A study of weather types at Athens and Thessaloniki and their relationship to circulation types for the cold-wet period, part I: two-step cluster analysis

This paper presents a method for grouping weather types that occur over an area, which combines meteorological parameters, reflecting air mass characteristics at the surface, with synoptic conditions prevailing over an area. Five quantitative meteorological parameters are used in the procedure: temperature, precipitation, relative humidity, wind velocity and sunshine duration. In addition, two qualitative variables related to the prevailing circulation type and whether it is cyclonic or anticyclonic are also included. The study period is 43 years (1958–2000) and is restricted to the cold and wet sub-period of the year, December–March. Weather types are defined using a relatively new method of cluster analysis, two-step cluster analysis, which allows the simultaneous use of both quantitative and qualitative variables. The aim of the present study is to distinguish primary weather patterns so that the investigation into the relationship between weather patterns and circulation types will be more effective. For Athens, six weather types are created, whereas for Thessaloniki five are produced. For both stations, only two weather types are related to anticyclonic situations. The majority of the identified weather types correspond to a distinctive and well-defined synoptic situation. Each weather type differs from the others, not only in terms of the circulation conditions referring to it, but also with reference to meteorological variables such as temperature and precipitation. The results of the evaluation of the aforementioned procedure are considered to be highly satisfactory.     

Anatomy of the synoptic conditions occurring over southern Greece during the second half of the 20th century. Part II. Autumn and spring

Summary ?A new procedure for grouping the circulation types occurring during autumn and spring over an area is introduced in this paper. The procedure combines surface air mass characteristics affecting the area with the prevailing synoptic conditions. Factor and Cluster Analysis are used to derive the circulation types, based on surface meteorological data, as well as on surface pressure grid data. The methods are applied to Athens, Greece, using data covering the period 1954–1999. Eight circulation types for the autumn (September, October, November) and eight for the spring (March, April, May) are derived. Clusters presenting low-pressure systems are found to be generally fast moving, while anticyclones generally remain over the area for more than two days. Finally an interesting seasonal distribution is found from some weather types. Received February 20, 2002; revised December 23, 2002; accepted January 9, 2003 Published online May 26, 2003     

北方麦收期间连阴雨天气环流特征

利用1980-2004年5月下旬至6月中旬北方麦收区30个代表站降水实况资料，连阴雨期间亚欧范围500hPa逐日形势图和500hPa高度平均图等，分析总结了近25年来北方麦收期间连阴雨的天气气候和环流形势特征；对北方麦收期间出现的连阴雨天气过程与环流形势和影响系统的关系进行初步探讨，确定连阴雨天气的概念模型。分析表明，阻塞高压形势且贝加尔湖附近伴有冷涡是造成连阴雨天气最主要的环流特征；在500hPa地转风υ场上，麦收区多处于南北风交界处；长连阴雨期间，850hPa东亚地区中低纬度盛行南风为主要特征。     

Synoptic and local scale atmospheric circulation associated with air pollution episodes in an urban Mediterranean area

Air pollution episodes in urban coastal areas follow certain pre-determined patterns, being associated with certain local meteorological conditions and emission of primary pollutants. In this study, the synoptic and local scale atmospheric circulation that prevails during air pollution episodes in a coastal major city in Greece, Thessaloniki, is examined for a period of 15 years (1989–2004). The study signifies the importance of studying air pollution meteorological patterns between coastal areas with different terrain characteristics. For Thessaloniki, it was found that the episodes occur mainly during the cold period of the year, while four types of synoptic scale circulation were recognized (I, II, III, IV) and five patterns of the local scale circulation (A1, A2, B1, B2 and B3). The highest percentage of episodes is associated with the presence of an anticyclone over the northern Greece (types I and IV), being characterized by weak or very weak surface pressure gradient intensity, according to the position and extension of the anticyclone. Moreover, a temperature increase of at least 1°C during the previous 3 days is required in the lower troposphere. Consistent with the synoptic conditions, the development of the sea breeze plays a crucial role in the occurrence of the episodes, even in the cold period of the year, when the sea breeze can still develop with smaller frequency and intensity. Finally, it was found that a small number of episodes is related with the advection of polluted air masses from the industrial area in the northwest of the city and from the Eordaia area in the west, which is the largest lignite producing area of Balkans.     

