The numerical simulation of storm surges along the Bangladesh coast

Using a set of basic hydrodynamic equations governing the motion in a sea, a vertically-integrated coastal zone numerical model has been developed for the simulation of storm surges generated by the tropical stroms striking the Bangladesh coast. The model is fully nonlinear and uses a conditionally stable semi-explicit finite difference scheme for solving the relevant equations. In this model the analysis area extends from 87°E to 93°E along the south coast of Bangladesh and there are three open-sea boundaries situated along 87°E, 93°E and 19°N. With this analysis area it is possible to record, on average, 24 h of the surge generating capacity of most of the severe cyclones before landfall at the Bangladesh coast.Numerical experiments are performed with the help of this model to simulate the surges generated by several severe cyclonic storms which struck the Bangladesh coast durin the past 25 y. To compare the model predicted surges with the observed sea-surface elevations, three cyclonic storms have been chosen for which reliable observations regarding storm characteristics and the associated surges were available. The predicted peak sea-surface elevations compare well with the observed values at Chittagong port.     

Cold front propagation in the Loisach valley: Simulations with a simple model

Summary A simple, basically two-dimensional multilevel model is used to simulate the passage of a cold front in the Loisach valley as described by Müller and Sladkovic (1990). It is found that the front moves essentially like a density current on its way from the mouth of the Loisach valley towards Garmisch. However, a quasi-stationary state is attained soon after the front passed the branching point of the valley near Garmisch where the cold air is then flowing east- and westward. This flow state is characterized by downward motion in the Loisach valley proper and rapid ascent above Garmisch. This circulation becomes less intense as the valleys are filled with cold air. The agreement of model results and observations is satisfactory given the limitations of the model.With 3 Figures     

台风与冷空气对重庆\

利用地面逐小时观测资料、NCEP/NCAR（1°×1°）再分析资料、FY-2F卫星加密观测（6 min分辨率）的云顶亮温（TBB）和常规观测资料,对1710号台风\     

Numerical simulation of typhoon surges along the east coast of Zhejiang and Jiangsu Provinces

Using conventional two-dimensional nonlinear storm model, the storm surges caused by the six ty-phoons with different kinds of tracks which hit the coast of Zhejiang and Jiangsu Provinces including Shanghai during the period from 1953 to 1974 are computed numerically. To avoid the nonlinear and linear computational instabilities, a finite-difference scheme with the quadratic conservation (or the semi-momentum) is adopted and a criterion for linear computational stability is derived. The basic parameters used in this computation are: the space step Δs=30km, the time step Δt=200sec, the coefficient of the wind stress = 2.6 "10-> and the coefficient of the bottom friction v&2=2.0x 10-'. The pressure and surface wind distribution for typhoon are assumed to be characterized by Fujita's pressure formula and Uerto's wind model, respectively.     

Use of a synoptic classification scheme to define seasons

Summary This paper presents a synoptic surface weather map classification scheme, and uses the categorization technique to meteorologically define seasons. Winter is defined as that period of the year in which a location is most frequently poleward of the polar front, while summer occurs when the site is most commonly equatoward. Fall and Spring are the transition periods when, respectively, increasingly more frequent and less frequent periods of time poleward of the polar front occur.Using 10 years of data, the application of this definition of seasons to the Gulf and Atlantic coasts of the United States is presented. While the frequency of the specific types of major synoptic weather features varied with latitude, the meteorological definitions of season are comparatively invariant with latitude (differing by no more than a month) for this geographic area. Using the meteorological definitions of season, the average winter for this region occurs from late October or early November to late March or early April. Summer is from late May to early June until late August or late September.

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Zusammenfassung Diese Arbeit stellt ein synoptisches Klassifikationsschema nach Bodenwetterkarten vor, das eine Kategorisierungstechnik verwendet, um die Jahreszeiten meteorologisch zu definieren. Der Winter ist als die Periode des Jahres festgelegt, in der ein Ort meist polwärts der Polarfront, während der Sommer jene Periode ist, in der er äquatorwärts der Polarfront liegt. Herbst und Frühjahr sind die Übergangsperioden, in denen zunehmend häufigere bzw. weniger häufige Abschnitte polwärts der Polarfront vorkommen.Die Anwendung dieser Jahreszeitendefinition wird mit Hilfe der Daten aus 10 Jahren für die Atlantik- und Golfküste der Vereinigten Staaten vorgestellt. Während die Häufigkeiten bestimmter Wetterlagen mit der geographischen Breite schwanken, erwies sich diese meteorologische Jahreszeitendefinition demgegenüber als vergleichbar invariant (mit einem maximalen Unterschied von einem Monat) innerhalb des ausgewählten Gebiets. Der so definierte Winter dieser Region beginnt zwischen Ende Oktober und Anfang November und dauert bis Ende März bzw. Anfang April. Sommer ist zwischen Ende Mai/Anfang Juni und Ende August/Ende September.

