“浣熊”强度变化的环境背景和卫星观测分析

利用实况观测定位和强度资料、NCEP的1°×1°全球最终分析资料、NOAA周平均1°×1°海温再分析资料和FY卫星水汽图像,分析台风浣熊(0801)的环境背景条件和内核结构演变与强度变化的关系.结果表明:(1)环境风垂直切变增大至10m·s~(-1)左右与南海北部海表温度逐步减小导致最大可能强度不断降低是"浣熊"快速发展过程中断并减弱的重要外部条件;(2)卫星水汽图像显示内核结构存在眼壁破裂和复原现象,该现象精确地反映其强度变化.眼壁破裂(复原)过程导致"浣熊"减弱(增强).     

应用AMSU B微波资料分析0509号Matsa台风水汽场分布特征

热带气旋的形成、发展、移动以及消亡与大范围水汽输送和集中息息相关.此研究以2006年7-8月的AMSU-B的183.3±1、183.3±3、183.3±7GHz三个水汽通道亮温为主要资料源,利用三个通道所在的权重高度差异,分别建立了对流层高、中、低层水汽量与三个通道亮温的e指数统计回归关系.在此基础上,选取西北太平洋2005年09号麦莎台风,利用AMSU-B亮温通过回归关系式计算出的高、中、低层水汽量,就其水汽输送与强度变化关系进行了分析.结果表明,其一,微波水汽通道亮温能够很好地表征热带气旋环境场不同高度的水汽分布和水汽输送,尤其是对流层中层亮温的变化显示出的水汽量的变化与热带气旋强度密切相关;其二,热带气旋发展过程中,除了西南季风形成的西南水汽通道是水汽维持的重要输送带外,东南水汽通道也经常成为登陆热带气旋强度维持的主要水汽供给.     

Beta gyres in global analysis fields

A three-component decomposition is applied to global analysis data to show the existence of a beta gyre, which causes Tropical Cyclone (TC) to drift from a large-scale environmental steering current. Analyses from the Global Data Assimilation and Prediction System (GDAPS) of the Korea Meteorological Administration (KMA), the Global Forecast System (GFS) of NCEP, and the Navy Operational Global Atmospheric Prediction System (NOGAPS) are used in this study. The structure of the beta gyre obtained in our analyses is in good agreement with the theoretical structure, with a cyclonic circulation to the southwest of the TC center, an anticyclonic circulation to the northeast, and a ventilation flow directed northwestward near the center. The circulation of the beta gyre is strongest at the 850-hPa level where the cyclonically swirling primary circulation is strongest, and decreases with height, in a pyramid shape similar to the primary circulation. The individual structure of the beta gyre is case- and model-dependent. At a certain analysis time, one model may clearly reveal a well-defined beta gyre, but the other models may not. Within one model, the beta gyre may be well defined at some analysis times, but not at other times. The structure of the beta gyre in the analysis field is determined by the nature of the vortex initialization scheme and the model behavior during the 6-h forecast in the operational data assimilation cycle.     

