The Use of Dual-Doppler Radar Data in the Study of 1998 Meiyu Frontal Precipitationin Huaihe River Basin

During the Meiyu period in June and July of 1998, intensified field observations have been carried out for the project “Huaihe River Basin Energy and Water Cycle Experiment (HUBEX)”. For studying Meiyu front and its precipitation in Huaihe River basin, the present paper has performed analysis on the middle and lower level wind fields in the troposphere by using the radar data obtained from the two Doppler radars located at Fengtai district and Shouxian County. From June 29 to July 3 in 1998, the continuous heavy precipitation occurred in Huaihe River basin around Meiyu front. The precipitation process on July 2 occurred within the observation range of the two Doppler radar in Fengtai district and Shouxian County. The maximum rainfall of the Meiyu front was over 100 mm in 24 h, so it can be regarded as a typical mesoscale heavy precipitation process related to Meiyu front. Based on the wind field retrieved from the dual Doppler radar, we find that there are meso-γ scale vertical circulations in the vertical cross-section perpendicular to Meiyu front, the strong upward motion of which corresponds to the position of the heavy rainfall area. Furthermore, other results obtained by this study are identical with the results by analyzing the conventional synoptic data years ago. For example: in the vicinity of 3 km level height ahead of Meiyu front there exists a southwest low-level jet; the rainstorm caused by Meiyu front mainly occurs at the left side of the southwest low-level jet; and the Meiyu front causes the intensification of the low-level convergence in front of it. Key words Dual Doppler radar - Meiyu front - Meso—γ scale vertical circulation This research was supported by Project HUBEX (Project Number: 49794030) which is funded by the National Natural Science Foundation of China (NSFC).

THE DYNAMICAL ANALYSIS OF THE STRUCTURE OF Β MESO-SCALE DOUBLE RAINBANDS IN MEIYU FRONT

In this paper, an idealized perturbation following the "surge-flow conceptual model" for typical Meiyu frontal structure is designed to explain the β meso-scale structure of rainbands in the Meiyu front using a non-hydrostatic, full-compressible storm-scale model including multi-phase microphysical parameterization. In addition, sensitivity numerical experiment on the vertical distribution of the ambient meridional wind is conducted to investigate the generation mechanism of β meso-scale double rainbands. The results of numerical experiments show that the cool and dry downdraft invading strengthened by the environmental aloft northerly wind plays a very important role to the generation and maintenance of the β meso-scale double rainbands. Moreover, the intensity and scale of the dry and cool downdraft invading are related to the intensity of the second circumfluence induced by mass adjustment when the acceleration of the westerly jet aloft occurs.

Convection Initiation of a Heavy Rainfall Event in the Coastal Metropolitan Region of Shanghai on the South Side of the Meiyu Front

Accurate prediction of the convection initiation(CI)in urban areas is still a challenge.A heavy rainfall event,missed by the 9-km regional operational modeling system,occurred in the coastal urban area of the Shanghai metropolitan region(SMR)in the late morning on 28 July 2020 on the warm side to the south of the Meiyu front.In this study,observational analyses and convection-permitting simulations with a resolution of 3 km were conducted to investigate the CI mechanism of this rainfall event.The results showed that the CI was due to the interaction of urban heat island(UHI),northwesterly outflows from the Meiyu front precipitation system(MFPS),and northeasterly sea winds.First,the UHI created a lifting condition producing adiabatic cooling and the vertical moisture transport in the urban region.Then,the mesolow generated by the UHI induced and enhanced local low-level convergence near the CI region and accelerated the northwesterly outflows and the northeasterly sea winds as they converged to the UHI.The convection was triggered as a result of the strengthened low-level convergence when the enhanced northwesterly outflows and northeasterly sea winds approached the updraft zone caused by the UHI center.Sensitivity experiments with either the urban area of the SMR removed or the MFPS suppressed further revealed that the enhancement of the low-level convergence was mainly contributed by the UHI.The outflows and sea winds transported cold and moist air to the CI region and partly offset the negative contribution of the urban drying effect to the low-level relative humidity to facilitate the development of the deep moist absolute unstable layer during the CI.In addition,the MFPS also contributed to the enhancement of the northeasterly sea winds by influencing the land–sea pressure contrast on the north of the SMR.