Anatomy of the synoptic conditions occurring over southern Greece during the second half of the 20th century. Part I. Winter and summer

Summary ?In this study a methodology for grouping seasonal circulation types occurring over an area is introduced. This procedure combines the surface air mass characteristics affecting the area with the synoptic conditions prevailing over it. Factor Analysis and Cluster Analysis are used to derive the circulation types, based on surface meteorological data and surface pressure grid data. The methods are applied to Athens, Greece, using data over the period 1954–1999 for winter (December, January, February) and summer (June, July, August) seasons. The daily circulation types are analyzed at surface level and their temporal evolution is examined via transition matrices. 315 grid points are used covering the area between 25° N to 60° N and 10° W to 40° E. This analysis derives 8 circulation types for the winter and 4 for the summer. A reduction in cyclonic activity and an increase in anticyclonic activity in the Central Mediterranean are detected in the late 1980s and early 1990s during the winter period. During summer the etesian winds and the local flows are dominant over Athens. Received February 20, 2002; accepted January 9, 2003 Published online May 26, 2003     

Future heat vulnerability in California, Part I: projecting future weather types and heat events

Excessive heat significantly impacts the health of Californians during irregular but intense heat events. Through the 21st century, a significant increase in impact is likely, as the state experiences a changing climate as well as an aging population. To assess this impact, future heat-related mortality estimates were derived for nine metropolitan areas in the state for the remainder of the century. Here in Part I, changes in oppressive weather days and consecutive-day events are projected for future years by a synoptic climatological method. First, historical surface weather types are related to circulation patterns at 500mb and 700mb, and temperature patterns at 850mb. GCM output is then utilized to classify future circulation patterns via discriminant function analysis, and multinomial logistic regression is used to derive future surface weather type at each of six stations in California. Five different climate model-scenarios are examined. Results show a significant increase in heat events over the 21st century, with oppressive weather types potentially more than doubling in frequency, and with heat events of 2?weeks or longer becoming up to ten times more common at coastal locations.     

Interannual horizontal heat advection in the surface mixed layer over the equatorial Pacific Ocean: assimilation versus TAO analysis

This study is motivated by an interest in obtaining a new automated classification scheme of daily circulation types suitable for use throughout Europe. The classification scheme is performed on 500 hPa geopotential height anomalies (NCEP Reanalysis data, 2.5°×2.5°). Nine grid points represent the study area. Five anticyclonic types (Anw, Ane, A, Asw and Ase) and seven cyclonic types (C, Cnnw, Cwnw, Cwsw, Cssw, Cse, Cne) are defined. Each of the circulation types has a distinct underlying synoptic pattern that produces the expected type and direction of flow over the study area. The classification scheme is applied to three different case studies in the Mediterranean Basin: Greece, Cyprus and central Italy. The precipitation percentage of the cyclonic type and the mean seasonal correlation coefficients for all circulation types are the two criteria used to evaluate the performance of the classification scheme. The ability of the HadAM3P general circulation model to reproduce the mean pattern and frequency of circulation types at the 500 hPa level in comparison to the NCEP dataset for the period 1960–1990 is also evaluated. The percentage of rainfall that corresponds to the cyclonic circulation types is greater than 85% for the three study regions. Furthermore, the correlation coefficients for the three classifications are very encouraging, for nearly all days of the study period. Compared to observations, the GCM is able to capture the mean patterns but not able to replicate exactly the observed variability of the circulation types over the three study regions.     

An assessment of the Jenkinson and Collison synoptic classification to a continental mid-latitude location

A weather-type catalogue based on the Jenkinson and Collison method was developed for an area in south-west Russia for the period 1961–2010. Gridded sea level pressure data was obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. The resulting catalogue was analysed for frequency of individual types and groups of weather types to characterise long-term atmospheric circulation in this region. Overall, the most frequent type is anticyclonic (A) (23.3 %) followed by cyclonic (C) (11.9 %); however, there are some key seasonal patterns with westerly circulation being significantly more common in winter than summer. The utility of this synoptic classification is evaluated by modelling daily rainfall amounts. A low level of error is found using a simple model based on the prevailing weather type. Finally, characteristics of the circulation classification are compared to those for the original JC British Isles catalogue and a much more equal distribution of flow types is seen in the former classification.     

