NUMERICAL SIMULATION OF THE GENERATION OF MESOSCALE CONVECTIVE SYSTEMS IN LARGE-SCALE ENVIRONMENT

The generation of mesoscale convective systems is simulated by a 7-level primitive equation model. The large-scale parts of observed data at 1200 Z June 11, 1983, which are obtained by low-pass filter, are used as the initial data. The results show that the generation of mesoscate convective systems can be simulated from fields of meteorological variables on the large-scale background. When the low-level southwest jet stream is very moist, mesoscale convective systems can develop ahead of the wind speed maximum in the warm sector of Jiang-Huai (Changjiang-Huaihe Rivers) cyclone, where the potential stability tends to remain negative. Furthermore, they are similar to the mesoscale convective complex (MCC), which appears frequently in the central part of the United States during the warm season (March to September), in dynamical and thermal structure, distribution of precipitation and the process of generation and development.     

A NUMERICAL SIMULATION OF TYPHOON GENERATION

By using a two-dimensional axisymmetrical PEM in which two physical processes (the Ekman pumping and the vertical transportation of cumulus momentum) are included, the genesis and development of typhoons have been simulated. The results of numerical simulation show that the generation and structure of the typhoon simulated by the model involving both the physical processes are much close to a real one in the atmosphere as compared with that involving either the Ekman pumping or the cumulus momentum transport. Therefore, it can be suggested that the cumulus momentum transport and Ekman pumping together play an important role in the genesis and development of typhoons through the CISK mechanism.     

A NUMERICAL EXPERIMENT WITH THE EFFECT OF A COMPLICATED TERRAIN ON THE MESOSCALE SYSTEMS

Improvement in two aspects is done of the one-level mesoscale numerical model of Mass et al.(1985)and the re-vised model is used to make a simulation of a severe convective weather process in North China,with the result showingthe pronounced effects of the topography upon the mesoscale systems.     

NUMERICAL SIMULATION OF DIMETHYLSULFIDE IN THE REMOTE MARINE BOUNDARY LAYER

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NUMERICAL SIMULATION OF THE SURFACE RADIATION FIELD IN A HILLY COUNTRY

Based on the simulations of the topographic parameters(mean orientation,slope,and terrain screenangle)in a hilly land,discussion is made term by term of the technique for modelling of all components isdone in the surface radiation balance over a rugged terrain,thus presenting a computer model for each com-ponent.In terms of experimental data,the components is calculated for the mesh grids of 100m×100 meach over 3.0×3.5 km~2 in the Zhaogongling,southern Dabie Mountains and a map is prepared showingthe distribution of these components for January and July.Results show that the hilly-land surface radiationfield matches the terrain element field quite well,which reveals the determining function of the latter,withorientational effect dominant in winter and terrain screen effect most significant in summer.The simulationtechnique presented is in principle applicable to the calculation for a radiation field in any kind of topogra-phy,thus providing a means for further exploration of hilly-land climate resource.     

A NUMERICAL SIMULATION OF THE DISTRIBUTION OF ACID PRECIPITATION IN CHONGQING AREA OF CHINA

A numerical model for the study of the regional acid precipitation is developed. The model consists of five parts: the distribution patterns of SO2 concentration, the mesoscale flow fields, the parameterization of SO2 transformation into SO4-, the parameterization of precipitation scavenging process, and the relationship between SO2 content in precipitation and ground level concentration of SO2 in the air. The distribution of SO2, SO2- and pH for all precipitations in Chongqing area during the period of July to October 1982 are simulated with the model. A comparison of the simulated results with experimental data shows that high SO2 concentration centres correspond to low pH centres. The source of the acid rain in Chongqing area is local air pollution which is due to the lower effective stack height, low wind velocity in the area, basin topography, and the use of coal with high sulphur content. The mechanism for the formation of the acid precipitation here may be different from that in the United States     

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.     