STRUCTURAL FEATURES OF THE MEIYU FRONT SYSTEM

A new subtropical front near the periphery of the West Pacific subtropical anticyclone is found,which is never revealed in previous studies.The coupling of the subtropical front and Meiyu front forms a Meiyu front system (MFS) and is the most direct synoptic system for the Meiyu precipitation along the Mid-lower Reaches of Yangtze River (MRYR) in China.In this paper.The detailed structural features and cloud features of the MFS in 1998 and 1999 are analyzed,which manifests that the MFS is an objective phenomenon over the period of Meiyu along MRYR and the Southwest Japan.Generally.the subtropical front is mainly located between 850 hPa and 500 hPa.The moist southwest monsoon is transported in the passageway between the Meiyu front and the subtropical front.The vertical motion ascends in the passageway and descends on both sides of the MFS.forming the MFS's secondary circulation.A lower TBB band indicated that obvious convective activities are also located in the passageway of MFS.The horizontal wind of MFS is vertically asymmetric.

A Simulation Study of the Mesoscale Convective Systems Associated with a Meiyu Frontal Heavy Rain Event

In this study, evolution of the mesoscale convective systems （MCSs） within a Meiyu front during a particularly heavy rainfall event on 22 June 1999 in East China was simulated by using a nonhydrostatic numerical model ARPS （Advanced Regional Prediction System）. Investigations were conducted with emphasis on the impact of the interaction among multi-scale weather systems （MWSs） on the development of MCSs in the Meiyu frontal environment. For this case, the development of MCSs experienced three different stages. （1） The convections associated with MCSs were firstly triggered by the eastward-moving Southwest Vortex （SWV） from the Sichuan Basin, accompanying the intensification of the upper-level jet （ULJ） and the low-level jet （LLJ） that were approaching the Meiyu front. （2） Next, a low-level shear line （LSL） formed, which strengthened and organized the MCSs after the SWV decayed. Meanwhile, the ULJ and LLJ enhanced and produced favorable conditions for the MCSs development. （3） Finally, as the MCSs got intensified, a mesoscale convective vortex （MCV）, a mesoscale LLJ and a mesoscale ULJ were established. Then a coupled-development of MWSs was achieved through the vertical frontal circulations, which further enhanced the MCV and resulted in the heavy rainfall. This is a new physical mechanism for the formation of Meiyu heavy rainfall related to the SWV during the warm season in East China. In the three stages of the heavy rainfall, the vertical frontal circulations exhibited distinguished structures and played a dynamic role, and they enhanced the interaction among the MWSs. A further examination on the formation and evolution of the MCV showed that the MCV was mainly caused by the latent heat release of the MCSs, and the positive feedback between the MCSs and MCV was a key characteristic of the scale interaction in this case.

Typical Structure,Variety,and Multi-Scale Characteristics of Meiyu Front

Meiyu front plays an important role in summer rainfall in central China. Based on the GMS-5 satellite images, NCEP reanalyses （2.5°×2.5°） and final analyses （1°×1°） data, and meteorological conventional sounding observations, the horizontal and vertical structures of the Meiyu front were summarized using multiple diagnostic variables, including winds, temperature, jet stream, front, pseduo-equivalent potential temperature, divergence, vertical motion, static instability, etc. In this paper, four cases were selected and analyzed, two of which are in 26-28 June and 23 July 2002 during the Experiment on Heavy Rain in the Meiyu period in the lower reaches of the Yangtze River, and the others are in May and July 1998. The two cases in July 1998 and July 2002 are the secondary Meiyu front cases. The results show that the structures and characteristics of the Meiyu front are different for various cases, or at various places and time, or at various stages of one case, and the frontal characteristics can be converted from the polar front to the equatorial front. Because of the interaction of the different scale circulations in the high and low latitudes, the horizontal structure of the Meiyu front has various forms. The results in this paper also show that the typical Meiyu front consists of a narrow band with a high gradient of potential equivalent temperature below 500 hPa, south of which is warm and moist air mass, and north of which is the transformed air mass from the midlatitude ocean or polar continent. Below the mid troposphere, south of the front blows southwesterlies, while north blows easterlies. The ascending motion and precipitation usually occur ahead of the Meiyu front. In the upper troposphere, the subtropical front is above the Meiyu front, but two fronts are separated. In addition, the upper westerly jet stream and the easterlies to the south of the Meiyu front result in the upper divergent flow field. The multi-scale characteristics of the horizontal structure of the Meiyu front can be summarized as follows： in the upper troposphere, there exist the subtropical westerly jet, the easterlies to the south of the Meiyu front, and the South Asian high; at the mid troposphere, 500 hPa, the subtropical high over the West Pacific is the main weather system, to the northwest of which there are some short-wave troughs; in the lower troposphere, the planetary-scale southwesterly monsoon, the large-scale low-level southwesterly jet, and the mesoscale vortex or wave in the shear line are closely associated with the Meiyu front.

