The synoptic setting of a thundery low and associated prefrontal squall line in western Europe

Summary A case study of a thundery low and associated prefrontal squall line in western Europe is presented. It is shown that the prefrontal squall line is linked to the vertical circulation associated with an intensifying cold front and a propagating midtropospheric jetstreak, with maximum wind speeds at levels between 700 and 500 hPa. The squall line is triggered in the updraught of the cross-frontal circulation, which can be observed at the earth's surface as a line of mass convergence or confluence stretching for more than 1000 km from southern Iberia to northern France. The intensification of the front and the destabilization of the atmosphere are interpreted by using the slope of isentropes as indicator of frontal intensity. An equation is derived for the rate of change of frontal intensity, which predicts a forward tilt of the cold front with increasing height due to the cross-frontal circulation in the area of warm air advection (the so-called Spanish plume) and, associated with this, a destabilization of the atmosphere at midtropospheric levels.With 17 Figures     

The behavior of total lightning activity in severe Florida thunderstorms

The development of a new observational system called LISDAD (Lightning Imaging Sensor Demonstration and Display) has enabled a study of severe weather in central Florida. The total flash rates for storms verified to be severe are found to exceed 60 fpm, with some values reaching 500 fpm. Similar to earlier results for thunderstorm microbursts, the peak flash rate precedes the severe weather at the ground by 5–20 min. A distinguishing feature of severe storms is the presence of lightning ‘jumps' — abrupt increases in flash rate in advance of the maximum rate for the storm. The systematic total lightning precursor to severe weather of all kinds — wind, hail, tornadoes — is interpreted in terms of the updraft that sows the seeds aloft for severe weather at the surface and simultaneously stimulates the ice microphysics that drives the intracloud lightning activity.     

Characteristics of cloud-to-ground lightning in warm-season thunderstorms in the Central Great Plains

In July 2005, a field campaign was conducted in the Central Great Plains to obtain 60-field/s video imagery of lightning in correlation with reports from the U.S. National Lightning Detection Network (NLDN) and broadband electric field waveforms from the Los Alamos Sferic Array (LASA). A total of 342 GPS time-stamped cloud-to-ground (CG) flashes were recorded in 17 different sessions, and 311 (91%) of these were correlated with reports from the NLDN. Only 6 of the 17 recording sessions were dominated by flashes that lowered negative charge to ground, and 11 were dominated by positive CG flashes. A total of 103 flashes recorded on video were correlated with at least one NLDN report of negative CG strokes, 204 video flashes were correlated with one or two positive stroke reports, and 4 had bipolar reports. In this paper, we will give distributions of the estimated peak current, I_p, as reported by the NLDN, of negative and positive first strokes that were recorded on video, the multiplicity of strokes that were recorded on video, and the number of ground contacts per flash that were resolved on video. 41 (40%) of the negative flashes produced just a single-stroke on video, and 62 (60%) showed two or more strokes. The observed multiplicity of negative flashes averaged 2.83, which becomes about 3.14 after correcting for the finite time-resolution of the video camera. 195 (96%) of the positive flashes produced just a single-stroke on video, and 9 (4%) showed two strokes; therefore, the observed multiplicity of positive flashes averaged 1.04. Five out of 9 (56%) of the positive subsequent strokes re-illuminated a previous channel, and 4 out of 9 (44%) created a new ground contact. Simultaneous video, LASA, and NLDN measurements also allowed us to examine the classification of NLDN reports during 3 single-cell storms (one negative and two positive). Based on the LASA waveforms, a total of 204 out of 376 (54%) NLDN reports of CG strokes were determined to be for cloud pulses. The misclassified negative reports had |I_p| values ranging from 3.8 kA to 29.7 kA, but only 58 (24%) of these had |I_p| > 10 kA, and only one misclassified positive report had I_p > 20 kA. Radar analyses showed that most of the negative and positive CG strokes that were recorded on video were produced within or near the convective cores of storms. The radar imagery also showed that single-cell storms tended to produce one polarity of CG flashes at a time, and that such storms could switch rapidly from negative to positive CG flashes when the reflectivity was near maximum. Multiple-cell storms produced both negative and positive flashes over a broad region, but each polarity tended to cluster near regions of high-reflectivity.     

