Evolution Mechanism of a Severe Squall Line Triggered by the Coupling of a Sea Breeze Front and a Gust Front

In the present study, a severe squall line(SL) was analyzed by using intensive observational surface data and radar monitoring products. In this process, mesoscale convergence lines, such as the sea breeze front(SBF), gust front and dry line, served as the main triggering and strengthening factors. The transition from convection triggering to the formation of the initial shape was mainly affected by the convergence line of the SBF, which combined with thermal convection to form the main parts of the SL. In the later stage, the convergence line of the gust front merged with other convergence lines to form a series of strong convective cells. The SBF had good indicative significance in terms of severe convective weather warnings. The suitable conditions of heat, water vapor and vertical wind shear on the Shandong Peninsula were beneficial to the maintenance of the SL. Before SL occurrence, tropopause folding strengthened, which consequently enhanced the baroclinic property in the middle and upper troposphere. The high sensible heat flux at the surface easily produced a positive potential vorticity anomaly in the low layer, resulting in convective instability, which was conducive to the maintenance of these processes. In the system, when precipitation particles passed through the unsaturated air layer, they underwent strong evaporation, melting or sublimation, and the cooling effect formed negative buoyancy, which accelerated the sinking of the air and promoted the sustained development of the surface gale. Together with the development of lowlevel mesocyclones, the air pressure decreased rapidly, which was conducive to gale initiation.

Assimilating Surface Observations in a Four-Dimensional Variational Doppler Radar Data Assimilation System to Improve the Analysis and Forecast of a Squall Line Case

This paper examines how assimilating surface observations can improve the analysis and forecast ability of a four- dimensional Variational Doppler Radar Analysis System （VDRAS）. Observed surface temperature and winds are assimilated together with radar radial velocity and reflectivity into a convection-permitting model using the VDRAS four-dimensional variational （4DVAR） data assimilation system. A squall-line case observed during a field campaign is selected to investigate the performance of the technique. A single observation experiment shows that assimilating surface observations can influence the analyzed fields in both the horizontal and vertical directions. The surface-based cold pool, divergence and gust front of the squall line are all strengthened through the assimilation of the single surface observation. Three experiments--assimilating radar data only, assimilating radar data with surface data blended in a mesoscale background, and assimilating both radar and surface observations with a 4DVAR cost function--are conducted to examine the impact of the surface data assimilation. Independent surface and wind profiler observations are used for verification. The result shows that the analysis and forecast are improved when surface observations are assimilated in addition to radar observations. It is also shown that the additional surface data can help improve the analysis and forecast at low levels. Surface and low-level features of the squall line-- including the surface warm inflow, cold pool, gust front, and low-level wind--are much closer to the observations after assimilating the surface data in VDRAS.

EFFECTS OF DIABATIC PHYSICAL PROCESSES ON THE SIMULATION OF MESO-β SCALE STRUCTURE OF A SQUALL LINE SYSTEM

The effects of different convective parameterization,explicit moisture schemes and surface heat- ing on the meso-β scale structure of a squall line system are investigated,by using an improved mesoscale model.It is found that the successful prediction of mesoscale convective systems hinges up- on not only the sub-grid scale convection,but also the resolvable scale phase change processes and the diurnal variation in the boundary layer.The simultaneous operation of the Fritsch-Chappell convective scheme with parameterized moist downdrafts and the prognostic equations for cloud water (ice) and rainwater (snow) seems to be essential in simulating realistically MCSs and reducing or eliminating the unrealistic development of the CISK-like instability associated with the squall line system.

The Predictability of a Squall Line in South China on 23 April 2007

This study investigated the predictability of a squall line associated with a quasi-stationary front on 23 April 2007 in South China through deterministic and probabilistic forecasts. Our results show that the squallline simulation was very sensitive to model error from horizontal resolution and uncertainties in physical parameterization schemes. At least a 10-km grid size was necessary to decently capture this squall line. The simulated squall line with a grid size of 4.5 km was most sensitive to long-wave radiation parameterization schemes relative to other physical schemes such as microphysics and planetary boundary layer. For a grid size from 20 to 5 km, a cumulus parameterization scheme degraded the squall-line simulation （relative to turning it off）, with a more severe degradation to grid size -10 km than 〉10 km. The sensitivity of the squall-line simulation to initial error was investigated through ensemble forecast. The performance of the ensemble simulation of the squall line was very sensitive to the initial error. Approximately 15% of the ensemble members decently captured the evolution of the squall line, 25% failed, and 60% dislocated the squall line. Using different combinations of physical parameterization schemes for different members can improve the probabilistic forecast. The lead time of this case was only a few hours. Error growth was clearly associated with moist convection development. A linear improvement in the performance of the squall line simulation was observed when the initial error was decreased gradually, with the largest contribution from initial moisture field.

