Study on Moist Potential Vorticity and Symmetric Instabilityduring a Heavy Rain Event Occurred inthe Jiang-Huai Valleys

In the light of the theory on moist potential vorticity (MPV) investigation was undertaken of the 700 hPa vertical (horizontal) component MP1 (MPV2) for the heavy rain event occurring in July 5–6, 1991. Results show that the distribution features of the two components were closely related to the development of a mesoscale cyclone as a rainstorm-causing weather system in the lower troposphere in such a way that the ambient atmosphere of which MPV1 > 0 and MPV2 < 0 with |MPV1| ≥ |MPV2| favored the genesis of conditional symmetric instability (CSI) and that, as indicated by calculations, a CSI sector was really existent in the lower troposphere during the heavy rain happening and contributed greatly to its development.

Impacts of Cloud-Induced Mass Forcing on the Development of Moist Potential Vorticity Anomaly During Torrential Rains

The impacts of cloud-induced mass forcing on the development of the moist potential vorticity (MPV) anomaly associated with torrential rains are investigated by using NCEP/NCAR 1? × 1? data. The MPV tendency equation with the cloud-induced mass forcing is derived, and applied to the torrential rain event over the Changjiang River-Huaihe River Valleys during 26–30 June 1999. The result shows that positive anomalies are located mainly between 850 hPa and 500 hPa, while the maximum MPV, maximum positive tendency of the MPV, and maximum surface rainfall are nearly collocated. The cloud-induced mass forcing contributes to the positive tendency of the moist potential vorticity anomaly. The results indicate that the MPV may be used to track the propagation of rain systems for operational applications.

Sensitivity of Cyclone Tracks to the Initial Moisture Distribution:A Moist Potential Vorticity Perspective

In this study, the characteristics of moist potential vorticity （MPV） in the vicinity of a surface cyclone center and their physical processes axe investigated. A prognostic equation of surface absolute vorticity is then used to examine the relationship between the cyclone tracks and negative MPV （NMPV） using numerical simulations of the life cycle of an extratropical cyclone. It is shown that the MPV approach developed herein, i.e., by tracing the peak NMPV, can be used to help trace surface cyclones during their development and mature stages. Sensitivity experiments are conducted to investigate the impact of different initial moisture fields on the effectiveness of the MPV approach. It is found that the lifetime of NMPV depends mainly on the initial moisture field, the magnitude of condensational heating, and the advection of NMPV. When NMPV moves into a saturated environment at or near a cyclone center, it can trace better the evolution of the surface cyclone due to the conservative property of MPV. It is also shown that the NMPV generation is closely associated with the coupling of large potential temperature and moisture gradients as a result of frontogenesis processes. Analyses indicate that condensation, confluence and tilting play important but different roles in determining the NMPV generation. NMPV is generated mainly through the changes in the strength of baroclinicity and in the direction of the moisture gradient due to moist and/or dry air mass intrusion into the baroclinic zone.

A Diagnostic Study of Moist Potential Vorticity Generation in an Extratropical Cyclone

Moist potential vorticity (MPV) and its generation may be important in the development of mesoscale structures such as rainbands within cyclones. In an adiabatic and frictionless flow, MPV generation is possible if the flow is three-dimensional and the air is unsaturated. Moist potential vorticity can be generated through the combined effects of gradients in the potential temperature and moisture fields. The diagnosis of MPV generation in an extratropical cyclone was performed with the ECMWF objectively analyzed fields for a system that developed during February 1992. It was found that at various stages during the development of the cyclone, negative MPV was generated: at the north end of the cold front; along the occluded front and the cold front; and in the region of the warm core. This pattern of negative MPV generation is in excellent agreement with the predictions of previous theoretical and numerical studies. After the cyclone ceased to deepen, the region of negative MPV generated in the cyclone was horizontally advected into a saturated area. The area of negative MPV generated both along the occluded front in this case study and in the region of the bent-back warm front in a numerical simulation showed a mesoscale structure with a width of about 200-500 km. It was found that the intrusion of moist or dry air into baroclinic zones was important for MPV generation. In addition, baroclinicity increase (adjacent to the area of condensation) in the regions of high moisture gradients led to significant MPV production.

