ON THE MAINTENANCE OF BLOCKING ANTICYCLONES OF NORTHERN HEMISPHERE—PART I:QUASI-GEOSTROPHIC POTENTIAL VORTICITY ANALYSIS

Four observed blocking anticyclones in different regions of the Northern Hemisphere are in-vestigated.Analyses show that there exist distinct differences in the maintenance of the time-meanquasi-geostrophic potential vorticity(PV)low in 300 hPa within blocking areas.In two Pacificblocking cases,the PV advection by time-mean flow tends to flow the PV low to northwestern partof the blocking highs,and thus is beneficial to the maintenance of the blockings'strength.Thetransfer by transient eddies acts to balance the effect of the time-mean flow.In the Atlantic andAlaska blocking cases,however,the advection of mean flow tends to flow the PV low eastward.The PV transfer by transient eddies acts to flow potential vorticity low to the western part of theblocking ridges and also to balance the time-mean flow's effect.Thus,in the latter two cases,it isthe transfer by the transient eddies that acts to maintain the blockings.     

ISENTROPIC POTENTIAL VORTICITY ANALYSIS ON THE MESOSCALE CYCLONE DEVELOPMENT IN A TORRENTIAL RAIN PROCESS

The mesoscale model MM4 is used to simulate the torrential rain associated with Meiyu frontoccurring on 5—6 July.1991 in the Changjiang-Huaihe Basin.Based on the outputs of the model,the cause of the mesoscale cyclogenesis on the lower troposphere is investigated in terms of thepotential vorticity principle.The results show that because of the favorable pattern of moistisentropic surface,the absolute vorticity increases when cold air with high moist potential vorticityvalue rapidly slides down southwards along the moist isentropic surface,and then causes thecyclonic vortex development.     

EFFECTS OF TROPOPAUSE AND POTENTIAL VORTICITY ANOMALY ON FRONTOGENESIS

A two-dimensional,semi-geostrophic numerical model incorporating the tropopause andstratosphere is used to investigate the effects of a positive potential vorticity anomaly and latentheat release on the frontogenetic process and the structure of the resulting frontal zone.It isdemonstrated that(1)the inclusion of tropopause and stratosphere significantly changes the frontalstructure only in the upper levels;(2)a clearly defined quasi-equivalent barotropic structure and aregion of upward motion of finite width appear when a positive potential vorticity anomaly exists onthe warm side of the maximum baroclinity in the lower troposphere,especially when it is locatedon the south edge of the baroclinic zone;(3)the above mentioned structure deteriorates as thefrontogenesis proceeds in a dry atmosphere but can be maintained in a moist frontogenetic processwith condensational heating;(4)the combination of a positive potential vorticity anomaly and thelatent heat release is able to accelerate the frontogenesis significantly with the time needed to forman intense frontal zone reduced to less than 15 h.The results have significant theoreticalimportance in understanding the complex nature of frontal structure and frontogenesis,especiallyin understanding the dynamic structure of the subtropical frontal zone observed during earlysummer over East Asia.     

ON THE GENERATION AND MAINTENANCE OF ATMOSPHERIC DISTURBANCES

Blocking is one of the intensive atmospheric disturbances which can maintain for a long time. In this paprer we investigate the general conditions favorable for the generation and maintenance of the intensive disturbances. First, the evolutional process of disturbances superimposed on a jet-like zonal flow is studied by using the wave-packet representation and the WKBJ method. Second, the mechanism for generation and maintenance of disturbances is investigated by using the nonlinear equations and the general physical laws. Finally, some numerical experiments are given for illustration, showing the rapid absorption of disturbances by the jet-like zonal flow in one case and the maintenance of disturbances for a long time in the other case.     

POTENTIAL VORTICITY STRUCTURE OF SUMMER CYCLONES OVER THE CHANGJIANG-HUAIHE VALLEY

Based on the theory of Ertel potential vorticity,the isentropic potential vorticity maps and vertical pro-files of potential vorticity for two summer cyclones over the Changjiang-Huaihe Valley are analysed.Afterdiscussing a possible mechanism for the genesis and development of such systems and their differences fromtypical extratropical cyclones,a conceptual model for their activities is proposed:A weak disturbance in the mid-level of troposphere originated from around the Qinghai-Xizang Plateau may cause heavy precipitation underfavourable conditions and latent heat release in the mid-troposphere leads to downward extension of cycloniccirculation and a wave on the quasi-stationary front.This weak cyclone can develop substantially and becomea typical extratropical cyclone only when air from the lower stratosphere flows downslope along isentropic sur-faces into the region of interest.     

