A Preliminary Analysis of Disturbance Tracksover the Mediterranean Basin

Summary  The Mediterranean basin experiences considerable cyclone activity mostly during fall, winter and spring and diminished activity during summer. In this study we present results of synoptic disturbance track analysis for two contrasting winter months and two, near average, summer months over the eastern Mediterranean. The surface and 500 hPa disturbance tracks were subjectively analyzed from two points of view. First, looking at tracks of conventionally defined cyclone centers (eddies) based on actual pressure and height distribution and second, looking at tracks of transient cyclonic disturbances (TRADs), defined as centers of negative deviations from the time mean. The second type of analysis demonstrated a considerable increase in the number of detectable tracks. Over the Mediterranean and vicinity the ratio between the number of surface TRAD tracks to cyclone tracks is, about 2, whereas at 500 hPa the ratio is much higher, about 5. However, the average life span of transient disturbances was only slightly longer than that of conventional cyclones (mainly at 500 hPa). At the surface and at 500 hPa about 50% of the cyclone tracks coincided to a certain extent with TRAD tracks. In summer, when conventional analysis over the eastern Mediterranean yields mostly quasi‐stationary low pressure centers associated with the Persian Gulf Trough, we detected clear signs of transient disturbances. Some interpretations of the differences between cyclones and TRADs in terms of weather in the eastern Mediterranean are also made. Received January 19, 1999Revised June 23, 1999     

Analyzing the genesis, intensity, and tracks of western Mediterranean cyclones

Midlatitude cyclones are analyzed on a selected region covering most of southern Europe and the western part of the Mediterranean Sea (35–50°N, 10°W–25°E). On the basis of mean sea level pressure fields of the ECMWF (European Centre for Medium-range Weather Forecast) Reanalysis Dataset (ERA-40), detailed evaluation of Mediterranean cyclones is accomplished for the period between 1957 and 2002 on a 1° horizontal resolution grid. Cyclone centers are identified and their paths are tracked with a 6-h time step (using 00 UTC, 06 UTC, 12 UTC, and 18 UTC). Decadal, annual, and seasonal statistical analysis of cyclone tracks includes the study of the genesis, frequency, and activity of the Mediterranean cyclones as well as the variability of cyclone tracks. The results suggest that the cyclone frequency in the western Mediterranean region increased in summer and autumn, and decreased in winter and spring. A special belt-shaped area is identified, which plays a special role in cyclogenesis, and also, the cyclone tracks often remain within this belt. An overall decreasing trend is detected in winter and spring in the entire Mediterranean belt, while cyclone frequency increased in autumn. The largest positive and negative trend coefficients are identified in summer.     

Three-Dimensional Structure and Low-Level Features of the North Pacific Storm Track from 1999 to 2005

A storm track is a region in which synoptic eddy activities are statistically most prevalent and intense. At daily weather charts, it roughly corresponds to the mean trajectories of cyclones and anticyclones. In this paper, the recent QuikSCAT （Quick Scatterometer） satellite sea winds data with a 0.5°×0.5° horizontal resolution, and the NCEP （National Centers for Environmental Prediction） 10-m height Gaussian grid wind data and pressure-level reanalysis data, are employed to document the spatial structure of the North Pacific storm track in winter （January） and summer （July） from 1999 to 2005. The results show that in winter the North Pacific storm track is stronger, and is located in lower latitudes with a distinct zonal distribution. In summer, it is weaker, and is located in higher latitudes. Based on the horizontal distributions of geopotential height variance at various levels, three-dimensional schematic diagrams of the North Pacific storm track in winter and summer are extracted and presented. Analyses of the QuikSCAT wind data indicate that this dataset can depict the low-level storm track features in detail. The double storm tracks over the Southern Oceans found by Nakamura and Shimpo are confirmed. More significantly, two new pairs of low-level storm tracks over the North Pacific and the North Atlantic are identified by using this high-resolution dataset. The pair over the North Pacific is focused in this paper, and is named as the ＂subtropical storm track＂ and the ＂subpolar storm track＂, respectively. Moreover, statistical analyses of cyclone and anticyclone trajectories in the winters of 1999 to 2005 reveal as well the existence of the low-level double storm tracks over the North Pacific.     

