Seasonal variation of mesoscale eddy intensity in the global ocean

Mesoscale eddies are a prominent oceanic phenomenon that plays an important role in oceanic mass transport and energy conversion.Characterizing by rotational speed,the eddy intensity is one of the most fundamental properties of an eddy.However,the seasonal spatiotemporal variation in eddy intensity has not been examined from a global ocean perspective.In this study,we unveil the seasonal spatiotemporal characteristics of eddy intensity in the global ocean by using the latest satellite-altimetry-derived eddy trajectory data set.The results suggest that the eddy intensity has a distinct seasonal variation,reaching a peak in spring while attaining a minimum in autumn in the Northern Hemisphere and the opposite in the Southern Hemisphere.The seasonal variation of eddy intensity is more intense in the tropical-subtropical transition zones within latitudinal bands between 15°and 30°in the western Pacific Ocean,the northwestern Atlantic Ocean,and the eastern Indian Ocean because baroclinic instability in these areas changes sharply.Further analysis found that the seasonal variation of baroclinic instability precedes the eddy intensity by a phase of 2–3 months due to the initial perturbations needing time to grow into mesoscale eddies.

Mesoscale and Microphysical Characteristics of a Double Rain Belt Event in South China on May 10–13,2022

A second rain belt sometimes occurs ahead of a frontal rain belt in the warm sector over coastal South China,leading to heavy precipitation.We examined the differences in the mesoscale characteristics and microphysics of the frontal and warm sector rain belts that occurred in South China on May 10–13,2022.The southern rain belt occurred in an environment with favorable mesoscale conditions but weak large-scale forcing.In contrast,the northern rain belt was related to low-level horizontal shear and the surface-level front.The interaction between the enhanced southeasterly winds and the rainfall-induced cold pool promoted the persistent growth of convection along the southern rain belt.The convective cell propagated east over the coastal area,where there was a large temperature gradient.The bow-shaped echo in this region may be closely related to the rear-inflow jet.By contrast,the initial convection of the northern rain belt was triggered along the front and the region of low-level horizontal shear,with mesoscale interactions between the enhanced warm-moist southeasterly airflow and the cold dome associated with the earlier rain.The terrain blocked the movement of the cold pool,resulting in the stagnation of the frontal convective cell at an early stage.Subsequently,a meso-γ-scale vortex formed during the rapid movement of the convective cell,corresponding to an enhancement of precipitation.The representative raindrop spectra for the southern rain belt were characterized by a greater number and higher density of raindrops than the northern rain belt,even though both resulted in comparable hourly rainfalls.These results help us better understand the characteristics of double rain belts over South China.

Mesoscale study on explosion-induced formation and thermochemical response of PTFE/Al granular jet

The dynamic formation,shock-induced inhomogeneous temperature rise and corresponding chemical reaction behaviors of PTFE/Al reactive liner shaped charge jet(RLSCJ)are investigated by the combination of mesoscale simulation,reaction kinetics and chemical energy release test.A two-dimensional granular model is developed with the randomly normal distribution of aluminum particle sizes and the particle delivery program.Then,the granular model is employed to study the shock-induced thermal behavior during the formation and extension processes of RLSCJ,as well as the temperature history curves of aluminum particles.The simulation results visualize the motion and temperature responses of the RLSCJ at the grain level,and further indicate that the aluminum particles are more likely to gather in the last two-thirds of the jet along its axis.Further analysis shows that the shock,collision,friction and deformation behaviors are all responsible for the steep temperature rise of the reactive jet.In addition,a shock-induced chemical reaction extent model of RLSCJ is built based on the combination of the Arrhenius model and the Avrami-Erofeev kinetic model,by which the chemical reaction growth behavior during the formation and extension stages is described quantitatively.The model indicates the reaction extent highly corresponds to the aluminum particle temperature history at the formation and extension stages.At last,a manometry chamber and the corresponding energy release model are used together to study the macroscopic chemical energy release characteristics of RLSCJ,by which the reaction extent model is verified.

