Temporal distribution of weather catastrophes in the USA

The temporal distributions of the nation’s four major storm types during 1950–2005 were assessed, including those for thunderstorms, hurricanes, tornadoes, and winter storms. Storms are labeled as catastrophes, defined as events causing $1 million or more in property losses, based on time-adjusted data provided by the insurance industry. Most catastrophic storms occurred in the eastern half of the nation. Analysis of the regional and national storm frequencies revealed there was little time-related relationship between storm types, reflecting how storm types were reported. That is, when tornadoes occurred with thunderstorms, the type producing the greatest losses was the one identified by the insurance industry, not both. Temporal agreement was found in the timing of relatively high incidences of thunderstorms, hurricanes, and winter storms during 2002–2005. This resulted in upward time trends in the national losses of hurricane and thunderstorm catastrophes, The temporal increase in hurricanes is in agreement with upward trends in population density, wealth, and insurance coverage in Gulf and East coastal areas. The upward trends in thunderstorm catastrophes and losses result from increases in heavy rain days, floods, high winds, and hail days, revealing that atmospheric conditions conducive to strong convective activity have been increasing since the 1960s. Tornado catastrophes and their losses peaked in 1966–1973 and had no upward time trend. Temporal variability in tornado catastrophes was large, whereas the variability in hurricane and thunderstorm catastrophes was only moderate, and that for winter storms was low.     

Temporal and spatial distributions of wind storm damages in the United States

High wind caused catastrophes, storms causing property losses >$1 million, during 1952–2006 averaged 3.1 events per year in the U.S. The average loss per event was $90 million, and the annual average loss was $354 million. High wind catastrophes were most frequent in the Northeast, Central, and West Coast areas. Storm losses on the West Coast were the nation’s highest, averaging $115 million per event. High wind losses are the nation’s only form of severe weather that maximizes on the West Coast. High wind catastrophes were most frequent in winter, and were infrequent in the late spring and early fall seasons. Loss areas were frequently confined to one state. Losses in the western U.S. and nationally have increased during the 1952–2006 period, both with statistically significant upward trends.     

Long-Term Fluctuations in Thunderstorm Activity in the United States

Thunder-day occurrences during a 100-year period based on data from carefully screened records of 86 first-order stations distributed across the United States were assessed for temporal fluctuations and trends during 1896–1995. Short-term (<10-year) fluctuations of adjacentstations were often dissimilar reflecting localized differences in storm activity in a few years, making spatial interpretations difficult. But, temporal fluctuations based on 20-year and longer periods exhibited regional coherence reflecting the control of large, synoptic-scale weather systems on the distribution of thunderstorms over broad areas. Classification of station fluctuations based on 20-year periods revealed six types of distributions existed and they formed 12 discrete areas across the nation. One type present in the lower Midwest and the South had a peak in storm activity in 1916–1935 followed by a general decline to 1976–1995.A second type maximizing at the same time had its minimum earlier, in 1956–1975. Another distribution found at stations in the upper Midwest and Northeast had a mid-century peak (1936–1955) with a recent minimum in1976–1995. A fourth distribution also peaked in 1936–1955 but had an early minimumin 1896–1915, and it mainly occurred in the northern plains and Rocky Mountains. A fifth distribution peaked during 1956–1975 and was foundat stations in four areas including the central High Plains, Southwest, northern Great Lakes, and Southeast. The sixth temporal distribution showed a steady increase in storm activity during the 100-year period, peaking in 1976–1995, and covered a large area extending from the Pacific Northwestacross the central Rockies and into the southern High Plains. The national average distribution based on all station values peaked in mid century. The national distribution differs markedly from several regional distributions illustrating the importance of using regional analysis to assess temporal fluctuations in severe weather conditions in the nation. The 100-year linear trends of the 86 stations defined six regions across the U.S. Significant upward trends existed over most of the western two-thirds of the nation, unchanging trends existed in the northern plains and Midwest, and downward trends were found in most of the nation's east. The up trends in storm-day frequencies in the southern plains occurred where storm damage is greatest and where demographic changes have added to storm losses over time. The national patterns of trends and storm distributions were similar to those found for hail. The temporal distributions of storm activity helped explain recent increases in major storms and their losses, conditions which have increased in the west and south.     