The effect of surface friction on the development of tropical cyclones

When tropical cyclones (hereafter referred as TCs) are over the ocean, surface friction plays a dual role in the development of TCs. From the viewpoint of water vapor supply, frictional convergence and Ekman pumping provide a source of moisture for organized cumulus convection and is propitious to the spin-up of TCs. On the other hand, surface friction leads to a dissipation of kinetic energy that impedes the intensification of TCs. Which role is dominant in the developing stage of TCs is a controversial issue. In the present work, the influence of surface friction on the growth of TCs is re-examined in detail by conducting two sets of numerical experiments initialized with different cyclonic disturbances. Results indicate that, because of the inherent complexities of TCs, the impact of surface friction on the evolution of TCs can not be simply boiled down to being positive or negative. In the case that a TC starts from a low-level vortex with a warm core, surface friction and the resultant vertical motion makes an important contribution to the convection in the early developing stage of the TC by accelerating the build-up of convective available potential energy (CAPE) and ensuring moisture supply and the lifting of air parcels. This effect is so prominent that it dominates the friction-induced dissipation and makes surface friction a facilitative factor in the spin-up of the TC. However, for a TC formed from a mesoscale convective vortex (MCV) spawned in a long-lasting mesoscale convective system (MCS), the initial fields, and especially the low-level humidity and cold core, enable the prerequisites of convection (i.e., conditional instability, moisture, and lifting), to be easily achieved even without the help of boundary-layer pumping induced by surface friction. Accordingly, the reliance of the development of TCs on surface friction is not as heavy as that derived from a low-level vortex. The positive effect of surface friction on the development of TCs realized through facilitating favorable conditions for convection is nearly cancelled out by the friction-induced dissipation. However, as SST is enhanced in the latter case, the situation may be changed, and different development speeds may emerge between model TCs with and without surface friction considered. In short, owing to the fact that TC development is a complicated process affected by many factors such as initial perturbations, SST, etc., the importance of surface friction to the intensification of TCs may vary enormously from case to case.     

ANALYSIS OF RAINSTORMS ASSOCIATED WITH SIMILAR TRACK TROPICAL CYCLONES HAITANG (0505) AND BILLS (0604)

It is generally thought that the influence of comparable track typhoons is approximately similar,but in fact their wind and especially their rainstorm distribution are often very different. Therefore,a contrastive analysis of rainstorms by tropical cyclones (TCs) Haitang (0505) and Bilis (0604),which are of a similar track,is designed to help understand the mechanism of the TC rainstorm and to improve forecasting skills. The daily rainthll of TC itaitang (0505) and Bills (0604) is diagnosed and compared.The result indicates that these two TCs have similar precipitation distribution before landtall but different precipitation characteristics after landthll. Using NCEP/GFS analysis data,the synoptic situation is analyzed; water vapor transportation is discussed regarding the calculated water vapor flux and divergence.The results show that the heavy rainfall in the Zhejiang and Fujian Provinces associated with Haitang (0505) and Bitis (0604) before landlhll results ti'om a peripheral easterly wind,a combination of the tropical cyclone and the terrain. After landthll and moving far inland of the storm,the precipitation of Haitang is caused by water vapor convergence carried by its own circulation; it is much weaker than that in the coastal area. One of the important contributing thctors to heavy rainstorms in southeast Zhejiang is a southeast jet stream,which is maintained over the southeast coast. In contrast,the South China Sea monsoon circulation transports large amounts of water vapor into Bills - when a water-vapor transport belt south of the tropical cyclone significantly strengthens - which strengthens the transport. Then,it causes water vapor flux to converge on the south side of Bilis and diverge on the north side. Precipitation is much stronger on the south side than that on the north side. After Bilis travels far inland,the cold air guided by a north trough travels into the TC and remarkably enhances precipitation. In summary,combining vertical wind shear with water vapor transportation is a good way to predict rainstorms associated with landing tropical cyclones.     

