MAJOR PROGRESS OF MESOSCALE METEOROLOGICAL RESEARCH IN CHINA
FROM 1999 TO 2002
ZHOU Xiaoping1, LU Hancheng3 , GAO Shouting1 , NI Yunqi2 and TAN Zhemin2
1. Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
2. Department of Atmospheric Sciences, Nanjing University, Nanjing 210093, China
3. Meteorological College of Air Force, Nanjing 211101, China
Studies done by Chinese scientists in the mesoscale fields during past four years have been reviewed. In theory, the mesoscale quasi-balance and half-balance equations are derived through scale analysis and pertubation method, which is suitable for describing mesoscale vortex motion. The subcritical instability, vortex-sheet instability and the meso-a scale heterotrophic instability are probed into, and their instability criteria are given. Adjustment frontgenesis mechanism and the effect of topography on frontgenesis are investigated. Both up and down slantwise vorticity development theories, slantwise vortex equation and moist potential vorticity (MPV) anomaly with heat and mass forcings and its impermeability theorem are well studied. From the viewpoint of the relativity of the moist potential vorticity conservation, the transformation mechanism between different scale weather systems is analyzed. Based on the data diagnosis and 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, some events associated with torrential rain systems and severe storms are analyzed and diagnozed. For numerical simulation, the configurations of meso-a, meso-b vortex, mesoscale downburst and precipitation produced by deep convective systems are simulated by MM5 and other mesoscale models.
Since 1999, mesoscale meteorological research in China has obtained grand advance. The obtainment of this advance is attributed to, on the one hand, the evident improvement of the computing facilities and progresses of mesoscale model development, on the other, the fast increase of the available observational data which come from satellite, Doppler radar and other remote sensing observations. Besides that, an important reason is that recent years severe floods and other local weather disasters, which are mainly caused by mesoscale synoptic systems, have quite often occurred in China. Thus, public attention has been paid to those mesoscale synoptic phenomena and promotes Chinese meteorologists to start realizing the importance and necessity of the mesoscale meteorological research. Especially, the fast growing of national economics in China makes more research funds in mesoscale fields put by government become possible. For these reasons above, Chinese meteorologists engaged in many aspect studies in the mesoscale fields relating to mesoscale dynamics, numerical simulation and observational data diagnoses etc. In this paper, the research on mesoscale dynamics is put in section two, the results from mesoscale numerical simulation and data diagnostic analyses are put in section three, and brief summery is given in section four.
II. THE RESEARCH ON MESOSCALE DYNAMICS
Through a long time research experience, many Chinese meteorologists more and more aware the research importance of mesoscale dynamics for better understanding of trigger mechanism of the abrupt development of mesoscale system, formation cause and evolution process of mesoscale systems. So many Chinese meteorologists are involved in studying mesoscale dynamics. The main achievements are as follows:
1. The Study of Slanting Vorticity Development Theory
The vertical vorticity has been paid much attention from the large-scale to the mesoscale meteorological research for many years. Based on statistical analysis, generally, the abrupt development of the vertical vorticity associates with the severe weather and storm. The classical vertical vorticity equation is the dot product of the unit vector in vertical direction and the curl of the momentum equation. However, it is more a description of how vorticity is changed than a useful constraint on that change. And the equation does not include thermodynamic elements, frictional dissipation and diabatic heating which affect the vertical vorticity development. Ertel (1942) derived a beautiful and unusually useful theorem: the potential vorticity is conserved in a frictionless and adiabatic dry atmosphere. This theorem provides a constraint on the vorticity that is free from many of the difficulties described above. From then on, the theorem of potential vorticity has been improved and applied to many research fields. Hoskins et al. (1985) summarized the application of the Ertel potential vorticity in the diagnosis of atmospheric motion. The concept of isentropic potential vorticity was also introduced. Generally, IPV is indicative of some aspects of the movement and development of weather systems in middle and high latitudes. However, in the lower troposphere, especially in low latitudes, IVP becomes very weak. Besides, it does not include the effects of moisture, Bennetts and Hoskins (1979) used an equation set with Boussinesq approxiation and deduced an equation for the variation of the so-called “wet-bulb potential vorticity”.