THE NUMERICAL SIMULATION OF THE THREE-DIMEN-SIONAL TELECONNECTIONS IN THE SUMMER CIRCULATION OVER THE NORTHERN HEMISPHERE

In this paper, a quasi-geostrophic, 34-level spherical coordinate model with Rayleigh friction, Newtonian cooling and the horizontal eddy thermal diffusivity is used to simulate the three-dimensional telecon nection in the summer circulation over the Northern Hemisphere.The computed results show that the change of the heat source over the Tibetan Plateau may cause the change of the atmospheric circulation over the middle and high latitudes in the Northern Hemisphere. When the heal source over the Tibetan Plateau is enhanced, it may cause the Tibetan high to enhance over South Asia and cause the change of the atmospheric circulation over East Asia and North America, i. e., Northeast China and North Japan will be controlled by a trough, which brings about a cold summer in this area. In the same way, an anticyclone will be enhanced over the Okhotsk sea. Moreover, another trough will be formed over Alaska, while another ridge will develop to the northeast of North America. Besides, the Pacific subtropical high will be weakened. These results are in good agreement with those obtained from the observed data.     

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.     

NONHYDROSTATIC NUMERICAL SIMULATION FOR THE“96.8”EXTRAORDINARY RAINSTORM AND THE DEVELOPING STRUCTURE OF MESOSCALE SYSTEM

During the period of 3—5 August 1996(for short “96.8”),an extraordinary rainstorm eventoccurred in Henan,Hebei and Shanxi Provinces in China,resulting in severe flood catastrophe.Synoptic analyses indicated that the stable gross col field and the interaction between a northwardmoving typhoon(down into low pressure)and its east lateral Pacific subtropical high were thelarge-and meso-scale circulation conditions of the “96.8” extraordinary rainstorm.The mesoscaletyphoon-low and its specific dynamical and thermodynamical structures were directly related to thisrainstorm event.The nonhydrostatic version of mesoscale numerical model MM5 was used toconduct investigation of numerical simulation for this case.The simulation with the full physicalprocesses of nonhydrostatic version MM5 was basically possessed of a capability to reproduce thegenesis,development and evolution of the large-scale and meso-α scale synoptic systems.Thesimulative results using a two-way interactive nesting procedure revealed that the typhoon-low waspossessed of an intensive coupled mechanism between the dynamical and thermodynamical fields,namely,the developing typhoon-low was possessed of a structure of the.cyclonic vorticity columnwith warm center and high humidity,the vorticity column on the lower levels was the moistconvective instability and negative moist potential vorticity structure:the intensive ascendingvertical motion and the intense divergence on upper levels and intensive convergence on the lowerlevels as well as the development of the convective cloud cluster were intercoupling:the intensesouthern wind jet companied by the typhoon-low was not only the interaccompanying andintercoupling condition of the development and maintenance of the typhoon-low and convectivecloud cluster,but also was the transportable belt of the moisture source and heat energy of the“96.8”extraordinary rainstorm.The analysis of simulative results of precipitation indicated thatthe distribution of the rainfall belt and rainfall rate was basically consistent with that of theobservation in spite of some rainfall centers less or larger than those of the observation for coarseor fine mesh domain,respectively.     

大尺度环境中中尺度对流系统生成的数值模拟试验

对1983年6月11日20时(北京时)实测资料进行低通滤波处理,取其大尺度部分作为初值,用七层原始方程模式对江淮气旋内中尺度对流系统的生成过程进行数值模拟试验.结果表明,从大尺度背景场出发,可以模拟出中尺度对流系统的生成.在西南低空急流十分潮湿的情况下,中尺度对流系统可以在江淮气旋暖区、低空急流大风速中心的前部、位势稳定度倾向维持负值的地区生成和发展.它的动力、热力结构,降水分布,以及发生、发展过程等,都与暖季(3—9月)在美国中部频繁出现的中尺度对流复合体(MCC)十分相似.     