NUMERICAL STUDY ON EFFECTS OF THE QINGHAIXIZANG PLATEAU ON FORMATION OF THE URAL BLOCKING HIGH

By employing the improved T42L5 spectral model and utilizing the ECMWF data covering the period from 1 Julyto 7 July 1982,a numerical research on the formation of the Ural blocking system has been made.The results show thatthe model forecasts for the upstream U ral area turn out to be worse if the dynamic effect of the Qinghai-Xizang Plateauis not considered.The correlation coefficient between the model forecasts and observed 500 hPa geopotential heightanomaly decreases by 9% for the 5-day mean,and their averaged root mean square (RMS) error increases 15 m.Due tothe dynamic effect of the Plateau,the trough being on the northwest of the Plateau is barricaded and turns to be atransversal trough.Consequently southwest flow occurs along the northwest of the Plateau in front of the trough,whilenortheast flow prevails over the west of the trough,causing the formation of the blocking high over the Ural area.Whenthe dynamic effect of the Plateau is not taken into consideration,the trough develops and moves southeastward and theUral blocking high changes into a migratory high.All these result in the failure of the simulation.The dynamic effect ofthe Plateau helps to increase the negative vorticities over the Plateau and its north periphery as well as the Ural area,andalso helps to increase the positive vorticities over the Black Sea and the Caspian Sea area.On the other hand,thethermodynamic effect mainly influences the Plateau and its downstream area and plays an less important role in theformation of the blocking high over the upstream Ural area.     

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 completeset of primitive equations.It was shown that motion in a saturated atmosphere without diabatic heat-ing and friction conserves moist potential vorticity.This property was then used to investigate the de-velopment of vertical vorticity in moist baroclinic processes.Results show that in the framework ofmoist isentropic coordinate,vorticity development can result from reduction of convective stability,orconvergence,or latent heat release at isentropic surfaces.However,the application of the usual analy-sis 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 tothis theory,whether the atmosphere is moist-symmetrically stable or unstable,or convective stable orunstable,the reduction of convective stability,the increase of the vertical shear of horizontal wind ormoist baroclinity may result in the increase of vertical vorticity,so long as the moist isentropic surfaceis slantwise.The larger the declination of the moist isentropic surface,the more vigorous the develop-ment of vertical vorticity.In a region with a monsoon front to the north and the warm and moist airto the south,or by the north of the front,the moist isentropes are very steep.The is the region mostfavorable for development of vorticities and formation of torrential rain.For a case of persistent torrential rain occurring in the middle and lower reaches of theChangjiang and Huaihe Rivers in June 11-15,1991,moist potential vorticity analysis,especially theisobaric analysis of its vertical and horizontal components,i.e.MPV1 and MPV2,respectively,iseffective for identifying synoptic systems not only in middle and high latitudes,but also in low lati-tudes and in the lower troposphere.It can serve as a powerful tool for the diagnosis and prediction oftorrential rain.     

A NUMERICAL EXPERIMENT ON THE INFLUENCES OF THE QINGHAI－XIZANG PLATEAU ON THE UPSTREAM BLOCKING EVENT

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BLOCKING EFFECT OF HIGH PRESSURE SYSTEM ON THE FORMATION OF EXTRA-INTENSE HEAVY RAIN

Commonly the centre of an intense heavy rain occurs in a very limited area,but for the three extra-intense heavy rains of the present study,63-8 in North China,75-8 in central China and 77-8 in the desertregion of Inner Mongolia,which all appeared under the environments of“Western Low and Eastern Blocking”(EB)pattern.From this study,the following effects of the EB are found:(1)It affects the precipitationsystems staggering in a local place and/or changes the trajectories of low votices and urges them into thesame raining areas intermittently.(2)It transports water vapour into raining areas.The air flows by thewest side of EB produce strong cyclonic vorticity behind EB frequently,which transports water vapour andforms mesoscale precipitation systems more favourably than the low level jets.(3)Air flows behind EB con-jugate with adequate topographic relief,which enhances the precipitation and makes the raining areas over-lapped.So that extra-intense heavy rains could occur in higher latitudes of semi-aird areas,and occasionallyeven in the desert region of North China.Such extra-intense heavy rains could not be explained by static local humidity and temperature only.This is also a principal discrimination between the prolonged extra-intense heavy rain and the short-rangeconvective precipitation and/or the common precipitation.     

THE PHYSICAL EFFECTS OF TOPOGRAPHY AND HEAT SOURCES ON THE FORMATION AND MAINTENANCE OF THE SUMMER MONSOON OVER ASIA

The physical effects of topography and heat bources on the formation and maintenance of the summer monsoon over Asia are discussed ia this paper by using the transformed Eularian-mean equations and a quasi-geostrophic 34-level spherical coordinate model.The computed results of the divergence of the E-P flux, the induced meridional circulation and the perturbation geostrophic wind speed induced by the forcing of topography and heat sources show that the diabatic heating effect over the Tibetan Plateau may play an important role for the formation and maintenance of the summer monsoon over Asia, which is much greater than the dynamical effect of topography.The computed results also show that, of the physical effects of topography and heat sources on the formation and maintenance of the summer monsoon over Asia, the effect of forced meridional circulation is larger than that of the divergence of E-P flux of the induced waves.     