Numerical Study of a Mesoscale Vortex in the Planetary Boundary Layer of the Meiyu Front

It was found that the heavy rainfall event along the Meiyu front in the lower reaches of the Yangtze River on 23 June 2009 was connected with a mesoscale disturbance vortex, which originated from the planetary boundary layer （PBL） and developed upward later and was discovered by using the Shuman-Shapiro filtering method. The mesoscale disturbance vortex in the PBL （PMDV） in this process corresponded well to the short-time rainstorm in the Doppler radar echo. Analysis of the high-resolution simulation results from the Advanced Weather Research and Forecasting Model （ARW） showed that there were several surface disturbances along the southern warm section of the Meiyu front prior to the generation of the PMDV. The PMDV interacted with the mesoscale convective system （MCS） and intensified the local convective precipitation. The north and southwest flows in the PBL converged at the time of the PMDV formation. Meanwhile, a southwesterly jet on the top of the PBL to the south side of the vortex reinforced the ascending motion and convergence. Hence, it is concluded that the PMDV was generated when the strong cold air flows north of the shear line encountered the southwest flow south of the shear line. The convergence line in the PBL, the intensification of the southwest wind, and the southward aggression of the north wind were critical for the development of the PMDV. The release of latent heat was found crucial for the formation of the PMDV as it facilitated the convergence at low levels.

A Case Study on a Quasi-Stationary Meiyu Front Bringing About Continuous Rainstorms with Piecewise Potential Vorticity Inversion

A 4-day persistent rainstorm resulting in serious flooding disasters occurred in the north of Fujian Province under the influences of a quasi-stationary Meiyu front during 5-8 June 2006. With 1°× 1° latitude and longitude NCEP reanalysis data and the ground surface rainfall, using the potential vorticity （PV） analysis and PV inversion method, the evolution of main synoptic systems, and the corresponding PV and PV perturbation （or PV anomalies） and their relationship with heavy rainfall along the Meiyu front are analyzed in order to investigate the physical mechanism of the formation, development, and maintenance of the Meiyu front. Furthermore, the PV perturbations related to different physics are separated to investigate their different roles in the formation and development of the Meiyu front. The results show： the formation and persistence of the Meiyu front in a quasi-WE orientation are mainly due to the maintenance of the high-pressure systems in its south/north sides （the West Pacific subtropical high/ the high pressure band extending from the Korean Peninsula to east of North China）. The Meiyu front is closely associated with the PV in the lower troposphere. The location of the positive PV perturbation on the Meiyu front matches well with the main heavy rainfall area along the Meiyu front. The PV inversion reveals that the balanced winds satisfying the nonlinear balanced assumption represent to a large extent the real atmospheric flow and its evolution basically reflects the variation of stream flow associated with the Meiyu front. The unbalanced flow forms the convergence band of the Meiyu front and it mainly comes from the high-pressure system in the north side of the Meiyu front. The positive PV perturbation related to latent heat release in the middle-lower troposphere is one of the main factors influencing the formation and development of the Meiyu front. The positive vorticity band from the total balanced winds is in accordance with the Meiyu front band and the magnitude of the positive vorticity from the balanced wind is very close to that from real winds. The PV perturbation in the boundary layer is to a certain degree favorable for the formation and development of Meiyu front. In general, the lower boundary potential temperature perturbation is not beneficial to the formation and development, which is attributed to the relatively low surface temperature due to surface evaporation and solar short-wave radiation reduction shaded by clouds on the Meiyu front band, however, it has some diurnal variation. The effect of PV perturbation in the upper troposphere on the formation and development of the Meiuyu front is relatively weaker than others＇ and not beneficial to the formation and development of the Meiyu front, but it is enhanced in the period of Meiyu front＇s fast southward movement when the deep North China trough develops and moves southeastward. Rest PV perturbation unrelated to latent heat release in the middle-lower troposphere plays a certain role in the Meiyu front＇s fast southward movement. Lastly, it should be pointed out that the different PV perturbations maybe play a different role in different stages of the Meiyu front development.