Climatology of extratropical cyclones over the South American–southern oceans sector

A climatology of extratropical cyclones is presented. Extratropical cyclones, their main characteristics and their predominant tracks, as well as their interannual variability, affect weather in South America. For that purpose, a storm track database has been compiled by applying a cyclone tracking scheme to six-hourly sea level pressure fields, available from the National Center for Environmental Prediction–National Center for Atmospheric Research reanalyses II for the 1979–2003 period. The spatial distribution of the cyclogenesis frequency shows two main centers: one around Northern Argentina, Uruguay, and Southern Brazil in all seasons and the other near to the North Antarctic Peninsula. The lifetime of extratropical cyclones in the South American sector exhibits small seasonality, being typically of the order of 3.0 days during most of the year and slightly higher (3.5 days) in austral summer. The distance travelled by the cyclones formed in the South American sector tends to be smaller than the total paths found in other areas of the Southern Hemisphere. A k-mean clustering technique is used to summarize the analysis of the 25-year climatology of cyclone tracks. Three clusters were found: one storm-track cluster in Northeast Argentina; a second one west of the Andes Cordillera; and a third cluster located to the north of the Antarctic Peninsula (around the Weddell Sea). The influence of the Antarctic Oscillation (AAO) in the variability of extratropical cyclones is explored, and some signals of the impacts of the variability of the AAO can be observed in the position of the extratropical cyclones around 40°S, while the impacts on the intensity is detected around 55°S.     

A cyclone statistics for the Arctic based on European Centrere-analysis data

Summary  Based on the six-hourly re-analysis sea-level pressure data of the European Centre for Medium-Range Weather Forecast (ECMWF) a cyclone statistics for the Arctic region north of 60° is elaborated for the period 1 November 1986 to 31 October 1991. For each low pressure center on a weather map its location, central pressure and horizontal pressure gradients in E, W, N, and S direction are determined. Furthermore, cyclone centers are followed with time to calculate trajectories, pressure tendencies, and lifetimes. A horizontal grid of 300 km × 300 km is used as unit area for the statistical computations. A unit area experiences about 20 cyclone passages per year (range 5–40). On the average, six cyclones occur simultaneously in the Arctic region. Lifetimes vary from 6 h to 15 days. The annual cyclone activity over the 5-year period is nearly the same. Cyclones are more frequent in summer (about 94 per month) than in winter (77 per month). In general summer cyclones are weaker than winter cyclones. On the average, the minimum central pressure during the lifetime of a cyclone is about 1000 hPa (typical range 980–1020) in summer and about 988 hPa (typical range 940–1030) in winter. In winter, a zone of high cyclone frequency extends from the region near Iceland over the Greenland Sea, Barents Sea, and Kara Sea to the Laptev Sea while the interior of the Arctic shows little cyclone frequency. In summer, the region near Iceland and the interior of the Arctic are separate centers of high cyclone frequency. Both in winter and summer very high cyclone frequencies are observed over the northern Baffin Bay. The regional distribution of mean central pressures and maximum pressure gradients roughly follows the distribution of cyclone frequencies except for the Baffin Bay cyclones which are generally weak. Cyclolysis dominates cyclogenesis over largest parts of the Arctic. Regions of high cyclone frequency are also regions of frequent cyclogenesis and frequent cyclolysis. One third of all cyclones is generated in a region with an already existing cyclonic circulation. Cyclones in the Fram Strait are studied in more detail because of their special impact on the ice export from the Arctic Ocean to the Atlantic Ocean. On the average, there are 5 cyclones per month. the cyclone frequency in the Fram Strait is higher during the winter period than during the summer period. This is in contrast to the overall Arctic frequency which is higher in summer than in winter. Cyclogenesis predominates in winter and cyclolysis in summer in the Fram Strait. The most frequent direction of motion is from SW to NE. Received November, 1999 Revised June 22, 2000     

