The effects of precipitation schemes and horizontal resolution on the major rainband in typhoon Flo (1990) predicted by the MRI mesoscale nonhydrostatic model

Summary ?This paper describes a numerical study of the major spiral rainband in typhoon Flo (1990) using the Meteorological Research Institute Mesoscale Nonhydrostatic Model (MRI-NHM). The effects of precipitation schemes and horizontal resolution on the representation of the simulated rainband are discussed. Dynamic and thermodynamic structures of the simulated major rainband to the north of the storm center are well represented in the model with a 5 km horizontal resolution. The structures are consistent with observational results reported for other tropical cyclones. Among the realistic features are: a cold pool and convergence on the inner side of the band; convergence above low-level inflow layers; and the outward slope of the updraft with height. The band is caused by the motion of the storm through its surroundings where horizontal wind has vertical shear. The simulation of the structure and precipitation pattern associated with the major rainband depends on the precipitation scheme rather than the horizontal resolution. The band appears more realistic when using explicit cloud microphysics as a precipitation scheme, rather than moist convective adjustment. This result is attributable to the difference in scheme triggering. In the simulation with moist convective adjustment, the elimination of vertical instability in low-level atmosphere is excessive, suppressing band formation. The overall structure of the band is also more realistic in the simulation using explicit cloud microphysics, because a cold pool exists in the lower layers and the vertical axis of upward flow tilts outward. This result suggests that prediction will partly depend on variables associated with cloud microphysics, such as the mixing ratio of cloud water. The horizontal grid distance, which varied between 5 and 20 km, quantitatively influenced the rainfall amount, although the large-scale band structure remained unchanged. The rainfall amount increased as the grid interval was reduced from 20 to 10-km but decreased as the interval was further reduced from 10 to 5 km. Received March 20, 2001; revised August 20, 2001     

AN OBSERVATIONAL ANALYSIS OF A TORRENTIAL RAINSTORM IN THE WARM SECTOR OF SOUTH CHINA COASTAL AREAS

On May 20 th 2007, a brief but severe downpour rainstorm occurred in the coastal areas of Maoming and Yangjiang with rainfall of 115 mm per hour. Data from NCEP/NCAR reanalysis with 1°×1° resolution, Doppler weather radar, conventional surface observations, high-altitude radiosonde and wind profiler radar were used to analyze characteristics and contributions of synoptic scale and mesoscale systems during this torrential rainstorm. The results showed that:(1) the storm was caused by a quasi-linear mesoscale convective system(MCS) and the slow-movement of this system was the primary trigger of the torrential downpour;(2) water vapor was abundant, nearly saturated and in steady state throughout the atmosphere before the storm; intrusion of the weak dry and cold air in the middle level and a striking "dry above and wet below " structure had increased the atmospheric instability;(3) low-level southwesterly airflow from a low pressure(trough) at the Beibu Gulf provided abundant water vapor at the onset of the rainstorm; a deep dry layer was formed by dry and cold air behind the high-level trough, which facilitated latent heat release;upper-level divergence and low-level convergence circulations also provided vertical uplift for warm and moist air at the lower level;(4) Topography only played a minor role as the MCS developed and strengthened over relatively flat coastal terrain. Low level density flow induced by convection triggered new convective cell generation at the leading edge of the convective system, thereby playing a key role in the change of temperature gradient at lower layers, and resulting in strengthening atmospheric instability.     