Increasing major hail losses in the U.S.

Property losses due to hailstorms on April 13–14, 2006, resulted in Midwestern property losses that totaled $1.822 billion, an amount considerably more than the previous record high of $1.5 billion set by an April 2001 hail event. The huge April 2006 loss was largely due to multiple severe storms with frequent large hail hitting major metropolitan areas. A highly unstable air mass that developed on April 13 led to several supercell storms and they then produced large hailswaths across portions of Iowa, Illinois, Indiana, and Wisconsin during a 30-h period. This storm event and prior recent major hail losses occurred when several major hailstorms developed and then traveled for hundreds of kilometers. The nation’s top ten loss events during 1950–2006 reveal a notable temporal increase with most losses in the 1992–2006 period. Causes for the increases could be an increasing frequency of very unstable atmospheric conditions leading to bigger, longer lasting storms, and/or a greatly expanded urban society that has become increasingly vulnerable to hailstorms.     

2002年和2003年春季中国沙尘暴形成和输送的对比分析

2002年春季中国华北和东北发生了数场强沙尘暴天气.而2003年春季沙尘暴发生次数较少且强度也弱.为了解这两年沙尘暴差异的原因,作者用NCEP/NCAR再分析资料对两年中3、4月份候距平进行了分析.从侯距平图可看出,2002年负距平区主要位于华北和东北,并与发生大范围沙尘暴天气的区域相吻合,这与东亚大槽的发展与演变有关.在2003年高度场的距平图中,高度场的负侯距平多位于我国西部,这是因中纬度长波槽是在西部地区发展形成的.从月平均风资料分析,2002年风速大于6 m s-1的区域也与强沙尘暴区一致.2003年两个月中,我国大部地区风速都偏小.虽然2003年4月在东北有大于6 m s-1的风区,但不在我国主要的沙漠化地区.通过对大气环流及地表参数的分析,作者认为沙尘暴的易发地区多是干旱与半干旱地区,植被的生长能力弱;在降水没有比常年显著增加并使地面植被生长有明显改善的情况下,大气动力因素相对地面参数而言是主导沙尘暴年际变化的主要因子.     

我国南方冬季低温雨雪冰冻事件的大气扰动信号分析

利用中国549个站点的温度和降水资料确定了1960～2008年发生在我国南方冬季的23次低温雨雪冰冻事件。对全球大气变量再分析资料做物理分解得到天气尺度扰动分量用于这些事件的早期信号分析。结果表明,在低温事件发生时,南方地区上空300 hPa对应有最大负高度扰动,850 hPa有负温度扰动。这些扰动的传播特性可以作为低温事件发生的早期信号。在低温雨雪站日数大于10的11次事件中,它们的扰动信号平均可提前11.2天追踪到。     

暴雨信号发布思路

深圳2000年4月份出现了4次暴雨和大暴雨，其中“4.14”为打破历史纪录的特大暴雨。深圳市气象台所发布的黄、红、蓝色暴雨信号是历史上次数最多，最频繁之一。本文通过对这几次不同量级的暴雨过程中暴雨信号发布情况以及短期天气预报和云图、雷达和自动站等中尺度资料在暴雨预警信号发布中所发挥作用的分析，总结发布暴雨信号的发布思路；并得到十分有益的经验。     