Evaluation of Two Momentum Control Variable Schemes and Their Impact on the Variational Assimilation of Radar Wind Data:Case Study of a Squall Line

Different choices of control variables in variational assimilation can bring about different influences on the analyzed atmospheric state. Based on the WRF model＇s three-dimensional variational assimilation system, this study compares the be- havior of two momentum control variable options-streamfunction velocity potential （ψ-χ） and horizontal wind components （U-V）-in radar wind data assimilation for a squall line case that occurred in Jiangsu Province on 24 August 2014. The wind increment from the single observation test shows that the ψ-χ control variable scheme produces negative increments in the neighborhood around the observation point because streamfunction and velocity potential preserve integrals of velocity. On the contrary, the U-V control variable scheme objectively reflects the information of the observation itself. Furthermore, radial velocity data from 17 Doppler radars in eastern China are assimilated. As compared to the impact of conventional observation, the assimilation of radar radial velocity based on the U-V control variable scheme significantly improves the mesoscale dynamic field in the initial condition. The enhanced low-level jet stream, water vapor convergence and low-level wind shear result in better squall line forecasting. However, the ψ-χ control variable scheme generates a discontinuous wind field and unrealistic convergence/divergence in the analyzed field, which lead to a degraded precipitation forecast.

Analysis of the Structure and Propagation of a Simulated Squall Line on 14 June 2009

A squall line on 14 June 2009 in the provinces of Jiangsu and Anhui was well simulated using the Advanced Regional Prediction System （ARPS） model. Based on high resolution spatial and temporal data, a detailed analysis of the structural features and propagation mechanisms of the squall line was conducted. The dynamic and thermodynamic structural charac- teristics and their causes were analyzed in detail. Unbalanced flows were found to play a key role in initiating gravity waves during the squall line＇s development. The spread and development of the gravity waves were sustained by convection in the wave-CISK process. The squall line＇s propagation and development mainly relied on the combined effect of gravity waves at the midlevel and cold outflow along the gust front. New cells were continuously forced by the cold pool outflow and were enhanced and lifted by the intense upward motion. At a particular phase, the new cells merged with the updraft of the gravity waves, leading to an intense updraft that strengthened the squall line.

Moisture Analysis of a Squall Line Case Based on Precipitable Water Vapor Data from a Ground-Based GPS Network in the Yangtze River Delta

A squall line swept eastward across the area of the Yangtze River Delta and produced gusty winds and heavy rain from the afternoon to the evening of 24 August 2002. In this papers the roles of moisture in the genesis and development of the squall line were studied. Based on the precipitable water vapor （PWV） data from a ground-based GPS network over the Yangtze River Delta in China, plus data from a Pennsylvania State University/National Atmospheric Center （PSU/NCAR） mesoscale model （MM5） simulation, initialized by three-dimensional variational （3D-VAR） assimilation of the PWV data, some interesting features are revealed. During the 12 hours prior to the squall line arriving in the Shanghai area, a significant increase in PWV indicates a favorable moist environment for a squall line to develop. The vertical profile of the moisture illustrates that it mainly increased in the middle levels of the troposphere, and not at the surface. Temporal variation in PWV is a better precursor for squall line development than other surface meteorological parameters. The characteristics of the horizontal distribution of PWV not only indicated a favorable moist environment, but also evolved a cyclonic wind field for a squall line genesis and development. The ＂＋2 mm＂ contours of the three-hourly PWV variation can be used successfully to predict the location of the squall line two hours later.

