EVOLUTION OF LARGE SCALE DISTURBANCES AND THEIR INTERACTION WITH MEAN FLOW IN A ROTATING BAROTROPIC ATMOSPHERE——PART II

In part I of this paper, we have discussed two problems: the general properties of two-dimensional baro-tropie motion and the evolution and structure of both Rossby wave packet and inertio-gravity wave packet, In this part, we shall continue our discussion. Third, normal modes and continuous spectra of both quasi-geostrophic and non-geostrophic models, their different behaviour, and the comparison of normal mode approach to the wave packet approach. Fourth, weakly nonlinear theory of interaction based on the analysis of eddy transports, A nonzonal basic flow as well as non-geostrophic model is also included in the consideration, The last, the fully nonlinear theory, making emphasis on the conditions for the maintenance of nonzonal disturbances and the conditions for their continuous and complete absorption by the zonal flow. A comparison of Rossby wave absorption to energy cascade in the two-dimensional turbulence is also given.     

EVOLUTION OF LARGE SCALE DISTURBANCES AND THEIR INTERACTION WITH MEAN FLOW IN A ROTATING BAROTROPIC ATMOSPHERE——PART Ⅰ

The problems on evolution of large-scale disturbances and their interaction with mean flow recently attract much effort of meteorologists due to their practical importance in weather and climate predictions. In this paper, some theoretical results obtained in current investigations of these problems will be reviewed. A barotropic atmosphere is taken in this paper, and the baroclinic atmosphere is left in our second paper.The following aspects are reviewed: First, the general properties of two-dimensional barotropic motion both in the nonlinear and linearized equations and both in the quasi-geostrophic and non-geostrophic models. Second, the evolution and the structure of Rossby wave packet superimposed on a zonal or non-zonal basic flow.In this part, only the above two problems are reviewed. The remanent problems, i.e., the normal modes and continuous spectra of both quasi-geostrophic and non-geostrophic models, weakly nonlinear theory and the fully nonlinear theory will be discussed in part Ⅱ (another     

INTERACTION OF TOPOGRAPHY WITH SOLITARY ROSSBY WAVES IN A NEAR-RESONANT FLOW

An inhomogeneous KdV equation including topographic forcing is derived by usingperturbation expansions and stretching transforms of time and space.The generation of forcedsolitary Rossby waves by topography in a near-resonant flow and their interactions with freesolitary waves are discussed,and some interesting results are obtained.The numerical resultsshow that the topography has obvious effect on enhancing the amplitude of disturbances,and itmay explain to some degree the formation of blocking by localized topography.     

THE LARGE-SCALE TOPOGRAPHY AND THE STABILITY OF ROSSBY WAVE IN THE BAROTROPIC ATMOSPHERE

The effects of the large-scale topography on the stability of Rossby wave are discussed by usingthe method of the multiple scale analysis. The results show that the influences of the topographywith north-south or west-east slopes on Rossby wave and energy propagation are different.     

WAVE-MEAN FLOW INTERACTION AND FORMATION OF BLOCKING——PERSISTENT ANOMALOUS WEATHER IN CHINA IN THE SUMMER OF 1980

In the summer of 1980,serious persistent abnormal weather occurred over vast areas in China.While record-break-ing cold and flood were observed in the reaches of the Changjiang and Huaihe Rivers,severe hot wave and drought dom-inated the entire northern China.The long-lasting disastrous weather is mainly due to the stable development and main-tenance of blocking anticyclone over the northeastern Asia.This study aims at the understanding of the roles oftime-varying weather system transport in the formation of the blocking.It was shown that during this period,there appeared continuous generation of synoptic-scale perturbations alongthe strong baroclinic zone over Europe and the western Asia.While such perturbations propagated eastward,energyconversion occurred.At equivalent barotropic layer,with weak dissipation,such energy conversion was subjected to theso-called bi-directional principle:while the energy of the synoptic-scale system cascaded to smaller scale system,amuch larger portion was transferred to the blocking system with larger scale.Potential vorticity diagnoses also revealedthat the transient weather systems played the roles of maintaining the mean anticyclonic vorticity to the south,and meancyclonic vorticity to the north,of the westerly jet,and exciting strong anticyclonic vorticity growth and correspondinggeopotential height increase in high latitude area downstream of the westerly diffluence region.The research also showed that,the intensity of the forcing of the blocking formation via wave-mean flowinteraction in this Asian case was much stronger than that occurring in the western Europe in the summer of 1976.It wastherefore concluded that when persistent abnormal weather in the northern China was studied,in addition to thesubtropical weather systems,attention should also be drawn to the development of baroclinic zone over Europe and thewestern Asia,and the propagation and transfer properties of the synoptic systems embedded in the baroclinic zone.     