Review on the mesoscale characterization of cement-stabilized macadam materials

The base layer constructed by cement-stabilized macadam(CSM)has been widely used in highway construction due to its low elasticity deformation and high carrying capacity.As a bearing layer,the CSM base is not exempt from fatigue cracking under cyclic loading in the service process.Cracks in the base will create irreversible structural and functional deficiencies,such as the potential for reflective cracking of subsequently placed asphalt concrete overlays.The fracture of the base will shorten the service life of the pavement.The quality of the CSM base is directly related to the bearing capacity and integrity of the whole pavement structure.It is of practical significance to further study the fatigue failure behavior of CSM material for the long-term performance of the pavement.The CSM material is a typical heterogeneous multiphase composite.On the mesoscale,CSM consists of aggregate,cement mortar,pores,and the interface transitional zone(ITZ).On the microscale,the hardened mortar contains a large number of capillary pores,unhydrated particles,hydrated crystals,etc.,which makes the spatial distribution of its material properties stochastic.In addition,cement hydration,dry shrinkage,and temperature shrinkage can also produce micro-crack defects in cement mortar.These microcracks will have crossscale evolution under load,resulting in structural fracture.Macroscopic complex deformation and mechanical response are the reflections of its microscopic and even mesoscale composition and structure.This study summarized the existing studies on the mesoscopic properties of CSM materials,respectively from the three aspects of mesostructure,structural characterization,and mesoscale fatigue damage analysis,to help the development of long-life pavement.The future research direction is to explore the mesoscale characteristics of CSM using multiscale representation and analysis methods,to establish the connection between mesoscale characteristics and macroscopic mechanical properties.

The Initial Mesoscale Vortexes Leading to the Formation of Tropical Cyclones in the Western North Pacific

A statistical analysis of the initial vortexes leading to tropical cyclone(TC)formation in the western North Pacific(WNP)is conducted with the ECMWF ERA5 reanalysis data from 1999 to 2018.It is found that TCs in the WNP basically originate from three kinds of vortexes,i.e.,a mid-level vortex(MV),a low-level vortex(LV),and a relatively deep vortex with notable vorticity in both the lower and middle troposphere(DV).Among them,LV and DV account for 47.9%and 24.2%of tropical cyclogenesis events,respectively,while only 27.9%of TCs develop from the MV,which is much lower than that which occurs in the North Atlantic and eastern Pacific.Such a difference might be ascribed to the active monsoon systems in the WNP all year round.Due to the nearly upright structure of mid-level convergence in the early pre-genesis stage,TC genesis efficiency is the highest in DV.Compared with MV,LV generally takes a shorter time to intensify to a TC because of the higher humidity and the stronger low-level cyclonic circulation,which is related to air-sea interaction and boundary-layer convergence.Further examination of the relationship between tropical cyclogenesis and large-scale flow patterns indicate that the TC genesis events associated with LV are primarily related to the monsoon shear line,monsoon confluence region,and monsoon gyre,while those associated with MV are frequently connected with easterly waves and wave energy dispersion of preexisting TC.Compared with other flow patterns,tropical cyclones usually form and intensify faster in the monsoon confluence region.

The Major Research Advances of Mesoscale Weather Dynamics in China Since 2003

This paper reviews the main theoretical progress of mesoscale weather dynamics since 2003, including： （1） The dynamic mechanisms of balanced and unbalanced flow are applied to study the genesis and development problems of mesoscale circulation. The symmetric instability and transverse-wave instability are analyzed in line and vortex atmosphere convection, and further research has been done on nonlinear convective symmetric instability. The interaction between forced convection and unstable convection and the wave characteristics of mesoscale motion are also discussed. （2） Intermediate atmosphere dynamic boundary layer models are developed. The complicated nonlinear interaction is analyzed theoretically between the atmospheric boundary layer and the free atmosphere. The structure of the topography boundary layer, atmospheric frontogenesis, the structure and circulation of the low-level front and other boundary layer dynamic problems are discussed. （3） The formation and development of meso-β-scale rainstorms under the background of the East-Asia atmosphere circulation are diagnosed with the variation of MPV （moist potential vorticity） anomalies. And some physical vectors are modified and applied in the moist atmosphere.

Study on a Mesoscale Convective Vortex Causing Heavy Rainfall during the Mei-yu Season in 2003

The strong heavy rainfall on 3–5 July 2003 causing the severe flooding in Huaihe River basin （HRB）, China is studied. It is noted that there are sometimes mesoscale convective vortex （MCV） in East Asia during the mei-yu season. Simulation results from the ARPS （Advanced Regional Prediction） data analysis system （ADAS） and WRF model were used to study the development of the mesoscale convective system （MCS） and mesoscale convective vortex （MCV）. It is confirmed that the MCV formed during the development of a previous severe MCS. A closed vortex circulation can be found below 600 hPa with a vorticity maximum in the middle troposphere. The evolution process of the MCV can be divided into three stages： initiation, maturation, and dissipation. During the mature stage of the MCV, a downdraft occurred in the center of the MCV and new convection developed in southeast of the MCV. The convergence and the tilting in the lower troposphere convergence and vertical advection in the middle troposphere were the main vorticity sources in the MCV initiation stage. Finally, a conceptual model between the mei-yu front and the embedded MCS and MCV is proposed. The mei-yu front was the background condition for the development of the MCS and MCV. A low level jet （LLJ） transported moisture and the weak cold air invasion via a trough aloft in the middle troposphere and triggering the severe convection. Furthermore, the intensified jet was able to result in the initiation of new ＂secondary＂ areas of convection in the eastern part of the MCV.