对流层低层偏东风对北京局地暴雨的作用

本文利用风廓线仪、地面自动站观测资料及NCAR/NCEP 1°×1°分析资料等,对北京地区两次局地暴雨天气过程的对流层低层偏东风进行了对比分析研究,重点分析了浅薄和深厚两次偏东风的形成机制、偏东风的垂直结构特征和温、湿特性,以及偏东风在北京局地暴雨中的作用等。主要结论如下:(1)浅薄偏东风活动在距地面高度500 m 以下,水平尺度约250 km,时间尺度约12 h, 地面风速平均约1 m/s;深厚偏东风活动在距地面高度3000 m 以下,水平尺度大于600 km,持续时间大于24 h,地面风速平均约4 m/s。(2)浅薄偏东风由边界层内浅薄的次天气尺度暖性低涡引起,深厚偏东风由天气尺度地面暖性低压倒槽的发展引起。(3)偏东风具有高相当位温的属性,其源地是北京东部或东南部的暖湿气团;在暖湿偏东风上方800～600 hPa 存在干冷空气活动,形成了有利的对流不稳定层结;浅薄偏东风暖湿能量的局地集中特征更为显著,而深厚偏东风在水汽和能量的持续输送方面,以及与500 hPa 偏西风形成较强的、有利于强对流风暴发展的低层垂直风切变方面作用更为显著。(4)浅薄偏东风在时间和空间上与近地面层辐合中心对应较好,与中高空辐散有较好配合,动力作用明显,直接起到了对流风暴的触发机制作用;深厚偏东风与辐合中心对应较差,与垂直运动及上层辐散也没有很好的配合,与对流风暴的触发没有直接的关系。但是深厚偏东风在对流层低层对应厚达3000 m 的潮湿空气层,削弱了雨滴下落过程中产生的蒸发降温作用,有利于对流的发展和维持。     

Squall Line and Its Vertical Motion Under Different Moisture Profiles in Eastern China

The impacts of different moisture profiles on the structure and vertical motion of squall lines were investigated by conducting a set of numerical simulations. The base state was determined by an observational sounding, with high precipitable water representing moist environmental conditions in the East Asian monsoon region. To reveal the impact of moisture at different levels, the moisture content at the middle and low levels were changed in the numerical simulations. The numerical results showed that more convective cells developed and covered a larger area in the high moisture experiments, which was characteristic of the convection during the Meiyu season in China. In addition, high moisture content at low levels favored the development of updrafts and triggered convection of greater intensity. This was demonstrated by the thermodynamic parameters, including Convective Available Potential Energy (CAPE), Lifted Index (LI), Lift Condensation Level (LCL), and Level of Free Convection (LFC). Dry air at middle levels led to strong downdrafts in the environment and updrafts in clouds. This could be because dry air at middle levels favors the release of latent heat, thereby promoting updrafts in clouds and downdrafts in the environment. Therefore, high relative humidity (RH) at low levels and low RH at middle levels favors updrafts in the cloud cores. Additionally, moist air at low levels and dry air at middle levels promotes the development of convective cells and the intensification of cold pool. The squall line can be organized by the outflow boundary induced by cold pool. The balance of cold pool and environmental wind shear is favorable for the maintenance and strengthening of squall lines.     

一次华南海岸带暖区暴雨过程的观测分析

2007年5月20日粤西茂名与阳江海岸带地区发生一场小时降水量达到115 mm的短时大暴雨。采用NCEP/NCAR FNL 1 °×1 °再分析资料,结合多普勒天气雷达、高空探空、自动站资料和风廓线雷达观测资料,分析此次暴雨过程的天气尺度系统与中尺度系统特征,探讨不同尺度系统对这次暴雨天气过程的作用。分析结果表明:（1）此次暴雨过程是由准线状中尺度对流系统导致的,对流系统的移动速度缓慢是导致出现暴雨的主要原因;（2）在暴雨发生前,整层大气水汽充沛,处于近饱和稳定状态;暴雨发生期间局地大气处于条件不稳定状态;中层弱冷槽过境及“上干下湿”的温湿结构增加了环境大气的不稳定性;（3）北部湾低压（槽）发展导致的强盛低层西南气流为暴雨发生提供了充沛的水汽;高空槽后干冷空气南下形成的深厚干层,有助于对流凝结潜热的释放;高层辐散、低层辐合环流为低层暖湿空气提供了垂直上升动力;（4）中尺度对流系统在地势相对平缓的沿海地区发展和加强,地形的动力抬升和辐合作用不大。对流活动诱发的低层密度流在对流带前缘不断激发出新的对流单体,对对流系统的维持和发展起关键作用;（5）对流单体的风暴传播效应使对流系统具有逆风传播的特征,移速缓慢;层云降水的蒸发冷却有可能改变其低层的温度梯度,使环境大气的不稳定性加强。      