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With 21 Figures     

The interface effect and the formation of a low-level jet along the east side of the rocky mountains

Based upon the analysis of several different causes for the low-level jet along the east side of the Rocky Moun-tains, the concept of “Interface Effect” is established. The basic mechanism for the formation of the low-level jet in North America has been found to be the compression and divergence, under the driving of ageostrophic winds, of the air columns between two surfaces-the ground and the bottom of inversion-which slope with different patterns in a cross-section normal to the jet stream. As a result, the air parcel is accelerated along the stream-line and the anticyclonic shear of the current increased. Also, the diurnal variation of the jet is determined by the interface effect.     

2020年梅雨期上海一次强降水过程垂直结构的综合观测分析

利用2010—2020年上海地区79个自动气象观测站逐小时观测资料和2016—2020年上海中心气象台和上海各区气象台发布的暴雨预警信号资料,采用趋势分析、累积频率等统计方法,分析了上海地区暴雨的时空分布特征,并且在对不同级别暴雨预警信号发布频次、预警时效进行统计分析的基础上,对不同天气形势下暴雨预警时效的差异进行了研究。结果表明: 上海地区6 h雨量标准的暴雨呈现显著上升趋势,其中暴雨蓝色预警上升趋势最为明显;7月6 h雨量标准的暴雨出现了明显的低谷,而1 h雨量标准的暴雨较6月仍然有所增加,显示出7月副高控制下上海地区多短时暴雨的特征。上海午后暴雨一般持续时间相对较短,而持续时间较长的暴雨多出现在上午。暴雨频次在中心城区和浦东靠近黄浦江地区存在大值中心,这可能与城市热岛效应有关。上海地区暴雨预警信号发布时间呈现双峰型变化特征,第一峰值出现在中午12∶00,12∶00—17∶00预警信号发布相对集中,第二峰值出现在早晨06∶00,各级别预警信号持续时间平均为5.9 h。上海地区暴雨预警命中率平均为82.4%,暴雨预警时效平均为1.3 h;副高边缘强对流型暴雨预警时效最短,台风本体或外围螺旋雨带型暴雨较易出现漏报,需要特别关注。

     

一次梅雨锋上中尺度气旋波引发的特大暴雨过程分析

利用新一代天气雷达、卫星、自动气象站、风廓线雷达、PWV/GPS、GFS/NCAR再分析场和常规天气资料,针对2011年6月10日发生于武陵山东侧鄂湘交界(通城附近)的特大暴雨过程,重点分析了梅雨锋上中尺度扰动系统的发生、发展过程以及与之联系的中尺度对流系统结构特征。结果表明:(1)特大暴雨过程与两个中尺度气旋波扰动发展形成的中尺度对流系统共同影响有关,而锋前暖低压倒槽内中尺度对流系统后向传播对暴雨形成和加强起到非常重要的作用。(2)武陵山东侧鄂湘交界区域是梅雨锋上中尺度气旋波新生或加强的重要源地之一,长江中游复杂地形下有利的动力和热力因素起到重要作用。(3)中尺度对流系统的发生、发展与中尺度气旋扰动有密切关系,雷暴单体集中在冷、暖锋附近和涡旋中心附近强烈发展,并在气旋波系统组织下,降水回波整体具有冷暖锋结构特征的逗点涡旋、涡旋、"S"和"人"字等形态特征。     

Cold air outbreaks along a non-frozen sea channel: effects of wind on snow bands

Wintertime cold air outbreaks along a non-frozen sea channel or a long lake can become destructive if the related bands of heavy snowfall hit onto land. The forcing for such bands is studied with a 2D numerical model set across an east–west sea channel at 60^oN (‘Gulf of Finland’), varying the basic geostrophic wind V _g. Without any V _g opposite coastal land breezes emerge with convergence. This results in a quasi-steady rising motion w _max ~ 7.5 cm/s at 600 m in the middle of the gulf, which can force a snow band. During weak V _g, the rising motion is reduced but least so for winds from 60^o to 80^o (~ENE), when modest alongshore bands could exist near the downstream (Estonian) coast. During V _g of 4–6 m/s from any direction, the land breezes and rising motions are reduced more effectively, so snow bands are not expected during moderate basic flow. In contrast, during a strong V _g of 20–25 m/s from 110^o to 120^o (~ESE) the land breeze perturbations are intense with w _max up to 15–18 cm/s. The induced alongshore bands of heavy snowfall are located in these cases at the sea but quite close to the downstream (Finnish) coast. They can suddenly make a landfall if the basic wind turns clockwise.     