中尺度强对流云系相互作用与热带气旋形成的数值模拟

本文是热带气旋形成的多尺度组合理论的续篇它以新的方法——数值模拟的结果支持了这一理论。特别是它进一步证实了“热带大气涡旋增幅效应”的存在,也进一步定量地解释了热带气旋前期低压环流的形成。     

Numerical Simulation of Microphysics in Meso-β-Scale Convective Cloud System Associated with a Mesoscale Convective Complex

Numerical simulation of meso-β-scale convective cloud systems associated with a PRE-STORM MCC case has been carried out using a 2-D version of the CSU Regional Atmospheric Modeling System (RAMS) nonhydrostatic model with parameterized microphysics. It is found that the predicted meso-γ-scale convective phenomena are basically unsteady under the situation of strong shear at low-levels, white the meso-β-scale convective system is maintained up to 3 hours or more. The meso-β-scale cloud system exhibits characteristics of a multi-celled convective storm in which the meso-γ-scale convective cells have lifetime of about 30 min. Pressure perturbation depicts a meso-low after a half hour in the low levels. As the cloud system evolves, the meso-low inten-sifies and extends to the upshear side and covers the entire domain in the mid-lower levels with the peak values of 5-8 hPa. Temperature perturbation depicts a warm region in the middle levels through the entire simulation period. The meso-γ-scale warm cores with peak values of 4-8oC are associated with strong convective cells. The cloud top evapo-ration causes a stronger cold layer around the cloud top levels.Simulation of microphysics exhibits that graupel is primarily concentrated in the strong convective cells forming the main source of convective rainfall after one hour of simulation time. Aggregates are mainly located in the stratiform region and decaying convective cells which produce the stratiform rainfall. Riming of the ice crystals is the predominant precipitation formation mechanism in the convection region, whereas aggregation of ice crystals is the predominant one in the stratiform region, which is consistent with observations. Sensitivity experiments of ice-phase microphysical processes show that the microphysical structures of the convective cloud system can be simulated better with the diagnosed aggregation collection efficiencies.     

Life cycle of mesoscale convective systems

Radar observations of the evolution and hierarchy of Cb clouds in different regions of the Earth are generalized to present the concept of a life cycle of a mesoscale convective system; during the life cycle, the separate Cb clouds develop into hierarchic coordinated mesoscale clusters, which recurrently originate and are localized in fixed places in the system which moves as a unit. The rising dominant clusters generate quasi-periodic oscillations of maximum intensity and wavelike space structure of the precipitation field. The methodological principles of the concept are used for objective classification of precipitation systems based on morphological and evolutional features. They can be used in techniques of very-short-range forecasting of hazardous convective weather.     

Analysis and simulation of mesoscale convective systems accompanying heavy rainfall: The goyang case

We investigated a torrential rainfall case with a daily rainfall amount of 379 mm and a maximum hourly rain rate of 77.5 mm that took place on 12 July 2006 at Goyang in the middlewestern part of the Korean Peninsula. The heavy rainfall was responsible for flash flooding and was highly localized. High-resolution Doppler radar data from 5 radar sites located over central Korea were analyzed. Numerical simulations using the Weather Research and Forecasting (WRF) model were also performed to complement the high-resolution observations and to further investigate the thermodynamic structure and development of the convective system. The grid nudging method using the Global Final (FNL) Analyses data was applied to the coarse model domain (30 km) in order to provide a more realistic and desirable initial and boundary conditions for the nested model domains (10 km, 3.3 km). The mesoscale convective system (MCS) which caused flash flooding was initiated by the strong low level jet (LLJ) at the frontal region of high equivalent potential temperature (θ_e) near the west coast over the Yellow Sea. The ascending of the warm and moist air was induced dynamically by the LLJ. The convective cells were triggered by small thermal perturbations and abruptly developed by the warm θ_e inflow. Within the MCS, several convective cells responsible for the rainfall peak at Goyang simultaneously developed with neighboring cells and interacted with each other. Moist absolutely unstable layers (MAULs) were seen at the lower troposphere with the very moist environment adding the instability for the development of the MCS.     

