A DYNAMICAL STUDY OF THE FLOW FIELD IN THE PBL OVER LARGE-SCALE TOPOGRAPHY

In this paper, the eddy coefficient and the treatment for dividing layers of Estoque's numerical model in modernPBL dynamics have been applied to derive order-zero and order-one analytic solutions over large-scale topography,the solutions are used to obtain the divergence and vertical velocity, improving the previous results.     

大地形上边界层流场的动力学研究

本文应用边界层气象学中Estoque数值模式关于湍流交换系数及分层的处理方法,求得了大地形存在时定常边界展方程的零、一级解析解,并用来得到大地形存在时边界层的散度场、垂直速度场,改进了前人的结果。     

DYNAMICAL CHARACTERISTICS OF THE STREAM FIELD OF COLD FRONT IN BOUNDARY LAYER*

The motion equation of atmospheric boundary layer with cold front surface under geostrophic momentum approximation is solved and some characteristics of the stream field for the cold front surface in the boundary layer are derived,for example,the slope of the cold front surface increases with the increases of geostrophic vorticity and the temporal tendency of geostrophic wind speed,and also increases with the decrease of the component of thermal wind speed along the motion direction of the front:the stream field above and below the cold front surface in the boundary layer depends on the slope of the front surface,geostrophic wind speed and its temporal and spatial distributions.A common characteristic is that there exist updraft motion above the cold front surface and downdraft motion below it.     

A STUDY ON PHYSICAL MECHANISM OF INTERANNUAL VARIATIONS OF LARGE-SCALE FLOW PATTERNS*

The thermal forcings of annual and interannual periodic variations are introduced into the barotropic vorticity equation,by using low order spectral model of the equation,more than 40 numerical experiments whose integration time is larger than 100 model years are performed in order to investigate variations of large-scale flow patterns arising from both external interannual thermal forcing and internal dynamical processes.In certain parametric range,when the frequency of the forcing term with interannual period equals to the frequency which is created by the internal dynamical processes alone,the amplitude of interannual variations of flow patterns increases obviously,and the period becomes double.In other parametric range,the amplitude of interannual variations of flow patterns shows abrupt changes and other nonlinear behavior,along with gradual changes of interannual forcing parameters.     

THE DYNAMICAL BASIS OF GEOSTROPHIC ADJUSTMENT MOTIONS IN TROPICS*

Some dynamic constrains in the process of the tropical baroclinic geostrophic adjustment are discussed.The dispersion equation of three-dimensional inertia-gravitational wave and the equation of temporal conservation of potential vorticity are given,without considering the gradient of planetary potential vorticity.It indicates that the motions will be horizontal,meaning that Taylor-Proudman theorem is still right for tropics.At the same time,the semi-geostrophic balance is easier to appear in the tropical belt.Therefore the motions are generally horizontal and non-divergent,but still are stratified after the geostrophic balance establishes.     

A DYNAMICAL ANALYSIS OF BASIC FACTORS OF LOW-FREQUENCY OSCILLATION IN THE TROPICAL ATMOSPHERE*

Based on dynamical characteristics of the tropical atmosphere,a mathematical model of low-frequency oscillation (LFO hereinafter) in low-latitudes has been developed.The analysis shows that the distributive features (shear of wind speed) of easterlies and westerlies in low-latitudes,the divergence and convergence of meridional and zonal flow,the vertical structure of diabatic heating and the Coriolis parameter f are the basic factors resulting in the LFO,while quasi-periodical baroclinity development or index cycle of westerly waves in middle-latitudes of the Southern Hemisphere is an external forcing to the LFO in the tropical atmosphere.The resonance on adequate condition makes LFO suddenly enhanced.     