A Review of Major Progresses in Mesoscale Dynamic Research in China since 1999

Mesoscale research conducted by Chinese meteorologists during the past four years is reviewed. Advances in theoretical studies include (a) mesoscale quasi-balanced and semi-balanced dynamics, derived through scale analysis and the perturbation method which are suitable for describing mesoscale vortices; (b) subcritical instability and vortex-sheet instability; (c) frontal adjustment mechanism and the effect of topography on frontgenesis; and (d) slantwise vorticity development theories, the slantwise vortex equation, and moist potential vorticity (MPV) anomalies with precipitation-related heat and mass sinks and MPV impermeability theorem. Prom the MPV conservation viewpoint, the transformation mechanism between different scale weather systems is analyzed. Based on the data analysis, a new dew-point front near the periphery of the West Pacific subtropical high is identified. In the light of MPV theory and Q-vector theory, some events associated with torrential rain systems and severe storms are analyzed and diagnosed. Progress in mesoscale numerical simulation has been made in the development of meso-α, meso-β vortices, meso-γ-scale downbursts and precipitation produced by deep convective systems with MM5 and other mesoscale models.

Vorticity Budget Investigation of a Simulated Long-Lived Mesoscale Vortex in South China

A vorticity budget investigation is performed using the output data from a numerical simulation of a typical MCV (mesoscale convectively generated votex) case in South China. Results suggest that the divergence caused by convection in the low troposphere is the main producer of positive vorticity, while vertical vorticity transferred by the tilting term from the horizontal vorticity compensates the upward output of cyclonic vorticity. Scale analyses of the vorticity equation suggest that the advection of planetary vorticity can be neglected owing to the low latitude, which is di?erent from the larger scale systems in high latitude areas. In addition, the distribution of relative vorticity tendency on pressure level is not uniform. A vortex will move along the vector from the negative to the positive vorticity tendency region. The mechanism of the phenomenon—that nearly all of the convectively ascending region is located southward/southeastward of the vortex center—is also discussed. Convergence with regard to latent heat release would be in favor of the spin-up of meso-vortex, however, the horizontal vorticity caused by wind shear is tilted by vertical motion due to convection. Consequently, the negative and positive vorticity tendencies are located symmetrically about the convective center, which suggests that the vortex southward movement is dynamically driven by convection.

Three-dimensional properties of mesoscale cyclonic warm-core and anticyclonic cold-core eddies in the South China Sea

In general,a mesoscale cyclonic(anticyclonic)eddy has a colder(warmer)core,and it is considered as a cold(warm)eddy.However,recently research found that there are a number of"abnormal"mesoscale cyclonic(anticyclonic)eddies associated with warm(cold)cores in the South China Sea(SCS).These"abnormal"eddies pose a challenge to previous works on eddy detection,characteristic analysis,eddy-induced heat and salt transports,and even on mesoscale eddy dynamics.Based on a 9-year(2000–2008)numerical modelling data,the cyclonic warm-core eddies(CWEs)and anticyclonic cold-core eddies(ACEs)in the SCS are analyzed.This study found that the highest incidence area of the"abnormal"eddies is the northwest of Luzon Strait.In terms of the eddy snapshot counting method,8620 CWEs and 9879 ACEs are detected,accounting for 14.6%and 15.8%of the total eddy number,respectively.The size of the"abnormal"eddies is usually smaller than that of the"normal"eddies,with the radius only around 50 km.In the generation time aspect,they usually appear within the 0.1–0.3 interval in the normalized eddy lifespan.The survival time of CWEs(ACEs)occupies 16.3%(17.1%)of the total eddy lifespan.Based on two case studies,the intrusion of Kuroshio warm water is considered as a key mechanism for the generation of these"abnormal"eddies near the northeastern SCS.