北太平洋风暴轴的三维空间结构

文中利用最新的0.5°×0.5°分辨率QuikSCAT(QuikBird Satellite Microwave Scatterometer Sea Winds Data)海面风场资料、NCEP(National Center for Environmental Prediction)的10 m高度风场资料和全球客观再分析资料,对1999-2005年冬季(1月)和夏季(7月)北太平洋风暴轴的三维空间结构进行了分析,发现冬季北太平洋风暴轴的强度较强,呈明显的纬向拉伸带状分布特征,位置偏南.夏季北太平洋风暴轴的强度较弱,位置偏北.根据不同高度上位势高度方差的水平分布特征,绘制了北太平洋风暴轴的三维结构示意图.利用高分辨率QuikSCAT资料对风暴轴特征的刻画更为细致,不但验证了Nakamu-ra在南大洋发现的双风暴轴现象,而且还发现在北太平洋和北大西洋下层分别存在"副热带风暴轴"和"副极地风暴轴"两个风暴轴.对1999-2005年冬季北太平洋气旋和反气旋的移动路径进行的统计分析,为北太平洋"双风暴轴"的存在提供了强有力的证据.     

北半球温带气旋活动和风暴路径的年代际变化

基于欧洲中心再分析数据ERA40的海平面气压场和高度场,本文分别采用拉格朗日和欧拉方法研究分析了1958～2001年北半球的不同季节温带气旋活动和风暴路径的年代际变化,以及可能的原因.以客观判定和追踪温带气旋为基础的拉格朗日方法得到了北半球的两个温带气旋主要活动中心,即北太平洋地区和北大西洋/北美地区,同时以500 hPa位势高度天气尺度滤波方差为基础的欧拉方法得到了同主要气旋活动中心相吻合的两条风暴轴.研究表明,44年中北大西洋/北美地区温带气旋活动北移加强,以春季最为显著.风暴轴也同样存在着向极移动并加强的特征,并且温带气旋和风暴路径两者移动趋势的相关性很高.作为一个典型地区,北大西洋/北美地区的气旋活动体现了风暴路径的北移,以及温带地区向极地的扩展.但有意思的是北太平洋的情况完全不同,即北太平洋地区的温带气旋活动和风暴轴向低纬度偏移并加强,以春季的南移趋势最为显著.对于此结论,两种方法也有很高的统计相关性.虽然大量研究表明北半球整体上呈现出风暴路径北移的变化特征,但对于具体地区情况有明显差异.另外,400 hPa最大Eady增长率和气旋活动频率的经验正交展开函数 (EOF) 第一模态的空间分布和时间序列非常相似,北太平洋地区和北大西洋地区风暴路径相反的变化趋势很可能同其大气斜压性的同位相的变化有着密切的关系.这也从另一个方面支持了本文对温带气旋和风暴路径年代际变化的分析.     

North Atlantic cyclones in CO2-induced warm climate simulations: frequency, intensity, and tracks

The effect of CO₂-induced climate change on the North Atlantic storm and cyclone tracks in winter is analysed using time slice experiments of the Hamburg atmospheric general circulation model (ECHAM3) with triangular truncation at wave number 42 (T42) and 19 levels. The sea surface temperature (SST) and sea ice boundary conditions for these experiments are taken from a transient Intergovernmental Panel on Climate Change (IPCC) scenario A run of ECHAM1/LSG at the times where the 1×CO₂ (control run), the 2×CO₂ and the 3×CO₂ concentrations are reached. Using a cyclone identification and tracking scheme, we detect the low pressure systems as relative minima in the 1000 hPa geopotential height field and connect them to cyclone tracks. The results of the Eulerian analysis of the storm track using filtered variances and the Lagrangian analysis of the cyclone trajectories from the three climate runs are discussed and compared with each other. In the 2×CO₂ experiment, the storm track shifts eastward, whereas the cyclone density shifts northeastward. In the 3×CO₂ experiment the storm track shows a southeastward shift, whereas the cyclone density shifts northward. The variability of the cyclone tracks is determined by a cluster analysis of their relative trajectories considering the first three days of the cyclones. The relative cyclone tracks are grouped into stationary, zonal and northeastward travelling cyclones. This analysis provides a method to assess the model quality and to detect changes of the cyclone trajectories in different climates. In the 2×CO₂ (but not in the 3×CO₂) run the occupation number of northeastward cyclones increases. Received: 27 January 1998 / Accepted: 19 May 1998     