MESOSCALE INSTABILITY OF A BAROCLINIC BASIC FLOW——PART I:SYMMETRIC INSTABILITY

This is the first part of“Mesoscale Instability of a Baroclinic Basic Flow”,which deals with the stabilityof a baroclinic flow against mesoscale perturbations parallel to it and the possibility for occurrence of abandwise mesoscale disturbance along with its effects on triggering and organizing deep convections in astratified atmosphere of weak static stability.The atmospheric perturbation equations with anelastic assumption are formed in the way that theyare suitable to be solved by generalized matrix method and by shooting method.The mesoscaIe instabilityspectra and the structures of the characteristic perturbations are calculated for linear,hypertangent profilesand realistic wind profile sounded before a squall line event.By energy analysis the effects of baroclinityof the basic flow on generating such kind of ageostrophic inertial convection are also discussed.     

MESOSCALE INSTABILITY OF A BAROCLINIC BASIC FLOW——PART I: SYMMETRIC INSTABILITY

This is the first part of "Mesoscale Instability of a Baroclinic Basic Flow",which deals with the stability of a baroclinic flow against mesoscale perturbations parallel to it and the possibility for occurrence of a bandwise mesoscale disturbance along with its effects on triggering and organizing deep convections in a stratified atmosphere of weak static stability.The atmospheric perturbation equations with anelastic assumption are formed in the way that they are suitable to be solved by generalized matrix method and by shooting method.The mesoscaIe instability spectra and the structures of the characteristic perturbations are calculated for linear,hypertangent profiles and realistic wind profile sounded before a squall line event.By energy analysis the effects of baroclinity of the basic flow on generating such kind of ageostrophic inertial convection are also discussed.     

THE OSCILLATION OF CERTAIN ZONAL MEAN CHARACTERISTICS OF MOTION ON A SPHERIC EARTH'S ATMOSPHERE——PART 2:ANISOTROPIC QUASI-EDDY MOTION

This paper is an extension of the auther's works (1980,1982,1987).Simple anisotropic distribution of hori-zontal kinetic energy is assumed,i.e.,the zonal kinetic energy is twice that of the meridional kinetic energy.The large-scale atmospheric motion is considered as that consisting of quasi-horizontal eddies and regular flow.The techniques of quasi-eddy,quasi-steady,quasi-geostrophic and quasi-adiabatic approximations are usedin order to get the analytical solutions of the system of equations governing the variation of the zonal meancharacteristics.The results show that the zonal mean characteristics of the atmospheric motion are a combination of dif-ferent periods ranging from a few days to a few weeks depending on the components of Chebyshev polynomi-als of the sine of the latitude.When more general anisotropy of horizontal kinetic energy is assumed,Gegenbauer polynomials appearinstead of Chebyshev polynomials.When the distribution of horizontal kinetic energy is isotropic,the polyno-mials retrograde to Legendre polynomials.     

ABRUPT CHANGE OF FLOW PATTERN IN BAROCLINIC ATMOSPHERE FORCED BY JOINT EFFECTS OF DIABATIC HEATING AND OROGRAPHY

Based on the catastrophic theory, the possible causes of abrupt change in the atmospheric circulation over the Northern Hemisphere during June and October have been explored by Li and Luo (1983) and Miao and Ding (1985). However these studies are confined to the barotropic atmosphere without consideration of orography. The purpose of this paper is to further study the physical mechanism of the abrupt change of flow pattern within the baroclinic atmosphere in the presence of orography. Results show that the abrupt change of flow pattern can be stimulated by the gradual variation of a diabatically heating parameter, which is similar to the observed fact about the rapid shift of position of the subtropical high center in the upper troposphere along the zonal direction during seasonal transition from the summer half year to the winter one.     

EFFECTS OF OROGRAPHY AND HORIZONTAL DIFFUSION ON THE GENERATION OF THE ASYMMETRIC MOTION IN THE BAROTROPIC FILTERED MODEL ATMOSPHERE

By utilizing the barotropic vorticity equation including effects of orography and horizontal diffusion,the linearized equations describing symmetric and antisymmetric motions and their analytic solutions are pre-sented.It can be found from the solutions that no matter what kind of motion may be,each solution consistsof three waves,namely,Rossby wave related to initial values,marching wave propagating at Rossby wavevelocity and stationary wave.The latter two are closely related to orography and horizontal diffusion.However,if the motion is symmetric at the initial instant,then the antisymmetric components of orographyand of horizontal diffusion are likely to lead to the generation of antisymmetric motion.In the steady state,the symmetric flow is connected with symmetric orography and horizontal diffusion and the antisymmetricflow with antisymmetric orography and horizontal diffusion.Further,in order to verify the above analysis,three numerical experiments have been made.The results show that antisymmetric orography can produceantisymmetric motion.Finally,the atmospheric interactions between Northern and Southern Hemispheresare discussed.     