一次冷锋降水云系结构和人工增雨条件模拟分析

利用中尺度模式ARPS对2013年10月13—14日华北南部到河南一次冷锋降水过程的数值模拟结果和卫星、雷达、飞机等观测资料,分析了冷锋云系不同部位宏微观结构和多种增雨潜力要素分布特征,初步探讨了冷锋云系增雨潜力区判别方法及分布特征。结果表明:此次降水过程冷锋云系结构具有不均匀性,云中含水量及其变化梯度自云系前部到后部逐步减小。冷锋云系不同位置垂直结构特征不同,云系前部自云底到云顶为整层上升气流区,云中存在典型的"催化-供给"结构,动力辐合和水汽条件较好,对应区域地面出现较大降水。云系后部上升气流区集中在中高层,4.0 km以下为下沉气流,云中冰相粒子丰富,但中低层液态水含量少,"催化-供给"结构不明显,动力辐合和低空水汽条件差,对应区域地面降水微弱或不产生降水。利用模式模拟结果逐步判别云系增雨潜力条件,结果显示:增雨潜力区主要位于冷锋云系前部、地面冷锋与700 hPa切变线之间约150 km宽的狭长带状区域,云体是具有"催化-供给"结构的冷暖混合云,可催化层高度为3.5—7.0 km。     

梅雨锋上两类中尺度对流系统形成的边界层特征

采用具有较高时空分辨率的地面观测资料以及WRF（Weather reasearch and forecasting）模式输出资料,分析了2009年6月29一-30日梅雨锋暴雨过程中两类不同的中尺度对流系统（rnesoscale convective system,MCS）边界层特征及边界层对两类MCS的触发维持机理,重点分析了海平面气压场特征、边界层冷池、干线及其在MCS中的影响。结果表明：两类中尺度对流系统的海平面气压特征存在着明显的差异,对流爆发阶段地面风场存在辐合线,再次激发阶段气压场呈“跷跷板”型的中尺度扰动,即由前置中低压和后置中高压组成,最强的对流带位于中低压和中高压之间的过渡区内;边界层辐合线是第一类中尺度对流系统（MCSl）维持的重要因素;MCSl爆发后边界层冷池生成,冷池前的冷出流与低层环境风产生的强辐合触发了第二类中尺度对流系统（MCS2）;存在于中低压和中高压之间的中尺度干线是MCS2的重要特点之一。     

梅雨锋暴雨中尺度对流系统的组织特征和触发条件分析

利用常规气象观测、地面加密自动站和多普勒天气雷达资料,结合WRF(Weather Research and Forecast)模式模拟资料,对2015年6月26—28日长江中下游的一次梅雨锋暴雨过程中尺度对流系统(Mesoscale Convective System,MCS)的组织特征和对流触发条件进行分析。结果表明:1)暴雨过程线状MCS在发展初期表现为东西向雨带不断的"后部建立"以及随后对流单体的"列车效应";在发展成熟期,对流单体向东北—西南向发展,形成多个近乎平行的东北—西南向短雨带。呈现2种尺度的对流组织方式:新生对流单体沿着单个雨带向东北方向的"列车效应"和短雨带沿着线状M CS向东平流的"列车带"效应。2)低空急流的持续加强为对流的发生发展提供了条件性不稳定和对流有效位能,偏南暖湿气流在向东北推进的过程中,在风速辐合处被强迫抬升至自由对流高度,释放不稳定能量,触发对流。3)对流雨带内近地面向南的冷出流与低层西南暖湿气流的持续交汇和相互作用有利于新单体生成发展,使雨带得以维持。     

