EASTWARD MOVEMENT OF MIDLATITUDE LOW FREQUENCY WAVES AND THEIR INTERACTION WITH SYSTEM DOWNSTREAM*

Within the framework of forcing/dissipation KDV dynamics, the effect is numerically studied of successive east-travelling troughs on quasi-stationary high's ridges downstream. Results show that two types of low-frequency transiency are available, depending on the intensities of down-drifting troughs. One is that transiency occurs in strength, i.e., the ridge undergoes transient change in intensity with unchanged position; the other is that transiency takes place in pattern, viz., the breakdown of the quasistationary character of the ridge's posi-tion, meaning that the ridge is replaced by a trough, followed by multiple transformation between ridge and trough.     

MERIDIONAL PROPAGATION OF EAST ASIAN LOW-FREQUENCY MODE AND MIDLATITUDE LOW-FREQUENCY WAVES*

Based on June to September 1981 ECMWF grid datasets analysis is done of the characteristics of thepropagation and structure of low-frequency(quasi 40 day) oscillation over eastern Asia. Results show aseparating(confluence) belt for the meridional propagation of low-frequency zonal(meridional) winds at higher(lower) levels over subtropical latitudes at 120°E, revealing that the oscillation of the zonal winds is quasi-geostrophic in pature and in phase in the high- and low-level. It is also found that the eastward propagationof the high-level zonal winds around 35°N in East Asia is the result of eastward march of midlatitude low-frequency waves with 60-90 longitude wavelength and speed of 1.5-2.0 longitudes per day. In addition, suchlow-frequency vortices, when moving over the coastwise region, tend to develop, accompanied by sharp oscil-lation in the westerly jetstream over eastern Asia.     

大气中行星尺度孤立波的动力特征

大气中的非线性波一直是气象学界所关心和重视的问题,Long[1]和Benney[2]等人最早把孤立波理论应用到大气中来,成功地解释了大气中的一些天气现象,后来我国学者巢纪平[3]、刘式达、刘式适[4]等又进行了这方面的研究工作,取得了一些有意义的结果。本文利用行星大气中的相当正压模式和Burger模式,在一定的条件下分别导出了它们所满足的KdV方程,其解是椭圆余弦波和孤立波,并讨论了孤立波的凸凹性,找到了孤立波西移的判据,指出了中高纬度大气中的阻高和切断低压等天气系统具有“孤立波”的性质。     

THE INTERLINKAGE OF LARGE-SCALE SYSTEMS BETWEEN HIGH AND LOW LATITUDES RELATED TO PROPAGATION OF THE PLANETARY WAVE AROUND EAST ASIA IN SUMMER*

This study has reviewed recent studies about impact of the Rossby wave propagation on the weather of East Asia.Then,we have focused on the northward progression of the subtropical high affected by the propagations and their interaction.The following results are obtained:(1) The two kinds of meridional Rossby wave propagations often affect the East Asian weather in summer,which propagate in two conflicting directions(northwest-southeastward and southwest-northeast-ward).This is considered as the interaction of the large-scale systems between high and low latitudes with wave propagation,especially to the circulation systems around East Asia.(2) The northward progression of the subtropical high is largely restrained,aecompanied by the southward shift of the convective activity in middle latitudes,under the strong and persistent Rossby wave propagation from high to low latitudes.And the opposite role could be found in the Rossby wave propagation from low to high latitudes.     

LOW-FREQUENCY SUMMER MONSOON IN INDONESIA-NORTHERN AUSTRALIA AND ITS RELATION TO CIRCULATIONS IN BOTH HEMISPHERES*

In terms of the expansion by extended empirical orthogonal function(EEOF) with data of ECMWF/WMO and of outgoing longwave radiation(OLR) furnished by the NOAA polar-orbiting satellite,a study is made of November 1981 to March 1982 low-frequency(30-60 day) summer monsoon activity in the Indonesian-North Australian zone(INAZ) together with its relation to atmospheric circulations in the Northern and Southern Hemispheres. Results show that at 850 hPa the southward blowing low-frequency NE(SW) wind from the eastern Asian coast changes, after crossing the euqator, to summer NW(SE) monsoon in the INAZ, which, when converging with(diverging from) the western-Australian enforced low-frequency SW(NE) wind, brings about strengthened(weakened) convection in the summer monsoon area and its eastward advance, with corresponding low-frequency variation shown in 850 hPa geopotential height. These outcomes are similar to those from the study of non-filtered actual observations, leading to the conclusion that the component of low-frequency variation illustrates major features characteristic of the variation in the tropical circulation.     