A MESO-α SCALE STUDY OF MEIYU FRONT HEAVY RAIN——PART I:OBSERVATIONAL STUDIES

Jn this paper, the data collected during the Mesoscale Weather Experiments in East China are utilized to study the meso-a scale rain-bands of meiyu front heavy rain, its structural features as well as the mechanism of its development. It has been revealed that the precipitation band during the meiyu season is in the shape of ribbon, which is parallel to the surface quasi-stationary front. Sometimes two meso-a scale rain-bands are present. The meso-a scale rain-band is associated with meso-a scale convergence line. As shown by the two dimentional disturbance circulation, calculated through band-pass filtering, the single rain-band is quite different from the double rain-bands. The former is, to some extent, akin to the frontogenetical circulation in the vicinity of the high- and low-level frontal zones; the latter features roller-like circulations at middle and low-levels with their axes parallel to the rain-bands while at higher levels they run in the opposite direction. This kind of disturbance may be     

A STUDY OF CIRCULATION DIFFERENCES BETWEEN EAST-ASIAN AND INDIAN SUMMER MONSOONS THEIR INTERACTION

Primarily based on the 1979 FGGE data an analysis is made of the circuktion differences between the East-Asian and Indian summer monsoons together with their oscillation features and also the interplay between various monsoon systems originating from the fact that the Asian monsoon area is divided into the East-Asian and Indian regions, of which the former is demarcated into the Nanhai (the South China Sea) and the Mainland subregions.     

THE OSCILLATION OF CERTAIN ZONAL MEAN CHARACTERISTICS OF MOTION ON A SPHERIC EARTH’S ATMOSPHERE——PART 2:ANISOTROPIC QUASI-EDDY MOTION

This paper is an extension of the auther's works (1980,1982,1987).Simple anisotropic distribution of horizontal kinetic energy is assumed,i.e.,the zonal kinetic energy is twice that of the meridional kinetic energy.The large-scale atmospheric motion is considered as that consisting of quasi-horizontal eddies and regular flow.The techniques of quasi-eddy,quasi-steady,quasi-geostrophic and quasi-adiabatic approximations are used in order to get the analytical solutions of the system of equations governing the variation of the zonal mean characteristics.The results show that the zonal mean characteristics of the atmospheric motion are a combination of different periods ranging from a few days to a few weeks depending on the components of Chebyshev polynomials of the sine of the latitude.When more general anisotropy of horizontal kinetic energy is assumed,Gegenbauer polynomials appear instead of Chebyshev polynomials.When the distribution of horizontal kinetic energy is isotropic,the polynomials retrograde to Legendre polynomials.     

A DIAGNOSTIC ANALYSIS OF 1997/1998 ENSO EPISODE AND ROLE OF INTRA-SEASONAL OSCILLATION IN TROPICAL ATMOSPHERE

On the basis of comparison of 1982/1983 strong ENSO processes, key characteristics such as rapid development, immense intensity and abrupt retreat are revealed with regard to the warm episode in 1997/1998, features governing the intraseasonal oscillation for the tropical Indian Ocean and the western Pacific during its onset and evolution of the ocean and atmosphere for the process are diagnostically studied in detail.     

A MESO-α SCALE STUDY OF MEIYU FRONT HEAVY RAIN－PART I: OBSERVATIONAL STUDIES

In this paper, the data collected during the Mesoscale Weather Experiments in East China are utilized to study the meso-α scale rain-bands of meiyu front heavy rain, its structural features as well as the mechanism of its development. It has been revealed that the precipitation band during the meiyu season is in the shape of ribbon, which is parallel to the surface quasi-stationary front. Sometimes two meso-α scale rain-bands are present. The meso-α scale rain-band is associated with meso-α scale convergence line. As shown by the two dimentional disturbance circulation, calculated through band-pass filtering, the single rain-band is quite different from the double rain-bands. The former is, to some extent, akin to the frontogenctical circulation in the vicinity of the high- and low-level frontal zones; the latter features roller-like circulations at middle and low-levels with their axes parallel to the rain-bands while at higher levels they run in the opposite direction. This kind of disturbance may be caused by the symmetric instability in the moist atmosphere.     