Anomaly of the Moist Potential Vorticity Substance with Mass Forcing and Its Application in Diagnosing Mei-Yu Front Rainfall

The effects of precipitation on the moist potential vorticity substance(MPVS) are investigated by analyzing the MPVS with precipitation mass forcing and its impermeability in daily 1°× 1° data of the National Centers for Environmental Prediction/National Center for Atmospheric Research(NCEP/NCAR) over the Yangtze River Basin from 21 June to 2 July 1999. The results show that the positive MPVS anomalies appear mainly along the Mei-yu front, where the maximum MPVS collocates with the maximum surface rainfall. Rain case diagnoses indicate that the MPVS anomaly may be used as a dynamical signal to detect the location and shift of the rain band when its impermeability is considered.

Tracking Surface Cyclones with Moist Potential Vorticity

Surface cyclone tracks are investigated in the context of moist potential vorticity (MPV). A prognostic equation of surface absolute vorticity is derived which provides a basis for using negative MPV (NMPV) in the troposphere as an alternative approach to track surface cyclones. An observed case study of explosive lee cyclogenesis is performed to test the e?ectiveness of the MPV approach. It is shown that when a surface cyclone signal is absent due to the blocking of the Rocky Mountains, the surface cyclone can be well identi?ed by tracing the peak NMPV.

AN EAST ASIAN SUBTROPICAL SUMMER MONSOON INDEX DEFINED BY MOIST POTENTIAL VORTICITY

Based on consideration of both thermodynamic and kinetic features of the subtropical summer monsoon in East Asia,a new index is defined by the moist potential vorticity (MPV) for this monsoon.Variation features of the subtropical summer monsoon over 60 years are analyzed using National Centers for Environmental Prediction/National Center for Atmospheric Research (USA) data from 1948 to 2007.Results show that the new index can well reflect the seasonal,interannual,and interdecadal variations of the East Asian subtropical summer monsoon.Correlation analysis of the new index and precipitation data from 160 stations in China shows that in high-index years,the summer monsoon is strong,and more rain falls in eastern North China,southwestern China,and along the coast of South China and less rain falls in the Yangtze-Huaihe R.basin.In low-index years,the opposite occurs.Lastly,the new index is compared with four established monsoon indices.The new index is found to have an advantage in representing summer rainfall in the Yangtze-Huaihe R.basin.

Moist Potential Vorticity Vector for Diagnosis of Heavy Rainfall Events in Tanzania

In this paper, we modify the convective vorticity vector (CVV) defined as a cross product of absolute vorticity and gradient of equivalent potential temperature to moist potential vorticity vector (MPVV) defined as a cross product of absolute vorticity () and the gradient of the moist-air entropy potential temperature (). The patterns of (MPVV) are compared with the patterns of heavy rainfall events that occurred over different regions in Tanzania on 20th to 22nd December, 2011 and on 5th to 8th May, 2015. Moreover, the article aimed at assessing the relative contributions of the magnitude, horizontal and vertical components of (MPVV) detecting on the observed patterns of rainfall events. Dynamic and thermodynamic variables: wind speed, temperature, atmospheric pressure and relative humidity from numerical output generated by the Weather Research and Forecasting (WRF) model running at Tanzania Meteorological Agency (TMA) were used to compute MPVV. It is found that MPVV provide accurate tracking of locations received heavy rainfall, suggesting its potential use as a dynamic tracer for heavy rainfall events in Tanzania. Finally it is found that the first and second components of MPVV contribute almost equally in tracing locations received heavy rainfall events. The magnitude of MPVV described the locations received heavy rainfall events better than the components.

Numerical Simulation and Moist Potential Vorticity Analysis of Torrential Rain in Jiangxi Province during June 2010

Based on the conventional ground observational data,a numerical simulation and moist potential vorticity( MPV) analysis has been carried on heavy rainfall event over Jiangxi province from 19 June to 20 June 2010,with a meso-scale rainstorm model. The results show that this rare rainstorm is a typical heavy rainfall over Meiyu front. The cold air flow behind North China vortex joined up the southwestern flow located in the northwest part of the strong and stable subtropical high,thus the cold air and warm air converged and maintained over the northern part of Hunan and Jiangxi province. The simulated precipitation of the high resolution model is very similar to the observational rainfall. The model has a good predictive skill for the location,intensity and center of heavy rainfall. By moist potential vorticity analysis,it is found that the distribution characteristic of MPV which heavy rainfall happens ahead has an obvious indication for precipitation forecast. The vertical overlapping of the positive and negative MPV1 areas is favorable to the generation and development of rainstorm. This zone is also the conjoint area of convective instability and baroclinic instability.