Many studies of Chinese meteorologists contribute to the development of the potential vorticity theorem. Based on the precise primitive equations, a similar variation equation for MPV was also obtained by Wu et al. (1998, 1995, 1997, 1999), which shows that in a frictionless and adiabatic saturated atmosphere, MPV is conserved, and the theory of Slantwise Vorticity Development (SVD) was proposed to study the development of the vertical component of relative vorticity in a moist baroclinic condition; According to the theory, vorticities are apt to develop near steep isentropic surfaces. In fact, since many kinds of weather systems in atmosphere do occur and develop near slantwise (moist) isentropic surfaces, it is applicable and necessary to investigate the evolution of these systems in the context of slantwise isentropic surfaces. Wu et al. (1999) deduced a complete form of vertical vorticity equation, based on the definition of PV and MPV. Compared to the traditional vertical vorticity equation, the new one has many advantages and is more applicable for diagnosis (Wu, 2001). Later on, some researchers found (e.g., Zhang et al., 1999) that up-sliding slantwise movements are always observed during the development and movement of oceanic frontal cyclones and rainstorms in Jiang-Huai (Changjiang-Huaihe Rivers) Valley. Cui et al. (2002) improved the theory of SVD to be suitable for the up-sliding slantwise motions, that is up-sliding slantwise vorticity development (USVD). They show that when an air parcel slides up along a slantwise isentropic surface, its vertical component of relative vorticity is developed. And the numerical simulation and isentropic analysis of frontal cyclones over the western Atlantic Ocean show that the SVD theory can well explain the development and propagation of such cyclones, and the downstream slantwise up-sliding movement and declination of isentropic surface make vorticity develop (USVD) under favorable conditions, then, result in the moving and development of cyclones.
2. The Study of Moist Potential Vorticity Unconservation
In the research process of large-scale phenomena, one has recognized that the potential vorticity conservation is destroyed and potential vorticity anomalies occur under the condition of external-source forcing (Hoskins and Berridford, 1988; Keyser and Rotunno, 1990). Based on large-scale potential vorticity anomalies, Fritsch and Maddox (1981) first pointed out that the negative anomalies of the potential vorticity in the upper troposphere are associated with anticyclone outflow from the mesoscale cloud anvil. Dynamical adjustment to the redistribution in the mass field brought about by convection has been shown to be capable of producing such an anomaly (Shutts and Gray, 1994; Fulton et al., 1995). The positive anomaly often appears as a mid-level cyclonic vortex (MCV). Davis and Weisman (1994) and Skamarock et al. (1994) have shown that MCV formation is favored by system-scale convergence in the presence of the earth's rotation. Gray et al. (1998) suggested that a combination of mass sinks from the system updraughts and downdraughts could be responsible for this convergence. Gray (1999) used a mass-forcing model to investigate how the potential vorticity anomalies formed, as a response to changes in the mass field due to convection in the rotating, stratified fluid. Although meteorologists abroad have paid attention to study the potential vorticity anomalies in the mesoscale convective systems, they implied that the potential vorticity anomalies are associated with lower level inflow and upper level outflow. They do no t realize in the deep convective system, especially, torrential rain system, condensation and precipitation may cause deficiency of mass and lead to mass unconservation in the continuity equation. This mass forcing may result in the moist potential vorticity (MPV) anomalies (Gao et al., 2002). They diagnosed the amount of MPV anomalies from intensive observation data (Gao et al., 2002). Meanwhile Wang et al. (1999) set up a set of non-uniform moist saturated equations and derived the potential vorticity equation of non-uniform saturated moist air and used this potential vorticity equation to diagnose and predict heavy events
(Wang et al., 2000). They found that quickly intends to zero is premise condition of occurrence of abrupt torrential rain.
