Richardson Number in a Moist Atmosphere and Its Application in the Analysis of Heavy Rainfall Events

To better describe the stability of a moist atmosphere,a new Richardson number,the Richardson number in a non-uniformly saturated moist atmosphere(Ri~*),is defined in this paper.Ri~* is the same as the Richardson number(Ri) except that the generalized potential temperature introduced by Gao et al.in 2004 is used to calculate the Brunt-Vaisala frequency.Then,with outputs from simulations of two heavy rainfall events,Ri~* is applied to diagnosing instabilities of rainfall areas.The results show that convec...     

Analyses of dry intrusion and instability during heavy rainfall event occurred in Northern China

The 1°×1° National Center for Environmental Prediction/National Center for Atmospheric Research （NCEP/NCAR） data and mesoscale numerical simulation data are analyzed to reveal a mechanism for the formation of heavy rainfall in Northern China; this mechanism is the non-uniformly saturated instability induced by a dry intrusion. The dry intrusion and the accompanying downward transport of air with a high value of potential vorticity （PV） are maintained during the precipitation event. As the dry air intrudes down into the warm and moist sector in the lower troposphere, the cold, dry air and the warm, moist air mix with each other, and, as a result, the atmosphere becomes non-uniformly saturated. On the basis of this non-uniform saturation, a new Brunt-Vaisaila frequency （BVF） formula is derived and applied to the precipitation event. It is shown that, compared to the conditions of either a dry or a saturated atmosphere, the BVF in a non-uniformly saturated, moist atmosphere （BVF） may be more appropriate for depicting the atmospheric instability in rainy regions.     

广义湿位涡理论及其应用研究

押重点介绍了诊断暴雨落区与强度的广义湿位涡理论研究方面的两个内容,一是由于暴雨系统中强降水引起的质量亏空导致的暴雨系统中质量强迫下的湿位涡异常理论,二是非均匀饱和大气中的广义湿位涡理论;对暴雨系统中质量强迫的物理意义和非均匀饱和大气中的广义位温引入的思路与意义作了详细说明,并对位涡理论作了细致推导。在此基础上,针对暴雨个例,利用质量强迫的湿位涡异常和非均匀饱和广义湿位涡异常诊断了暴雨落区,从理论和诊断上论证了利用这两种湿位涡异常判断暴雨落区的可行性。     

A Review of Potential Vorticity and its Possible Application in the moist Atmospheric Flow

Potential vorticity (PV) has been served as a powerful and useful dynamic tracer for the understanding of the large-scale dynamics and synoptic variations in the atmosphere and oceans. Significant progress has been made on the application of PV. In recent decades there has been a substantial amount of work done on PV in a general moist atmosphere. In this paper PV and the generalized moist potential vorticity (GMPV) and their application in the tropical cyclones and mesoscale meteorological field are reviewed. The GMPV is derived for a real atmosphere (neither totally dry nor saturated) by introducing a generalized potential temperature instead of the potential temperature or equivalent potential temperature. Such a generalization can depict the moist effect on PV anomaly in the non-uniformly saturated atmosphere. A new convective vorticity vector (CVV) is introduced in connection with GMPV in order to diagnose the development of tropical deep convections.     

广义湿位涡与暴雨落区预报的诊断分析

探讨了广义湿位涡理论及其倾向方程在暴雨落区预报方面的可应用性，为非均匀饱和广义湿位涡理论的业务应用提供了新的思路。 资料诊断结果表明， 非均匀饱和大气中的广义湿位涡虽然不具有守恒性， 但由于其体现了实际大气的非均匀饱和特性，尤其是水汽梯度效应的引入，使得广义湿位涡的异常在对流层低层能比较清楚地反映出暴雨发生时期的高水汽集中的特性和机制，有效地反映出暴雨区以及暴雨落区的变化。      