Analysis of Three Supercell Storms with Doppler Weather Radar Data

Three supercell storms on 24 June 2004 (0624), 28 June 2003 (0628), and 27 September 2002 (0927) induced different damages in Shandong Province. Storm 0927 was inferior in size and intensity to storms 0628 and 0624. The structure and evolvement of the three storms were analyzed in detail based on the WSR-98D radar data in combination with weather charts. The results show that mesoscale surface convergence triggered release of instable energy, which resulted in severe convection. During the development stage, storms 0927, 0628, and 0624 displayed multi-cell propagation, single-cell evolution, and multi-cell mergence, respectively. The storm tracks were similar: they were all right-moving supercell storms, i.e., moving at an angle of 30°–70° to the right of the mean wind and at a speed of about 45%-70% of the mean wind speed. In the mature stage, the maximum reflectivity appeared at the low level in storm 0927, mid level in storm 0628, and mid-upper level in storm 0624. These storms possessed almost all typical features of supercell storms: weak echo region (WER), bounded weak echo region (BWER), and mesocyclone. An organized mesocyclone formed at the middle height of an updraft, deepened gradually downward and upward, and became a typical mid-level mesocyclone with strong updrafts. The vertical structures of airflows in the three storms were similar, i.e., significant convergence at low level, nearly pure rotation at mid level, and divergent rotation at upper level. However, signatures of mid-level horizontal airflows in the three storms were different: at mid level, there was a single vortex in storm 0628, but a double-vortex flow pattern was seen in storms 0927 and 0624. The horizontal structure of the double-vortex flow was hard to be blown away by the environmental airflow, and thus the storms could persist for a longer period of time than the single vortex storm.     

Damaging Weather Conditions in the United States: A Selection of Data Quality and Monitoring Issues

The limitations of observational data available for the study of damaging weather conditions (e.g., storms and extreme temperature events) are discussed. Crop and property insurance loss records are advocated as a potential supplement to traditional weather observations, as they integrate specific information about the spatial dimension of damaging weather conditions and the cost of damage they cause. Insurance loss data may also be analyzed in combination with meteorological data sets to derive indicator variables for the detection of damaging weather events.Two sets of insurance data are described. One record provides adjusted property losses associated with "catastrophic" weather events since 1949, and the other is an index of the amount of crop-hail losses per year since 1948. Additionally, an example of the benefits of the combination of insurance and meteorological data is presented through a selection of results from a recent study of freezing temperatures in the southeastern United States and associated insurance claims related to pipe bursting.If insurance data are to be applied in the future in similar studies of damaging weather conditions, it is essential that the insurance industry continues to collect and adjust loss data and periodically confirm that adjustment factors are temporally consistent.     

三次超级单体风暴雷达产品特征及气流结构差异性分析

2002年9月27日、2003年6月28日和2004年6月24日山东部分地区遭受了不同程度的灾害性天气,雷达观测分析表明是3次超级单体风暴所致,0927风暴尺度和天气现象次于0628和0624风暴.利用济南多普勒雷达探测资料,结合天气形势,对这3次典型超级单体强度结构、流场结构及其演变过程进行了仔细的分析,结果表明:地面中尺度辐合触发了不稳定能量的释放,引发了强对流天气发生;风暴形成阶段表现为不同的演变特征,0927风暴表现为多单体传播型,0628风暴表现为单体自身发展型,0624风暴表现为群发单体合并型;移动路径相似,都属于右移风暴,偏离风暴承载层平均风右侧30°-70°,移动速度约为风暴承载层平均风速的45%-70%;发展成熟阶段最大强中心高度表现不同,0927风暴位于单体底部,0628风暴位于单体中下层,0624风暴位于单体中层以上,最大反射率因子和垂直积分液态含水量(VIL)表现也有差别,0624风暴最强,0628风暴次之,0927风暴相对较弱.风暴旺盛成熟阶段表现为典型的超级单体特征,有界弱回波区(BWER)和中气旋;风暴旺盛成熟阶段风暴垂直流场结构有相似性,低层气旋性辐合,中层近似于气旋性旋转上升,高层气流辐散;中层水平流场结构存在较大差异,0927和0624.风暴为双涡管式旋转结构,0628风暴为单涡式的气旋旋转结构.     