3. The Study of Frontogenesis and Interaction of Orographic Disturbance with Front
Since Margules (1906) first gave the slop formula of front from dynamic point of view, more and more meteorologists have paid their attention to study frontogenesis from different aspects. During early period, Sawyer and Stone (1966), Williams and Plotkin (1968) gave their balance frontogenesis models, later Hoskins (1971) and Hoskins and Bretherton (1972) pointed out frontogenesis models and semi-geostrophic mathematical theory of frontogenesis. Although fronogenesis theory has been studied from different angles, some problems about fronogenesis still remain, such as the relationship between the geostrophic adjustment and frontogenesis. For solving some frontogenesis questions remained in the past, and getting a better understanding of frontogenesis, Wang and Wu (1999) investigated interaction of orographic disturbance with front by numerical simulation. It is shown that the front is dominated mainly by the orographic disturbance if the front is weak. As the cold front moves across the mountain, its intensity decreases on the upwind slop and increases on the downwind side due to the thermal superposition. Wang et al. (2000) probed into evolution and frontogenesis of an imbalanced flow. The basic conclusion is that if the initial fields are not in geostrophic balance, the adjustment and evolution will occur in the stratified flow and fronogenesis will be generated under suitable conditions— suitable vapor distribution and orographic forcing. Wu and Fang (2001a; 2001b) studied the relationship between the geostrophic adjustment and frontogenesis by invoking the minimum principle of energy and found the final state of geostrophic adjustment is determined by the initial potential vorticity. Further, they studied the relationship between balanced flow and frontogenesis and the effect of topography on geostrophic adjustment and frontogenesis. It is found that for the two-dimensional, inviscid, rotating and nonlinear model, the final state of the flow depends on the initial conditions. If the initial potential vorticity of the flow is non-uniform, the final state is not necessarily geostrophic and discontinuity will occur. Meanwhile, the horizontal distribution of the initial potential temperature and its position relative to the mountain play important roles during the geostrophic adjustment and local frontogenesis. Wang et al. (2002) found that there is interaction of diabatic frontogenesis and moisture processes in the cold-frontal rain-band. Attention is also paid by Tan and Wu (2000a; 2000b) to study the effect of orography and boundary layer friction on the structure and circulation of surface cold front (SCF) and surface warm front (SWF). The results show that the inclination of SCF mainly depends on the geostrophic wind in the warm sector, frontal moving speed and the position of SCF relative to orography and the vertical motion in SCF zone while the inclination of SWF chiefly depends on the frontal moving speed and the geostrophic flow in the warm sector that is decreased with the frontal speed increasing, and the vertical motion in SWF zone is more complicated than that in SCF. There are three chief upward motion zones around the frontal zone.
In addition, the structure and dynamical features of the Meiyu front system are also investigated. Gao et al. (2002) found that a subtropical front exists near the northwest periphery of the West Pacific subtropical high and put forward that coupling of the subtropical front and Meiyu front forms a Meiyu front system (MYFS) and is the most direct synoptic system for the Meiyu precipitation along the mid-lower reaches of the Yangtze (Changjiang) River in China. The importance of the subtropical front lies in that it can promotes the transportation of the warm and moist southwest monsoon flow along the passageway of MYFS and block the warm and moist flow to penetrate southwest widely.
4. The Study of Instability
As an important study of atmospheric dynamics, the instability problem has attracted the attention of many scientists to study on such theme, which reveals the physical essence of atmospheric motion and provides the theoretical foundation for the improvement of operational prediction. The main study domains of mesoscale dynamic instability focus on two aspects—symmetry instability and transversal wave instability.
Symmetry instability is also called slantwise ascending instability or slantwise convective instability. The study of such instability aims initially at the explanation of large-scale convection in planetary circulation and then it is applied into the research of trigger mechanism of some mesoscale weather phenomenon (such as mid-latitude squall line, frontal precipitation, rain belt and blizzard etc.). Ooyama (1966) investigated the stability of the baroclinic circular vortex and put forward a sufficient criterion for instability. Later on, many meteorologists (Hoskins,1974; Bennetts and Hoskins,1979; Emanuel,1979,1982,1983a,1983b,1985; Ogura et al.,1982; Xu et al.,1985; Xu,1986a,1986b,1989) have studied the symmetry instability problem and revealed that such instability may be of great importance in organizing and starting up band convection giving a rational explanation to multiple rain bets appearing in fronts and cyclones.
The studies of Chinese meteorologists also promote the development of symmetric instability theory. Gao and Sun (1986) studied the high-order approximation of the symmetric instability by invoking the criterion of Richardon number. Zhang (1988a,b) obtained the dispersion relation with rigid boundary condition and achieved the conclusion that the symmetry instability can be caused only in the case of 0<Ri<1 (Ri is Richardson number) excluding the horizontal shear of basic stream. From the early study on relation between WAVE-CISK and symmetry instability subjected to non-static equilibrium (Zhang and Zhang, 1992), it is found that propagating unstable perturbation will appear once where heating takes place. Furthermore, with a non-static model of quasi-two dimensions, numerical method is applied in the study of the effect of heating feedback to linear and non-linear symmetry instability (Zhang and Zhang, 1995). Wang et al. (2000) deduced a modified symmetry instability criterion for thermal wind non-equilibrium basic flow by using two-dimensional linear equations with Bousinesq approximation of moist air. It is found that the thermal wind non-equilibrium of the basic flow is favorable for the development of symmetry instability under certain conditions.