一种客观的天气气候分类方法

本文将广州逐日4次观测多要素值作主因子分析,对前几个主因子得分进行聚类,划分出冬季有5种天气型,夏季有6种。各型有鲜明的天气特征及一定的环流背景。天气型的出现频率和转换频率刻划出气候的结构和动态。此法有综合性、客观性和优化的特点,纵然分型采用的是常规变量,但它与生物、环境现象有较好的对应关系,显示出这种方法在天气气候分析和应用气象上的潜力。      

低温雨雪冰冻灾害天气与大范围霜冻天气对比分析

利用ECMW F客观分析格点资料对2008年1~2月历史罕见的低温雨雪冰冻天气过程与183例广西大范围霜冻天气过程前一天20时的环流、气象要素作了合成后的对比、统计分析,结果表明:造成两种低温寒害过程的南下冷空气都在相同强度以上,都有一定程度的东亚槽引导。但由于形成条件不同,两种低温寒害过程在500hPa高度场、地面气压场、低层850hPa风场、温度场、湿度场存在较大差异。     

2006年4月11-12日寒潮天气成因

利用天气图和数值预报产品,对2006年4月11-12日河南省寒潮天气过程的环流形势、单站气象要素变化及成因等进行了分析,结果表明:500 hPa横槽转竖是寒潮天气产生的重要影响系统;850 hPa冷平流是强降温的主要原因。     

2006年4月11—12日寒潮天气成因

利用天气图和数值预报产品，对2006年4月11—12日河南省寒潮天气过程的环流形势、单站气象要素变化及成因等进行了分析，结果表明：500hPa横槽转竖是寒潮天气产生的重要影响系统；850hPa冷平流是强降温的主要原因。     

西江流域面雨量与区域大气环流型关系

利用Lamb-Jenkinson大气环流分型方法,对西江流域1971—2015年逐日平均850 hPa和500 hPa高度场进行环流客观分型,分析流域降水天气环流型出现概率及主导环流型变化特征,探讨主导环流型对西江流域总面雨量和子流域面雨量的贡献率及环流型配置与降水的关系。结果表明:当850 hPa为西南风型、500 hPa为西风型时,流域出现降水天气的概率最大;850 hPa气旋型和500 hPa西风型对年总面雨量和各子流域面雨量的贡献率均为最大,且对东部子流域面雨量的贡献率大于西部子流域,850 hPa南风型与500 hPa反气旋型的环流配置是西部子流域秋季降水偏多的主导环流型配置;春季850 hPa气旋型与500 hPa西风型、夏季850 hPa气旋型与500 hPa西风型、秋季850 hPa南风型与500 hPa反气旋型、冬季850 hPa西南风型与500 hPa西风型的环流配置时,出现强降水天气的概率分别为18.7%,21.1%,4.0%和2.0%,即夏季最大,其次为春季,冬季最小。近45年,850 hPa气旋型、500 hPa西风型对流域年总面雨量的贡献率呈增加趋势,是西江流域面雨量呈偏多趋势的主导环流型。     

台风“摩羯”（1814）的路径特点与预报分析

2018年8月13—16日台风“摩羯”（1814）及其残余低压环流影响山东,造成区域性暴雨或大暴雨。利用常规气象观测资料和区域自动气象站、天气雷达、气象卫星等资料及数值天气预报产品对此过程进行分析,总结了“摩羯”的路径特点。此次台风“摩羯”路径特殊,并出现突变。通过对“摩羯”的路径预报仔细分析,得出台风路径预报着眼点和部分有指示意义的预报指标。环流形势分析是预报台风路径的一个重要前提,大陆高压显著增强是环流形势调整的一个信号,是预报“摩羯”路径的一个关键因素;台风前进方向的对流层温度脊线和500 hPa正涡度轴线对台风未来路径有良好的指示作用。分析结果能够为提高山东台风路径预报的准确率提供参考。     

The exceptionally wet year of 2014 over Greece: a statistical and synoptical-atmospheric analysis over the region of Thessaloniki

The highest rainfall totals (912.2 mm) and the largest number of raindays (133 days), since 1958, were recorded in Thessaloniki during the year of 2014. Extreme precipitation heights were also observed on a seasonal, monthly and daily basis. The examined year presented the highest daily rainfall intensity, the maximum daily precipitation and the largest number of heavy precipitation days (greater than 10 mm), and it also exceeded the previous amounts of precipitation of very wet (95th percentile) and extremely wet (99th percentile) days. According to the automatic circulation type classification scheme that was used, it was found that during this exceptionally wet year, the frequency of occurrence of cyclonic types at the near surface geopotential level increases, while the same types decreased at a higher atmospheric level (500 hPa). The prevailing type was type C which is located at the centre of the study area (Greece), but several other cyclonic types changed during this year not only their frequency but also their percentage of rainfall as well as their daily precipitation intensity. It should be highlighted that these findings differentiated on the seasonal-scale analysis. Moreover, out of the three teleconnection patterns that were examined (Scandinavian Pattern, Eastern Mediterranean Teleconnection Pattern and North Sea-Caspian Pattern), the Scandinavian one (SCAND) was detected during the most of the months of 2014 meaning that it was highly associated with intense precipitation over Greece.     