MESOSCALE INSTABILITY OF A BAROCLINIC BASIC FLOW—PART II:TRANSVERSAL INSTABILITY

This is the second part of “Mesoscale Instability of a Baroclinic Basic Flow” which discusses theinstability of a basic flow against mesoscale perturbations of transversal type.A bi-mode instability spectrum is obtained by generalizing the Eady model to ageostrophic regime inan f-plane:Eady modes present at the synoptic and subsynoptic scales,while the ageostrophic baroclinicmesoscale modes present at the inertial scales of a few tens to hundreds kilometers.The mesoscale modeis featured by an asymmetric "eat eyes" pattern in the vertical cross section and by an alternative distri-bution of divergence and vorticity in the horizontal direction.The growth rates of the mesoscale modesare about four times larger than those of Eady modes in magnitudes for the same wind profile.The majorenergy source for development both Eady mode and mesoscale mode is the baroclinic available energy storedin the rotational basic flow.     

VARIATIONS OF 30—60 DAY OSCILLATION IN ATMOSPHERE BEFORE AND DURING 1982 EL NINO

By using the upper-wind data from July 1980 to June 1983,the variations of the low-frequency oscillation(LFO)in the atmosphere before and during 1982 El Nino have been investigated.Before the El Nino,the LFOpropagates from west to east over the equator of the Eastern Hemisphere and from east to west over 20°N.The eastward propagating LFO over the equator consists of zonal wavenumber 1 propagating eastward andzonal wavenumber 2 with a character of stationary wave.The oscillation of zonal wavenumber 2 can modulatethe oscillation strength.After the onset of the El Nino,the propagating directions of the LFO over the equatorand 20°N of the Eastern Hemisphere change to be westward and eastward,respectively.The LFO over thewestern Pacific weakens rapidly and one coming from middle and high latitudes propagates to the equator.From the phase compositions of streamline fields for the zonal wavenumber 1 of equatorial westward propa-gatirg LFO,it is found that the atmospheric heat source in the equator of the eastern Pacific(EEP)excites aseries of the equatorial cyclones and anticyclones which move northward and westward and form the westwardpropagating LFO over the equator.With the wavelength of 20000km,this kind of equatorial wave is similarto the mixing Rossby-gravity wave.In its westward and northward movement,the circulation in East Asiais modified.This may be the mechanism of the influence of El Nino on the climate of China.     

ROLE IN MOISTURE TRANSPORTATION OF SEASONAL MEAN AND INTRASEASONAL OSCILLATIONS IN ASIAN SUMMER MONSOON AREA——LONG-TERM AVERAGE CHARACTERISTICS

By use of the May—September 1980—1986 ECMWF daily data of u,v,r and T at 850 hPa,a comparative analysisis performed of basic features of moisture transportation at seasonal mean,quasi-40-day,-biweekly,and-weeklyoscillations,indicating that the seasonal mean transfer plays a decisive role in the moisture flux over the Asian monsoonregion,displaying the integer of the monsoon systems there in character;that the transport related to these tropicalintraseasonal oscillations are of equal importance in the monsoon period except the difference in their behaviors,i.e.,thetransfer shows considerable relative independence in the South-and East-Asian systems;and that the transport at allthese intraseasonal oscillations is found to be feeble at equatorial latitudes with little or no influence on each other forboth hemispheres.     

THE ESTIMATION OF THE LONG-RANGE TRANSPORT OF AIR POLLUTANTS IN CHINA——PART I:TRAJECTORY ANALYSIS

The trajectories of air parcels through the ten big cities of China(Beijing,Xi'an,etc.)have been com-puted by using the data of wind fields at 1500 m elevation.The data based on these trajectories and endpoints of trajectories have been used to calculate the long-range transport of pollutants emitted from thesecities and to estimate the geographic position of distant sources affecting these cities.An advantage of thestatistical trajectory model is eXpected that the error of the individual trajectories tends to be averaged outin the long run.     

THE ESTIMATION OF THE LONG-RANGE TRANSPORT OF AIR POLLUTANTS IN CHINA——PART I: TRAJECTORY ANALYSIS

The trajectories of air parcels through the ten big cities of China(Beijing,Xi'an,etc.)have been computed by using the data of wind fields at 1500 m elevation.The data based on these trajectories and end points of trajectories have been used to calculate the long-range transport of pollutants emitted from these cities and to estimate the geographic position of distant sources affecting these cities.An advantage of the statistical trajectory model is eXpected that the error of the individual trajectories tends to be averaged out in the long run.