Diagnosis Analysis on the Moist Potential Vorticity of a Regional Rainstorm on May 27,2013 in Eastern Jiangxi Province

Based on conventional meteorological data and NCEP 1°×1° DEG reanalysis data, reasons for a regional rainstorm in Fuzhou, Jiangxi Province on May 27, 2013 was analyzed from the aspects of weather situation, influencing systems, water vapor, dynamic and thermal instability. The results showed that the regional rainstorm happened in the warm area while the Jianghuai cyclone moved eastwards, and it was a short-dura- tion strong convective rainstorm. Jianghuai cyclone, 500 hPa trough, low vortex and southwesterly jet were the main influencing systems of the rain- storm. The precipitation was mainly the result of release of convective unstable energy in the lower troposphere. MPVl was negative in the lower troposphere and positive in the middle and upper troposphere. Moist potential vorticity in the upper troposphere moved downwards, which was ad- vantageous to the release of the unstable energy and then increased precipitation. The negative moist potential vorticity center in the lower tropo- sphere can reflect the position and intensity of the rainstorm, and the intensity and duration of precipitation were consistent with the increase of the negative MPVl. The distribution of MPV2 showed that as atmospheric baroclinicity increased, heavy rain always occurred in the lower troposphere where baroclinicity was strong. The areas with negative baroclincity in the lower troposphere matched with rainstorm center very well.

EVOLUTION OF MOIST POTENTIAL VORTICITY DURING A WARM-ZONE HEAVY RAINFALL EVENT IN THE PEARL RIVER DELTA

First,based on routine meteorological data,the synoptic characteristics of a heavy warm-sector rainfall that occurred on June 13,2008 in the Pearl River Delta were analyzed.Second,a mesoscale numerical model,Weather Research and Forecasting(WRFV2.2),was used to simulate the heavy rainfall. Diagnostic analyses were done of moist potential vorticity(MPV)for its horizontal components(MPV2) and vertical components(MPV1)based on the simulation results of WRFV2.2 to identify the mechanism of the rainfall development.The results showed that the heavy rainfall occurred when there were high MPV1 in the upper levels and low MPV1 and high MPV2 in the lower levels.Disturbances of high MPV1 in the upper levels came from the southwest or northwest,those of low MPV1 in the lower levels came from the southwest,and those of high MPV2 came from the south.Disturbances of low MPV1 at low levels were the direct cause of convective instability.Enhanced vertical shear of meridional wind led to increased MPV2 at lower levels,strengthened baroclinicity,and active warm and wet flows.These distributions of MPV helped to trigger the release of unstable energy and produce warm-sector heavy rainfall.As it integrates the evolution of dynamic and thermal fields,MPV is able to reveal the development of this heavy rainfall effectively.

Analysis on the Moist Potential Vorticity and Conditional Symmetric Instability of a Rainstorm

[Objective] The research aimed to analyze the moist potential vorticity (MPV) and conditional symmetric instability (CSI) of a rainstorm in Guangxi in the prior flood season. [Method] Based on the conventional observation data, precipitation data from the automatic station and 1°×1° six-hourly reanalyzed data from NCEP, MPV and CSI in a rainstorm process which occurred in the prior flood season in Guangxi were analyzed. The characteristics of MPV component and role of CSI before and after the rainstorm occurrence were discussed. [Result] The dense belt of isoline in the southeast of MPV1 positive-value area (MPV2 negative-value area) at 925 hPa, the underneath of sloping MPV1 positive-value column (MPV2 negative-value column) on the profile map and the underneath of dense belt of θe isoline had the good corresponding relationships with the falling zone of strong precipitation. After the system developed strongly, the enhancements of gradient and intensity of MPV1 and MPV2 predicted the rainstorm increase. In the beginning period of strong precipitation occurrence, the convective instability at 850-650 hPa co-existed with CSI. The convective instability and vertical movement played the main role. In the middle and latter periods of strong precipitation, the high-altitude weak cold air further invaded southward. It triggered the release of unstable energy, and compelled that the original sloping θe dense belt became steep. It was favorable for the development of sloping vorticity, and the atmosphere gradually turned into the neutral state of convection. CSI and ramp motion played the major role. Then, the rainstorm continued. [Conclusion] The research provided the theory basis for the application of MPV and CSI into the local routine business.