夏季青藏高原上影响MCS东移的动力场特征研究

运用空间数据挖掘中的聚类方法（K-均值法和CLARANS法）,对影响夏季青藏高原上中尺度对流系统（MCS）东移的动力场空间分布特征进行了研究,得到了有利于MCS移动和传播的动力学条件。研究成果不仅为预测高原上MCS的移动趋势,揭示其发生、发展的机理提供了一种新的方法和思路,且对于提高长江流域暴雨和强对流天气预报的准确性具有重要的实际应用价值。     

APPLICATION OF THE VARIATIONAL METHOD OF TOVS DATA OVER THE TIBETAN PLATEAU IN IMPROVEMENT OF THE INITIAL FIELD OF NUMERICAL MODELS*

In this paper,TOVS satellite data are used through variational method on the data-sparse plateau area.Diagnoses are carried out to find a way to solve the large error problem of model initial field.It is put forward that TOVS retrieval data can be used to improve the initial field of numerical prediction model on Tibetan Plateau area.Through variational method,TOVS data are processed and the liability of the initial information on the plateau is improved.Diagnostic results confirm further that the application of TOVS retrieval data can improve our capability to describe the dynamic system features on the plateau and the objectivity of related initial information such as the distribution of water vapor channel and stratification stability.     

STUDY OF THE RANDOMNESS INHERENT IN THE TYPHOON MOVEMENT*

A difference scheme is adopted to perform numerical integrations of a deterministic barotropic vorticity equation for four designed experiments,and the 1-D and 2-D sequences are obtained for the velocity of the typhoon center with the aid of model output,together with the correlation dimension and K entropy investigated.Results show that there exists randomness inherent in the center movement in the model atmosphere.Eventually the influence of localized heat sources and basic flow upon the stochasticity and predictability of the track of typhoon is discussed.     

AN EXPERIMENT STUDY OF FRONTAL CYCLONES OVER THE WESTERN ATLANTIC OCEAN*

By using PSU/NCAR MM5 mesoscale model,a 60-h simulation is performed to reproduce a frontal cyclogenesis over the Western Atlantic Ocean during March 13-15 1992.The model reproduces well the genesis,track and intensity of the cyclone,its associated thermal structure as well as its surface circulation.The major cyclone (M) deepens 45 hPa in the 60-h simulation and 12 hPa in 6 hours from 36 h to 42 h (model time) and 27 hPa in 24 hours from 36 h to 60 h (model time).Cross-section and isentropic analysis tell us that the cyclogenesis is in very close relation with slantwise isentropic surfaces;the cyclone is always superposed on the core of neutral convective stability with nearly vertical isentropic surfaces,which coincides with what the theory of Slantwise Vorticity Development (SVD) says.Beginning with the theory of SVD,the development and propagation of the oceanic frontal cyclone are studied by using high-resolution model output in the context of slantwise isentropic surfaces.The results show that the frontal cyclone deepens rapidly by the interaction with the large-scale environment after occurring over the ocean with weak static stability;and the theory of SVD can well interpret the development and propagation closely related with slantwise isentropic surfaces,The downstream slantwise up-sliding movement along canting isentropic surfaces makes vorticities develop (USVD) under favorable condition (C_D<0,where CD is SVD index),and results in the moving and development of the cyclone.     

THE MODELING STUDY OF DOWNBURST*

Downburst is a very dangerous weather phenomenon for aeroplane taking off or landing. In order to understand the initial formation and evolution of downburst and to study the effects of atmospheric environment condition and the microstructure of cloud-precipitation particles on the downburst development processes, we have designed and carried out a modeling scheme by making use of our own non-hydrostatic compressible mesoscale-γ model including necessary cloud-precipitation processes. The initial conditions of temperature, humidity and wind are from an observation case in which the downburst occurred. The results of computations demonstrate the evolution of downburst and show the variation of various environmental and microphysical parameters. Some of the mechanisms about the downburst occurrence have been obtained. Computation results may help airport forecasters to determine the occurrence of downburst better.     

