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

The motion equation of atmospheric boundary layer with cold front surface under geostrophicmomentum approximation is solved and some characteristics of the stream field for the cold frontsurface in the boundary layer are derived,for example,the slope of the cold front surface increaseswith 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 motiondirection of the front:the stream field above and below the cold front surface in the boundary layerdepends on the slope of the front surface,geostrophic wind speed and its temporal and spatialdistributions.A common characteristic is that there exist updraft motion above the cold frontsurface and downdraft motion below it.     

边界层内冷锋流场的动力学特征

文中求解了锋面存在时地转动量近似下的大气边界层运动方程,得到了边界层内冷锋流场的一些特征,如冷锋坡度随地转涡度增加而增加,随地转风速时间倾向的增加而增加,随沿锋面传播方向的热成风分量的减少而增加。而边界层内冷锋面上下的流场与锋面坡度、地转风及其时空变化特征有关,共同特点是在冷锋面高度以下有下滑运动,而其上有一层上滑运动区。     

ON THE PDF MODELS FOR ATMOSPHERIC DIFFUSION IN BOUNDARY LAYER*

In this paper,taking its turbulent exchange coefficient as a function of the Lagrangian time scale and standard variance of the turbulence in atmosphere,the atmospheric dispersion PDF models are obtained on the basis of atmospheric diffusion K-theory.In the model the statistics of wind speed are directly used as its parameters instead of classic dispersion parameters.The bi-Gaussian PDF is derived in convective boundary layer (CBL),from the statistics of vertical velocity in both of the downdraft and updraft regions that are investigated theoretically in the other part of this paper.Giving the driven parameters of the CBL (including the convective velocity scale ω^* and the mixing depth h_i) and the time-averaged wind speed at release level,the PDF model is able to simulate the distribution of concentration released at any levels in the CBL.The PDF's simulations are fairly consistent with the measurements in CONDORS experiment or the results brought out by some numerical simulations.     

THERMODYNAMIC PROPERTIES AND ROLE OF THERMALS IN THE STRATUS-TOPPED BOUNDARY LAYER*

From results of two large-eddy simulation of stratus-topped boundary layer,the structure,thermodynamic properties and role of thermals are investigated by using conditional sampling methods,which divided the thermals into the warm/moist,cool/dry,warm/dry and cool/moist events.The results show that the main turbulent circulation in the stratus-topped boundary layer is composed of the warm/moist updraft and cool/dry downdraft.Below entrainment region,the warm/moist updrafts and cool/dry downdrafts are,respectively,positively and negatively buoyant and contribute most to total fluxes and variances.Evaporative cooling has important effect on the structure and thermodynamic properties of thermals in stratus-topped boundary layer.     

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.     

STRUCTURAL FEATURES OF THE MEIYU FRONT SYSTEM*

A new subtropical front near the periphery of the West Pacific subtropical anticyclone is found,which is never revealed in previous studies.The coupling of the subtropical front and Meiyu front forms a Meiyu front system (MFS) and is the most direct synoptic system for the Meiyu precipitation along the Mid-lower Reaches of Yangtze River (MRYR) in China.In this paper.The detailed structural features and cloud features of the MFS in 1998 and 1999 are analyzed,which manifests that the MFS is an objective phenomenon over the period of Meiyu along MRYR and the Southwest Japan.Generally.the subtropical front is mainly located between 850 hPa and 500 hPa.The moist southwest monsoon is transported in the passageway between the Meiyu front and the subtropical front.The vertical motion ascends in the passageway and descends on both sides of the MFS,forming the MFS's secondary circulation.A lower TBB band indicated that obvious convective activities are also located in the passageway of MFS.The horizontal wind of MFS is vertically asymmetric.     

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.     

