A MESO-β SCALE SIMULATION OF THE EFFECTS OF BOREAL FOREST ECOSYSTEM ON THE LOWER ATMOSPHERE*

Based on the Intensive Field Campaign(IFC-1)data of Boreal Ecosystem-Atmosphere Study (BOREAS),a three-dimensional meso-β scale model is used to simulate the effect of boreal forests on the lower atmosphere.A fine horizontal resolution of 2 km×2 km is used in order to distinguish the vegetative heterogeneity in the boreal region.A total of 20×25 grid points cover the entire sub-modeling area in BOREAS' South Study Area(SSA).The ecosystem types and their coverage in each grid square are extracted from the North American Land Cover Characteristics Data Base (NALCCD) generated by the U.S.Geographical Survey (USGS) and the University of Nebraska-Lincoln(UNL).The topography of the study area is taken from the Digital Elevation Map(DEM)of USGS.The model outputs include the components of the energy balance budget within the canopy and at the ground,the turbulence parameters in the atmospheric boundary layer and the wind,temperature and humidity profiles extending up to a height of 1500 m.In addition to the fine time and spatial step,the unique feature of the present model is the incorporation of both dynamic and biological effects of the Boreal forest into the model parameterization scheme.The model resultscompare favorably with BOREAS' IFC-1 data in 1994 when the forest was in the luxuriant growing period.     

EFFECTS OF DIABATIC PHYSICAL PROCESSES ON THE SIMULATION OF MESO-β SCALE STRUCTURE OF A SQUALL LINE SYSTEM

The effects of different convective parameterization,explicit moisture schemes and surface heat-ing on the meso-β scale structure of a squall line system are investigated,by using an improvedmesoscale model.It is found that the successful prediction of mesoscale convective systems hinges up-on not only the sub-grid scale convection,but also the resolvable scale phase change processes and thediurnal variation in the boundary layer.The simultaneous operation of the Fritsch-Chappell convectivescheme with parameterized moist downdrafts and the prognostic equations for cloud water (ice) andrainwater (snow) seems to be essential in simulating realistically MCSs and reducing or eliminatingthe unrealistic development of the CISK-like instability associated with the squall line system.     

NUMERICAL STUDY OF A MESO-β SCALE RAINSTORM EVENT

Nanjing Air Force Institute of Meteorology,Naniing 211101     

MAINTAINABLE MECHANISM OF MESO-β SCALE CONVECTIVE SYSTEM

By means of the Penn State-NCAR Mesoscale Model Version 5(MM5)with a horizontalresolution of 20 km.the maintainable mechanism of Meso-β scale Convective System(Mβ CS)hasbeen investigated on the basis of simulation of the temporal and spatial thermodynamics structureof the MβCS which occurred in Wuhan and its surroundings on 21 July 1998.The occurrence of thesignificant warm-core in the center of the MβCS happened in Changjiang River Basin between theMufu Mountain and the Dabie Mountain.To the southern side of the MβCS,there exist thesouthwest low-level jet(LLJ)and the vertical secondary circulation in the low and middletroposphere respectively.In respect to the northern of the MβCS,the northwest jet emerges in theupper troposphere,accompanied with cold and dry atmosphere downdrafts,resulting in anothersecondary circulation.The foregoing mentioned vertical wind shear provides a favorable dynamicalenvironment for the intensification and maintenance of the MβCS.Equally important,the latentheat release associated with the MβCS produces the warm center in the middle troposphere and thepressure falls.The pressure drop then accelerates air parcels toward the low leading to strongconvergence as well as the intensified convection,establishing a positive feedback between theconvection and the latent heat release,which is the thermodynamic mechanism of the developmentand maintenance of the MβCS.     

AN OBSERVATIONAL STUDY OF THE MESO-β SCALE MOVING SYSTEMS IN THE MEIYU FRONT

The new concept and analysis method for the rainfall peak are introduced in this paper,and an obser-vational study of a heavy rain case in the Meiyu front has been made with finer radiosonde and precipitationdata.It has found in this case that there are a lot of meso-β scale systems associated with the rainfall peaksin the Meiyu front.Meso-β scale systems can be divided into two kinds,i.e.,the moving and standing types.The moving type is characterized by the unstable gravity wave and has a path corresponding to the meso-α scalerain belts in the direction.The discussion about the meso-β systems is made by using the symmetric andtransversal wave instability theory.     