地形和边界层摩擦对登陆热带气旋路径和强度影响的研究

采用准地转的正压模式, 研究了无非绝热加热时地形和边界层摩擦对登陆热带气旋路径和强度的影响.结果表明: 地形作用对登陆热带气旋西北移动路径的影响比较明显, 而对登陆热带气旋强度的影响不明显; 边界层摩擦可以通过改变热带气旋X方向上和Y方向上的移动速度以及改变热带气旋水平环流结构对登陆热带气旋西北移动路径产生一定的影响, 边界层摩擦对登陆热带气旋强度的影响非常明显, 其中摩擦是造成登陆热带气旋强度迅速减弱的一个重要因素.     

Characteristics of Arctic synoptic activity, 1952–1989

Summary Synoptic activity for the Arctic is examined for the period 1952–1989 using the National Meteorological Center sea level pressure data set. Winter cyclone activity is most common near Iceland, between Svalbard and Scandinavia, the Norwegian and Kara seas, Baffin Bay and the eastern Canadian Arctic Archipelago; the strongest systems are found in the Iceland and Norwegian seas. Mean cyclone tracks, prepared for 1975–1989, confirm that winter cyclones most frequently enter the Arctic from the Norwegian and Barents seas. Winter anticyclones are most frequent and strongest over Siberia and Alaska/Yukon, with additional frequency maxima of weaker systems found over the central Arctic Ocean and Greenland.During summer, cyclonic activity remains common in the same regions as observed for winter, but increases over Siberia, the Canadian Arctic Archipelago and the Central Aretic, related to cyclogenesis over northern parts of Eurasia and North America. Eurasian cyclones tend to enter the Aretic Ocean from the Laptev Sea eastward to the Chukchi Sea, augmenting the influx of systems from the Norwegian and Barents seas. The Siberian and Alaska/Yukon anticyclone centers disappear, with anticyclone maxima forming over the Kara, Laptev, East Siberian and Beaufort seas, and southeastward across Canada. Summer cyclones and anticyclones exhibit little regional variability in mean central pressure, and are typically 5–10 mb weaker than their winter counterparts.North of 65°N, cyclone and anticyclone activity peaks curing summer, and is at a minimum during winter. Trends in cyclone and anticyclone activity north of 65°N are examined through least squares regression. Since 1952, significant positive trends are found for cyclone numbers during winter, spring and summer, and for anticyclone numbers during spring, summer and autumn.With 11 Figures     

Characteristics of the internal and external sources of the Mediterranean synoptic cyclones for the period 1956–2013

This paper investigates the main sources and features of the Mediterranean synoptic cyclones affecting the basin, using the cyclone tracks. The cyclones’ tracks are identified using sea level pressure (SLP) from the NCEP/NCAR reanalysis data for the period 1956–2013. The identified cyclones are classified into two categories: basin affected and basin non-affected. Most of the basin-affected (non-affected) cyclones are internal (external), i.e., generated inside (outside) the Mediterranean basin. This study reveals four (five) main sources of internal (external) cyclones. These four (five) main sources generated about 63.76% (57.25%) of the internal (external) cyclones. Seasonal analysis shows that most of the basin-affected internal (external) cyclones were generated in the winter (spring) season. The lowest number of cyclones were found in the summer. Moreover, the synoptic study of the atmospheric systems accompanied the highest- and lowest-generated years demonstrates that the deepening of the north Europe cyclones and the relative positions of Azores- and Siberian-high systems represent the important factors that influence the number of internal cyclones. Essential factors influencing the external cyclones are the strength of the maximum upper wind, Azores high, Siberian high, and orientations of their ridges.     