SENSITIVITY TESTS OF INTERACTION BETWEEN LAND SURFACE PHYSICAL PROCESS AND ATMOSPHERIC BOUNDARY LAYER*

In this paper,an interactive model between land surface physical process and atmosphere boundary layer is established,and is used to simulate the features of soil environmental physics,surface heat fluxes,evaporation from soil and evapotranspiration from vegetation and structures of atmosphere boundary layer over grassland underlying.The sensitivity experiments are engaged in primary physics parameters.The results show that this model can obtain reasonable simulation for diurnal variations of heat balance,soil volumetric water content,resistance of vegetation evaporation,flux of surface moisture,and profiles of turbulent exchange coefficient,turbulent momentum,potential temperature,and specific humidity.The model developed can be used to study the interaction between land surface processes and atmospheric boundary layer in city regions,and can also be used in the simulation of regional climate incorporating a mesoscale model.     

一次干冷锋过程的边界层风场结构和尺度分析

本文利用南京北郊164米气象塔实测资料对一次非降水弱冷锋过程的边界层风场结构一三维风速、横锋向水平散度、湍流通量和能量的时空分布，以及多尺度时空分布进行了计算分析。结果表明，在冷暖空气的过渡区（锋区）风场各参量具有明显的梯度，锋区前线的强上升气流与强冷锋相近，但锋区宽度较强冷锋宽得多，其时间尺度为30分钟，水平距离为35公里。锋面过境对风温场的影响时段为24小时，在这期间，出现阵风浪涌和多种中、小尺度（时间尺度为3－4小时、1－20分钟）的湍涡活动。     

梅雨锋结构的数值模拟

利用1999年6月下旬持续性梅雨锋降水过程的全程四维同化模拟结果,深入分析梅雨锋结构的时空不均匀变化特征及其与低涡降水强度的密切关系。结果表明,梅雨锋呈现明显的中层锋和边界层锋两段锋的特征,中层梅雨锋区对降水的影响比边界层锋更为关键,中层锋的加强、锋坡增大趋于垂直、锋区垂直环流的加强和与高空急流锋区的上下贯通,有利于梅雨锋降水的加强,强降水并不出现于中层锋区最强的时段,而是发生于大范围锋区强度达峰值之后约16—24 h。中低层总变形加强与梅雨锋的加强有密切关系。组成低空急流的中低层u,v分量呈现不同的分布和演变特征,强南风中心位于900—800 hPa,呈明显的低空急流状特征,贴近暴雨区还可能出现较小尺度的急流;而强西风中心出现于中层锋前700—500 hPa,表现为高空强西风区沿锋区上界的向下延伸;低空南风急流通常与总变形同时加强。强锋段的锋前饱和高湿高能气柱、锋前中低层急流状南风区和中层西风均匀大值区等要素场呈现高度组织化的特征。梅雨锋的低层特性,如辐合、锋区强度、总变形和南风分量及降水强度等要素呈现显著的中尺度扰动特征,有明显的日变化且受长江中下游中尺度地形影响,扰动特征有随时间上传的趋势。     

EFFECTS OF BOUNDARY LAYER PARAMETERIZATION ON THE MONTHLY MEAN SIMULATION*

Treatments of the boundary layer in a five-layer primitive equation model with mixed P-σ coordinates are improved by incorporating similarity theory,which is widely used at present.Two kinds of parameterization schemes of the boundary layer are designed,and static test and numerical simulation are performed.It is found that the iterative scheme can fairly improve the simulation of the mean climate field in July.The abnormally strong heat low over the Tibetan Plateau is weakened to some extent.     

SPECTRAL CHARACTERISTICS OF SURFACE-LAYER TURBULENCE OVER THE SUBURBS OF TIANJIN*

The paper computed the spectra of velocity and temperature,and the cospectra of velocity and temperature by the observational data of July,1990 in the suburbs of Tianjin.The results show that the characteristics of the atmospheric turbulence spectra over the suburbs are basically in accordance with some typical results over the flat terrain.But the scale,on which turbulence can satisfy the isotropic condition,over the suburbs is larger than over the flat terrain.The spectrum peak frequency range is a bit narrower.The feature of the spectrum range in low frequency(LF) is out of accordance with that of the flat terrain either.     

THE COMBINATORY METHOD FOR DETERMINATION OF THE TURBULENT FLUXES AND UNIVERSAL FUNCTIONS IN THE SURFACE LAYER*

A combinatory method of determining the turbulent fluxes in the surface layer has been developed and their general representations have been thus obtained.The universal functions of the (M-O) similarity in the surface layer can be determined by the method.The results calculated by using the ITCE's data indicate that the method is feasible.     