A PRELIMINARY SIMULATION OF EFFECTS OF CLIMATE CHANGE ON RICE IN THE MIDDLE AND LOWER REACHES OF CI-IANGJIANG RIVER AND WINTER WHEAT PRODUCTION ON THE HUANG-HUAI-HAI PLAIN

In this paper,based on the analysis of the variation of effective accumulated temperature and precipita-tion in the last 100 years,together with the potential impacts of greenhouse effects on the climate in Chinasimulated with the current major climatic models(GFDL,GISS,NCAR,OSU and UKMO),the impacts ofclimate change on rice in the middle and lower reaches of Changjiang River and winter wheat production on theHuang-Huai-Hai Plain~* are simulated with the available weather-yield statistic models on the assumption that te-mperature variation ranges 1—4℃ and precipitation variation ranges ±10—20%.The result is that under thecurrent planting systems and agrotechniques,the impact of climate change on wheat production is more signifi-cant than that on rice;the climatic conditions of agricultural production in the north of China will become morefavourable while those in the south of the middle and lower reaches of Changjiang River remain the same;butthose in the southern part of North China and the basins between Changjiang River and Haihe River are ratherunstable.In general,it is difficult to reach the conclusion that advantages or disadvantages dominate in suchclimate changes because of uncertainties.     

THE EFFECTS OF MECHANICAL FORCING ON THE MEAN MERIDIONAL CIRCULATION AND TRANSFER PROPERTIES OF THE ATMOSPHERE

Experiments using a quasi-geostrophic model and the ECMWF T21 spectral model with and without orography are performed to investigate the effects of mechanical forcing on the mean meridional circulation. Results show that mechanical forcing intensifies the horizontal poleward heat flux and redistributes the eddy angular momentum in the vertical, and that this changes significantly the intensity and location of the mean meridional circulation centres.It is shown how the mean meridional circulation is set up in such a way to satisfy both the dynamical and thermodynamical transport requirements of the model atmosphere. Whenever external forcing changes the eddy fluxes, the Coriolis torques from the upper horizontal branches of the mean meridional circulations change to balance the extra divergence of eddy momentum flux, and additional adiabatic heating is produced by the vertical branches of the toroids to balance the extra divergence of eddy heat flux. The mean meridional circulation is, therefore, confirm     

NUMERICAL SIMULATION FOR THE EFFECTS OF PBL AND THE SURFACE ON POLLUTANT CONCENTRATIONS

On the basis of theoretical and experimental results of study of planetary boundary layer (PBL), the physical parameters describing the structure of PBL are calculated by using the data obtained from a meteorological tower and the effects of PBL and the surface on pollutant concentrations are numerically simulated with a time-dependent two-dimensional advection and diffusion equation.It is shown that the diurnal variation of PBL results in that of concentration. The height of mixing layer is an important factor to determine the ground-level concentration. As for an elevated point source, the height of mixing layer, growing from lower to higher than the releasing height is a necessary condition for the phenomenon of fumigation. It is also shown that the surface may be considered as a boundary with perfect reflection when Vd ≤0.001 m s-1, but has an important effect on concentration and must be carefully dealt with when Vd≥0.01 m s-1.     

NUMERICAL STUDY OF A MESO-β SCALE RAINSTORM EVENT*

Diagnosis is performed of a thunderstorm rainstorm event occurring in the summer of 1996 at Nanjing and numerical simulation undertaken in the context of hydrostatic equilibrium framework of MM5 as the fifth version of the PSU/NCAR mesoscale model. Analyses show that the rainstorm-associated thermal condition was the accumulation of unstable potential energy and the dynamic condition was vigorous convergence updrafts. And the simulation within the hydrostatic framework indicates the significant role of latent heat release in the rainstorm occurrence: that even for a 30 km grid spacing horizontally of great importance to the successful modeling of the meso-p event was a convection parameterization scheme that led to less rainfall in our run based only on its explicit version but to the prediction in closer agreement with the observed when its implicit version was used in combination: for the thunderstorm-accompanied torrential rain. the Grell scheme was superior to the version of Kuo and the improved Arakawa-Schubert parameterization scheme (Grell 1993:Anthese and Kuo 1987; Arakawa and Scherbt 1974: Grell et al. 1991).Moreover, better results came from the simulation in the context of hydrostatic framework of the MM5 compared to those from the run within the nonhydrostatic equilibrium framework, a problem that awaits further efforts.     