A Modified Algorithm for Identifying and Tracking Extratropical Cyclones

In this study, a modified identification and tracking algorithm for extratropical cyclones is developed. This identification scheme is based on triangular-mesh contouring techniques combined with a connected-component labeling method in order to detect the outer boundaries and spatial domain characteristics of individual cyclones. A new tracking method allowing for the identification of cyclone merging and splitting events, as well as short-lived windstorms, is developed to reduce the uncertainty in the tracking of extratropical cyclones. I also show that this method excludes the tracks of open systems that would have been unnecessarily detected using conventional NCP methods. The climatological features of the distribution of cyclone frequencies are substantially larger over the traditional storm track regions compared to those seen in previous studies. Interestingly, a significant increase in the cyclone density in the Arctic occurs during all four seasons(up to 19%in summer) compared to that seen with a latitude–longitude gridded mesh analysis. I develop two new regional intensity indices(depth and vorticity) based on the cyclonic domain to better quantify the cyclonic activity in the Arctic region, and find that the interannual variabilities in these two indices are highly consistent. The results of this analysis may shed light on high-latitude cyclonic behavior studies via the newly detected 2D cyclone atlas derived from this cyclonic-domain-based algorithm.     

夏季进入北极温带气旋的活动特征及其影响因素北大核心

进入北极地区的气旋在移动过程中往往伴有大风、强降水等特征,对北极气候变化有深刻的影响。基于NCEP2再分析资料,识别并跟踪了北半球夏季(6—8月)从中纬度进入北极的温带气旋,考察了其年际变化特征和影响因素。结果表明:1979—2019年夏季进入北极的温带气旋共867个,其中消失在北极边缘区域和中心区域的数量分别为688个和161个,且后者平均强度更大、平均持续时间更长。分区域研究发现,夏季从陆地进入极区的气旋个数较多,而从海洋进入极区的气旋强度更大,活动更为剧烈。对进入北极的气旋年际时间序列进行分析发现,夏季进入北极的气旋个数和强度均存在年际变率,其中气旋个数的年际变率尤为显著。气旋个数年际变率主要周期为5 a,强度的主要周期约为2.7 a。进一步分析发现,引导气流是影响气旋向北移动的重要因素。此外,夏季北大西洋气旋强度与同期北大西洋涛动(North Atlantic Oscillation,NAO)指数存在较好相关。研究还表明,进入极区气旋活动的年际变化受大气斜压不稳定性的影响,在北太平洋地区区域平均的Eady增长率与气旋个数和强度的相关性均最强,相关系数分别为0.4和0.5。     

Statistical and Comparative Analysis of Tropical Cyclone Activity over the Arabian Sea and Bay of Bengal (1977–2018)

A statistical comparative analysis of tropical cyclone activity over the Arabian Sea and Bay of Bengal (BoB) has been conducted using best-track data and wind radii information from 1977 to 2018 issued by the Joint Typhoon Warning Center. Results have shown that the annual variation in the frequency and duration of tropical cyclones has a significant increasing trend over the Arabian Sea and an insignificant decreasing trend over the BoB. The monthly frequency of tropical cyclones in both the Arabian Sea and the BoB shows a notable bimodal character, with peaks occurring in May and October–November, respectively. The maximum frequency of tropical cyclones occurs in the second peak as a result of the higher moisture content at mid-levels in the autumn. However, the largest proportion of strong cyclones (H1–H5 grades) occurs in the first peak as a result of the higher sea surface temperatures in early summer. Tropical cyclones in the Arabian Sea break out later during the first peak and activity ends earlier during the second peak, in contrast with those in the over BoB. This is related to the onset and drawback times of the southwest monsoon in the two basins. Tropical cyclones in the Arabian Sea are mainly generated in the eastern basin, whereas in the BoB the genesis locations have a meridional (zonal) distribution in May–June (October–November) as a result of the seasonal movement of the low-level positive vorticity belt. The Arabian Sea is dominated by western and northwestern tropical cyclones by that track west and NW, accounting for about 74.6%, whereas the tropical cyclones with a NE track account for only 25.4%. The proportions of the three types of tracks are similar in the BoB, with each accounting for about 33% of the tropical cyclones. The mean intensity and size of tropical cyclones over the Arabian Sea are stronger and larger, respectively, than those over the BoB and the size of tropical cyclones over the North Indian Ocean in early summer is larger than that in autumn. The asymmetrical structure of tropical cyclones over North Indian Ocean is affected by the topography and the longest radius of the 34 kt surface wind often lies in the eastern quadrant of the tropical cyclone circulation in both sea areas. FAN Xiao-ting (樊晓婷), LI Ying (李 英), et al.     