A Study on the Dynamic Mechanism of the Formation of Mesoscale Vortex in Col Field

The mesoscale vortex associated with a mesoscale low-level jet （mLLJ） usually causes heavy rainfall in the col field. The col field is defined as a region between two highs and two lows, with the isobaric surface similar to a col. Using a two-dimensional shallow water model, the meso-β scale vortex couplets （MβVCs） induced by eight types of mesoscale wind perturbations in an ideal col field were numerically simulated. With the sizes of -100 km, the MβVCs induced by northerly perturbation （NP） and southerly perturbation （SP） moved toward the col point. The sizes of MβVCs induced by southwesterly perturbation （SWP）, southeasterly perturbation （SEP）, northwesterly perturbation （NWP）, and northeasterly perturbation （NEP） were relatively small for the perturbations moving toward dilatation axis. The MβVC induced by easterly perturbation （EP） and westerly perturbation （WP） could not develop because they quickly moved away from the col point, before the circulation could form. The size of the circulation was determined by the distance between the vortex and the col point. The closer to the col point the vortex was, the larger the size of vortex. The comparisons of maximum vorticity and vorticity root mean square error （RMSE） of the NP, the SWP, and the WP show that the maximum vorticity and the vorticity RMSE of the NP decreased slower than other perturbations. Therefore, the weak environment of the col field favors the maintenance of vorticity and the formation of vortex. When a mesoscale vortex forms near the col point or moves toward the col point, it may maintain a quasitationary state in the stable col field.

Initiation and Evolution of Long-Lived Eastward-Propagating Mesoscale Convective Systems over the Second-Step Terrain along Yangtze-Huaihe River Valley

Based on the previous statistical analysis of mesoscale convective systems(MCSs)over the second-step terrain along Yangtze-Huaihe River Valley,eight representative long-lived eastward-propagating MCSs are selected for model-based sensitivity testing to investigate the initiation and evolution of these types of MCSs as well as their impact on downstream areas.We subject each MCS to a semi-idealized(CNTL)simulation and a sensitivity(NOLH)simulation that neglects condensational heating in the formation region.The CNTL experiment reveals convection forms in the region downstream of a shortwave trough typified by persistent southwesterly winds in the low-to midtroposphere.Upon merging with other convective systems,moist convection develops into an MCS,which propagates eastward under the influence of mid-tropospheric westerlies,and moves out of the second-step terrain.The MCS then merges with pre-existing local convection over the plains;the merged convection reinforces the cyclonic wind perturbation into a mesoscale vortex at 850 hPa.While this vortex moves eastward to regions with local vortex at 850 hPa,another vortex at 925 hPa is also intensified.Finally,the vortices at 850 and 925 hPa merge together and develop into a mesoscale convective vortex(MCV).In contrast,MCSs fail to form and move eastward in the NOLH experiment.In the absence of eastward-propagating MCSs,moist convection and mesoscale vortices still appear in the plains,but the vortex strength and precipitation intensity are significantly weakened.It is suggested the eastward-propagating MCSs over the second-step terrain significantly impact the development and enhancement of moist convection and vortices in the downstream areas.

ANALYSIS OF MESOSCALE CONVECTIVE SYSTEMS ASSOCIATED WITH A WARM-SECTOR RAINSTORM EVENT OVER SOUTH CHINA

With multiple meteorological data, including precipitation from automatic weather stations, integrated satellite-based precipitation (CMORPH), brightness temperature (TBB), radar echoes and NCEP reanalysis, a rainstorm event, which occurred on May 26, 2007 over South China, is analyzed with the focus on the evolution characteristics of associated mesoscale-β convective systems (Mβcss). Results are shown as follows. (1) The rainstorm presents itself as a typical warm-sector event, for it occurs within a surface inverted trough and on the left side of a southwesterly low-level jet (LLJ), which shows no obvious features of baroclinicity. (2) The heavy rainfall event is directly related to at least three bodies of Mβcss with peak precipitation corresponding well to their mature stages. (3) The Mβcss manifest a backward propagation, which is marked with a new form of downstream convection different from the more usual type of forward propagation over South China, i.e., new convective systems mainly form at the rear part of older Mβcss. (4) Rainstorm-causing Mβcss form near the convergence region on the left side of an 850-hPa southwesterly LLJ, over which there are dominantly divergent air flows at 200 hPa. Different from the typical flow pattern of outward divergence off the east side of South Asia High, which is usually found to be over zones of heavy rains during the annually first rainy season of South China, this warm-sector heavy rain is below the divergence region formed between the easterly and southerly flows west of the South Asian High that is moving out to sea. (5) The LLJ transports abundant amount of warm and moist air to the heavy rainfall area, providing advantageous conditions for highly unstable energy to generate and store at middle and high levels, where corresponding low-level warm advection may be playing a more direct role in the development of Mβcss. As a triggering mechanism for organized convective systems, the effect of low-level warm advection deserves more of our attention. Based on the analysis of surface mesoscale airflow in the article, possible triggering mechanisms for Mβcss are also discussed.