A Simulation Study on the Characteristics of Cloud Microphysics of Heavy Rainfall in the Meiyu Front

A heavy rainfall in the Meiyu front during 4-5 July 2003 is simulated by use of the non-hydrostatic mesoscale model MM5 （V3-6） with different explicit cloud microphysical parameterization schemes. The characteristics of microphysical process of convective cloud are studied by the model outputs. The simulation study reveals that： （1） The mesoscale model MM5 with explicit cloud microphysical process is capable of simulating the instant heavy rainfall in the Meiyu front, the rainfall simulation could be improved signifi- cantly as the model resolution is increased, and the Goddard scheme is better than the Reisner or Schultz scheme. （2） The convective cloud in the Meiyu front has a comprehensive structure composed of solid, liquid and vapor phases of water, the mass density of water vapor is the largest one in the cloud; the next one is graupel, while those of ice, snow, rain water and the cloud water are almost same. The height at which mass density peaks for different hydrometeors is almost unchangeable during the heavy rainfall period. The mass density variation of rain water, ice, and graupel are consistent with that of ground precipitation, while that of water vapor in the low levels is 1-2 h earlier than the precipitation. （3） The main contribution to the water vapor budget in the atmosphere is the convergence of vapor flux through advection and convection, which provides the main vapor source of the rainfall. Besides the basic process of the auto-conversion of cloud water to rain water, there is an additional cloud microphysical process that is essential to the formation of instant heavy rainfall, the ice-phase crystals are transformed into graupels first and then the increased graupels mix with cloud water and accelerates the conversion of cloud water to rain water. The positive feedback mechanism between latent heat release and convection is the main cause to maintain and develop the heavy precipitation.

Q Vector Analysis of Torrential Rainfall from Meiyu Front Cyclone:A Case Study

Following similar derivation of quasi-geostrophic Q vector （Q^C）, a new Q vector （Q^N） is constructed in this study. Their difference is that the geostrophic wind in quasi-geostrophic Q vector is replaced by the wind in Q^N vector. The diagnostic analysis of Q^N vector is compared with that of Q^G vector in the case study of a typical Meiyu front cyclone （MYFC） occurred over Changjiang-Huaihe regions during 5-6 July 1991. The results show that the Q^N vector has more diagnostic advantages than Q^G vector does. Convergence of Q^N vector at 700 hPa is found to be a good indicator to mimic the horizontal distribution of precipitation. Q^N vector is further partitioned into four components： Q^Nalst （along-stream stretching）,Q^Ncurv （curvature）,Q^Nshdv （shear advection）, and Q^Ncrst （cross-stream stretching） in a natural coordinate system with isohypse （PG partitioning）. The application of Q^N PG partitioning in the MYFC torrential rain indicates that PG partitioning of Q can identify dominant physical processes. The horizontal distribution of 2V·Q^Nalst is similar to that of 2V·Q^N and mainly accounts for 2V·Q^N during the entire period of Meiyu. The effects of Q^Ncurv on rainfall enhancement fade from the mature stage to decay stage. Qshdv enhances precipitation significantly as the MYFC develops, and the effect weakens rapidly when the MYFC decays during its eastward propagation. Q^Ncrst shows little impacts on rainfall during the onset and mature phases whereas it displays significant role during the decay phase.Q^N alst and Q^Nshdv and Q^Ncrst show cancellation only during the decay period.