超强台风韦帕（0713）螺旋雨带中尺度结构双多普勒雷达研究

超强台风＂韦帕＂（Wipha）是2007年登陆中国大陆最强的台风,在浙江省造成了特大暴雨。利用宁波和舟山双多普勒天气雷达同步观测资料,对＂韦帕＂的两条螺旋雨带进行了双雷达三维风场反演;并综合利用组网雷达拼图数据等资料,对螺旋雨带的三维精细结构进行了分析。研究表明：（1）两条螺旋雨带的三维结构有很多相似之处。螺旋雨带内部低层有多个强回波区,水平速度大值区主要分布在强回波带上;强回波带的低层有较强的上升气流,最强上升气流超过4 m/s。在螺旋雨带中存在多个辐合辐散对、上升下沉气流对,这对于螺旋雨带的维持和进一步发展具有重要作用。在沿着台风中心的垂直剖面内,螺旋雨带内部的强回波区向雨带外侧倾斜。雨带外侧2 km高度以下的低层有较强的内流,最大值为5 m/s;雨带内侧有较强的外流,2 km高度以上均受外流控制;内流和外流在雨带中部低层汇合抬升。切向速度的强中心出现在3 km高度,速度值随高度增加而逐渐减小。（2）两个时段的螺旋雨带也存在差异。前一个时段的螺旋雨带对流发展更旺盛,45 dBZ的回波高度为4.8 km,而后一个时段的螺旋雨带45 dBZ的回波高度仅3.2 km。垂直剖面内,前一个时刻螺旋雨带低层辐合更强,最强辐合值超过-15×10^-4s^-1,正是由于低层的强辐合和充足的水汽供应,才使得雨带内部中低层的回波发展旺盛。     

A Numerical Study of Mesoscale-Topography Influence on the Heavy Rainband of Typhoon Hato (2017)

During the movement of Typhoon Hato (2017) over land, heavy rainfall occurred when the spiral rainband which was about 100 km distance away from the center of the typhoon passed the Dayao Mountain (with an elevation of 1.2 km). In this study, the structures and forming mechanism of the heavy rainband along the mountain range are investigated by using high-resolution model simulations. The results show the importance of topography in causing the heavy rainband. Upslope of the steep terrain lifts the cyclonic flow to produce strong upward motion when the rainband passes across with high wind speed. At the same time, the warm and humid air is lifted to the steep slope, causing unstable energy to accumulate over the windward slope, which is conducive to the occurrence of rainfall. In particular, the convective cells generated upstream of rainband will further strengthen and develop due to the uplift when they move close to the mountain foot. Some precipitation particles in the convective cells fall to the ground while others move downstream with the intense updrafts, forming heavy rainfall near the summit. As a result, the largest accumulative rainfall coincides well with the orientation of the mountain ridge.     

黄淮流域一次飑线天气的数值模拟与分析

利用1°×1°NCEP再分析资料和探空资料,对2009年6月3日发生在黄淮流域的强飑线天气过程进行诊断分析,并采用WRF模式进行天气过程模拟和进一步研究。结果表明：本次飑线天气是东北冷涡后部横槽引导冷空气南下与南方暖湿气流相遇引发的。地面干线附近是雷暴和飑线的高发区。这次飑线天气发生在高空急流减弱之际、低空急流建立之前。高低空急流的U、V风分量变化对飑线有一定的指示意义。不同阶段的飑线降水和大风出现位置不同。     

一次多弓状中尺度雨带的成因机理及其与水平涡度的关系

用WRF中尺度数值模式、NCEP/NCAR再分析资料、多普勒雷达观测资料等,对2018年5月5日发生在我国华中地区的一次多弓状雨带降水过程的形成机理及其与水平涡度的关系进行分析发现:雨带发生在切变线南侧的西南气流中,多弓状雨带出现前,大尺度高低层气旋式曲率的水平涡度和对流有效位能为降水提供了有利于上升运动的背景场。弓状雨带最初形成在对流不稳定和低层气流辐合条件下,局地强降水引发的下沉运动使中低层大风出现,大风中心南侧反气旋式的环流与背景场中的西南气流汇合构成了短波槽,尾部雨带出现在短波槽中,弓状头部生成于北侧的气旋式风场切变中,大风中心相较南北两侧更快的移速使雨带中部向前侧凸起。流场上的短波槽发生在700 hPa以下,在西南气流的背景场下,该槽向前后两侧的双向传播是多弓状雨带形成的重要触发因子。中尺度弓状雨带附近低层的水平涡度在强盛期、减弱期和消散期有着明显不同的结构特征;而在高层,雨带发展时和强盛期都为气旋式水平涡度控制,减弱期多为反气旋式水平涡度控制。      