中纬和热带气旋路径可预报性的差异

应用非线性系统分析方法，对比研究了中纬度欧洲地区（０～９０°Ｅ，２５～６０°Ｎ）冬季１８０个气旋路径和西太平洋地区（１１０～１７０°Ｅ，１０～４５°Ｎ）夏季１８０个台风路径。通过计算独立气旋“对”的关联函数和气旋路径的结构函数，得到了代换相空间中气旋（台风）路径吸引子维数为２～２．５（１．４～１．９）和路径的自相似性。由此推断初始误差ｅ的折指率均为２～３天左右，分区的结果亦类似。对气旋路径的时间标度分析表征着系统的无标度性，关联方差谱遵从频率的２～３次方幂律。     

MESOSCALE NONLINEAR GRAVITATIONAL WAVES AND THEIR INTERACTION IN VERTICAL SHEAR FLOW*

Interaction equations of two nonlinear gravitational waves in baroclinic atmosphere are presented via multi-scale perturbation method,which can be classified into coupling nonlinear Schrodinger equations.In particular,the interaction course of two nonlinear gravitational waves of basic flow in vertical linear and quadratic shear is illustrated.Numerical calculation displays that wave amplitude enlarges and wave width narrows when two solitary gravitational waves meet and chase;that basic flow with single shear is more beneficial than that with quadratic shear to the interaction of two nonlinear wave packets;and that the interaction of two wave packets makes wave shape change more greatly and energy more dispersive,which contributes to the occurrence of changeable weather.Therefore,one of the probable mechanisms for the appearance of strong convection weather is the interaction between mesoscale nonlinear gravitational waves.     

STUDY OF NONLINEAR ROSSBY WAVES IN THE ATMOSPHERE UNDER SEMI-GEOSTROPHIC APPROXIMATION*

Under semi-geostrophic approximation the nonlinear ordinary differential equations are obtained for the motion in the barotropic and baroclinic atmospheres with the effects of zonal shear basic flow and topographic forcing included.Two constraints are acquired of finite-amplitude periodic and solitary waves in the original model with the aid of the phase-plane geometric qualitative theory of a dynamic system defined by the differential equation.The explicit solution of the nonlinear waves is found by means of the approximation method and some significant results are achieved.     

ON THE BAROTROPIC INSTABILITY OF LARGE SCALE VORTICES*

By using the linearized barotropic vorticity equation in polar coordinates the stability of perturbations on a large scale circular basic flow is transformed into a generalized eigenvalue problem,yielding the relationship between the growth rate of the amplitude of perturbations and the azimuthal wave number. Then, numerical experiments whose integration time is 60 model hours are performed in terms of a quasi-geostrophic barotropic model in Cartesian coordinates using the perturbation stream function field of unstable mode superimposed on a strong and weak circular basic flows as the initial fields. The experimental results reveal that the amplitudes of the initial perturbations in the model atmosphere grow with time. The amplitude of the perturbations superimposedon the strong circular basic flow grows quicker and forms a spiral-band-like structure.     

OBSERVATIONAL STUDY OF MERIDIONAL VARIATION OF UV-B RADIATION DURING VOYAGES TO THE ARCTIC AND ANTARCTIC REGIONS*

Based on 1999-2000 observations made by the first Arctic and sixteenth Antactic scientific voyages,a study is undertaken about the meridional surface UV-B (B band ultraviolet rays) variations in 75°N-70°S.It is mitigated as a function of latitudes and marked by lower radiation averaged over the Northern Hemisphere (NH) than over the Southern Hemisphere (SH),with its daily course basically similar to that of total radiation.Around polar summer noon hours (localtime) and where ice albedo is maximum,the strongest UV-B irradiance on the surface perpendicular to sun's beams as found at equatorial latitudes is measured sometimes.In the areas near Zhongshan Station the increase of surface UV-B radiation shows a close relation to the decrease of ozone in the higher atmosphere but it has a less intimate relation with its concentration at ground.     