ON THE STOCHASTIC ANALYSIS OF A SIMPLE CLIMATE MODEL

By using the first thermodynamic law and stochastic function,a stochastic nonlinear climate model isproposed and the model analytical solution is obtained.The result indicates that the global temperature prob-ability density function has two peaks.If the solar radiation varies,a sudden change in the number of peakwill happen.In addition,we find that there are different sea temperature responses at different latitudes.     

ON THE DYNAMICS OF SUBTROPICAL UPPER-TROPOSPHERIC QUASI-STATIONARY VORTICES IN NORTHERN SUMMER

In this paper the dynamic mechanism of the subtropical quasi-stationary vortices in the upper troposphere,such as the southern Asia high and the mid-oceanic troughs,are studied with a highly truncated spectral model,in which the thermal forcing and the nonlinear interaction between wave and zonal components,as well asthe basic zonal flow changing with latitudes are incorporated.The results show a stable equilibrium statewith an upper level fiow field similar to the observed subtropical systems.It is found that the thermal forc-ing has some important controlling effects on these planetary-scale vortices.     

NUMERICAL EXPERIMENTS ON DYNAMIC MECHANISM OF BLOCKING

By using a two-level quasi-geostrophic spectral model, the numerical experiments on blocking process affected by thermal forcing are conducted. The dynamic mechanisms of blocking, including the resonance between wave and forcing, the control effects of thermal forcing on the position and intensity of the highs and the instabilities of two kinds of equilibrium states are studied by controlled experiments.     

ANALYSIS ON CHARACTERISTICS OF ATMOSPHERIC DUCT AND ITS EFFECTS ON THE PROPAGATION OF ELECTROMAGNETIC WAVE

On the basis of introducing the basic categories of atmospheric refraction and their existingconditions,the forming processes of three kinds of atmospheric ducts are expounded.Several maincharacteristics of atmospheric duct are summarized and analyzed,and field sounding data from theWEstern North-Pacific cloud-radiation EXperiment(WENPEX)and meteorological data aroundthe Xisha sea area are used to validate these characteristics.Meanwhile the sensitivities of theevaporation duct height to the variations of atmospheric humidity.the air-sea temperaturedifference.and horizontal wind speed are examined.With the analysis of the effect of atmosphericduct on the propagation of electromagnetic wave.the maximum trapped-wavelength and the criticalemitting angle of elevation for electromagnetic wave which can form duct propagation are derived.At the same time the four kinds of necessary conditions for electromagnetic wave to form ductpropagation are brought forward.The effects of atmospheric duct on ultrashort wave propagation,radar observation.short wave communication etc.are also discussed.     

ANALYSIS OF SPHERICAL FUNCTION SPECTRAL STRUCTURE OF THE NORTHERN HEMISPHERE 500 HPA MONTHLY MEAN HEIGHT

Study is undertaken of spherical function spectral structures of long-term mean and anomaly patterns of theNorthern Hemisphere 500 hPa monthly mean geopotential height together with the seasonal and interannual variationsinvestigated.Results show that they are marked by low dimensions and low orders,and the mean and anomaly fields canbe described in terms of 20 and 50 spherical function components,respectively.     

THE EFFECTS OF TOPOGRAPHY ON THE SUMMER ATMOSPHERIC ENERGETICS OF THE NORTHERN HEMISPHERE IN A LOW-RESOLUTION GLOBAL SPECTRAL MODEL

An analysis is made of the effects of topography on the summer atmospheric energetics of the Northern Hemisphere in a low-resolution global spectral model. The numerical model is a global, spectral, primitive equation model with five equally spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. The model includes comparatively full physical processes.Each term of the energy budget equations is calculated in four specific latitudinal belts (81.11°S-11.53°S; 11.53°S-11.53°N; 11.53°N-46.24°N; 46.24癗-81.ll癗) from a five-year simulation with mountains and a one-year simulation without mountains, respectively. Differences between them are compared and statistically tested. The results show that synoptical scale waves transport available potential energy and kinetic energy to long waves and increase conversion from available potential energy of the zonal flow to eddy's and from the eddy kinetic energy to the zonal kinetic energy in region 3 (11.53°N-46.24°N     

PRELIMINARY ANALYSIS ON THE STRUCTURE AND INTENSITY OF CONCENTRIC DOUBLE-EYE TYPHOONS

Typhoon eye, one of the most outstanding features in the evolution of typhoon, has various complex configurations: circular, elliptic, oval, square, polygonal, or irregular shapes. It is very interesting to note that some typhoons show the concentric double-eyes encircled with two concentric cloud towers.The concentric double-eye structure and intensity of typhoons are investigated and discussed by using reconnaissance aircraft data, radar echo photos and satellite images, 76 double-eye typhoons were observed during 1949－1983. The results indicate that the concentric double-eye structure, generally appearing in intense typhoons over the northwestern Pacific Ocean, is closely related to typhoon’s intensity. This kind of structure has apparent characteristics in geographical and seasonal distributions.Based on the observational data, an idealized structure model for the concentric double-eye typhoons is proposed.     