CLIMATIC CHARACTERISTICS OF NET RADIATION IN THE EARTH-ATMOSPHERE SYSTEM OVER CHINA*

Investigated are effects of the total cloudiness and other factors on earth-atmosphere net radiation(EANR) and analyzed is its relation to other components and ground surface net radiation in the context of ERBE and ISCCP.Evidence suggests that planetary scale albedo and earth-atmosphere short wave absorption radiation have maximum effect on the net radiation under study,with the influence of cloud and latitude displayed predominantly through the two factors;OLR has relatively weak effect;the earth-atmosphere net radiation is well correlated with surface net radiation.Analysis is also performed of the geographic distribution of the earth-atmosphere net radiation throughout China,and the annual curve of the net radiation on a local basis is marked by high(low) value in summer(winter) with the impact of factors.including total cloudiness responsible largely for the shift of the months with maximum.     

A NUMERICAL STUDY ON FORMATION OF THE NEGATIVE VORTICITY REGION OVER THE NORTHEAST SIDE OF THE QINGHAI-XIZANG PLATEAU*

In order to investigate the formation of the negative vorticity region over the northeast side of the Qinghai Xizang (Tibetan) Plateau,four sets of numerical experiments have been performed in this paper with a quasigeostrophic barotropical model considering large-scale topography,diabatic heating and dissipation.The diabatic heating in the model contains a constant forcing and timevarying forcing.The time-varying characters are determined by the continuous evolution of the sensible heat flux at Damxung Station (30°29'N,91°06'E) from 31 May to 4 June 1998.Results suggest that there are three types of processes significantly contributing to the formation of the negative vorticity region over the northeast side of the Qinghai Xizang Plateau,and they are the advection of the anticyclonic vortex at the upstream by the basic flow,the energy dispersion of the cyclonic vortex over the south side of the Plateau,and the strengthening of anticyclonic systems produced by the thermal forcing of the Plateau.     

TRAJECTORY ANALYSES OF THE DUSTSTORMS OVER EAST ASIA*

The transports of dust are calculated using 3-dimensional(3-D) trajectory method for three cases of duststorms in the terrain-following coordinate system,and the synoptic processes are also discussed for each case.The case of 17-20 April 1980,a severe duststorm was associated with the rapid development of a cyclone over the Mongolia Plateau.The dust moved from west to east across several deserts,formed a typical dust path in spring.The other two were weaker and the dust was triggered by the strong wind behind the cold front from northwest or north.Because the vertical velocity is considered in 3-D trajectory analysis,trajectories calculated should better reveal the transport rule of the dust particles and the results seem to be more consistent with the synoptic processes.The trajectory analysis on the 2-D isobaric surfaces is simpler but can be used only in the conditions with weak vertical wind shear and weak vertical velocity.The difference of trajectories at lower levels between two methods may be caused by the different treatment of orography.     

大尺度大气运动年际变化的数值研究

大尺度大气运动的流型和区域性气象要素随时间的演变,均具有比较清楚的年际变化。流型和要素年际变化的若干统计规律,正在长期天气预报和气候预报的业务中被广泛运用。目前,关于年际变化形成机理的问题已日益受到重视,这将为相应的预报方法提供动力学的理论基础。     

A NUMERICAL STUDY OF THE VARIATIONS AND DISTRIBUTIONS OF TROPOSPHERIC OZONE AND ITS PRECURSORS OVER CHINA*

By means of a three-dimensional meteorological model (MM5) and a chemical model,the distributions of tropospheric ozone and its precursors over China have been simulated in summer and winter time,16-18 August 1994 and 7-9 January 1995.The distribution of ozone over the Tibetan Plateau in summer time is deeply discussed.The simulated results indicate that thedistributions of surface ozone and NO_x are in good agreement with observed results,and human activities and photochemical reactions are the main factors controlling the surface ozone and NO_x concentrations.In addition,higher ozone concentrations are coincided with the air convergence,and the lower concentrations are related to the air divergence.In summer,over the Tibetan Plateau the strong flow convergence results in higher ozone concentrations in the lower troposphere:and the strong flow divergence results in lower ozone concentrations in the upper troposphere.In winter time ozone concentrations show large-scale characteristics controlled by westerly flow,and in the jet area they are lower than those outside the jet.     