湿空气动热力学理论及应用研究进展

回顾了湿空气热动力学的研究进展,对未饱和湿大气、饱和湿大气及非均匀饱和湿大气的动热力方程、能量方程、连续方程等进行了梳理,指出饱和湿空气动量方程与非均匀饱和湿空气动量方程的最大区别在于对凝结过程的处理不同。饱和湿大气中,由于大气均是饱和的,由饱和造成的水凝物可处处出现,不能区分真正发生水汽凝结的区域。而非均匀饱和湿空气中,凝结发生与相对湿度的幂次方有关(即与凝结概率函数的分布有关),在相对湿度较小的区域不会出现水汽凝结,凝结区与非凝结区可自动区别,其描述的凝结过程与实际大气更接近。同时,总结了湿大气水汽凝结饱和非均匀分布的动热力物理量在高影响天气分析中的应用,最后讨论了未来推进湿空气动力学研究需重点考虑的一个内容。     

A generalized frontogenesis function and its application

With the definition of generalized potential temperature, a new generalized frontogenesis function, which is expressed as the Lagrangian change rate of the magnitude of the horizontal generalized potential temperature gradient, is derived. Such a frontogenesis function is more appropriate for a real moist atmosphere because it can reflect frontogenesis processes, in which the atmosphere in a frontal zone is typically characterized by neither completely dry nor uniform saturation. Furthermore, by derivation, the expression of generalized frontogenesis function includes both temperature and humidity gradients, which is different from and superior to the traditional frontogenesis function in moist processes, which also uses equivalent potential temperature. Diagnostic studies of real cases are performed and show that the generalized frontogenesis function in non- uniformly saturated moist atmosphere indeed provides a useful tool for frontogenesis, compared to using the traditional frontogenesis function. The new frontogenesis function can be used in situations involving either a strong temperature or moisture gradient and is closely correlated with precipitation.     

两次陕北暴雨过程热力动力机制诊断

利用常规气象观测资料、NCEP FNL分析资料（水平分辨率为1°×1°,时间分辨率为6 h）,对2013年7月21-22日和2014年7月8-9日两次陕北暴雨过程成因进行热力动力诊断,结果表明:两次陕北暴雨与高低空急流关系密切,暴雨带位于低空急流左侧的水汽辐合区,“0721”过程低空急流更强,在高低空急流耦合的强上升运动区（延安）出现大暴雨。降水前期,两次过程大气均存在对流不稳定,切变线触发对流,产生强降水,而其释放的凝结潜热加热形成中低层大气的热力不连续面,湿斜压性及锋生增强,造成整层饱和大气的抬升,维持强降水。“0721”过程前期对流降水的潜热释放更大,由此反馈的低空急流及锋生更强,出现大暴雨天气。广义对流涡度矢量垂直分量很好地描述了两次暴雨过程高低空急流耦合作用以及凝结潜热释放增强的锋生作用,其变化趋势能够反映降水的发展和减弱过程。暴雨出现在湿热力平流参数垂直积分大值中心及南侧的高梯度区,大值中心出现后约6 h会产生强降水,这对于强降水落区的预报有一定指示意义。     

非均匀饱和大气中广义位温物理意义的图像解释

Compared to potential temperature in the dry atmosphere and equivalent potential temperature in the saturated atmosphere, generalized potential temperature has already proven a better thermodynamic parameter in describing the non-uniformly saturated real atmosphere. To add otherwise absent graphic explanations, this paper first presents the physical definition of * through a tephigram. Then, the utility of the measurement in identifying and forecasting the locations of precipitation maxima and heat wave areas with diagnostic comparison studies and traditionally used thermodynamic parameters is shown.     