THE RELATIONSHIP BETWEEN HORIZONTAL VORTICITY INDUCED BY VERTICAL SHEAR AND VERTICAL MOTION DURING A SQUALL LINE PROCESS

The horizontal vorticity equation used in this study was obtained using the equations of motion in the pressure coordinate system without considering friction, to reveal its relationship with vertical shear. By diagnostically analyzing each term in the horizontal vorticity equation during a squall line process that occurred on 19 June 2010, we found that the non-thermal wind term had a negative contribution to the local change of upward movement in the low-level atmosphere, and that its impact changed gradually from negative to positive with altitude, which could influence upward movement in the mid-and upper-level atmosphere greatly. The contribution of upward vertical transport to vertical movement was the largest in the low-level atmosphere, but had negative contribution to the upper-level atmosphere. These features were most evident in the development stage of the squall line. Based on analysis of convection cells along a squall line, we found that in the process of cell development diabatic heating caused the subsidence of constant potential temperature surface and non-geostrophic motion, which then triggered strong convergence of horizontal acceleration in the mid-level atmosphere and divergence of horizontal acceleration in the upper-level atmosphere. These changes of horizontal wind field could cause a counterclockwise increment of the horizontal vorticity around the warm cell, which then generated an increase of upward movement. This was the main reason why the non-thermal wind term had the largest contribution to the strengthening of upward movement in the mid-and upper-level atmosphere. The vertical transport of large value of horizontal vorticity was the key to trigger convection in this squall line process.

Convective Scale Structure and Evolution of a Squall Line Observed by C-Band Dual Doppler Radar in an Arid Region of Northwestern China

A long-lived and loosely organized squall line moved rapidly across U¨ru¨mqi, the capital city of Xinjiang Uygur Autonomous Region of China on 26 June 2005, generating hail and strong winds. The squall line was observed by a dual Doppler radar system in a field experiment conducted in 2004 and 2005 by the Chinese Academy of Meteorological Sciences and the local meteorological bureau in northwestern China. The 3D wind fields within the squall line were retrieved through dual Doppler analyses and a variational Doppler radar analysis system （VDRAS）. The formation and structure of the squall line as well as the genesis and evolution of embedded convective cells were investigated. During its life period, the squall line consisted of six storm cells extending about 100 km in length, and produced hail of about 25 mm in diameter and strong surface winds up to 11 m s^-1. Radar observations revealed a broad region of stratiform rain in a meso-β cyclone, with the squall line located to the west of this. Two meso-γ scale vortices were found within the squall line. Compared to typical squall lines in moist regions, such as Guangdong Province and Shanghai, which tend to be around 300–400 km in length, have echo tops of 17–19 km, and produce maximum surface winds of about 25 m s^-1 and temperature variations of about 8-C this squall line system had weaker maximum reflectivity （55 dBZ）, a lower echo top （13 km） and smaller extension （about 100 km）, relatively little stratiform rainfall preceding the convective line, and a similar moving speed and temperature variation at the surface.

A Numerical Study of a TOGA-COARE Squall-Line Using a Coupled Mesoscale Atmosphere-Ocean Model

An atmosphere-ocean coupled mesoscale modeling system is developed and used to investigate the interactions between a squall line and the upper ocean observed over the western Paci?c warm pool during the Tropical Ocean/Global Atmosphere Coupled Ocean and Atmosphere Response Experiment (TOGA-COARE). The modeling system is developed by coupling the Advanced Regional Prediction Sys- tem (ARPS) to the Princeton Ocean Model (POM) through precipitation and two-way exchanges of mo- mentum, heat, and moisture across the air-sea interface. The results indicate that the interaction between the squall-line and the upper ocean produced noticeable di?erences in the sensible and latent heat ?uxes, as compared to the uncoupled cases. Precipitation, which is often ignored in air-sea heat ?ux estimates, played a major role in the coupling between the mesoscale convective system and the ocean. Precipitation a?ected the air-sea interaction through both freshwater ?ux and sensible heat ?ux. The former led to the formation of a thin stable ocean layer underneath and behind the precipitating atmospheric convection. The presence of this stable layer resulted in a more signi?cant convection-induced sea surface temperature (SST) change in and behind the precipitation zone. However, convection-induced SST changes do not seem to play an important role in the intsensi?cation of the existing convective system that resulted in the SST change, as the convection quickly moved away from the region of original SST response.