梅雨期3类不同形成机制的暴雨

通过对1998—2007年观测资料的分析,根据影响梅雨期暴雨发生、发展机制的不同,将梅雨期暴雨分为外强迫型、自组织型和非组织化局地型3种类型。外强迫型主要包括冷槽推进型、西南涡移出型和北槽南涡型,该类型暴雨主要是由大尺度环流的动力强迫抬升和冷空气侵入形成的不稳定层结共同作用而产生;在3类外强迫型中,高低空急流相互作用和冷槽的影响形式有很大不同。在自组织型中,暴雨对流系统具有较长生命周期,并以合并增长、上下游发展和新生中尺度涡旋等形式而传播、发展,是在切变线、水汽辐合带和低空急流等弱环境强迫下形成的一类暴雨。在非组织化局地型中,主要有山区午后强对流和副热带高压边缘对流不稳定两种形式,局地对流不稳定是暴雨产生的主要原因。不同的形成机制,导致出现了不同类型的梅雨期暴雨,相应地这些不同类型的梅雨期暴雨具有不同的预报难度。     

一次强降水超级单体风暴的动力特征分析

利用NCEP/NCAR(0.25°×0.25°)逐6 h再分析资料、常规观测资料、多普勒雷达资料对2018年9月19日晚发生在四川盆地东北部的强降水超级单体风暴进行诊断分析。研究表明：此次强降水超级单体风暴发生在较强的不稳定能量、很低的抬升凝结高度、低层深厚湿层、较弱的对流抑制能量及中等到强的垂直风切变的背景条件下,低层冷空气的侵入最终触发了本次过程。在强降水超级单体风暴发展演变过程中,中低层较强垂直风切变的重要作用是产生水平涡管。水平涡管又在上升气流的作用下抬升为垂直涡管,最后产生垂直涡度。而通过对大气垂直涡度方程的分析发现：垂直涡管在随高度增加的上升气流的拉伸作用下,不断加强,致使上升气流更强烈旋转,水平旋转又反过来加强了上升气流。上升气流与水平涡旋持续不断的正反馈机制是形成中气旋的重要原因。     

一次强雹云结构的双多普勒雷达观测分析

利用石家庄SA多普勒天气雷达和饶阳X波段双偏振雷达探测资料，结合常规综合观测资料，对2018年6月13日下午发生于太行山东麓的雹云的天气背景、降雹特征、雷达回波演变特征及回波三维立体结构进行了综合分析，重点利用双多普勒雷达径向速度资料反演出格点的三维风速（流场），并结合回波特征分析了雹云云体结构。结果显示，高空强劲的偏北风急流促使涡后横槽转竖，槽后冷空气沿偏北气流南下，形成上干冷、下暖湿的不稳定层结，在低空低涡附近及地面辐合线上发展造成这次风雹天气；双多普勒雷达反演风场表明，雹云的中层强回波中心呈明显的“S”形水平流场和悬挂回波配置特征，构成了具有成雹的“0线”结构；不仅佐证了雹云降雹“0线”结构的存在，而且证明其呈现形式具有多样性。      

河北唐山一次飑线过程的中尺度天气分析

利用常规观测资料、自动气象站资料及雷达资料,对2013年8月4日影响唐山的一次飑线过程进行了中尺度分析。结果表明:500 hPa高空槽是产生这次飑线的主要影响系统,地面中尺度辐合线是这次过程的触发机制;对流层中层干冷空气入侵与低层暖湿气流的辐合增强了大气层结的不稳定;低层辐合、高层辐散进一步加强了垂直运动的发展;中低层垂直风切变则有利于飑线的发展、加强和维持。雷达回波图上可识别出中低空的中尺度辐合线、弓形回波、逆风区等中小尺度结构特征,对于此类强对流性天气的预报具有实际指示意义。     

Mechanism of high rainfall over the Indian west coast region during the monsoon season