Storm surges in the Western Baltic Sea: the present and a possible future

Globally-coupled climate models are generally capable of reproducing the observed trends in the globally averaged atmospheric temperature or mean sea level. However, the global models do not perform as well on regional/local scales. Here, we present results from four 100-year ocean model experiments for the Western Baltic Sea. In order to simulate storm surges in this region, we have used the General Estuarine Transport Model (GETM) as a high-resolution local model (spatial resolution ≈ 1?km), nested into a regional atmospheric and regional oceanic model in a fully baroclinic downscaling approach. The downscaling is based on the global model ECHAM5/MPI-OM. The projections are imbedded into two greenhouse-gas emission scenarios, A1B and B1, for the period 2000–2100, each with two realisations. Two control runs from 1960 to 2000 are used for validation. We use this modelling system to statistically reproduce the present distribution of surge extremes. The usage of the high-resolution local model leads to an improvement in surge heights of at least 10% compared to the driving model. To quantify uncertainties associated with climate projections, we investigate the impact of enhanced wind velocities and changes in mean sea levels. The analysis revealed a linear dependence of surge height and mean sea level, although the slope parameter is spatially varying. Furthermore, the modelling system is used to project possible changes within the next century. The results show that the sea level rise has greater potential to increase surge levels than does increased wind speed. The simulations further indicate that the changes in storm surge height in the scenarios can be consistently explained by the increase in mean sea level and variation in wind speed.     

Development of an establishment scheme for a DGVM

Environmental changes are expected to shift the distribution and abundance of vegetation by determining seedling establishment and success. However, most current ecosystem models only focus on the impacts of abiotic factors on biogeophysics (e.g., global distribution, etc.), ignoring their roles in the population dynamics (e.g., seedling establishment rate, mortality rate, etc.) of ecological communities. Such neglect may lead to biases in ecosystem population dynamics (such as changes in population density for woody species in forest ecosystems) and characteristics. In the present study, a new establishment scheme for introducing soil water as a function rather than a threshold was developed and validated, using version 1.0 of the IAP-DGVM as a test bed. The results showed that soil water in the establishment scheme had a remarkable influence on forest transition zones. Compared with the original scheme, the new scheme significantly improved simulations of tree population density, especially in the peripheral areas of forests and transition zones. Consequently, biases in forest fractional coverage were reduced in approximately 78.8% of the global grid cells. The global simulated areas of tree, shrub, grass and bare soil performed better, where the relative biases were reduced from 34.3% to 4.8%, from 27.6% to 13.1%, from 55.2% to 9.2%, and from 37.6% to 3.6%, respectively. Furthermore, the new scheme had more reasonable dependencies of plant functional types (PFTs) on mean annual precipitation, and described the correct dominant PFTs in the tropical rainforest peripheral areas of the Amazon and central Africa.     

A case study of mesoscale convective band (MCB) development and evolution along a quasi-stationary front

This paper presents a case study of mesoscale convective band （MCB） development along a quasi-stationary front over the Seout metropolitan area.The MCB,which initiated on 1500 UTC 20 September 2010 and ended on 1400 UTC 21 September 2010,produced a total precipitation amount of 259.5 mm.The MCB development occurred during a period of tropopause folding in the upper level and moisture advection with a low-level jet.The analyses show that the evolution of the MCB can be classified into five periods：（1） the cell-forming period,when convection initiated; （2） the frontogenetic period,when the stationary front formed over the Korean peninsula; （3） the quasi-stationary period,when the convective band remained over Seoul for 3 h; （4） the mature period,when the cloud cover was largest and the precipitation rate was greater than 90 mm h-1; and （5） the dissipating period,when the MCB diminished and disappeared.The synoptic,thermodynamic,and dynamic analyses show that the MCB maintained its longevity by a tilted updraft,which headed towards a positive PV anomaly.Precipitation was concentrated under this area,where a tilted ascending southwesterly converged with a tilted ascending northeasterly,at the axis of cyclonic rotation.The formation of the convective cell was attributed in part by tropopause folding,which enhanced the cyclonic vorticity at the surface,and by the low-level convergence of warm moist air and upperlevel divergence.The southwesterly flow ascended in a region with high moisture content and strong relative vorticity that maintained the development of an MCB along the quasi-stationary front.     

安徽沿长江东部春季一次大范围雾天气过程诊断分析

利用地面气象观测资料、高空探测资料、NCEP再分析资料、芜湖市边界层风廓线雷达资料和高速公路气象观测站资料,分析了2012年3月6日安徽省沿长江东部大范围雾天气过程形成的环流背景及雾生消的物理条件。结果表明：安徽沿江东部地区此次春季大范围雾的性质为辐射雾,雾发生时雾区上空为西到西南风为主,无明显冷空气影响,地面为高压控制的均压场,有利于雾的生成和维持。由雾生消的物理条件可知,近地面水汽条件较好和长波辐射降温造成的水汽凝结是此次大范围雾形成的重要原因。地面辐射降温形成的近地面逆温层有利于雾的维持,且随着近地面逆温层的抬升,雾层变厚并发展。低空的逆温层则形成稳定的层结,阻止水汽向上传输。近地面风速大小合适,风垂直切变小,低层有湍流,中层无明显上升运动,构成雾形成的有利动力条件;而湿层变厚又阻止了水汽向高层交换,有利于雾的生成和维持。日出后,太阳辐射增强,有利于雾发生和维持的地面辐射降温、逆温和动力条件逐渐消失,雾逐渐消散。     