INFLUENCE OF INTERACTION BETWEEN LOW AND MIDDLE LATITUDES ON ENSO VARIABILITY IN THE CANE-ZEBIAK MODEL*

In this paper,an error source in the atmospheric component of the CZ(Cane-Zebiak) model is discussed,which is missing a free mode in "the exact solutions".However,the improved scheme is proposed,which is the computational scheme with adjusted wind or observed u and v as lateral boundaries.The simulations show that the simulated surface wind by the improved scheme strongly bears resemblance to the observation except for the area near the west and the east boundaries of the integrated area.These results support the conclusion that the wind stress simulated by the improved scheme with lateral boundaries is much better than that simulated by the CZ model,and show that interaction between low and middle latitudes has an important influence on the ENSO variability in the CZ model.Therefore,considering its impact on the CZ model can improve capability of the CZ model for simulating ENSO variability.     

SURFACE OZONE CHARACTERISTICS AT THREE STATIONS IN GAW OF WMO IN CHINA*

The characteristics of surface O₃ on clear days at Waliguan Observatory,Lin'an regional station and Longfengshan regional station in China were analyzed in this paper.The three stations belong to Global Atmospheric Watch(GAW)of WMO.There was obvious daily variation on clear days at Lin'an.with maximum(42.9 ppb)and minimum(20.3 ppb)of daily range appearing in spring and summer,respectively.The daily variation was more regular at Lonfengshan than at Lin'an.The maximum(about 27 ppb)appeared in autumn at Longfengshan.There was no obvious daily variation and also daily range was smaller in other seasons except weaker daily variation in summer at Waliguan.But the surface O₃ concentration(SOC)in summer was higher than that in winter at Waliguan.The SOC on clear days of summer at Waliguan was over 20 ppb higher than at Longfengshan and Lin'an.The global radiation and NO_x concentration were the main factors which control the SOC on clear days at Longfengshan and Lin'an.and played important role in different seasons and areas.The transportation of air flow around the area of Qinghai-Xizang(Tibet)Plateau was the main cause for high SOC and weak daily variation in summer at Waliguan.The similar effect of transportation was obtained at the Mauna Loa Observatory.The distribution characteristics of SOC increasing with height in the troposphere determined the difference of SOC between East China and West China.     

EFFECT OF LOCAL BASIC FLOW STRUCTURE PATTERN IN FORMATION OF BLOCKING HIGHS*

The effect of local basic flow structure pattern (BFSP) on a blocking high formation is investigated within the framework of forced dissipation KDV dynamics.The zonal and meridional positions of the high's center excited by a heat source depend on the BFSP selected.     

EFFECT OF ENERGY DISPERSION ON THE STRUCTURE AND MOTION OF TROPICAL CYCLONE*

The energy dispersion of a typhoon vortex and its effect on the typhoon motion are studied using an analytical method of double-Fourier expansion as well as a numerical model in a β-plane nondivergent barotropic framework.The analytic model and the linear version of the numerical model give essentially the same result:the energy dispersion from a tropical cyclone can creat an L-H-L wave train to the east of the tropical cyclone.Three numerical experiments,integrated for 7 model days by the nonlinear model,indicate that the closed high in the wave train produces obvious influence on the structure and movement of the tropical cyclone.     

EFFECT OF THE LINEAR BETA TERM ON MOVEMENT AND DEVELOPMENT OF TROPICAL CYCLONE*

The dynamics of tropical cyclone is investigated in a nondivergent,barotropic model with no basic flow.The effect of linear beta term on the movement and development of tropical cyclone is emphatically demonstrated.The streamfunction tendency due to the symmetric component of linear beta term appears in a dipole-like pattern with an east-west symmetry,which maintains and intensifies the large-scale beta gyres and causes the tropical cyclone to have a westerly moving component.The streamfunction tendency due to the asymmetric component of linear beta term arises in an ellipse pattern with a north-south major axis,which weakens the tropical cyclone.The streamfunction tendency due to the asymmetric component of linear beta term and the intensity of large-scale cyclonic beta gyre synchronously vary in a fluctuating manner with time.     