江苏近10 a高架雷暴特征分析

对2007—2016年发生在江苏地区的冬半年雷暴进行特征分析,筛选出12次典型的高架雷暴天气过程,揭示江苏发生高架雷暴的时空分布特征和典型的环流形势,发现逆温层顶之上的不稳定浅层和上下层强垂直风切变分别为高架雷暴的发生提供弱热力不稳定和强动力不稳定条件。强垂直风切变、850 hPa附近强烈的锋生导致的锋面次级环流,高空槽前正涡度平流随高度增加以及高层辐散、低层辐合造成的抽吸作用,为高架雷暴的发生和维持提供逆温层之上的动力抬升条件。高架雷暴发生时高仰角反射率因子呈现出类似零度层亮带的环形特征,对流单体不断生成在圆环附近。初步归纳了江苏高架雷暴的预报着眼点:500 hPa先后高空槽东移,700 hPa有16 m·s~(-1)以上的西南急流,850 hPa切变线东伸,存在逆温层顶高于1. 5 km,逆温强度大于5℃的较强逆温,0～6 km垂直风切变超过18 m·s~(-1),700 hPa与500 hPa温度差在15℃以上以及700hPa的相对湿度高于80%,且比湿在5～6 g·kg~(-1)。     

Construction and assessment of a logistic regression model applied to short-term forecasting of thunderstorms in León (Spain)

Forecasting thunderstorms is one of the most difficult tasks in weather prediction due to the scarce knowledge on how to characterise the mechanisms taking part in the formation of thunderstorms. New tools based upon the objective recognition of appropriate conceptual models have been developed in the last years. A good example of this are the statistical models, based on variables that characterise the preconvective atmospheric conditions.This paper presents the results obtained by applying a short-term forecast model to thunderstorms during the summer periods in León (Spain). This model makes use of the logistic function as a binary forecasting technique determining storm/no storm. The logistic function used was a combination of 15 previously selected meteorological variables. The model has been constructed with the data collected on 152 occasions, and it has been verified on 77 other occasions.The skill scores obtained show that the use of statistical models combining the data, provided by variables characterizing the preconvective conditions and the triggering mechanisms, represent an important step in the difficult task of short-term thunderstorm forecasting.     

Advances in tornado and storm research in the United Kingdom and Europe: the role of the Tornado and Storm Research Organisation

The Tornado and Storm Research Organisation (TORRO) was formed in the UK in 1974 in order to determine realistic spatial, temporal and intensity distributions of tornadoes in the UK and, eventually, throughout Europe. Currently, TORRO's databases contain nearly 2000 tornadoes and over 550 waterspouts for the UK alone. In 1972, TORRO's founder, Terence Meaden, devised the Tornado Intensity Scale. This scale enables the wind speeds of tornadoes to be rated on a scale from T0 to T10, or more (since it is an open-ended scale). Using this scale, estimates have been made of the statistical return periods of differing intensities of UK tornadoes. TORRO's research into understanding the conditions of tornado development is ultimately intended to lead to issuing forecasts of tornadoes. In 1991, TORRO issued the first tornado watch in the UK for 12 November and this proved to be accurate—not only did three property-damaging tornadoes strike East Anglia but the forecast maximum intensity of T5 was attained. In 1975, TORRO expanded its activities to include conventional thunderstorm reporting and investigations. In 1982, TORRO incorporated the British and Irish thunderstorm data-collection organisation, the Thunderstorm Census Organisation (TCO), which was established in 1924. By the early 1980s, TORRO's network of voluntary thunderstorm observers, located throughout the UK and Ireland and increasingly in other European countries, numbered around 350 and the network continues to grow. In addition to TORRO's early focus on tornadoes and thunderstorms, its data collection and research has expanded to consider other whirlwinds (such as waterspouts and land devils), hailstorms, lightning (including ball lightning) and blizzards/heavy snowfalls. In 1996, TORRO began providing easy access to its databases (e.g. post-1995 UK and European severe weather events, especially tornadoes and hailstorms) and other information via its internet site at http://www.torro.org.uk/. Building on over 25 years of experience—and of the half a century of the TCO before it—TORRO continues to expand its European role in severe weather data collection and research, helped by the on-going appointment of European representatives and its increasing European membership.     

Climatology of severe hailstorms in Great Britain

The annual and seasonal frequency, geographical distribution, and intensity of British hailstorms are examined. In 1986, the Tornado and Storm Research Organisation (TORRO) developed a Hailstorm Intensity Scale to characterise around 2500 hailstorms known to have occurred in Great Britain since the first documented hailstorm event of 1141 AD. The most intense British hailstorm reached intensity H8 on the TORRO international scale which extends from intensities H0 to H10. This paper focuses on over 800 hailstorms that reached TORRO intensity of H3 or more, the “severe” category. Analyses are presented for the historical period and the most recent 50-year period, 1950 to 1999. Consideration is given to examining the 50 most intense hailstorms (TORRO intensity H5–6 or more) known to have occurred in Britain since 1650. These storms all occurred between the months of May and September with a well-defined peak during July. These exceptional storms typically followed a track from the S, SSW or SW to the N, NNE or NE with a swath length of 25 km or more (reaching 335 km in one case) and a swath width sometimes in excess of 10 km.     