一次超级单体分裂过程的雷达回波特征分析

2007年7月9日16—20时（北京时）在河北南部非常罕见地观测到了多个超级单体风暴在相近地点连续生成及分裂的过程。利用石家庄新乐SA型多普勒天气雷达资料、地面自动站及常规天气资料,对超级单体分裂过程及环境条件做了分析。表明这次的多个超级单体风暴是在强的对流有效位能和垂直风切变的环境条件下发生的。由于垂直风切变矢量方向随高度逆时针旋转,因此,分裂后左移的反气旋风暴得到加强,发展成为具有深厚中反气旋的左移超级单体风暴,而右移的气旋性风暴受到抑制,与理论研究结果一致。但也有不同之处,沿着地面高湿区内热力边界偏暖一侧移动的气旋性风暴没有受到明显抑制,有利的地面环境条件抵消了气旋性风暴受抑制的程度,使气旋性风暴能够持续更长的时间。该强烈发展的带有明显中反气旋的超级单体风暴具有低层钩状回波和入流缺口、中高层有界弱回波区及位于有界弱回波区之上的高层具有反射率因子核心和强烈风暴顶辐散,与经典的气旋式右移超级单体风暴的回波特征非常类似,除了是反气旋涡旋外,其回波特征与气旋式超级单体近似成镜像。风暴分裂是在单体形成不久的发展初期开始的。分裂先从中高层开始,然后迅速向下延伸。分裂后相对于0—6 km风切变矢量,左侧的单体为反气旋左移风暴,右侧的为气旋性右移风暴。     

华北地区夏季一次致雹强风暴的分析

文章对2007年7月9日下午华北中部地区一次区域性强对流性天气过程中风暴单体的短时临近预报方法进行研究。对实际探测资料和数值模式产品的分析发现以下特点：高空槽将由后倾槽转为前倾槽、底层不稳定层结会明显加大,在地面冷锋东移冲击下在沿锋面伸展的露点锋区内可能将有强雷暴系统发展;全球谱模式T213、中尺度MM5模式的产品对区域性对流天气发生、影响的区域有3h以上的预报时效,具有一定的区域预报能力,但落点预报能力明显有限。对多普勒雷达产品的分析表明：多普勒雷达产品对灾害性天气的落点、影响区域具有30分钟以上的预测时效,通过基本反射率、相对风暴速度等产品的特征判断一个对流风暴具有类似强降水超级单体特征,可据此预报该雷暴中心经过区域可能有冰雹、大风等灾害性天气;风廓线产品在3-7km高度层内垂直风切变矢量具有顺时针旋转特点,有利于风暴发展成强风暴;风暴追踪信息基本能反映风暴移动路径的变化,其路径预报时效最长达1h,在雷暴初期预报准确率随雷暴数目增多、移动异向性明显而越低,在雷暴中后期则明显提高并对临近预报具有明显的指示性。     

甘肃夏季特强沙尘暴分析

夏季沙尘暴的气候特征表明,夏季是甘肃省沙尘暴的次多发季节,主要集中在民勤、鼎新、金塔。通过对一次罕见的甘肃省夏季强沙尘暴天气分析发现:高空小槽、切变线、热低压是引发夏季沙尘暴的主要天气系统,而春季沙尘暴一般是大尺度天气系统造成的;夏季沙尘暴发生前期高空急流反映并不明显,急流风速的突然加大和沙尘暴几乎同时发生,这是夏季沙尘暴预报的难点之一;沙尘暴发生前8~12 h的螺旋度场对沙尘暴预报有较好的指示意义,正值越大,沙尘暴越强,但当沙尘暴与强降水同时发生时,沙尘暴区螺旋度值明显小于强降水中心螺旋度值。     

北京沙尘天气与源地积雪变化的关系

主要分析了北京沙尘天气变化规律及沙尘源区积雪变化与北京沙尘日数的关系,并探讨源区积雪变化影响北京沙尘天气的机制.研究表明55年来北京沙尘日数基本呈减少趋势,但1998～2000年又有所增加,沙尘暴日数也在减少,近10年北京没有出现强沙尘暴天气.而沙尘源区积雪深度和积雪面积与北京沙尘存在明显的负相关关系.冬季源区积雪减少(增加),很可能导致春季沙尘日数增加(减少),作者认为冬季积雪变化引起的土壤含水量变化是影响春季北京沙尘天气的原因之一.     