SEASONAL VARIATION OF LIDAR-DERIVED AEROSOL CONTENT IN THE LOWER LAYERS OF THE ATMOSPHERE

Results of the study of seasonal variation of total column content of aerosols in the 50—1100 m heightregion are presented,The columnar content of aerosols is computed by integrating the height profiles ofnighttime aerosol number density estimated from the Argon ion lidar observations collected at the ⅡTM,Pune for the one year period from October 1986 to September 1987.The results of the study indicated thatthe aerosol content was higher during winter than during monsoon months by over 78% with an annualmean columnar aerosol content of 1.66×10~8 cm~(-2).The observed seasonal variaticn of aerosol content wasfound to be associated with that of surface wind,temperature,relative humidity(R.H.)and rainfall datarecorded for the above one year period at the station.     

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.     

MAINTAINABLE MECHANISM OF MESO-β SCALE CONVECTIVE SYSTEM*

By means of the Penn State-NCAR Mesoscale Model Version 5(MM5)with a horizontal resolution of 20 km.the maintainable mechanism of Meso-β scale Convective System(MβCS)has been investigated on the basis of simulation of the temporal and spatial thermodynamics structure of the MβCS which occurred in Wuhan and its surroundings on 21 July 1998.The occurrence of the significant warm-core in the center of the MβCS happened in Changjiang River Basin between the Mufu Mountain and the Dabie Mountain.To the southern side of the MβCS,there exist the southwest low-level jet(LLJ)and the vertical secondary circulation in the low and middle troposphere respectively.In respect to the northern of the MβCS,the northwest jet emerges in the upper troposphere,accompanied with cold and dry atmosphere downdrafts,resulting in another secondary circulation.The foregoing mentioned vertical wind shear provides a favorable dynamical environment for the intensification and maintenance of the MβCS.Equally important,the latent heat release associated with the MβCS produces the warm center in the middle troposphere and the pressure falls.The pressure drop then accelerates air parcels toward the low leading to strong convergence as well as the intensified convection,establishing a positive feedback between the convection and the latent heat release,which is the thermodynamic mechanism of the development and maintenance of the MβCS.     

ON THE BAROTROPIC INSTABILITY OF LARGE SCALE VORTICES

By using the linearized barotropic vorticity equation in polar coordinates the stability of pertur-bations 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 az-imuthal wave number. Then, numerical experiments whose integration time is 60 model hours areperformed in terms of a quasi-geostrophic barotropic model in Cartesian coordinates using the per-turbation stream function field of unstable mode superimposed on a strong and weak circular basicflows as the initial fields. The experimental results reveal that the amplitudes of the initial pertur-bations 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.     

NUMERICAL SIMULATION STUDY OF THE EFFECTS OF TERRAIN AROUND TAIWAN ON TYPHOON DOT(1990)

With the use of analyzed TCM-90 data,Typhoon Dot(1990)and complex evolutions are successfullyreproduced in numerical sboulation as it travels over the island of Taiwan. The simulation includes theformation of secondary highs in both circulation and geopotential fields, trajectory jumps during mergence with main centers, anomalous northward movement of a low center west of Taiwan, evolutionalprocess of a low-level jet over the Taiwan Straits and significant deviations of circulation center fromgeopotentia1 center between upper and lower levels. By examining ev0lutions of 3-h interval simulatedresults, detailed processes of changes in structure and track before and after Dot's passage of Taiwanis given, whose evolutional images are otherwise impossible with conventionai observations at intervalsof 6 h. A number of control experboents are conducted in the end of the work for understanding causesand mechanisms behind various properties.     