ON THE MODELLING STUDY OF MONSOON*

The major advances and achievements of recent studies on monsoon simulation are reviewed.Four aspects are included in the paper and they are simulation of general feature and seasonal to interannual variation of monsoon.simulation of monsoon and ENSO relations,paleo-monsoon simulation,and the predictability study of monsoon and monsoon climate.Concluding remarks are made in the last section.     

AN ANALYSIS OF THE AEROSOLS IN THE LOWER-LEVEL ATMOSPHERE OVER BEIJING NORTHERN SUBURBS IN WINTER*

The physical and chemical characteristics of the aerosol samples were studied.The samples were collected on the 3 platforms (at levels 8 m,100 m and 320 m respectively) of IAP (Institute of Atmospheric Physics,Chinese Academy of Sciences) observation tower during the periods from January 1 to 16 and from February 20 to March 2,2001.The concentrations of TSP (total suspended particles),chemical components and size distribution for aerosols were calculated.The effects of the meteorological conditions on the aerosol concentrations and its distributions werestudied.The main aerosol sources from which the aerosols in the samples collected came were investigated.The conclusion was drawn preliminarily as follows.The TSP concentrations decreased with height.Over 70% of the aerosol mass were distributed in the fine particle group (d<2.1μm).The top 20 aerosol elements with high concentration are listed from high to low according to their concentrations,and they are S,Fe,Ca,K,A1,Na,Mg,Zn,Cl,Ba,P,Ti,Pb,Mn,As,Cu,V,Br,Cr,and Sb.The aerosol element concentrations decreased with height,too.The mass of volatile elements was mainly distributed in the fine particle group.The ion mass was distributed in the same way as the element mass.From the point of view of contribution to the air aerosols,both anthropogenic and natural sources are important for Beijing.     

QUASI-BIENNIAL OSCILLATION IN RAINFALL OVER CHINA*

Based on monthly rainfall data from 80 stations of China for the period of January 1951—February 1985 analysed in terms of the maximum entropy spectrum and EEOF(extended empirical orthogonal function) techniques,the spatial and temporal features of quasi-biennial oscillation(QBO)in the rainfall are investigated in a detailed manner,with some results of interest acquired.Highly evident is the QBO in rainfall over China,especially the mid and lower reaches of the Changjiang River,the NE side of the Qinghai-Xizang Plateau,North China,the southern part of NE China,areas south of the Changjiang River and of the Nanling mountain region,and most of Xinjiang.The spatial wave described by the first and second eigenvectors of EEOF for eastern China is a type of wavetravelling from NE to SW,indicating the cycle of patterns(in the south to north direction):dryness(D)-wetness(W)-dryness(D)→southern W-northern slight D→W-D-W→southern D-northern slight W→D-W-D again,with the flooded region between the Changjiang and Huanghe Rivers for the D-W-D occurrence and with the Changjiang River as the division for the southern W-northern slight D happening.The third and fourth eigenvectors show a standing wave-like feature for the same eastern part,indicating the cycle of D-W-D→transition→W-D-W→transition→D-W-D,again,where the transition means the general weakening of drought and flooding and their subsequent reversal development.The amplitudes of the time coefficients of the first two eigenvectors are opposite in the trend of their changes to those of the next two and show a period of 10—14 years.     

DYNAMICAL STUDY OF DIABATIC HEATING INFLUENCE ON RADIAL INHOMOGENEOUS STRUCTURE OF TROPICAL CYCLONE*

Based on the tropical cyclone (TC) asymmetric disturbance as the superposition of the symmetric environmental circulation,the analytical solution of travelling wave is given by using the barotropical nondivergent model with diabatic heating forcing and non-friction in a plane polar coordinate.Then,the TC radial inhomogeneous structure is analyzed on radial/tangential velocity and geopotential height.It is found that the different kinds of structures are influenced by the Coriolis parameter (f),TC intensity (Ω),disturbance circular frequency (ω),and TC angular wavenumber (m).And,the diabatic heating (Q₁) has significant impacts on the radial/tangential velocity distribution shaped like the inner-tight and outer-relaxed.