A Diagnostic Analysis of the Simulated Structure of a Meiyu Front System in 1999

A numerical simulation of a torrential rain event occurring in the Jiang-Huai Valley of China from 22-24 June 1999 is performed and analyzed by using the PSU/NCAR MM5 mesoscale non-hydrostatic model. The high-resolution model output data are utilized to diagnose the double front structure, and the distributions of potential temperature, equivalent potential temperature, and specific humidity in the vicinity of the Meiyu Front System （MYFS） in the Jiang-Huai Valley. The results show that both the potential temperature gradient and the specific humidity gradient have important impacts on the two strong equivalent potential temperature gradient zones associated with the double front structure of the MYFS, but the latter （moisture gradient） is more important. In addition, the tendency equation of specific humidity gradient is theoretically derived. It shows that variations of the specific humidity gradient are related to the advection, convergence/divergence, horizontal and vertical vorticities （secondary circulation） effects and the gradient of water vapor source/sink. As an example, the budget of the meridional component of the tendency equation is selected and diagnosed by using the above model simulation data of the torrential rain event. It is shown that the variation of the specific humidity gradient averaged throughout the simulation is mainly controlled by the convergence/divergence effect, the secondary circulation effect associated with the horizontal vorticities, and the water vapor source/sink effect. Since the water vapor source/sink is often formed from the phase change processes of water vapor in the air and thus directly associated with cloud and precipitation microphysics processes, the variation of the specific humidity gradient is closely related with cloud and precipitation microphysics and the distribution, development and evolution of cloud and rainfall systems. The double front structure of the MYFS provides an advantageous environmental condition for the development and movement of the mesoscale torrential rain system nearby. In turn, the development of the torrential rain exerts a signifiant impact on the MYFS through changing the thermal and moisture distributions.

Dynamic Trigger and Moisture Source of Two Typical Meiyu Front Rainstorms Associated with Eastward-Moving Cloud Clusters from the Tibetan Plateau

Eastward-moving cloud clusters from the Tibetan Plateau(TP)often trigger heavy rainfall events in the Yangtze River basin in summer.Forecasting these events in an operational environment remains a challenging task.Here,dynamical diagnosis and a Lagrangian trajectory model are used to analyze the background atmospheric circulation,maintenance mechanism,and moisture transport of two Meiyu front rainstorms(MYFR)during 30 June-2 July 2016 and 17-19 June 2018 associated with eastward-moving cloud clusters from the TP.It is shown that in both cases heavy rainfall is characterized by semi-continuous rainbelts extending from the eastern TP to the Yangtze River valleys with eastward-spreading convective clouds weakening and strengthening alternately from the eastern TP to downstream regions.Following the track of positive water vapor advection,centers of positive vorticity propagate downstream through the Sichuan basin.The baroclinic thermodynamic–dynamical interaction and the barotropic nonequilibrium force work against each other in the development of the MYFR.Specifically,during the early stage of precipitation development,the barotropic non-equilibrium force dominates,while during the period of heavy precipitation the baroclinic thermodynamic-dynamical interaction dominates.The convergence associated with the baroclinic thermodynamic-dynamical interaction guarantees the persistence of heavy precipitation.Compared to the climate mean state(1988-2018),both MYFR events associated with eastward-moving cloud clusters from the eastern TP are characterized by increased moisture transport from the southwest.One of the major paths of moisture transport in both cases is along the south side of the TP,directly connected to the eastward movement of cloud clusters.

DYNAMIC STUDY ON INFLUENCE OF INERTIAL GRAVITY WAVES INDUCED BY UNBALANCED FLOW ON MEIYU FRONT HEAVY RAIN

The Meiyu front heavy rain process in 1-3 June 2000 is numerically simulated in this paper, and results are then analyzed to show the effects of geostrophic balance collapse,unbalanced flow occurrence,low level jet (LLJ) development,and gravity waves genesis and propagation on the rainstorm.Analyses indicate that the sudden northwest movement of subtropical high may destruct the local geostrophic balance,leading to an increase in the local pressure gradient and the occurrence of ageostrophic flow,and meanwhile the adjustment of circulation starts to build a new balance.During the process,an LLJ and gravity waves appear correspondingly.The dispersion of unbalanced energy through the divergence/convergence of the geostrophic departure winds, promotes the propagation of strong wind cores along the LLJ,and the dispersion direction is influenced by the steering flow and the moisture concentration area.The development of LLJ is one of important conditions,which induces the heavy rain especially in the left front part of the jet where the convergence and shear of winds occur.It is also found that the genesis of disturbance, meso-vortex,and meso-convective system provides a favorable condition for the rainstorm.The above results are clearly illustrated by the high spatial and temporal resolution simulation data from a mesoscale numerical model.