SOME FEATURES OF SEVERE TROPICAL CYCLONE CHANGE IN THE COOL AND WANM SUMMER OVER EAST CHINA SEA

Based on the definition of cool summer and warm summer, statistical characteristics of severe tropical cyclones are studied over East China Sea using 1951-1994 typhoon and temperature data. It is foundthat the frequency, tracks, intensity and moving speed of the tropical cyclone between cold summer andwarm summer were much different. The backgtound features of the difference were also discussed incontrast.     

冬季北太平洋爆发性气旋的天气气候特征

本文利用1975—1984年12—2月北太平洋爆发性温带气旋资料以及500 hPa天气图，概括出气旋爆发式加强前12小时五种环流型，即北美高压型（NAH）、东太平洋高压型（EPH）、中太平洋高压型（MPH）、白令海阻高型（BBH）、太平洋低压型（PL）。以NAH型和EPH型出现的机会最多，稳定时间最长，并且在这两种天气形势下气旋最易发展。另外本文还分析了气旋爆发后的路径趋势，指出在形势稳定期，气旋爆发后的路径亦稳定；在形势调整期，气旋后期路径随太平洋长波系统的演变而变化。     

大地形上空偶极子东移对热带气旋路径的影响

With a quasi-geostrophic barotropic model on the β-plane with a topography term, 16 experiments were performed with an integration period of 6 days. The interaction between tropical cyclone tracks and 500 kin-scale vortices originating from the western part ora large-scale topography is investigated. It is suggested that this kind of interaction may have a significant impact on the moving speed and direction of tropical cyclones. Under certain conditions, this interaction may be a factor in causing an abnormal tropical cyclone track. Furthermore, the effect of large-scale topography plays an important role in the formation of unusual tropical cyclone tracks.     

热带气旋路径强度预报——SAPC法

SAPC——逐段相似气候持续性热带气旋路径、强度预报方法，既考虑了热带气旋的气候持续性规律，又考虑了与现时热带气旋状态特别相似的历史热带气旋路径、强度变化规律，用不同的相似条件和权重，把气候路径、相似路径和持续性路径作了有机的统一。该方法克服了预报路径的大曲率变化，考虑了随时间、地点变化的热带气旋的气候规律，根据热带气旋的现时状态，即可随时制作路径、强度预报。历史资料和实际应用表明该方法有实用价值。     

热带气旋路径趋势数值预报的试验研究

给出了一个嵌套于欧洲中期天气预报中心全球谱模式中的套网格模式，用于预报热带气旋路径趋势，提出了一种以欧洲中期天气预报中心提供的时距为24小时的预报场为基础的“接力预报”方法；设计了五种不同的“接力预报”方案；并对8507号和8509号两个热带气旋就各种不同预报方案进行了数值预报试验。结果表明，本模式对热带气旋路径趋势具有较强的预报能力，对疑难路径也具有一定的预报能力，为开展热带气旋的路径趋势预报提供了一个新的途径。文中还对影响热带气旋路径趋势的一些主要因子结合各预报结果进行了分析，为模式尽早投入业务提供了一定的依据。