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 modern PBL 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.     

NUMERICAL SIMULATIONS OF THE EFFECTS OF AEROSOLS IN THE ATMOSPHERIC BOUNDARY LAYER ON THE CLIMATE*

The climatic effects of the atmospheric boundary aerosols are studied by the use of a three-dimensional climate model.Simulated results show that the climate states both at the surface and in the atmosphere change remarkably when the aerosols with different optical thicknesses and properties are introduced into the atmospheric boundary layer of the model.The aerosols absorb and scatter the solar shortwave radiation,therefore,they reduce the solar energy reaching the ground surface and decrease the surface and the soil temperatures.The temperature in the boundary layer increase because of the supplementary absorption of radiation by the boundary aerosols.In the atmosphere,the temperatures at all isobaric surfaces rise up except for the 100 hPa level.The atmospheric temperatures below the 500 hPa level are directly influenced by the boundary aerosols,while the atmospheric temperatures above the 500 hPa level are influenced by the heating due to convective condensation and the changes in the vertical motion field.Cyclonic differential circulations appear over the desert areas at the low levels,and anticyclonic differential circulations exist at the upper levels in the horizontal flow fields.The vertical motions change in correspondence with the differential circulations.The changes in precipitation are directly related to that of vertical motions.The mechanisms of climate effects of the boundary aerosols are also discussed in this paper.     

梅雨锋暴雨的中尺度结构和边界层特征

对１９９０年两场梅雨锋暴雨的地面中尺度结构作了较详细的分析研究，并利用加密观测的铁塔和低探资料，揭示了梅雨锋暴雨各阶段边界层热通量和动量通量特征。     

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.     

AIR-SEA COUPLING FEATURES FROM THE WARM PHASE TO THE COLD PHASE OF ENSO CYCLES IN THE SEVENTIES TO THE EIGHTIES*

Using the sea surface temperature and wind anomalies(SSTA and SSWA for short) of the tropical Pacific from January 1970 to December 1989,main spatial patterns of tropical Pacific SSTA and SSWA coupling features in the transform course from the warm phase to the cold phase of El Nino-southern Oscillation(ENSO) cycles are discussed.The main conclusions are as follows:(1)air-sea coupling patterns at the mature stage of El Nino(La Nina) are main spatial ones of tropical Pacific SSWA and SSTA coupling:(2)at the mature stage of El Nino,the interaction of the anticyclonic anomaly wind,generated by the forcing of distinct meridional SSTA gradient in the Northern Hemisphere tropical central Pacific.with the California cold current and SSTA is mainly responsible for weakening of El Nino;(3)the second sea temperature increase along the South American coast in the decaying course of El Nino results from the eastward movement of the weakened positive SSTA in the tropical central-eastern Pacific forced by anomalous west wind stress:(4)La Nina results from the joint effect of Walker circulation,Ekman drift and negative SSTA in the tropical central-eastern Pacific.     

南京近郊地区初冬大气边界层风温场的探测和分析

利用现场低空探空探测所获取的实测资料,揭示了南京近郊初冬大气边界层风、温场在不同天气条件下的特征,得出了晴天温廓线日变化的模型,用谐波分析探讨了温度和风随高度分布和随时间演变的特征。     

涡动相关仪探测台风边界层动量通量特征分析

台风的发展维持受制于自身(内部结构)及与下垫面间的通量传输过程,利用台风“利奇马”登陆前后的涡动相关仪观测数据,分析台风过境前后地面要素变化及动量通量特征。分析结果表明：台风过境期间,地面气象要素在短时间内出现急剧的变化,动量通量在台风登陆过程中显示出“三峰”的结构,对应的摩擦速度随风速的变化以10 m/s为分界线,低风速下呈现明显的正相关性,高风速时则慢慢转为持平甚至回落;能谱分析中,峰值主要存在于0.000 2 Hz和0.06 Hz的位置上,另有一位于0.001 Hz外的较弱峰值;能量贡献上,大尺度系统占据主导作用,但中尺度对流系统与湍流尺度运动在能量贡献中同样不可忽视。