Q矢量旋度：一个与暴雨相关的新诊断量

As a powerful tool to diagnose vertical motion, frontogenesis, and secondary circulation, the Q vector and its divergence are widely used. However, little attention has been given to the curl of Q vector. In this paper, a new set of analyses combining the divergence of the Q vector (DQ) with the vertical component of the curl of the Q vector (VQ) is applied to a Northeastern cold vortex rainfall case. From the derivation, it was found that the expressions of the Q vectors and their divergences in saturated moist flow (DQm) differ from those of dry and unsaturated moist atmosphere (DQ), while the VQs of various background flows are exactly the same, which largely simplified the analyses. This case study showed that, compared with the DQ, not only can the DQm diagnose precipitation more effectively, but the VQ may also be indicative of precipitation (especially for heavy rainfall and strong convection) because of its direct, close relationship with ageostrophic motion. Thus, the VQ may be computed and analyzed with ease, and may serve as a useful tool for analyses of precipitation and strong convective systems. Citation: Yang, S., and D. H. Wang, 2008: The curl of Q vector: a new diagnostic parameter associated with heavy rainfall, Atmos. Oceanic Sci. Lett., 1, 36-39     

近年来我国暴雨中尺度动力分析研究进展

总结了近十年以来中国气象科研人员在暴雨中尺度动力分析研究领域的主要研究成果，从大尺度环流背景与中小尺度系统的相互作用、高低空急流、低涡、位涡与对流涡度矢量、螺旋度、不稳定等方面对它们进行了分类概括。对暴雨中尺度动力分析研究的回顾表明，以往常用的数值模拟、能量转换收支分析、涡度、散度、涡度收支、涡度平流、温度平流、各种不稳定指数等的诊断仍然是暴雨中尺度动力分析的主要手段，但是，近年来，除了位涡、螺旋度等动力变量在暴雨中尺度动力分析中的广泛应用以外，一些新的物理量，如非均匀饱和广义湿位涡、对流涡度矢量以及有限区域风场分解方法在暴雨中尺度动力分析中也得到了更多的应用。     

2003年8月“巴蜀夜雨”过程的模拟和分析研究

结合中尺度数值预报模式AREM的数值试验和观测资料分析,对2003年8月川西地区的9次夜雨过程进行了模拟研究和综合分析.结果表明,在一定环流背景下,川西地区特殊地形引起的沿坡地的辐合上升运动和下垫面提供给低层大气的热通量所导致的大气层结不稳定,对川西夜雨的形成和发展有重要影响.白天,随着陆-气通量交换的增加,低层大气的温度和湿度逐步升高,并在午后达到极值.与此同时,低层偏南暖湿气流在盆地西部由于气旋性弯曲而形成的东北风在午后逐渐加强,这支气流在盆地西部被地形阻挡,产生爬升运动.辐合上升将低层高温高湿的大气向上输送,使得大气不稳定层结的厚度以及强度都增加;日落以后,低层大气的相对湿度随着气温的降低而增大,容易饱和而形成凝结,同时大气中积累了相当可观的对流有效位能,低层辐合抬升等因素容易触发不稳定能量释放,造成对流性夜雨天气.强烈的对流辐合运动需要周围大气的入流补偿,促使偏东风气流增强且向高空伸展,这令辐合抬升作用进一步增强.     

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 heating and friction conserves moist potential vorticity.This property was then used to investigate the development 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 analysis 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,on 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 development 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 latitudes and in the lower troposphere.It can serve as a powerful tool for the diagnosis and prediction of torrential rain.     

Potential Deformation and Its Application to the Diagnosis of Heavy Precipitation in Mesoscale Convective Systems

Many observational studies have shown that deformation, like vertical vorticity and divergence, is closely related to the occurrence and distribution of strong precipitation. In this paper, to involve deformation in precipitation diagnosis, a new parameter called potential deformation(PD) is derived and then applied to precipitation detection within a simulated mesoscale convective system(MCS). It is shown that PD includes both stretching deformation and shearing deformation and shares similar characteristics with deformation insofar as it does not change with the rotating coordinate. Diagnosis of the simulated MCS reveals that PD performs well in tracing the MCS' precipitation. In terms of their distributional pattern, the large-value areas of PD are similar to the precipitation in the different development stages of the MCS. A detailed analysis of the physical processes contained within the PD shows that it can reflect the three-dimensional moisture variation,vertical wind shear and wind deformation within the MCS. These structures are usually a comprehensive reflection of the characteristics of the surface cold pool, rear inflow jet, downward cold air flow and upward warm moist flow within the precipitating convective cells. For this reason, the PD shows much stronger anomalies in the precipitating atmosphere than the non-precipitating atmosphere, which implies considerable potential for its application in detecting heavy precipitation within MCSs.     