The mechanism responsible for high rainfall over the Indian west coast region has been investigated by studying dynamical, thermodynamical and microphysical processes over the region for the monsoon season of 2009. The European Centre for Medium-Range Weather Forecasts wind and NCEP flux data have been used to study the large scale dynamical parameters. The moist adiabatic and multi-level inversion stratifications are found to exist during the high and low rainfall spells, respectively. In the moist adiabatic stratification regime, shallow and deep convective clouds are found coexisting. The Cloud Aerosol Interaction and Precipitation Enhancement EXperiment aircraft data showed cloud updraft spectrum ranging from 1 to 10 m s^?1 having modal speed 1–2.5 m s^?1. The low updrafts rates provide sufficient time required for warm rain processes to produce rainfall from shallow clouds. The low cloud liquid water is observed above the freezing level indicating efficient warm rain process. The updrafts at the high spectrum end go above freezing level to generate ice particles produced due to mixed-phase rainfall process from deep convective clouds. With aging, deep convection gets transformed into stratiform type, which has been inferred through the vertical distribution of the large scale omega and heating fields. The stratiform heating, high latent heat flux, strong wind shear in the lower and middle tropospheric levels and low level convergence support the sustenance of convection for longer time to produce high rainfall spell. The advection of warm dry air in the middle tropospheric regions inhibits the convection and produce low rainfall spell. The mechanisms producing these spells have been summarized with the block diagram.     

对流降水云中大气垂直运动反演及个例试验

为深入认识对流降水云结构及动力特征,基于降水频段调频连续波5520 MHz垂直指向雷达(VPR-CFMCW),使用地面至15 km高度的反射率因子及径向速度,建立对流降水云中大气垂直运动的反演方法,分析对流垂直结构及大气垂直运动随高度分布的演变特征.对在广东龙门测站探测的2019年4月20-22日前汛期4次对流降水进行...     

Mesoscale modeling study of severe convection over complex terrain

Short squall lines that occurred over Lishui, southwestern Zhejiang Province, China, on 5 July 2012, were investigated using the WRF model based on 1°× 1° gridded NCEP Final Operational Global Analysis data. The results from the numerical simulations were particularly satisfactory in the simulated radar echo, which realistically reproduced the generation and development of the convective cells during the period of severe convection. The initiation of this severe convective case was mainly associated with the uplift effect of mesoscale mountains, topographic convergence, sufficient water vapor, and enhanced low-level southeasterly wind from the East China Sea. An obvious wind velocity gradient occurred between the Donggong Mountains and the southeast coastline, which easily enabled wind convergence on the windward slope of the Donggong Mountains; both strong mid–low-level southwesterly wind and low-level southeasterly wind enhanced vertical shear over the mountains to form instability; and a vertical coupling relation between the divergence on the upper-left side of the Donggong Mountains and the convergence on the lower-left side caused the convection to develop rapidly. The convergence centers of surface streams occurred over the mountain terrain and updrafts easily broke through the lifting condensation level(LCL) because of the strong wind convergence and topographic lift, which led to water vapor condensation above the LCL and the generation of the initial convective cloud. The centers of surface convergence continually created new convective cells that moved with the southwest wind and combined along the Donggong Mountains, eventually forming a short squall line that caused severe convective weather.     

Observations of planview flux patterns within convective structures of the marine atmospheric surface layer

Air/sea flux variability on horizontal scales from 50 m to several km results, in part, from the presence of coherent convective structures within the atmospheric boundary layer. The horizontal distribution of fluxes within these convective updrafts and downdrafts is, therefore, central to studies of air/sea interaction and remote sensing of sea surface wind and wave fields. This study derives these flux patterns from observations of the Marine Atmospheric Surface Layer (MASL).Research aircraft flights through the MASL provide an optimal means for sampling large numbers of the above-mentioned coherent structures. The NCAR Electra flew numerous legs through the MASL at a height of 50 m during the 1987 stratocumulus phase of Project FIRE (First ISSCP (International Satellite Cloud Climatology Program) Regional Experiment).In situ measurements from these legs serve as the dataset for this paper. The data are processed in such a way as to retain only the turbulence fluctuations. Conditional sampling, based on the vertical velocity field, results in the isolation of convective updrafts and downdrafts. Compositing of the data for these two classes of convective drafts results in horizontal planviews of the vertical fluxes of buoyancy, absolute humidity, along-meanwind component of momentum, and vertical velocity. To ensure dynamical similarity, these horizontal planviews are oriented in a coordinate system aligned with the mean wind.     