Properties of the circulation classification scheme based on the rotated principal component analysis

Summary The study addresses some methodological issues of application of principal component analysis (PCA) to the classification of circulation patterns. The obliquely rotated PCA in T-mode (i.e. with time observations corresponding to variables and grid points to realizations) is applied to 500 hPa geopotential heights over Europe and adjacent parts of Atlantic Ocean. The solutions are examined for various numbers of principal components rotated, and for both raw and anomaly data, with the aim to find the way of determining the optimum number of circulation types. This is done, among others, by examining temporal and spatial stability of solutions, their compliance with simple structure requirements, and temporal behaviour of classifications. Some of the solutions that are pre-selected according to the rule based upon the separation between successive eigenvalues prove to perform considerably better than unselected ones; some of them do not. Which pre-selected solutions should be given preference is impossible to decide in advance, without a detailed scrutiny. Nevertheless, even after such a scrutiny is done, more than a single classification are acceptable. The final choice of the optimum solution depends on the aims of the intended study: It should balance the demands on statistical stability of types and on resemblance between types and daily patterns classified with them.With 6 Figures     

梅雨锋上的三类暴雨

利用常规气象观测资料、区域自动站资料、FY-2C云顶亮温（T_BB）资料及NCEP 1°×1°再分析资料,对2015年6月17-18日发生在黔东南地区的典型梅雨锋西段暴雨进行了诊断分析。结果表明（：1）在500 hPa两槽一脊单阻型梅雨形势下,冷空气沿贝加尔湖阻塞高压东侧南下与来自南海、孟加拉湾的暖湿气流在黔东南交汇,500 hPa短波槽东移促使低空切变线东移南压和地面梅雨锋发展,配合200 hPa南亚高压东部脊附近的\     

Cold-air cyclogenesis along the Gulf-Stream front: investigation of diabatic impacts on cyclone development, frontal structure, and track

Summary On 24–25 February 1989 a storm brought high winds and moderate to heavy snow to the U.S. East Coast. The storm is noteworthy for its rapid mesoscale development within a polar air mass at relatively low latitudes and for the difficulty experienced by operational NWP models and forecasters in predicting the storms impact. This paper investigates the mesoscale structure and evolution of the cold-air cyclone through analysis of enhanced data sets collected during the xperiment on apidly ntensifying yclones over the tlantic (ERICA). Results are presented from numerical sensitivity studies of the impact of diabatic heating on storm structure and track using the Mesoscale Atmospheric Simulation System (MASS) model.The following conclusions are drawn from the research. Differential surface fluxes in the vicinity of the Gulf Stream led to the development of a well-defined baroclinic zone at low levels that extended parallel to the axis of the Gulf-Stream front. The baroclinic zone strengthened and assumed the characteristics of a shallow warm front as the cyclone matured. Enhanced cyclonic vorticity, moisture-flux convergence, clouds, and precipitation accompanied the front. Early in the event a series of shallow, thermally forced vortices of small wavelength (200km) formed along the baroclinic zone in the area of maximum surface-heat fluxes offshore of the Carolinas. Baroclinic instability associated with a vigorous short-wave trough aloft resulted in the outbreak of deep convection surrounding and rapid intensification of the northernmost vortex.Numerical sensitivity experiments were conducted to investigate the nonlinear response of the mass field to the convection. The results show that latent heating in deep convection surrounding the surface low produced a mesoscale height perturbation aloft. The subsequent acceleration of the flow aloft substantially increased the integrated mass divergence above the surface cyclone, leading to deepening on the scale observed. The observed track of the low followed the axis of the warm front, which in turn followed the axis of maximum SST gradient associated with the Gulf-Stream front. Accurate simulation of the storm track required a high-resolution, full-physics run that included high-resolution SST data in the initial condition and moisture nudging during the early hours of the simulation.Current affiliation: NWS Office of Climate, Water, and Weather Services, 1325 East-West Highway, Silver Spring, MD 20910.Current affiliation: NWS UCAR/COMET, Boulder, CO 80307-3000.