The synoptic meaning of helicity

Summary Helicity, a scalar quantity resulting from the inner product of velocity and vorticity, has until now mostly been used in the field of mesoscale meteorology and boundary layer meteorology. Goals of this paper are the derivation of the flux form of the helicity equation in general form without neglection of friction and Coriolis force and the examination of helicity patterns of larger scales. The general helicity equation is approximated for the synoptic and frontal scale by use of the scale analysis. High helicity values are bounded to fronts, such that helicity determines their positions. Finally the helicity patterns at different heights and helicity sources and sinks are discussed for a case study of a cyclogenesis over the Atlantic.With 6 Figures     

The Use of Pre-Storm Boundary-Layer Baroclinicity in Determining and Operationally Implementing the Atlantic Surface Cyclone Intensification Index

The lateral motion of the Gulf Stream off the eastern seaboard of the United States during the winter season can act to dramatically enhance the low-level baroclinicity within the coastal zone during periods of offshore cold advection. The ralative close proximity of the Gulf Stream current off the mid-Atlantic coast can result in the rapid and intense destabilization of the marine atmospheric boundary layer directly above and shoreward of the Gulf Stream within this region. This airmass modification period often precedes either wintertime coastal cyclogenesis or the cyclonic re-development of existing mid-latitude cyclones. A climatological study investigating the relationship between the severity of the pre-storm, cold advection period and subsequent cyclogenic intensification was undertaken by Cione et al. in 1993. Findings from this study illustrate that the thermal structure of the continental airmass as well as the position of the Gulf Stream front relative to land during the pre-storm period (i.e., 24–48 h prior to the initial cyclonic intensification) are linked to the observed rate of surface cyclonic deepening for storms that either advected into or initially developed within the Carolina-southeast Virginia offshore coastal zone. It is a major objective of this research to test the potential operational utility of this pre-storm low level baroclinic linkage to subsequent cyclogenesis in an actual National Weather Service (NWS) coastal winter storm forecast setting.The ability to produce coastal surface cyclone intensity forecasts recently became available to North Carolina State University researchers and NWS forecasters. This statistical forecast guidance utilizes regression relationships derived from a nine-season (January 1982–April 1990), 116-storm study conducted previously. During the period between February 1994 and February 1996, the Atlantic Surface Cyclone Intensification Index (ASCII) was successfully implemented in an operational setting by the NWS at the Raleigh-Durham (RAH) forecast office for 10 winter storms. Analysis of these ASCII forecasts will be presented.     

东北一次雪飑事件的触发机制研究

2017年1月26日,中国东北地区发生了一次短时强降雪过程.本文利用ECMWF再分析数据诊断该过程的可能触发机制.分析表明,该过程可分两个阶段:初生阶段降雪远离高地形,低层锋生和有利的辐散场配置激发上升运动释放不稳定;增强阶段雪带接近长白山,低层锋生,地形环流以及与低空急流有关的风切变共同释放锋前不稳定.本文进一步计算...     

The influence of topography on the cloud-to-ground lightning density in South Brazil

A comparative analysis between cloud-to-ground (CG) lightning density and terrain parameters (altitude and terrain slope) in South Brazil is presented. This region is characterized by a contrasting topography, where a mountain chain separates lower (depression) and higher (plateau) landscapes. The altitude and terrain slope data were obtained from the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM). Two years of CG lightning data (from June 2005 to May 2007) obtained by the Brazilian Lightning Detection Network (BrasilDAT) were used in the study. To avoid intracloud contamination, positive CG flashes with peak currents less than 20 kA were removed from the data set. A relative detection efficiency model was used to correct the lightning data. The results indicate that, for localized areas (following the mountain shape) within this region of Brazil, the CG lightning density is correlated with the terrain slope and not the altitude. This suggests that terrain slope has more influence than altitude on the thunderstorm occurrence and lightning activity. In addition, a temporal analysis shows that over high altitude regions the diurnal variation (amplitude) of lightning activity is stronger and the peak occurs 1 h earlier than over low altitude regions.     