Spatial Propagation of Different Scale Errors in Meiyu Frontal Rainfall Systems

The spatial propagation of meso-and small-scale errors in a Meiyu frontal heavy rainfall event,which occurred in eastern China during 4-6 July 2003,is investigated by using the mesoscale numerical model MM5.In general,the spatial propagation of simulated errors depends on their horizontal scales.Small-scale（L 〈 100 km） initial error may spread rapidly as an isotropic circle through the sound wave.Then,many scattered convection-scale errors are triggered in moist convection zone that will spread abroad through the isotropic,round-shaped sound wave further more.Corresponding to the evolution of the rainfall system,several new convection-scale errors may be generated continuously by moist convection within the propagated round-shaped errors.Through the above circular process,the small-scale error increases in amplitude and grows in scale rapidly.Mesoscale（100 km 〈 L 〈 1000 km） initial error propagates up-and down-stream wavelike through the gravity wave,meanwhile migrating down-stream slowly along with the rainfall system by the mean flow.The up-stream propagation of the mesoscale error is very important to the error growth because it can accumulate error energy locally at a place where there is no moist convection and far upstream from the initial perturbation source.Although moist convection plays an important role in the rapid growth of errors,it has no impact on the propagation of meso-and small-scale errors.The diabatic heating could trigger,strengthen,and promote upscaling of small-scale errors successively,and provide ＂error source＂ to error growth and propagation.The rapid growth of simulated errors results from both intense moist convection and appropriate spatial propagation of the errors.

Case studies on mesoscale structures of heavy rainfall system in the Yangtze River generated by Meiyu front

A heavy rainfall system was observed over the Yangtze River during Meiyu season by using dual-Doppler radar systems in the field experiment conducted by the project of National Fundamental Research Planning “Research on formation mechanism and the prediction theory of hazardous weather over China”. The three-dimensional mesoscale kinematic structure and process of a heavy rain on 22 and 23 July, 2002 are investigated by using the radial velocity and dual-Doppler radar technique. The results show that a southwest-northeast oriented rain band with the length of about 1000 km involves numbers of meso-β or meso-γ-scale convective cells with the size of 20―50 km. The heavy rain band in the middle reaches of the Yangtze River ex-ists in a low-level wind shear. The interaction between southwest low-level jet on the southern side of wind shear and east wind on the northern side formed the updraft. The wind disturbance, wind shear and convergence generate the convective action. The new cell developed in right rear flank of rain band and moved to the southwest wind area, where the vapor is abundant. This kind of echo can last a long time and developed well. The strong convective echoes are often ac-companied by the meso-γ-scale vortex and convergence.

Study on the Dynamic Characteristics of an Eastward-offshore Mesoscale Vortex along the Meiyu-Baiu Front

The eastward-moving Meiyu-Baiu frontal mesoscale vortices (MBFMVs) appear frequently and often cause heavy rainfall events along their tracks. A move-off-shore MBFMV was selected to enhance our understanding of this type of vortex. Synoptic analyses indicate that the MBFMV is a type of meso-α vortex and mainly occurs in the lower troposphere. A short wave trough near the coastline is highly favorable for the formation, sustainment, and displacement of the MBFMV. Vorticity budgets indicate that at lower levels of the MBFMV, convergence is the dominant factor for the increase of positive vorticity, and at high levels of the MBFMV, the vertical transportation associated with convective activities is the most important factor. The hori-zontal transportation was the main factor decreasing the positive vorticity. The land and sea environments are crucial to the evolution of the MBFMV. The characteristics of the Meiyu-Baiu Front (MBF) are also vital to the variation of the vortex.