THE THEORY OF MOIST POTENTIAL VORTICITY AND ITS APPLICATION IN THE DIAGNOSIS OF TYPHOON RAINFALL AND INTENSITY

This paper tests the impacts of cloud-induced mass forcing on the moist potential vorticity （MPV） anomaly associated with torrential rains caused by Typhoon No.9914 （Dan） by using fine model simulation data outputted by the Fifth-Generation NCAR / Penn State Mesoscale Model （MMS）. The diagnostic results show that the positive MPV anomaly region, which is obtained by integrating the MPV from 600 hPa to 300 hPa in the vertical, roughly coincides with the precipitation at their synchronous stages either in position or in the distribution pattem, and the maximum positive MPV area of Dan is located mainly between 600 hPa and 300 hPa, which is much higher than torrential rain cases. Further analyses also showed that the value of positive MPV anomaly increased or decreased with the development of Dan, and the positive MPV anomaly may also be served as a tracer to indicate the evolution of tropical cyclone intensity.

On the Significance and Use of the Generalized Moist Potential Vorticity Equation

In this paper,the continuity and thermodynamic equations including moisture forcings were derived.Using these two equations and the basic momentum equation of local Cartesian coordinates,the budget equation of generalized moist potential vorticity(GMPV) was derived.The GMPV equation is a good generalization of the Ertel potential vorticity(PV) and moist potential vorticity(MPV) equations.The GMPV equation is conserved under adiabatic,frictionless,barotropic,or saturated atmospheric conditions,and it is closely associated with the horizontal frontogenesis and stability of the real atmosphere.A real case study indicates that term diabatic heating could be a useful diagnostic tool for heavy rainfall events.

CONSERVATION OF MOIST POTENTIAL VORTICITY AND DOWN-SLIDING SLANTWISE VORTICITY DEVELOPMENT

An accurate form of the moist potential vorticity(MPV)equation was deduced from a complete set of primitive equations.It was shown that motion in a saturated atmosphere without diabatic heat- ing and friction conserves moist potential vorticity.This property was then used to investigate the de- velopment of vertical vorticity in moist baroclinic processes.Results show that in the framework of moist isentropic coordinate,vorticity development can result from reduction of convective stability,or convergence,or latent heat release at isentropic surfaces.However,the application of the usual analy- sis of moist isentropic potential vorticity is limited due to the declination of moist isentropic surfaces. and a theory of development based on z-coordinate and p-coordinate was then proposed.According to this theory,whether the atmosphere is moist-symmetrically stable or unstable,or convective stable or unstable,the reduction of convective stability,the increase of the vertical shear of horizontal wind or moist baroclinity may result in the increase of vertical vorticity,so long as the moist isentropic surface is slantwise.The larger the declination of the moist isentropic surface,the more vigorous the develop- ment of vertical vorticity.In a region with a monsoon front to the north and the warm and moist air to the south,or by the north of the front,the moist isentropes are very steep.The is the region most favorable for development of vorticities and formation of torrential rain. For a case of persistent torrential rain occurring in the middle and lower reaches of the Changjiang and Huaihe Rivers in June 11-15,1991,moist potential vorticity analysis,especially the isobaric analysis of its vertical and horizontal components,i.e.MPV1 and MPV2,respectively,is effective for identifying synoptic systems not only in middle and high latitudes,but also in low lati- tudes and in the lower troposphere.It can serve as a powerful tool for the diagnosis and prediction of torrential rain.

Application of Generalized Moist Potential Vorticity to the Analysis of a Torrential Rain Case

Considering the non-uniform saturation of the real atmosphere,a diagnostic study of a torrential rain event is performed from the perspective of the anomaly of the generalized moist potential vorticity（GMPV） after introducing the generalized potential temperature into the thermodynamic frame of the real atmosphere. The results show that the anomaly appeared at mid-lower levels of the troposphere when the torrential rain happened.Analysis of the magnitudes and locations of maximum values of the tendency of the GMPV indicates that the anomaly of the GMPV was closely related to the occurrence of torrential rain. Therefore,the GMPV could be used to forecast and diagnose the occurrence of torrential rain in operational application.