用地球静止气象卫星云图分析我国几类强雷暴天气

夏季的雷暴、冰雹、大风等强对流天气,尺度小,生命史短,用常规的天气观测资料分析它们的活动存在着一定的困难。然而,地球静止气象卫星(GMS)云图为我们提供了连续观测这些对流活动的一种可能性。本文主要是利用三小时一次的GMS云图资料,分析了1978年6—8月份我国大陆上的六类雷暴天气,阐述了强雷暴活动的云图特征。指出,假若有30分钟一次的GMS云图,则对夏季大陆上强雷暴的分析和监视更为有利。     

WRF模式对弱强迫系统中雷暴预报个例研究

用WRF2．2版本,针对2006年7月13日洛阳市区,发生在副高边缘偏东南暖湿气流中的雷暴进行了模拟预报,结果表明WRF模式能描述弱强迫天气尺度系统中的中小尺度对流系统,能捕捉到常规天气图上难以分辨出来的雷暴单体。模式输出的中尺度要素场可以确定雷暴发生的地点;模式探空的对流有效位能、抬升指数、沙瓦特指数、K指数随时间演变曲线的拐点,能指示雷暴发生的时间。个例分析表明WRF模式在预报弱强迫天气系统雷暴时具有较好的性能,用WRF模式来作雷暴的分析预报早一条可行的徐径。     

利用锋生函数对2008年年初昆明准静止锋生消过程的诊断分析

利用ERA-Interim 0.125°×0.125°高分辨率逐6小时再分析数据,选取昆明准静止锋2008年1~2月长时间维持过程诊断分析了锋生函数及其各分项在锋面生消过程中的作用与贡献,并结合2016年1月20~27日锋面增强西进过程和1979年1月14~20日锋面减弱东退过程进行了比较分析。结果表明:（1）非绝热加热项对锋面生消作用较小,但存在显著的日变化。在辐射作用下,非绝热加热项日间表现为锋消,夜间为锋生。（2）西移冷气团在被高原大地形抬升过程中存在局地地形迫使冷气团下沉的情况,并导致垂直运动倾斜项表现为锋生。由于地形固定不变,局地锋生形成次级锋生带。次级锋生带与昆明准静止锋的形成、维持和东西摆动有密切的联系。（3）辐合辐散项和水平形变项是锋生函数变化的主要贡献项,其中水平形变项中的切变变形作用以锋生为主,而伸缩变形项在纬向产生锋生,在经向产生锋消。     

新津机场雷暴的特征

利用1986～1996年新津机场雷暴观测资料，分析了该区域雷暴天气发生的时间和空间分布特征。在此基础上结合典型的天气个例，用天气动力学方法，讨论了新津机场雷暴天气的物理成因。结果显示：新津机场雷暴集中在4～9月，以热雷暴为主。高温、高湿、不稳定能量的大气和动力触发是其形成的重要条件。     

新津机场雷暴的特征

利用1986～1996年新津机场雷暴观测资料,分析了该区域雷暴天气发生的时间和空间分布特征.在此基础上结合典型的天气个例,用天气动力学方法,讨论了新津机场雷暴天气的物理成因.结果显示:新津机场雷暴集中在4～9月,以热雷暴为主.高温、高湿、不稳定能量的大气和动力触发是其形成的重要条件.     

阳泉市主汛期极端气候事件演变特征

利用1972年-2008年阳泉6月-8月的珏最高气温、降水量、风和天气现象等观测资料,分别统计了高温、暴雨、冰雹、雷暴和大风等极端气候事件发生的概率及分布特点,以期为探索阳泉气候变化的规律和研究方法做出较好的尝试.     

On the future reduction of snowfall in western and central Europe

Large parts of western and central Europe face a 20–50 % future reduction in snowfall on Hellmann days (days with daily-mean temperatures below freezing). This strong reduction occurs in addition to the expected 75 % decrease of the number of Hellmann days near the end of the twenty first century. The result is insensitive to the exact freezing-level threshold, but is in sharp contrast with the winter daily precipitation, which increases under most global warming scenarios. Not only climate model simulations show this. Observational records also reveal that probabilities for precipitation on Hellmann days have been larger in the past. The future reduction is a consequence of the freezing-level threshold becoming a more extreme quantile of the temperature distribution in the future. Only certain circulation types permit these quantiles to be reached, and it is shown that these have intrinsically low precipitation probability.     

准地转Q矢量在河南省区域暴雨过程中的诊断应用

应用NCEP再分析资料及常规地面观测资料,对2005年7月21-24日河南省大暴雨过程中准地转Q矢量作了诊断分析,结果发现:在对流层低层850 hPa等压面上,Q矢量散度的负值区与暴雨落区有着较好的对应关系;Q矢量散度场在垂直方向上出现低层辐合、高层辐散配置时,利于暴雨的产生,暴雨区位于Q矢量辐合中心下方,降水的强弱与Q矢量的低层辐合及高层辐散强弱变化一致。Q矢量的y分量能较好地反映出锋生、锋消的作用,并能指示6-12 h后的暴雨落区。