A CLIMATOLOGY OF EXTRATROPICAL TRANSITION OF TROPICAL CYCLONES IN THE WESTERN NORTH PACIFIC

Based on best-track data and JRA-25 reanalysis, a climatology of western North Pacific extratropical transition (ET) of tropical cyclone (TC) is presented in this paper. It was found that 35% (318 out of 912) of all TCs underwent ET during 1979–2008. The warm-season (June through September) ETs account for 64% of all ET events with the most occurrence in September. The area 120°E–150°E and 20°N–40°N is the most favorable region for ET onsets in western North Pacific. The TCs experience ET at latitudes 30°N–40°N and have the greatest intensity in contrast to other latitude bands. The distribution of ET onset locations shows obviously meridional migration in different seasons. A cyclone phase space (CPS) method was used to analyze the TC evolution during ET. Except for some cases of abnormal ET at relatively high latitudes, typical phase evolution paths—along which TC firstly showed thermal asymmetry and an upper-level cold core and then lost its low-level warm core—can be used to describe the main features of ET processes in western North Pacific. Some seasonal variations of ET evolution paths in CPS were also found at low latitudes south of 15°N, which suggests different ET onset mechanisms there. Further composite analysis concluded that warm-season ETs have generally two types of evolutions, but only one type in cold season (October through next May). The first type of warm-season ETs has less baroclinicity due to long distance between the TC and upper-level mid-latitude system. However, significant interactions between a mid-latitude upper -level trough and TC, of either approaching or being absorbed into the trough, and TC’s relations with downstream and upstream upper-level jets, are the fingerprints for both a second type of warm-season ETs and almost all the cold-season ETs. For each type of ETs, detailed structural characteristics as well as precipitation distribution are illustrated by latitude.     

基于人工神经网络的热带气旋路径预报试验

利用气候持续性因子,分别采用神经网络法及最小二乘回归法,建立西北太平洋地区12、24、36和48h热带气旋路径预报模型。通过对1992～2002年资料的试报,人工神经网络方法优于回归方法,且这种优势在预报时效较长时更明显。人工神经网络法48h的预报平均绝对误差比回归法减小27．56km,预报水平提高7％。     

2010年8月大气环流和天气分析

2010年8月大气环流主要特征如下:极涡呈单极型,中心略偏于东半球。北半球高纬度环流呈4波型分布,欧洲西北部、新地岛附近为低于-40 gpm负距平所控制。西北太平洋副热带高压面积较常年同期偏大,强度显著偏强,位置异常偏西。2010年8月,全国平均气温为21.4℃,比常年同期(20.3℃)偏高1.1℃。全国平均降水量为108.1 mm,比常年同期(102.6 mm)偏多5.5 mm。月内出现了14次降水过程,共有5个热带风暴生成,其中1008号热带风暴南川在福建登陆。江淮、江南、华南北部、重庆、四川东部及新疆南部等地高温日数异常偏多。     

FY2卫星云图分析系统在热带气旋北冕过程中的应用

利用FY2卫星云图分析系统的各项分析功能,结合常规观测资料对热带气旋北冕的天气形势、云图演变以及路径变化、风雨情况等进行了分析。结果表明,副热带高压摆动,中纬度西风小槽东移,低层西南气流加强和减弱是"北冕"出现两次北抬和一次南折的直接原因;对流云团的发展和减弱对应低层西南气流的加强和减弱;强降水与云顶亮温的最低值中心及强度密切相关,而云团面积和云顶亮温与强降水也有很好的对应关系,其中发展的云系是强降水发生的信号。FY2卫星云图分析系统较好地分析了"北冕"整个过程的变化特点,对于短时临近预报有较好的辅助作用。     

西北太平洋热带气旋频数和强度变化趋势初探

利用1951—2006年西北太平洋 (含南海) 热带气旋资料, 研究了不同强度热带气旋的气候变化特征。结果表明:超强台风 (近中心最大风速≥58m/s, 简称超强台风Ⅱ) 频数、强度和初、终旋日期的变化特征都不同于其他级别热带气旋; 西北太平洋热带气旋的总频数有长期减少趋势, 主要由热带低压和超强台风Ⅱ的长期减少趋势引起; 随着热带气旋强度增强, 出现月最大频数的月份逐渐推迟; 超强台风月频数最大值发生在秋季; 超强台风Ⅱ频数的年变化与除了超强台风Ⅰ(近中心最大风速为51~58m/s) 外的其他级别热带气旋反相关; 受超强台风Ⅱ减少影响, 热带气旋年平均最大风速有减小的长期趋势; 热带气旋的初、终旋日期没有显著的长期变化趋势, 但超强台风Ⅱ的初旋日期有推迟趋势, 终旋日期有提前趋势, 发生时间缩短。