ON THE INTERACTIONS BETWEEN ENSO AND THE ASIAN MONSOON ON THE SCALE OF QUASI-BIENNIAL OSCILLATION

A diagnostic analysis is performed of the quasi-biennial oscillations during the ENSO cycle and the results are based to study the interactions between ENSO and the Asian monsoons. It shows that the Asian monsoons have significant influence on the ENSO cycle on the quasi-biennial scale. Materialized through the onset and southward progression of the winter monsoon, the influence appears in the tropical western Pacific to excite severe convection and to further affect the ENSO cycle. The phenomenon is not only reflected in the quasi-biennial mode but the annual variation of the Asian winter monsoon in reality.     

NUMERICAL SIMULATION OF EXTREMELY HEAVY RAIN AND MESO-β SCALE LOW VORTEX IN INVERTED TYPHOON TROUGH*

Large-scale and mesoscale analyses are made for extremely heavy rain(EHR) and meso-β scale low vortex(MSLV) in Jiading District of Shanghai Municipality during 6-7 July 2001.It is shown that the EHR forms in the situation of northern westerly trough linking together with southern inverted typhoon trough at northwest side of the West Pacific Ocean subtropical high. Numerical simulation is made using a 21-layer improved REM(regional η coordinate model) for this course.The results show that the precipitation forms earlier than MSLV.and the strong convergence in wind velocity mate(WVM) triggers the strong precipitation.The formative reasons of WVM.especially the weak wind velocity center are discussed,and the formative mechanisms of the MSLV and EHR are discussed using high spatial and temporal resolution modeloutput physical fields.The results show that the heavy rain releases latent heat and warms the air column,and enhances the low level positive vorticity that existed before.Then it causes the formation of MSLV.There is a positive feedback mechanism between low vortex and precipitation,so CISK must be an important mechanism.     

THE IMPACT OF A DIAGNOSTIC CLOUD SCHEME ON FORECASTS WITH A GLOBAL ATMOSPHERE MODEL

Experiments were conducted to test the impact of a cloud diagnosis scheme in place of prescribed zonalaverage cloud on medium and long range integrations with the Australian Bureau of Meteorology ResearchCentre(BMRC)global atmosphere model.The cloud scheme was shown to improve the temperature bias inthe lower troposphere but there was deterioration in the upper troposphere,especially in the tropics,asso-ciated with underestimation of high cloud amount.Thirty day mean fields in a January integration showed greater amplitude in the Northern Hemisphereplanetary waves and a deeper Antarctic circumpolar trough than the control experiment or a simulation withno cloud.The results for the diagnosed cloud case agree more closely with corresponding observed fields.There was also some reduction in the zonal average zonal wind component reflecting the additional zonalasymmetry introduced by the diagnostic cloud scheme.Similar trends were also noted in medium and longrange forecasts for January and July conditions,although the impact on predictive skill was slight and insome cases detrimental.Areas for improving the diagnostic cloud scheme are noted.     

SIMULATION STUDY OF THE EFFECT ON COLD SURGE OF THE QINGHAI-XIZANG PLATEAU AS A HUGE OROGRAPHY

Using a limited-area P-σ incorporated coordinate five-level primitive equation model fed with theECMWF 5°×5°grid data,a simulation is made of a large-scale cold surge of late December 1982.Resultsshow that the Qinghai-Xizang Plateau has no profound thermal but dynamic effect on the East-Asian wintermonsoon that forces cold air to go southwards by its east side,exciting Kelvin waves behind the cold frontto result in the maximum NE wind.Besides,gravitational waves independent of the Plateau occur aheadof the front advancing towards the south.They may be due to the excitation involved in the front itself.The cold surge is propagated under the Hadley cell,making it strengthened and moved southwards.     

EFFECTS OF THE ENVELOPE DEGREE OF OROGRAPHY ON THE SIMULATED CLIMATE PROPERTIES

A primitive equation model with a zonal domain and a p-σ incorporated vertical coordinate systemis used to study the effects of the envelope degree of orography on the simulated properties of bothwinter and summer climates.Results show that the orography with a larger envelope degree can im-prove the simulations to certain extent,especially in winter.It is found that the simulated propertiesof climate are mainly determined by the co-effects of the land-sea and the orographic distributions,and the envelope degree of orography has a secondary effect.However it should be taken properly inorder to improve simulations:otherwise,it would be harmful.Different envelope degrees of orogra-phy can be adopted in different areas.     

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.