西南地区一次典型冰冻雨雪复合极端灾害天气事件的环流特征及降水相态差异分析

在全球变暖气候背景下,复合型极端灾害天气事件频发。2008年以来我国南方冬季频发的雨雪冰冻,就是一种典型的复合型致灾极端天气事件。因此,本研究分析了西南地区一次典型大雪冻雨复合天气事件的大雪与冻雨期环流特征及降水相态差异,揭示了二者的关联特征。冻雨发生在贵州境内的云贵准静止锋锋面强斜压环境中,降雪主要发生在四川北部,位于静止锋以北的冷区。降雪区和冻雨区的垂直环流存在显著差异:降雪区以上升运动为主,温度基本随高度递减;冻雨区大气中低层存在逆温层,导致温度层结出现冷—暖—冷的分布,垂直运动呈两层环流模态,低层的上升运动受到中层下沉运动抑制,强上升运动不易发展。借助可综合表征环流特征和水汽相变的广义湿位涡理论,分别诊断大雪和冻雨发生发展时期的广义湿位涡分布特点,发现其斜压项的异常能更好体现准静止锋附近的大气斜压性,也可指示出大雪冻雨降水的落区及变化,可作为大雪冻雨区的动力识别特征量之一。通过气压扰动方程的计算分析,表明向下的扰动气压梯度力与浮力的平衡差异,是降雪与冻雨垂直环流特征差异的主要原因。本研究从环流特征入手开展雨雪冰冻复合极端灾害天气分析,可为复合降水相态预报和发电企业电力运行保障提供参考。

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.

一次江淮暴雨的MPV及对称不稳定研究

根据湿位涡（MPV）理论分析了1991年7月5～6目的暴雨过程中700hP高度上的湿位涡垂直部分MPV1和水平部分MPV2。结果显示，MPV1和MPV2的分布特征同对流层低层造成暴雨的天气系统──中尺度气旋的发展演变有很好的对应关系；MPV1＞0，MPV2＜0且｜DMPV1｜≥｜DMPV2｜的环境大气有利于对称不稳定出现；计算表明，这次过程的对流层中低层确有对称不稳定区域存在，它对暴雨及其系统的发展起了重要作用。

一次黄河气旋新生过程中湿斜压不稳定机制的数值模拟研究

作为温带气旋形成的主要机制,湿斜压不稳定在气旋动热力学研究中占据中心地位,其可进一步分为干斜压不稳定、湿不稳定、非绝热Rossby波和Type C气旋新生(对流层顶干侵入)四类。2016年7月18日黄河气旋快速生成后东移进入华北造成“7·20”特大暴雨,相比气旋成熟期,其初生阶段的动热力机制尚不清楚。本文利用ERA5再分析资料与WRF模式,对该气旋新生过程的湿斜压不稳定机制进行了数值模拟研究。模拟结果指出,对流层中低层呈非绝热Rossby波形态,即系统东移发展主要由垂直运动-非绝热效应的循环所推动,其中波动触发和传播所依赖的垂直运动更多由涡度平流提供;高层位涡汇与非地转风延缓了对流层顶干侵入位涡的东移,维持了高低层相位差,最终在干侵入前部发展出贯穿对流层的位涡柱。利用非线性片段位涡反演,分别从初始场中移除非平衡分量、对流层顶干侵入位涡、低层非绝热源位涡,设计了若干敏感性试验,结合广义垂直运动方程分析可得:本次过程斜压波必须在充足水汽条件下与非绝热过程耦合才能强烈发展,关闭潜热气旋新生将被抑制,干斜压不稳定无法解释本次过程;初始非平衡场的去除不影响本次斜压不稳定性质但将延后系统发展时间,受湿度条件和中尺度环流结构限制,低层非平衡风的活跃区域主要由干斜压动力学控制;该个例近地面位温梯度小,仅依赖低层初始位涡难以有效组织起非绝热Rossby波东传,同时有别于Type C气旋新生,高层位涡异常也不足以激发起强大的中低层非绝热加热。于本次黄河气旋新生而言,一方面要求初始低层位涡异常具有一定强度,以抵消高层干侵入前部伴随的冷却下沉对其的抑制;另一方面也需要高层位涡异常通过垂直渗透以涡度平流形式加强低层位涡东侧上升运动,在高低层初始相位差合适情况下,持续促使非绝热Rossby波东移发展,推动系统进入水汽条件更好的华北地区。干斜压不稳定、非绝热Rossby波和Type C气旋新生机制均不能独立解释本次事件,此次黄河气旋新生是在非绝热Rossby波和对流层顶干侵入混合作用下,初始时刻最优扰动增长形成的。