A NUMERICAL STUDY ON THE INFLUENCE OF WIND FIELDS AT UPPER LEVELS IN THE MEIYU PERIOD ON THE DEVELOPMENT OF LOW LEVEL JET AND MESOSCALE SYSTEMS*

Using a mesoscale model,a numerical study on a heavy rainfall case occurring in the Changjiang-Huaihe River Basin is made in this paper.The influence of the intensity of northeasterly wind in front of the Qinghai-Xizang high at upper level on the low level wind field and development of mesoscale systems as well as heavy rainfall is investigated.The model well reproduced the heavy rainfall process and the weather systems associated.And it indicates that the strong northeasterly flow around the high at upper troposphere will bring about not only the strengthening of low level southeasterly wind,but also the appearance of shear-line and mesoscale vortex at low level.The coupling of northerly wind at upper level and southerly wind at lower level constructs a vertical indirect circulation which is most favourable for the development of convective motions.Its ascending branch in the shear-line area is very strong and shows a pronounced mesoscale characteristic.     

THE CALCULATION AND ANALYSIS OF SLOPE NET RADIATION IN CHINA*

By using the climatological calculating method for each component of slope surface net radiation proposed by the authors,calculations and analyses are done of the distribution features of slope net radiation in China with emphasis on the discussion of variations of slope net radiation in typical stations and sites with slope direction,slope,latitude and season.The distribution features of net radiation on the north and south slopes are,for the first time,mapped and discussed,revealing the great difference on the national basis,and thus acquiringa new interesting result that the negative-value area of winter net radiation on the north slope(20°)can reach Yunnan and Guizhou Provinces and middle and upper reaches of the Changjiang River.     

EFFECTS OF OROGRAPHY AND AIR FLOW IN EKMAN BOUNDARY LAYER*

It is convenient to use σ-coordinates to discuss the dynamic effect of orography and the flow in Ekman boundary layer.In this paper,the theory of mixing length is generalized to the σ-coordinate system.Then the governing equations,describing the motion in the boundary layer over the mountain regions are derived.The features of flow in the boundary layer,especially the effects of Ekman pumping,are discussed indetails.It is pointed out that there are three factors affecting the vertical motion at the top of the boundary layer:(1) vorticity distribution in the boundary layer,which is directly related to the divergence and convergence of air flow caused by friction,(2) the upslope or downslope motion of flow over the mountain slopes,and(3) the mutual effect of orography and friction induced by the ageostrophic component climbing upward or downward in the boundary layer over mountain regions.     

BAROTROPIC AND BAROCLINIC PATTERNS OF ATMOSPHERIC CIRCULATIONS RELATED TO MONSOON TYPES*

In the context of 1982-1994 NCEP/NCAR wind at 12-level isobaric surfaces on a global basis calculation is made of the barotropic(mass-weighed vertical mean) and baroclinic components(difference between the actual wind at each level and barotropic component) of atmospheric flow fields,followed by dealing with the distribution features of barotropic and baroelinie patterns globally in winter and summer,alongside with the classification of global monsoons according to the surface barotropic/baroclinic patterns.Evidence suggests that the seasonal variation of both components will lead to the reversal of a prevailing wind between winter and summer,thus causing a related monsoon:the baroclinie flow pattern is indicative of a thermal circulation driven by atmospheric inhomogeneous heating chiefly from land-sea thermal contrast whilst the barotropic counterpart represents the result mainly from dynamic effects,which is helpful to the understanding of monsoon nature.And further study shows that the classical monsoon regions in tropical Asia,Africa and South America fall into a baroclinic category,those in the bi-hemispheric subtropical Pacific into a barotropic type and the East Asian subtropical monsoon generated underthe joint action of both the patterns falls into a mixed category.