Transversal perturbation is defined as perturbation propagating along the basic stream (its equal phase plane is perpendicular to basic stream). The initial study on the instability of such perturbation is mainly concentrated on synoptic and convective scale. Charney (1947) and Eady (1949) proposed the necessary condition of the atmospheric barotropic instability. Later on, the studies of instability are extended from linear or quasi-linear to non-linear, from conservative system to dissipative systems, and the work and papers on it are too a lot to be listed orderly. The contribution of Chinese meteorologists to the development of this kind of problem is hardly neglected. Zhang (1988a,b) achieved the double-mode unstable spectrum of baroclinic stream through extending eddy mode from f-plane to non-geostrophic case and defined the instability of mesoscale mode as transversal instability. Zhang and Zhang (1998) made numerical study on linear and non-linear transversal instability and investigated the effect of condensation heating on transversal stability. Then, Zhang and Zhang (1999; 2001) investigated the characteristics value of transversal disturbance at baroclinic shear. The results show that when the wind shear of basic flow is not zero, two kinds of gravitational inertial waves and one kind of vortex wave have continuous spectrum. And in the vertical direction, the waves mentioned above have respective critical layer. In addition, an interesting finding is that the continuous spectrums of the above waves can overlap at mesoscale frequency range. This figure indicates that the complication of mesoscale motion. Zhang et al. (2000, 2001, 2002) noted that many observed clouds and rain belts are neither parallel nor perpendicular but heterotropic to the basic stream. They investigated the unstable mechanism of such heterotropic perturbation (called heterotropic instability) and analyzed the characteristics and energy transformation in such process. The results show that if the value of Ri (Richaralson number) is less than 0.95, perturbation instability may occur in the whole mesoscale spectrum, and the symmetric instability and the baroclinic instability are the major kinds of instability in β-mesoscale and synoptic scale spectrum, respectively. While, the instability in of α-mesoscale wave is heterotropic instability.
Lu et al. (2001) studied the phenomenon of subcritical instability in baroclinic atmosphere by numerical experiment. The results show that when nondimensional parameterβ≥0.25, the phenomenon of subcritical instability can exist in two-layer baroclinic atmosphere. The occurrence of subcritical instability is relative to the selection of the initial disturbances in model. The latitudinal gradient of Coriolis Parameter is also an important factor that affects the strength of baroclinic stability, the larger its value is, the smaller strength of the baroclinic stability is.
Lu (2001) proposed a new generalized energy as Lyapunov function and deduced a new criterion of non-linear barotropic stability containing two inequalities. One of the inequalities is that the inertial disturbance amplitude is less than a critical value, and the other is that the frictional coefficient is more than another critical value. The former adds a strong restriction to the latter, which is a usual nonlinear stability criterion. This figure denotes that the subcritical instability easily occurs for a finite-amplitude disturbance in realistic situation.
In the existing studies, the vortex sheet instability along the shear line is rarely investigated. Although Scorer (1997) studied the vortex sheet instability in the region with the vertical wind shear and the instability of the steady vortex, the vortex sheet instability with horizontal wind shear is not studied yet. Gao (2000), Gao and Zhou (2001) studied the instability of the vortex sheet along the shear line and deduced a criterion of such instability, which indicates that the disposition of environment field restrains the disturbance developing along the shear line, there exist multi-scale interactions between this mesoscale disturbance and environment field.
Besides all above mentioned, other inspects of mesoscale dynamic are also paid attention by Chinese meteorologists. Chen and Tan (1999), Fei and Tan (2001) investigated the evolution of helicity in the development of tropical cyclone and the helicity dynamics of severe convective storms. Gao and Lei (2000) deduced a streamwise vorticity equation in three-dimensional natural coordinate. And the equation reveals that the total change and the local change of the streamwise vorticity both depend on the curvature of streamline, unsteady feature of streamline and magnitude of velocity. Gao and Chen (2000) studied the lee waves over a big topography, through the rotating tank experiments.
III. THE RESULTS FROM MESOSCALE NUMERICAL SIMULATION AND DATA DIAGNOSTIC ANNALYSES
With the evident improvement of the computing facilities, the development of mesoscale model and the fast increase of the available observational data, many numerical experiments and data diagnostic analyses for particular phenomena in China are made in recent years. The focus is torrential rain, which is a common meteorological disaster and may cause floods.