我国北方区域沙尘天气的时间特征分析

将我国北方沙尘主要影响区划分为3个区（西北区、华北区、东北区）,用网格面积加权计算区域平均的方法,比较了各区沙尘天气的时间演化特征。结果表明：西北部沙尘日数的量级明显多于东北部;沙尘暴的发生有比较明显的日变化特征,各区白天较夜间更易发生沙尘暴,2区（华北区）和3区（东北区）发生沙尘暴初始时刻的峰值出现在14：00,1区（西北区）出现在15：00～16：00;2区和3区出现沙尘天气的极值在4月,而1区在4、5两个月都是极值期;春季是各区沙尘天气的多发期,1区夏季沙尘天气发生的频次也较高;各区的沙尘日数均在20世纪80年代中期前后发生了由多到少的跃变,1区和2区的突变点在1987年,3区在1983年。我国北方3个区沙尘天气的日、月、季节变化有明显的区域特征,其中1区表现得较为独特,2区和3区则比较接近。     

1981—2013年四川省视程障碍类天气现象分析

利用四川省1981—2013年雾、轻雾、吹雪、雪暴、烟幕、霾、沙尘暴、扬沙和浮尘9种视程障碍天气现象资料,对其发生日数、发生概率和分布特征进行统计。结果表明:(1)各天气现象发生日数排序为:轻雾>雾>浮尘>霾>烟幕>扬沙>沙尘暴>吹雪>雪暴。(2)轻雾和雾年发生日数为分别为176d/a和29d/a,日发生概率分别为48%和8%,远高出其他天气现象。(3)季节变化方面,雾和轻雾主要出现在秋季和冬季;霾、吹雪和雪暴集中出现在冬季;浮尘发生春季;扬沙多发生在冬季和春季;而沙尘暴、烟幕主要发生在春季和秋季。(4)变化趋势上轻雾基本保持平稳;烟幕呈增加趋势;而雾、霾、沙尘暴、扬沙和浮尘呈下降趋势。(5)大气层结稳定、水汽充足、风速较小、人口集中和排放量较大,易于盆地雾、轻雾、霾和烟幕的形成;不合理利用水和土地资源,北方地区沙尘天气随冷空气南下,是沙尘天气发生的重要原因;而吹雪和雪暴均发生在冬季降雪量大且风速较大的川西高原。     

A climatic study of severe storms over Bulgaria produced by Mediterranean cyclones in 1990−2001 period

Over south-eastern Europe, severe weather events are often associated with Mediterranean cyclones. This paper presents a climatic study of severe storms over Bulgaria produced by synoptic-scale Mediterranean cyclones, which are the main high-impact weather systems for the region during the winter season. The study is based on a synoptically oriented data set that contains systematic information about the pronounced Mediterranean cyclones including their life and trajectories over the Mediterranean area and the severe storms over Bulgaria produced by them. The definition of a severe storm is a storm in which the wind speed and precipitation exceed thresholds of 15 m/s and 30 mm/24 h, respectively. The observed severe storms were split into two groups by the number of districts where they have caused damages. During the last years a trend of decreasing numbers of initiating synoptic-scale Mediterranean cyclones has been observed. However, the number of those producing a high-impact weather phenomenon over Bulgaria has increased. In these high-impact cases, the observed paths of the cyclones are connected to the specific circulation conditions over the region. In the most severe cases, almost 80% of the cyclones move through the southernmost parts of Balkan Peninsula and for a large portion of them, this motion is associated with a blocking regime in the mid-level mass field.The development of such Mediterranean cyclone cases has been studied and results are presented here. The results illustrate the synoptic-scale mechanisms for intensification of a feeding flow of Mediterranean air towards the area affected by extremely severe weather.     