A Modeling Study on the Mechanism of Convective Initiation of a Squall Line over Northeastern China

High-resolution numerical simulation results of a squall line initiated along a convergence zone in northeast China on 26 June 2014 were presented in this study.The simulation was performed by a convection-permitting model with coarse and fine grids of 4 and 1.33 km,respectively,and the simulation results were validated against the observation.Results showed that the simulation adequately reproduced the life cycle of the squall line,which allowed detailed investigation of the mechanism of convective initiation in this case.The synoptic condition was favorable for convective initiation and the convection was triggered in a convergence zone,where a branch of dry and cold air and a branch of moist and warm air collided.The water vapor flux divergence was inhomogeneous and some cores of water vapor convergence existed in the convergence zone.These cores were the spots where water vapor converged intensely and the air there was forced to rise,creating favorable spots where the convection was initially triggered.A series of quasi-equally spaced vortices near the surface,which themselves were the result of horizontal shear instability,were accountable for the inhomogeneity of the surface water vapor flux divergence.These vortices rotated the moist air into their north and dry air into their south,thus creating more favorable spots for convective initiation in their north.After initiation,the updraft turned the horizontal vorticity into vertical vorticity in the mid-level.The vortices near the surface collaborated with the vorticity maxima in the mid-level and enhanced the development of convection by providing water vapor.

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.

Correlations of the Evolution of a CCOPE Squall Line with Surface Thermodynamics and Kinematic Fields

A midlatitude squall line passed over the array of the Cooperative Convective Precipitation Experiment (CCOPE) on 1 August 1981. The structure and evolution of the squall line, and the correlations of the storm with surface thermodynamics and kinematic fields are investigated, mainly by using radar and surface mesonet data in CCOPE. The storm-wide precipitation efficiency is also estimated.The squall line was of an obvious process of metabolism. Thirty-four cells formed successively in front of the primary storm and eventually merged into it during the period 1700-2010 MDT. The newest cells formed near surface equivalent potential temperature maxima, and near surface moisture flux convergence zones or / and the 'temperature break lines'. The thunderstorm rainfall, with the precipitation efficiency of 54%, lags 25-30 min behind the moisture flux convergence on the average.

Characteristics of Cloud-to-ground Lightning during a Squall Line Process outside of the Subtropical High

Based on the monitoring data of cloud-to-ground( CG) lightning positioning network and Doppler weather radar as well as MICAPS1°× 1° objective analysis field,a squall line process outside of the subtropical high in low-latitude plateau on May 7,2010 was analyzed. The results showed that wind direction shear between low and high levels and low-level convergence zones provided favorable circulation background for the strong thunderstorm process,while high energy and high humidity,strong thermal instability and ascending motion at low and middle levels offered beneficial environmental conditions for the formation of the thunderstorm. 9 620 return strokes of cloud-to-ground lightning were monitored by the lightning positioning network,and cloud-to-ground lightning was distributed like bands between 584 and 586 hP a. The occurrence of cloud-to-ground lightning was mainly related to echo top and echo intensity at-10 ℃ stratification height,and it mainly appeared in zones where echo top height was larger than 13 km and echo intensity at-10 ℃ stratification height was 35-40 dB Z. Wind convergence and maintaining of high radial velocity were favorable for the development of convective echoes and occurrence of cloud-to-ground lightning.

The Effects of Strong Ageostrophic Outflows on the Formation of Surface Mesoscale Pressure Systems in Squall Lines

Based on the previous study of the streamline field triggered by singularities in a two-dimensional potential flow,the wind field caused by vorticity lines in an incompressible flow is deduced in this paper.The result shows an elliptic cyclonic（anticyclonic） circulation in association with a positive（negative） vorticity line.By use of the shallow-water model,the flow fields are simulated in a weak wind background under the influence of mesoscale vorticity lines.In the case of two vorticity line,one positive and the other negative,a mesoscale vortex couplet forms in the flow.When three vorticity lines are considered,three mesoscale circulations develop,and a mesohigh and two mesolows similar to the thunderstorm high,wake low and pre-squall mesolow of a mature squall line are produced.Theoretical analysis and numerical simulations show that the formation of the surface mesoscale pressure systems in squall lines may be partly attributed to the dynamical effects of the ageostrophic outflows.The strong downdrafts under the thundercloud base of the squall line lead to surface ageostrophic outflows,and produce positive-negative-positive arranged vertical vorticity bands（VBs） along the direction normal to the squall line,then the mesoscale circulations develop and mesoscale pressure systems form or strengthen during the geostrophic adjustment.By use of the scale separation method,this dynamic mechanism is confirmed by a case study of a severe storm passing over eastern China on 17 June 1974.

