征。越赤道气流ISO特征可以作为台风季节预报的参考量之一。     

Numerical Study on the Impacts of the Bogus Data Assimilation and Sea Spray Parameterization on Typhoon Ducts

The Weather Research and Forecasting model version 3.2 (WRF v3.2) was used with the bogus data assimilation (BDA) scheme and sea spray parameterization (SSP), and experiments were conducted to assess the impacts of the BDA and SSP on prediction of the typhoon ducting process induced by Typhoon Mindule (2004). The global positioning system (GPS) dropsonde observations were used for comparison. The results show that typhoon ducts are likely to form in every direction around the typhoon center, with the main type of ducts being elevated duct. With the BDA scheme included in the model initialization, the model has a better performance in predicting the existence, distribution, and strength of typhoon ducts. This improvement is attributed to the positive effect of the BDA scheme on the typhoon’s ambient boundary layer structure. Sea spray affects typhoon ducts mainly by changing the latent heat (LH) flux at the air-sea interface beyond 270 km from the typhoon center. The strength of the typhoon duct is enhanced when the boundary layer under this duct is cooled and moistened by the sea spray; otherwise, the typhoon duct is weakened. The sea spray induced changes in the air-sea sensible heat (SH) flux and LH flux are concentrated in the maximum wind speed area near the typhoon center, and the changes are significantly weakened with the increase of the radial range.     

0920号超强台风“卢碧”引起的强海上大气波导成因分析与数值模拟研究

基于中国台湾地区主持的侵台台风之飞机侦察及下投式探空仪观测实验（Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region,DOTSTAR）获得的高分辨率下投式探空仪探测资料,分析了2003年9月—2012年8月所有发生在中国台湾地区附近海域的台风型大气波导事件,遴选出一次由0920号超强台风“卢碧”引起的强台风型海上大气波导过程作为研究对象。利用欧洲中期数值预报中心（ECMWF）再分析资料（水平分辨率0.125°×0.125°）,对此次波导的生成原因进行了分析;基于WRF模式比较了不同初始化方法对台风强度、尺度和周围台风型大气波导的模拟能力。结果表明,此次强台风型大气波导发生在台风环流西北侧外围的弱下沉运动区,其形成与850 hPa高度附近北方强干空气平流导致湿度随高度锐减密切相关。在数值模拟中运用台风动力初始化方法,可以有效改进台风强度、路径和尺度的模拟效果,进而有利于改善台风型大气波导尤其是波导层所在高度的模拟效果。台风外围出现的大气波导通常以悬空波导为主,模拟效果与台风螺旋雨带和内核尺度的模拟关系密切,而与台风强度和眼墙结构关系不大。中尺度数值模式WRF具有模拟台风型大气波导的能力,是研究台风型大气波导的有力手段。      

The role of the land surface processes in the rainfall generated by a landfall typhoon: A simulation of the typhoon Sepat (2007)

Landfall tropical cyclones are a major kind of severe weather affecting China. The typhoon Sepat, declassified as tropical storm after its landfalling, caused a continuous heavy rainfall event over China mainland from 19th to 25th August, 2007. The storm cyclone resided over the Hunan province for 60 hours, causing observed accumulated precipitation larger than 300 mm in a large area of the Hunan province and leading severe flood events. This event was simulated using the Weather Research and Forecasting (WRF) model coupled with the surface layer scheme UTOPIA. The model was able to reproduce the main characteristics of the event, including the typhoon track and the rainfall field and timing. In addition, three sets of sensitivity experiments have been performed. In the first one, the effects of different land surface schemes (RUC, NOAH and UTOPIA) coupled with WRF on the precipitation, sensible and latent heat flux fields associated with the Typhoon Sepat (2007) were investigated. The second set of sensitivity experiments analyzed the role of the surface fluxes (sensible and latent heat flux) on the typhoon evolution. The third set of sensitivity experiments regarded the initialization of the soil moisture content. These experiments showed that both latent and sensible heat fluxes sustained this landfalling typhoon, maintaining the spiral structure of rain belt. Among the two fluxes, the latent heat one played a major role in determining the intensity, the track and the rainfall distribution of the typhoon. In addition, the correct initialization of the soil moisture content has reveled a fundamental parameter to be initialized in order to correctly evaluate the distribution and intensity of the rain field. The intercomparison between the three different land surface schemes coupled with WRF showed that the WRF-UTOPIA and WRF-NOAH outputs seem comparable between each other and physically most realistic than those of WRF-RUC. These analyses were helpful to understand the evolution and the development of the landfalling typhoon, and demonstrated that WRF-UTOPIA and WRF-NOAH could be considered a good tool for managing the risk evaluation connected with the occurrence of such events at regional scale.