2002年我国沙尘暴天气特征分析

利用逐日8个时次地面天气报告资料，分析了2002年全国沙尘暴的时空分布特征。结果表明：2002年出现的12次沙尘暴过程以3月18～22日的强沙尘暴过程影响范围最广、强度最强；我国北方地区浑善达克沙地及其周边地区的沙尘暴日数最多。另外，2002年我国沙尘暴高频期集中、早春多于晚春。主要原因是前期北方持续气温偏高、降水偏少，3—4月上中旬影响我国的冷空气十分频繁且强度较强，4月下旬—5月上旬我国北方地区出现了几次较大范围降水，5月中下旬影响我国的冷空气势力较弱且位置偏东。     

库尔勒市历年沙尘天气发生特性分析

本文根据1971~2010年库尔勒市气象实测资料,分析了库尔勒市沙尘天气40年变化特征,以及沙尘天气与平均风速,降水量,平均相对湿度,平均地面0cm温度和平均气温等气象因子之间的关系。结果表明:沙尘暴、扬沙、浮尘等沙尘天气在年际、季节与各月变化上具有一致性。20世纪的70年代沙尘天气发生日数最多,从1971~2005年沙尘天气发生日数呈波动下降趋势,1971年最高,共出现沙尘天气112d但不同沙尘天气发生日数最值出现的年份不同,2005~2010年沙尘天气发生日数又开始回升,到2010年沙尘天气的发生次数已接近70年代的平均水平。其次,沙尘天气呈现春夏季节发生日数多,秋冬发生日数少的季节变化趋势,每年的4月沙尘天气出现最多,1月沙尘天气出现最少。沙尘天气的发生与空气相对湿度、降水量呈现极显著的负相关,与风速呈现极显著的正相关,而与气温的变化关系不明显。根据2010年4月的沙尘颗粒物监测表明,沙尘天气使大气中的Ca、Mg、Fe等地壳元素含量增加。      

Temporal fluctuations in weather disasters: 1950–1989

A unique historical data set describing the 142 storms producing losses in excess of $100 million in the United States during the 1950–89 period were analyzed to describe their temporal characteristics. These weather disasters (WDs) caused $66.2 billion in losses, 76% of the nation's insured losses in this period. Disasters were most prevalent in the south, southeast, northeast, and central U.S., with few in and west of the Rockies. The incidence of WDs was high in the 1950s, low in the 1960s-early 1970s, and peaked in the 1980s. Losses due to WDs peaked in the 1950s, again in the late 1960s, and with a lesser peak after 1985. The areal extent of storm losses peaked after 1975 and was least in the 1960s. The temporal variations of the three storm measures (incidence, losses, and extent) had poor agreement, and agreed only when they peaked in the 1950s. Regionally-derived time distributions of WDs showed marked north-south differences with a U-shaped 40-year distribution in the northern half of the nation, whereas southern regions had a relatively flat trend until achieving a peak in the 1980s. The temporal distributions of hurricane-caused disasters differed regionally, with the distributions in the southern, southeastern, and northeastern U.S. each quite different. Temporal distributions of thunderstorm and winter storm-produced disasters were regionally more uniform. The national 5-year WD frequencies correlated moderately well with annual mean temperatures which explained 40% of the variability found in WDs during 1950–89. Weather disasters peaked in the relatively warm-dry 1950s and again in the warm-wet 1980s, and were least in the cool-wet 1960s and 1970s. The distribution of WDs during 1950–89 appears positively related to the temporal fluctuations in cyclonic activity.