东北冷涡过程中的飑线分析

利用哈尔滨站的飑线资料和常规高空、地面观测资料，对冷涡过程中的飑线从天气尺度环流背景、大尺度动力条件和中尺度天气系统等几个方面进行了分析。结果指出：①飑线发生在冷涡发展较强阶段，②飑线发生在冷涡温压场结构不对称性较强的锋区上，③层结不稳定、低层水汽输送与辐合、强烈的上升运动等是必要的天气尺度条件，④地面上有明显的中尺度系统，⑤飑线发生时天气尺度动能明显向中尺度系统转换。

京津冀一次飑线过程的精细时空演变特征分析

基于雷达资料快速更新四维变分同化(RR4DVar)分析系统,利用京津冀6部新一代多普勒天气雷达资料和区域(约700个)自动站资料对2013年7月30—31日京津冀地区一次飑线系统在移动过程中系统不同部位的热动力结构及其环境场时空演变特征进行了分析。结果表明,这次飑线过程是在有利的天气尺度背景形势下发生发展的。飑线形成初期,其中段和南段前部有强的暖湿空气辐合上升运动,并受到强的中层垂直风切变的影响,且在飑线中南段两侧水平正负涡度近似平衡,这种环境下,十分有利于飑线中南段的组织发展。而北段前部受弱的中层垂直风切变及辐散下沉运动影响,不利于北段系统有组织的发展;随着飑线移动下山,中段和南段前部在较强的中层垂直风切变控制下,出现强的暖湿空气辐合上升运动,且存在偏南暖湿气流稳定输送到系统上升运动区中,这对飑线系统中段和南段自身对流单体新生及高度组织化极为有利。随着飑线演变为弓形回波,中段环境场仍维持强的中层垂直风切变且位于冷池出流边界的控制之下,由于地形强迫效应,有利于弓形回波前沿低空偏南暖湿气流的辐合上升,是飑线下山迅速增强并发展成弓形回波,且稳定维持的主要原因。

“0613”华北飑线过程的多普勒雷达回波特征

利用常规气象观测和新型监测资料,分析了2016年6月13—14日华北飑线过程的卫星云图、雷达回波、自动站极大风速风场等特征。结果表明:(1)"0613"飑线过程发生在"上干下湿"的水汽垂直分布环境条件下;在蒙古冷涡影响背景下,河套地区生成的对流云团在前倾结构700 h Pa与850 h Pa冷式切变线之间发展合并,形成有组织的飑线系统。(2)在强盛阶段,飑线具有明显的弱回波通道,飑线西段为偏西风与偏东风形成的气旋式切变,东段为强的反气旋式切变;弓形回波顶点处风向、风速辐合显著,飑线内部后侧倾斜向下的入流急流将中层高动能的干冷空气向地面引导,加强了对流风暴的下沉运动,并与后侧倾斜向下的冷空气入流共同作用加强了飑线前侧的气压梯度,是地面大风形成的主要原因。(3)飑线前部低压暖区生成的对流云泡在自动站极大风速风场切变线附近发展合并形成超级单体风暴,其后侧中高层入流将高动能的干冷空气向地面引导,促使地面出流及风暴前沿辐合抬升运动增强,使得超级单体风暴生命维持较长,是山西长治大冰雹持续近4 h的主要原因。(4)同一飑线系统在不同环境条件下其垂直结构、移速及带来的强天气有明显差异。(5)自动站极大风速切变线的生成较雷暴大风带的出现提前30~40 min,这对飑线大风预警有指示意义。

西北地区强沙尘暴成因的中尺度分析

利用ＧＭＳ静止气象卫星红外资料，对我国西北地区１９９０年以来爆发的６次强沙尘暴天气的成因做了中尺度分析。结果表明，强沙尘暴是由一些中尺度强对流系统形成和发展而造成的。它们包括：中－α尺度飑线系统、中－β尺度强对流系统和中尺度对流复合体。前二者是锋前晴空干热区内形成的干飑线，而后者是由一系列与中尺度对流复合体相关联的飑线造成的。对这些中尺度系统的特征。