2012年夏季四川东部一次大暴雨过程分析

本文使用常规观测资料、四川省自动站降水资料、0.1°×0.1°的FY－2E云顶亮温资料和1°×1°的NCEP再分析格点资料对2012年7月20～23日四川东部强降水过程的主要影响系统、水汽源地、动力、热力条件等进行诊断分析,结果表明:(1)本次暴雨过程中伴有500hPa高空槽东移至四川并向南加深发展,槽后冷空气与槽前暖湿气流在四川汇合,低层有低涡发展,配以高低空急流耦合的有利形势;(2)暴雨前期水汽主要来源于孟加拉湾,随着南海台风西进,其外围偏东气流向西输送增强,西南暖湿气流北上受到抑制,使得雨带南压;(3)降水以对流性降水为主,暴雨期间水汽凝结潜热在对流层中低层起主要作用,强上升运动将低层的潜热加热向上输送,形成高空的热源中心,强降水期间大气的加热是与大气的垂直上升运动密切相关的;在本次暴雨过程垂直输送项是视热源Q1和视水汽汇Q2的主要贡献者,尤其是在强降水阶段;(4)在低涡在发展阶段,低层正涡度局地变化项首先得到发展,在低涡减弱阶段,正涡度局地变化项的峰值中心由低层向中低层抬升;(5)中尺度对流系统与小时降水分布一致,MCS的发展是触发降水的重要因素之一。      

一次飑线过程对流稳定度演变的诊断分析

针对2016年5月2日发生在华东地区的一次飑线过程,利用WRF模式进行高分辨率数值模拟。在成功模拟飑线发生、加强和移动的基础上,对此次过程中对流不稳定特征以及引起对流稳定度变化的原因进行诊断分析。结果表明:（1）在降水发生前,低层大气表现为对流不稳定;降水发生后,对流不稳定能量得到释放,大气趋于稳定。为了分析引起对流稳定度变化的原因,推导了局地直角坐标系中相当位温垂直梯度的倾向方程,其中位势散度是引起位势稳定度局地变化的主要强迫项。在弱降水区,低层位势散度为负值,有利于增强位势不稳定;强降水区及其前沿为位势散度正值区,倾向于抑制位势不稳定。在强降水区低层,位势散度的主要分量为垂直风切变项,代表垂直风切变和大气湿斜压性的综合作用;高层的主要分量为散度项,代表水平散度和位势稳定度的耦合作用。（2）位势散度能综合表征降水区上空垂直风切变、大气湿斜压性、水平辐合辐散和大气位势稳定度变化等特征,因而与降水联系紧密。本文利用位势散度对飑线降水进行预报,结果表明,位势散度与小时观测降水在时间和空间上吻合较好,对降水区有一定的指示意义,可以为飑线降水业务预报提供参考。     

相当湿位涡异常与梅雨锋暴雨分析

基于位涡异常与中尺度对流系统发展密切相关的分析事实,以及中尺度天气的发生发展的过程中水汽的重要作用,在引入水汽变化方程的基础上,导出了非均匀饱和大气中的相当湿位涡方程,进而分析了引起相当湿位涡异常的主要因子。依据相当湿位涡包含动力因子、热力因子和水汽因子的特点,利用相当湿位涡及增量对一次梅雨锋暴雨进行诊断和模拟分析,初步揭示了相当湿位涡及其增量对梅雨锋强降水的指示意义,相当湿位涡及其增量大值区对未来1～3小时的强降水具有较好的指示作用。针对梅雨锋暴雨,可以将梅雨湿度锋与相当湿位涡相结合作为强降水短时临近预报的一个指标。      