Comparative Studies of Different Mesoscale Convection Parameterization Schemes in the Simulation of Mei-Yu Front Heavy Rain

The mei-yu front heavy rainstorms occurred over Nanjing on 3 5 and 8 9 July 2003 and were simulated in this paper using the Weather Research and Forecasting Model (WRFv3.1) with various mesoscale convection parameterization schemes (MCPSs). The simulations show that the temporal and spatial evolution and distribution of rainstorms can be modeled; however, there was incongruity between the comparative simulations of four different MCPSs and the observed data. These disparities were exhibited in the simulations of both the 24-hour surface rainfall total and the hourly precipitation rate. Further analysis revealed that the discrepancies of vertical velocity and the convective vorticity vector (CVV) between the four simulations were attributed to the deviation of rainfall values. In addition, the simulations show that the mid-scale convection, particularly the mesoscale convection system (MCS) formation, can be well simulated with the proper mesoscale convection parameterization schemes and may be a crucial factor of the mei-yu front heavy rainstorm. These results suggest that, in an effort to enhance simulation and prediction of heavy rainfall and rainstorms, subsequent studies should focus on the development and improvement of MCPS.

Eddy diffusivity and coherent mesoscale eddy analysis in the Southern Ocean

The spatial distribution of eddy diffusivity,basic characteristics of coherent mesoscale eddies and their relationship are analyzed from numerical model outputs in the Southern Ocean.Mesoscale fluctuation information is obtained by a temporal-spatial filtering method,and the eddy diffusivity is calculated using a linear regression analysis between isoneutral thickness flux and large-scale isoneutral thickness gradient.The eddy diffusivity is on the order of O(103 m2/s)with a significant spatial variation,and it is larger in the area with strong coherent mesoscale eddy activity.The mesoscale eddies are mainly located in the upper ocean layer,with the average intensity no larger than 0.2.The mean radius of the coherent mesoscale cyclonic(anticyclonic)eddy gradually decays from(121.2±10.4)km((117.8±9.6)km)at 30°S to(43.9±5.3)km((44.7±4.9)km)at 65°S.Their vertical penetration depths(lifespans)are deeper(longer)between the northern side of the Subpolar Antarctic Front and 48°S.The normalized eddy diffusivity and coherent mesoscale eddy activity show a significant positive correlation,indicating that coherent mesoscale eddy plays an important role in eddy diffusivity.

The Effect of Mesoscale Flows on Regional Atmospheric Transport in a Complex Terrain

Model results simulated over a complex terrain under a synoptically calm condition, using a three-dimensional (3-D) regional-scale meteorological acid deposition model (RMADM), show that thermally induced mesoscale circulations (MCs): sea-and land-breeze circulations and up- and down-slope flow circulations play a fundamental role in determining how the pollutants being dispersed . Analysis showed that under synoptically calm condition, the role played by the MC would dilute the smoke released during the early stage of the emission; the accumulation, however, would become important if the synoptically calm condition lasts long. Since the structure and intensity of the MCs depend on geography and topographical allocation, land surface coverage, incoming solar radiation intensity and so on, it makes the estimates of source-reception relationship and long-range atmospheric dispersion more difficulty. It concluded that it is impossible for a pollution model to correctly simulate smokes transport using only the synoptic station data, since the mesoscale information can not be resolved from these datasets.

Influence of mesoscale eddies on primary production in the South China Sea during spring inter-monsoon period

Mesoscale eddies have been suggested to have an impact on biological carbon fixation in the South China Sea （SCS）. However, their overall contribution to primary production during the spring inter-monsoon pe riod is still unknown. Based on large-scale biological and environmental in situ observations and synchro nous remote sensing data, the distribution patterns of phytoplankton biomass and the primary production, and the role of mesoscale eddies in regulating primary production in different eddy-controlled waters were investigated. The results suggested that the surface chlorophyll a concentrations and water column inte grated primary production （IPP） are significantly higher in cyclonic eddies and lower in the anticyclonic eddies as compared to that in non-eddy waters. Although eddies could affect various environmental factors, such as nutrients, temperature and light availability, nutrient supply is suggested to be the most important one through which mesoscale eddies regulated the distribution patterns of phytoplankton biomass and pri mary production. The estimated IPP in cyclonic and anticyclonic eddies are about 29.5% higher and 16.6% lower than the total average in the whole study area, respectively, indicating that the promotion effect of mesoscale cold eddies on the primary production was much stronger than the inhibition effect of the warm eddies per unit area. Overall, mesoscale eddies are crucial physical processes that affect the biological car bon fixation and the distribution pattern of primary production in the SCS open sea, especially during the spring inter-monsoon period.