“东方之星”翻沉事件强对流天气分析及数值模拟

2015年6月1日21:32（北京时）左右,"东方之星"号客轮由南京开往重庆途中,行至湖北省荆州市监利县长江大马洲水道时遭遇狂风暴雨天气而翻沉。经调查分析,此次事故是由一次突发罕见的飑线天气伴随的下击暴流袭击所致。使用ARPS模式,同化常规资料及监利县周边4部雷达资料,综合多种观测分析飑线伴随下击暴流过程中系统结构及发展变化特点,结果表明:降水质点的拖曳和下沉气流的共同作用是强对流活动发生发展和下击暴流产生的重要原因,低层干燥、中层湿润的不稳定层结有利于动能向下传输及地面大风的生成。数值模拟表明:地面水平风场大值区、近地面水平和垂直风向风速变化、10 min累积降水量大值中心和组合反射率因子高值区走向呈一致的带状分布,与观测对应良好。受下击暴流直接影响,事故点附近的雷雨大风强度陡增,近地面出现狭窄的阵风锋,风切变明显;事故点附近主要受到超过10 m·s-1的下沉气流和超过18 m·s-1的强烈偏西风共同影响,降水中心分钟降水量超过10 mm。     

北京“623”大暴雨的强降水超级单体特征和成因研究

本文利用雷达、加密地面自动站等高时空分辨率的观测资料,结合NCEP 1°×1°再分析资料、常规观测等资料,对2011年6月23日发生在北京城区的极端强降水事件开展了细致的观测和诊断分析。结果表明,这次极端强降水事件,主要是由向东南移动的东北—西南走向的飑线右端的强降水超级单体（High Precipitation Supercell,简称HPS）造成的,这是目前已有文献记载的中国发生纬度最高的HPS。HPS在移动方向的右后侧和右前侧均有明显的“V”型入流,这不同于已有HPS模型,表明中、低层干冷空气和低层暖湿气流特征显著。在环境条件方面,存在对流层低层逆温层,其能量存储盖作用使得雷暴具有爆发性增强的潜势,但该逆温层是在08:00～14:00（北京时,下同）的6小时内形成的,对业务预报极具挑战性。相对其他大气层结热动力参数, 风暴相对螺旋度和粗理查逊数在14:00较08:00显著增大,对HPS的发生具有一定指示作用。高空偏西风急流和低层偏东风活动显著,使得北京地区的水平风垂直切变增强,形成上干下湿的对流不稳定以及次级环流圈。高空急流造成强烈的相当位温差动平流,促进对流不稳定度发展加强。结合复杂地形作用,在北京西部100 m地形高度线附近形成显著的平原暖湿空气与山地干冷空气的干湿分界线以及风场辐合线。水汽供应主要源自低层偏东风和本地水汽积累。当飑线从西北方向侵入北京并向东南方向移动时,在北部山区,由于条件不足,雷暴没有显著发展加强;然而,在西部山区,在湖面、城市热岛、低层偏东风、冷池出流共同作用下,加之其他有利的环境条件,飑线右端雷暴强烈发展加强,特别是当经过100 m地形高度线附近时发展成为HPS,进而造成石景山区模式口站的大暴雨中心。     

2011年7月黑龙江省西部罕见暴雨成因分析

利用区域自动站和常规观测资料及卫星云图、多普勒雷达资料,对2011年7月31日黑龙江省西部大区域暴雨过程进行分析。结果表明：黑龙江省西部暴雨区位于高空西风急流出口区左侧、低空和超低空急流的左侧,高空辐散和低空辐合作用为暴雨过程提供了动力条件,低空和超低空急流为暴雨提供了水汽条件;在对流层中层有较强的干冷空气侵入,短时强降水落区位于干侵入边界,并且与TBB大梯度区有较好的对应关系;而在雷达回波中,短时强降水落区位于速度图中的风场不连续线附近。