Evolution of instability before and during a torrential rainstorm in North China

NCEP-NCAR reanalysis data were used to analyze the characteristics and evolution mechanism of convective and symmetric instability before and during a heavy rainfall event that occurred in Beijing on 21 July 2012.Approximately twelve hours before the rainstorm,the atmosphere was mainly dominated by convective instability in the lower level of 900-800 hPa.The strong southwesterly low-level jet conveyed the moist and warm airflow continuously to the area of torrential rain,maintaining and enhancing the unstable energy.When the precipitation occurred,unstable energy was released and the convective instability weakened.Meanwhile,due to the baroclinicity enhancement in the atmosphere,the symmetric instability strengthened,maintaining and promoting the subsequent torrential rain.Deriving the convective instability tendency equation demonstrated that the barotropic component of potential divergence and the advection term played a major role in enhancing the convective instability before the rainstorm.Analysis of the tendency equation of moist potential vorticity showed that the coupled term of vertical vorticity and the baroclinic component of potential divergence was the primary factor influencing the development of symmetric instability during the precipitation.Comparing the effects of these factors on convective instability and symmetric instability showed some correlation.     

湿位涡和倾斜涡度发展

从完整的原始方程出发，在导出精确形式的湿位涡方程的基础上，证得绝热无摩擦的饱和湿空气具有湿位涡守恒的特性。并由此去研究湿斜压过程中涡旋垂直涡度的发展。结果表明，在湿等熵坐标中，涡旋的发展与对流稳定度的减少，等熵面上的辐合和潜热的释放有关。由于等熵位涡分析的应用受等熵面倾斜的限制，又进而发展了Ｚ坐标及Ｐ坐标中的倾斜涡度发展理论。指出无论是湿对称不稳定或对流不稳定大气，还是湿对称稳定或对流稳定大气，除对流稳定度的影响外，风的垂直切变的增加或水平湿斜压的增加均能因湿等熵面的倾斜而引起垂直涡度的增长。湿等熵面的倾斜越大，这种由干湿斜压性加强所引起的涡旋发展更激烈。在梅雨锋附近及其南侧暖湿区的北端，湿等熵面十分陡立，是涡旋发展及暴雨发生的重要地区。对１９９１年６月１２—１５日江淮流域暴雨过程的湿位涡分析表明，湿位涡分析，尤其是等压面上湿位涡量ＭＰＶ１和ＭＰＶ２的分析不仅在中高纬有效，在低纬度及低对流层也十分有效，是暴雨诊断和预报的有力工具。     

黔西南一次中尺度暴雨的数值模拟诊断研究

使用WRF模式 (Weather Research and Forecasting model) 模拟了2006年6月12日贵州省西南部一次典型的突发性强对流暴雨过程, 模式较真实地模拟了这次局地发展的中尺度暴雨天气过程。对流层低层的中尺度辐合线造成了初始的上升运动, β中尺度对流系统首先在地面锋线前不稳定的暖区中生长, 辐合线南侧的偏南气流对水汽和热量的输送是对流能够持续生长的最重要因素。通过非地转ω方程的诊断证明, 在降水开始后, 凝结加热的释放对β中尺度对流系统的发展最为重要, 它强迫产生的上升运动分量超过了低层暖平流强迫造成的上升运动分量。在相应的热力、 动力结构的调整作用下, 对流层低层出现中尺度低空急流、 中尺度涡旋等动力结构。到降水过程后期, 由于偏北气流的侵入, 降水区上空对流层低层转为对流稳定的层结, β中尺度对流系统无法获得不稳定能量以维持其发展, 降水也逐渐减弱直至终止。