The structure of mesoscale convective systems in central Russia

Described is the organization of 216 summertime mesoscale convective systems (MCSs) in Moscow region. Initially, the horizontal radioecho fields corresponding to the stage of MCS reflectivity maximum were divided into 92 lines with the length of more than 200 km and 124 nonlinear MCSs. Subsequently, three types of lines were separated: lines with solid segments of reflectivity of more than 40 dBZ of ∼100 km long, discontinuous convection bands representing the combinations of linear and node-like amorphous thunderstorms, and convex or concave bow-shaped systems frequently forming the families of spirals or occlusions. The nonlinear MCSs contain circular radar cells (similar to open mesoscale cells) or small bow segments of ∼30 km; however, the most intensive storms have a propagation axis connecting the major MCSs. Finally, an objective classification of MCSs is proposed based on the quantiles of climatological distributions of various parameters describing the intensity, morphology, and kinematics of MCSs.     

Numerical simulations of mesoscale flood environment

Summary The transition from a cold to a warm state of the E1 Niño-Southern Oscillation (ENSO) cycle is studied using Comprehensive Ocean-Atmosphere Data Sets (COADS) for the period 1950–1992.The onset of El Niño (November to December of the year preceding the El Niño) is characterized by an occurrence of minimum sea-level pressure anomalies in the subtropics around the node line of the Southern Oscillation. This pressure fall favors the formation of the anomalous cyclonic circulations over the western Pacific and leads to the establishment of anomalous westerlies in the western equatorial Pacific during the boreal spring of the El Niño year. The westerly anomalies then intensify and propagate into the central Pacific by the end of the El Niño year. This is an essential feature of the development of a basin-wide warming.It is argued that the development of the equatorial westerly anomalies over the western Pacific may result from the thermodynamic coupling between the atmosphere and ocean. In boreal winter and spring the mean zonal winds change from westerly to casterly over the western equatorial Pacific. A moderate equatorial westerly anomaly initially imposed on such a mean state may create eastward SST gradients via changing rates of evaporational cooling and turbulent mixing. The equatorial SST gradients would, in turn, induce differential heating and zonal pressure gradients which reinforce the westerly anomalies. The feedback between the eastward SST gradients and westerly anomalies promotes the eastward propagation of the westerly anomalies.With 9 Figures     

Passive microwave and infrared structure of mesoscale convective systems

Summary The precipitation structure of mature Mesoscale Convective Systems (MCS) is examined in both the midlatitudes and the tropics using SSM/I microwave measurements, geostationary satellite observations, and ground-based radar observations. Discussion includes qualitative comparisons between midlatitude and tropical MCS cases, with particular emphasis on the delineation of convective and stratiform regions and the characterization of microwave polarization difference temperatures in the MCSs. Implications are given regarding the importance of the vertical precipitation structure on top of the atmosphere (TOA) microwave temperatures and for rain retrieval algorithms using measurements from space.Some of the principle findings include the ability of passive microwave brightness temperature measurements to distinguish stratiform and convective regions of MCSs for both tropical and midlatitude cases and over land and ocean backgrounds. Convective regions typically had low differences between the vertical and horizontal brightness temperatures while the stratiform regions have larger differences, and these differences are likely related to the spatial microphysical variations in the upper levels of the precipitation region. Several cases were found in midlatitudes and one case in the tropics where the lowest infrared (IR) brightness temperatures were displaced into the anvil region and were not colocated with the coldest microwave temperatures. Life cycle dependence of the displacement is suggested, but the SSM/I measurements with a maximum of twice daily coverage over the same location were inadequate to answer this question.With 10 Figures     

The role of the convective “trigger function” in numerical forecasts of mesoscale convective systems

Summary Three-dimensional numerical model simulations of a mesoscale convective system are performed to evaluate the sensitivity of the simulations to differences in the convective trigger function. The Penn State/NCAR mesoscale model with the Kain-Fritsch convective parameterization scheme is used as the modeling system for the study. All simulations are performed on the June 10–11, 1985 squall line from the OK PRE-STORM field experiment. Individual simulations differ only in their specification of the trigger function within the Kain-Fritsch scheme. Comparison of results from 12 hour simulations indicates that the position, timing, and intensity of convective activity and mesoscale features vary substantially as a function of the trigger function formulation. The results suggest that the convective trigger function is an integral part of the overall convective parameterization problem, and that great care must be exercised is designing realistic trigger function formulations, especially as model resolutions approach the scale of individual convective clouds.With 7 Figures