2009年大连春季强对流雨雪天气过程分析

2009年4月15日大连出现了春季最晚的降雪天气。利用常规资料、自动气象站和雷达等多种资料及中尺度模式MM5对这次强对流雨雪天气过程进行了分析和模拟,结果表明,200hPa急流、500hPa贝加尔湖冷槽南压形成的冷涡、中低层南支槽前水汽输送以及地面冷锋是产生大连春季强对流雨雪天气的环流背景;中层干冷空气叠加在低层暖湿层...     

A novel scheme to derive optimized circulation pattern classifications for downscaling and forecast purposes

Summary Many synoptic classifications that have been introduced share the problem of a large within-type variability. Therefore, there is still the need for refinement, especially when aiming to downscale climate model outputs. The objective scheme proposed here has been developed to derive circulation pattern classifications that optimally distinguish between different values of regional weather elements. In comparison with methods that similarly take local and large-scale information into consideration (such as classification and regression trees [CART]), computational demand is relatively low and human input is practically unwarranted. Recognition of individual cases is based upon composites of the large-scale conditions of several different meteorological parameters such as geopotential heights, temperatures and relative humidities. For each local weather element a so-called screening discriminant analysis is conducted selecting those large-scale synoptic variables yielding optimal selectivity. The procedure is basically analogous to a stepwise screening regression. At each stage the predictor field minimizing the RMSE between forecasts and observations together with the previously chosen predictor fields is selected. The procedure is terminated either when a maximal number of predictor fields is reached or when the inclusion of another field does not result in a further decrease of the RMSE. Similarity between mean large-scale conditions and the individual cases that are to be assigned is determined by means of a slightly modified version of the widespread Euclidean distance.Using daily data from 51 climate stations located in the Elbe river catchment in the northeastern part of Germany and the western part of the Czech Republic, the attainable reduction of climatological variances was calculated for a variety of weather elements. Results prove the ability of the scheme to effectively discriminate large-scale circulation patterns with respect to local weather parameters, especially temperatures (skill scores of approx. 80%).When solely accurate discrimination is of interest, a fuzzy variant can be utilized producing even better results. It yields the percentage of each circulation type for a specific case, i.e. day. However, in this case physical insights can not be obtained.     

Synoptic Climatology of the Southern Beaufort Sea Troposphere with Comparisons to Surface Winds

Synoptic-scale atmospheric circulation patterns drive wind forcing of dynamic and thermodynamic processes in Arctic sea ice. Synoptic typing and compositing are common techniques used to identify a limited number of prevailing weather classifications that govern a region's climate. This work investigates atmospheric circulation patterns (surface to 250?hPa) for the southern Beaufort Sea and corresponding surface wind regimes within each synoptic type. Significant changes (p?<?0.05) in relative frequencies of a number of synoptic types were attributed to declining summer sea ice. Corresponding upper-level circulation anomalies show increasingly meridional atmospheric circulation. Synoptic Types 9 and 11 were identified as key October-November-December circulation features that represent deepening of the Aleutian low with concomitant strengthening of pressure gradients over the southern Beaufort Sea. Classification of coastal-based wind observations shows a shift towards increased easterly wind forcing. A case study of surface wind data from the CCGS Amundsen (2009–2011) provided a direct example of the surface wind regime within the marginal ice zone within each synoptic type during a period of reduced Arctic sea-ice cover.     

盐城市发生大暴雨的天气类型分析

按500 hPa形势,盐城市大暴雨的天气类型可划分为台风型和副高边缘型两大类。发生大暴雨时一般副高较强,高空有低槽或切变线,有明显的冷暖气流辐合区。系统深厚,高、中、低层配合较好。地面天气系统主要有冷锋、静止锋、气旋、低压倒槽。     

新疆强冰雹天气的分析

根据1961～2003年新疆90个气象观测站的地面观测资料计算得到新疆强冰雹天气81次，其持续时间在1～2d，出现在4～8月。普查43年历史天气图，新疆强冰雹天气的500hPa环流形势分为纬向、经向和南疆低涡3种类型，并对其进行了环流形势和物理量合成诊断分析。