ANALYSES OF THE DYNAMIC EFFECTS ON WINTER CIRCULATION OF THE TWO MAIN MOUNTAINS IN THE NORTHERN HEMISPHERE--Ⅱ.VERTICAL PROPAGATION OF PLANETARY WAVES

A linear,hemispheric and stationary spectral model with multilayers in the vertical was employed to simulate the vertical propagation of waves triggered by mountains.Results show that,in cooperation with the East Asia zonal mean flow,Tibetan Plateau can excite a strong wavenumber 1 perturbation in the stratosphere with its ridge and trough located over the Pacific and Atlantic Oceans respectively.On the other hand,the stratospheric wavenumber 1 perturbation caused by the mechanical forcing of the Rocky Mountains in cooperation with the North America zonal mean flow is very weak.Calculations from observational data of the vertical profile of critical wavenumber for vertically propagating waves imply that the tropospheric wavenumber 1 perturbation can hardly penetrate the North America tropopause upwards,whereas it can freely propagate through the East Asia tropopause into the stratosphere.Two-dimensional E-Pcross-sections obtained from both observational data and simulated results also demonstrate that waves excited by the Rocky Mountains are refracted towards low latitudes in the troposphere during their upward propagation:whereas,in addition to the above mentioned equatorward leaning branch,the wavenumber 1 and 2 planetary waves excited by the Tibetan Plateau possess another branch which is refracted to high latitudes during upward propagation and penetrates the tropopause into the stratosphere.It is therefore concluded that the difference in the horizontal and vertical wave propagations in the two hemispheres is a result of the different dynamical forcing induced by the two main mountains in the Northern Hemisphere.     

ANALYSES OF THE DYNAMIC EFFECTS ON WINTER CIRCULATION OF THE TWO MAIN MOUNTAINS IN THE NORTHERN HEMISPHERE--Ⅰ.RELATIONSHIP AMONG GENERAL CIRCULATION, TELECONNECTION AND STATIONARY WAVES

The distribution of troughs and ridges of geopotential height,the teleconnection patterns and the propagation patterns of stationary waves are the main features of the January mean geopotential height field at 500hPa.Data analyses and numerical experiments indicate that these three characteristics are associated to one another and closely related to the mechanical forcing of the Rocky Mountains and Tibetan Plateau.There exists a prominent negative correlation in the intensity variation between the American trough and the Asian trough at high and middle latitudes.Such negative correlation,in connection with the interannual variation of the intensity of the jets in front of the two troughs,leads to the existence of similar teleconnection patterns in North America and East Asia.On the other hand,the different propagation behaviour of quasi-stationary waves downstream of the two main mountains results in a fundamental difference in the distribution of correlation chains in North America and East Asia.     

A NUMERICAL STUDY FOR MECHANISM OF THE EFFECT OF NORTHERN SUMMER ARCTIC ICE COVER ON THE GLOBAL SHORTRANGE CLIMATE CHANGE

In terms of nine-layer global spectral model involving fuller parameterization of physical process,with arhomboidal truncation at wavenumber 15,experiments are conducted by virtue of two numerical schemes,one withlong-range mean coverage of Arctic ice,and the other with supercooled water at the same temperature as the ice,fol-lowed by an analysis of the difference field simulated by the two schemes.Results show that(1)the impact of Arctic iceon the northern short-range climate is realized through the change in polar ice coverage to cause local temperaturechange in the polar region to set up gradient difference in temperature from north to south,thus affecting the atmospher-ic circulations and,on the other hand,two trains of two-dimensional Rossby waves excited by the atmospheric heatsource anomaly have impacts on the Northern Hemisphere(NH)extratropical region,one of which is similar to the JPteleconnection pattern first presented by Nitta(1987);(2)The significant impact of Arctic ice anomaly on the southernshort-range climate change is accomplished with the aid of the anomaly of the equatorial heat source that excites a two-dimensional Rossby wavetrain propagating along a great circle route into the Southern Hemisphere(SH)extratropics,and the cross-equatorial propagation of the NH wavetrain also has effects on the SH atmosphere.Simulation indicatesthat with the 15-day integration the Arctic ice exerts an influence mainly on the NH and when the model atmosphere isgetting stabilized,the effect is dominantly on the SH short-range climate change.