Sun and Zhao (2000) made a diagnosis and simulation study of a strong heavy rainfall in South China in the mid June of 1994. The results of diagnosis show that the heavy rainfalls in Guangxi Autonomous Region may be caused by terrain or other factors, besides enough moisture.
The results of stimulations indicate that the hydrostatic scheme is better than non-hydrostatic scheme in simulating large-scale rainband; on the contrary, in reproducing the maximum centers of heavy rainfalls the opposite is true. Further, they (2002a, 2002b) analyzed the mesoscale convective system and its environmental fields during the June 1994 record heavy rainfall of South China. Results show that the convergence of low-level moisture flux is very strong, and that with the development of strong convective system, the maximum vertical motion extends upwards, so do the saturated layer and the neutral layer. The appearance of neutral layer in the middle troposphere is greatly different from that of strong rainstorms of North America. The analyses of the physical processes, initial environmental fields and topography on Meso-βconvective system show that the latent heat flux can affect the triggering and developing of strong convective systems. The effect of the relative humidity is the most significant to simulation of the strong convective systems. The effect of temperature of initial field is smaller than that of humidity field. The effect of the “trumpet”-shaped topography in the north of Guangxi Autonomous Region is very important to simulation of strong heavy rainfall.
Song and Lu (2001) designed a non-hydrostatic model for deep moist slantwise convection and made a numerical study of the conditional symmetric instability (CSI). It is concluded that a strong slantwise updraft layer appears accompanying downdraft layers; a slantwise cloud is formed and sustained along the updraft region of CSI; mixed-instability is formed because of the development of CSI. Zhang et al. (2002) analyzed the flash flood on 21 July 1998 in Wuhan of China. The diagnostic analyses show that the flash flood is caused by a meso-β convective system and the atmosphere is nearly saturated with a big store of convective available potential energy. Bei and Zhao (2002a, 2002b), and Bei et al. (2002) simulated successfully the low-level vortex and shear line and accompanying severe heavy precipitation in July 1998 by using non-hydrostatic mesoscale model. They also studied the effect factors of heavy rainfall and the possible way to improve forecast. It is found that a series of meso-βweather system occurred and developed along the middle reaches of the Yangtze River, and hence, caused severe heavy rainfall in this area. The occurrence and evolution of these weather systems have the characteristics of local phenomena.
Moist potential vorticity (MPV) theorem is widely applied to the studies and diagnoses of torrential rain and mesoscale cyclone. Shou and Li (1999), Shou et al. (Changjiang River and Huaihe River) (2001) and Wu et al. (2002) investigated the heavy rain occurring in July 5-6, 1991 in the Jianghuai Valleys, in the light of the MPV theorem. Results show that the distribution feature of MPV is closely related to the development of a mesoscale cyclone, and that the favorable pattern of the moist isentropic surface and the cold air sliding down along the moist isentropic surface rapidly were the favorable factors to increase the absolute vorticity and to cause the cyclogenesis. Fan et al. (2001) discussed the application of the PV conservation principle for meso-cyclogenesis. Gao et al. (2002) derived a moist potential vorticity (MPV) equation with both heat and mass forcing, and diagnosed the MPV anomaly along the Yangtze River from June 24-26 1999. Both dynamical and diagnostic methods reveal that moist potential vorticity anomaly regions correspond well to the regions of intensive precipitation. Lu et al. (2002) studied the cause of meso-β vortex system by using the high-resolution model output data. They found that the development of mesoscale vortex system is connected with the meoscale transportation, accumulation of various thermodynamic and dynamic variables, and abrupt vertical motion in regional area in the moist neutral stratification condition.
In a word, the mesoscale dynamics research of China in the past several years makes much progress and the studies we did reflect particular ideas, such as, the SVD theory, the mass forcing in a torrential rain system, the heterotropic instability, the instability of the vortex sheet along the shear line, the criterion of non-linear subcritical barotropic stability; various mesoscale numerical simulation and data diagnostic analyses throw light on some particular meteorological phenomena in China and deepen our understanding of some mesoscale systems, especially the meso-β system. However, we still have a long way to go to keep up with the advanced research level in the world. We believe that mesoscale meteorological research has a prospective future in China, with the development of our country's economy, the collective effort of all Chinese meteorologists, and the fast increase of the available observational data.
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