INTERACTION BETWEEN TROPICAL CYCLONE AND MEIYU FRONT

Generally speaking,the convection activities are inactive over western Pacific warm pool and tropical cyclone(TC)activity seldom occurs over the offshore of East Asia during the period of Meiyu rainfall.However,if a TC is active in this area,the Meiyu rainfall will often weaken or end up. Based on a statistical study with the data from 1980 to 1995,it is found that about 91% of 23 TC activities affected the intensity of Meiyu rainfall,and 50% of the end-up of Meiyu events were related to the active TCs and the change of subtropical high.The present paper simulates the effect of TC on Meiyu circulation by using MM4 model,and the results agree with the observations. From the point of view of vapor and energy transport,the landing of TC cuts not only the transport of the water vapor to Changjiang-Huaihe River basin from the Bay of Bengal but also the conversion of the mean flow energy to the Meiyu circulation because of the TC forcing to the zonal circulation.These two effects make the convection and perturbation existing in Meiyu region lack the supply of the vapor and energy for their maintenance and lead to the end of Meiyu rainfall.

MOIST POTENTIAL VORTICITY DIAGNOSIS OF A MEIYU FRONTAL HEAVY RAIN PROCESS SIMULATION DURING SUMMER,1991

In this paper,the heavy rain process from June 30 to July 2,1991,has been simulated by MM4. and three-dimensional moist potential vorticity distribution of the simulation results has been calculated.It is shown that moist potential vorticity is an important physical variable to reveal heavy rain structure and dynamic mechanisms.Negative moist potential vorticity corresponds to the Meiyu front-wind shear line system and the negative center corresponds to the heavy rain center.Negative moist potential vorticity mainly attributes to the effects of meridional baroclinic term and convective unstable term.The former is favourable to the maintenance of zonal precipitation and the latter is the mechanism of the heavy rain center propagating along the rain belt.The heavy rain is contributed by the cooperative effects of conditional convective instability,baroclinic instability and upper air inertial instability.

Persistence of Snow Cover Anomalies over the Tibetan Plateau and the Implications for Forecasting Summer Precipitation over the Meiyu-Baiu Region

The present reported study investigated the persistence of snow anomalies over the Tibetan Plateau(TP) from the preceding seasons to summer and the relationship between the previous snow cover anomaly and summer precipitation over East Asia. The results showed that, relative to other snow indices, such as the station observational snow depth(SOSD) index and the snow water equivalent(SWE) index, the snow cover area proportion(SCAP) index calculated from the SWE and the percentage of visible snow of the Equal-Area Scalable Earth Grids(EASE-grids) dataset has a higher persistence in interannual anomalies, particularly from May to summer. As such, the May SCAP index is significantly related to summer precipitation over the Meiyu-Baiu region. The persistence of the SCAP index can partly explain the season-delayed effect of snow cover over the TP on summer rainfall over the Meiyu-Baiu region besides the contribution of the soil moisture bridge. The preceding SST anomaly in the tropical Indian Ocean and ENSO can persist through the summer and affect the summer precipitation over the Meiyu-Baiu region. However, the May SCAP index is mostly independent of the simultaneous SSTs in the tropical Indian Ocean and the preceding ENSO and may affect the summer precipitation over the Meiyu-Baiu region independent of the effects of the SST anomalies. Therefore, the May SCAP over the TP could be regarded as an important supplementary factor in the forecasting of summer precipitation over the Meiyu-Baiu region.

Numerical Simulations of Intense Meiyu Rainfall in 1991 over the Changjiang and Huaihe River Valleys by a Regional Climate Model with p-σ Incorporated Coordinate System

Based on the primitive equation model with p- σ incorporated coordinate system originally developed by Qian et al., a one-way nested fine mesh limited area model is developed. This model is nested with ECMWF T42 data to simulate the extra-intensive rainfall event occurring in the Changjiang and Huaihe River valleys in summer of 1991. The results show that the model has certain capacity to fairly reproduce the regional distribution and the movement of the main rainfall belts. Therefore it can be used as a regional climate model to simulate and predict the short-range regional climate changes.