A statistical analysis of mesoscale eddies in the Bay of Bengal from 22–year altimetry data

Eddy properties in the Bay of Bengal are studied from 22 a archiving, validation and interpretation of satellite oceanographic（AVISO） data using a sea level anomaly（SLA）-based eddy identification. A geographical distribution and an eddy polarity, an eddy lifetime and propagation distances, eddy origins and terminations,eddy propagation directions and trajectories, eddy kinetic properties, the evolution of eddy properties,seasonal and interannual variabilities of eddy activities are analyzed in this area. Eddies exist principally in the western Bay of Bengal and most of them propagate westward. The polarity distribution of eddies shows cyclones prefer to occur in the northwest and south of the Bay of Bengal, while anticyclones mainly occur in the east of the bay. Five hundred and sixty-five cyclones and 389 anticyclones with the lifetime that exceeds 30 d are detected during the 22 a period, and there is a preference for the cyclones for all lifetime and propagation distances. The kinetic properties of all observed eddies show the average amplitude of the cyclones is larger than that of the anticyclones, whereas that is opposite for average radius, and their average velocities are basically the same. Moreover, the evolution of eddies properties reveals that the eddies with a long lifetime that exceeds 90 d have a significant double-stage feature of the former 50 d growth period and the dying period after 50 d. For the seasonal variability of the eddies, the cyclones occur more often in spring while the anticyclones occur more often in summer. The analysis of long-lived eddy seasonal distributions shows that there is the obvious seasonal variation of the eddy activities in the Bay of Bengal. The interannual variability of an eddy number shows an obvious negative correlation with the EKE variation.

The Influences of Boundary Layer Parameterization Schemes on Mesoscale Heavy Rain System

The mesoscale numerical weather prediction model (MM4) in which the computations of the turbulent exchange coefficient in the boundary layer and surface fluxes are improved, is used to study the influences of boundary layer parameterization schemes on the predictive results of the mesoscale model. Seven different experiment schemes (including the original MM4 model) designed in this paper are tested by the observational data of several heavy rain cases so as to find an improved boundary layer parameterization scheme in the mesoscale meteorological model. The results show that all the seven different boundary layer parameterization schemes have some influences on the forecasts of precipitation intensity, distribution of rain area, vertical velocity, vorticity and divergence fields, and the improved schemes in this paper can improve the precipitation forecast. Key words Boundary layer parameterization - Mesoscale numerical weather prediction (MNWP) - Turbulent exchange coefficient - Surface fluxes - Heavy rain This paper was supported by the National Natural Science Foundation of China (Grant No. 49875005 and No. 49735180).

Evolution of the Total Lightning Activity in a Leading-Line and Trailing Stratiform Mesoscale Convective System over Beijing

Data from the Beijing SAFIR 3000 lightning detection system and Doppler radar provided some insights into the three-dimensional lightning structure and evolution of a leading-line and trailing-stratiform （LLTS） mesoscale convective system （MCS） over Beijing on 31 July 2007. Most of the lightning in the LLTS-MCS was intracloud （IC） lightning, while the mean ratio of positive cloud-to-ground （＋CG） lightning to –CG lightning was 1：4, which was higher than the average value from previous studies. The majority of CG lightning occurred in the convective region of the radar echo, particularly at the leading edge of the front. Little IC lightning and little ＋CG lightning occurred in the stratiform region. The distribution of the CG lightning indicated that the storm had a tilted dipole structure given the wind shear or the tripole charge structure. During the storm’s development, most of the IC lightning occurred at an altitude of ～9.5 km; the lightning rate reached its maximum at 10.5 km, the altitude of IC lightning in the mature stage of the storm. When the thunderstorm began to dissipate, the altitude of the IC lightning decreased gradually. The spatial distribution of lightning was well correlated with the rainfall on the ground, although the peak value of rainfall appeared 75 min later than the peak lightning rate.