PRELIMINARY NUMERICAL STUDY OF TOPOGRAPHIC EFFECTS OF THE TIBETAN PLATEAU ON SURFACE DIRECT RADIATION*

To improve the simulating ability of a model,this paper presents a scheme of calculating direct radiation at land surface with topography in the model.A numerical study is conducted for the topographic effects of the Tibetan Plateau on the direct radiation using NCEP terrain data.Results show that,after taking account into the topographic radiation effect,the regional average of the radiation over the Plateau obviously increases in the local early morning and late afternoon,but changes less around noon.The effect is stronger in winter than that in summer.And heterogeneous topography has also affected the distribution of the radiation in this area.A simple numerical experiment shows that considering the effect will lead ground temperature to increase on the slope having more sunshine,and vice versa.     

A NUMERICAL MODEL FOR HILLY-LAND GLOBAL RADIATION

Based on the simulations of the hilly-land topographic parameters,i.e.,average orientation and slope,and topographic screen angle,a theoretical model is detailed to greater extent for global radiation overhilly countries in terms of the experimental data.Three main aspects are examined:the model construction;numerical run with the model parameters;discussion of simulations obtained locally in the model run for thesouthern Dabie Mountains.Results show that the significant effect of the topography on global radiation.The present model applies inprinciple to the numerical simulation of global radiation for any form of topography of various regions.     

地形非均匀性对网格区地面长波辐射通量计算的影响

从理论和数值试验两个方面证明地形的非均匀性(如海拔高度)对网格区地面长波辐射通量的计算有重要影响,海拔高度场的区域平均值及其变差系数是影响网格区地面长波辐射通量的主要因素,仅仅用地表均匀假定下的区域平均参量(如平均海拔高度和平均温度)所计算的网格区地表有效辐射通量值与其真实值之间存在着一定的误差。由于地表有效辐射通量是海拔高度的非线性函数,在特定情况下,其影响相当大,可产生不容忽略的误差。相对而言,海拔高度自身非均匀性对误差的影响远大于地表温度非均匀性项及其混合扰动项所产生的误差。对于不同的地形平均高度,地形非均匀性影响的程度并不相同。平均高度较小时,非均匀性的影响几乎可以忽略,但随着地形平均高度的增加,地形非均匀性的影响程度呈非线性增长趋势。因而,在复杂地形区域,考虑次网格地形的热力作用非常必要。     

TEST OF A TROPICAL LIMITED AREA NUMERICAL PREDIC-TION MODEL INCLUDING EFFECT OF REAL TOPOGRAPHY

In this paper, a tropical limited numerical prediction p.e. model in sigma coordinate is developed. The predicted variables are deviations from a rest reference atmosphere. This transformation is of benefit in reducing truncation error and guarantees computational stability after the very steep real topography is introduced into the model. The numerical tests with this model show that the large-scale obstacle of the Tibetan Plateau is not negligible in forecasting tropical systems like the South China Sea typhoon.     

SOLAR RADIATION AT GROUND IN EAST-ASIA FROM DATA OF THE GEOSTATIONARY SATELLITE GMS

A method(Rieland,1985)has been modified to determine the downward solar radiation at ground overEast Asia,using GMS-3 data of the satellite with a spatial resolution of 10×10 krn~2 and a time interval of 3hours.The distributions of the monthly mean global radiation are obtained for January,April,July and Octoberof 1985.These results illustrate that the topography effect of the Tibetan Plateau is not negligible.The valuesof global radiation over the high Tibetan Plateau during all seasons are higher than those in the lower-levelsurroundings.By comparing model results with ground observed data, the relative errors in monthly averagesof global radiation are 2.8％ for clear sky and 6.3％,5.3％,5.0％ and 4.5％ for cloudy sky in January,April,July and October,respectively.The relative error in daily global radiation during the snow-free season is lessthan 20％ and larger than 20％ in snow-covered winter.We,however,had no data for these comparisons whichwere measured directly at the Plateau during the year 1985.     

THE EFFECTS OF TOPOGRAPHY ON THE SUMMER ATMOS-PHERIC 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°N-81.11°N) 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) due to mountains; topography intensifies the atmospheric baroclinity in region 3, consequently the baroclinic conversion of atmosphere energy is increased. The seasonal characteristics associated with the summer atmospheric energy source in region 3 are caused by seasonal variation of the solar radiation and the land-ocean contrasts and independent of topographic effects. The mechanism of topographic effects on the increase of long wave kinetic energy is also discussed.     

THE DYNAMIC EFFECT OF THE TIBETAN PLATEAU ON THE FORMATION OF ZONAL TYPE CIRCULATION OVER EAST ASIA

From a statistical study of the atmospheric circulation types over the Northern Hemisphere it is found that the zonal type circulation predominates in East Asia. From a three-layer primitive equation model in σ-coordinate, the orographic effect on a westerly current over the Northern Hemisphere is studied. A distinguished deepening trough is observed in the lee-side slope of the Rocky Mountains. But a pronounced trough is over the East Asia coast far from the Tibetan Plateau. In a case of 48 h numerical forecast, it is found that the effect of the Tibetan Plateau tends to build up a rather zonal type circulation, which is con-sistent with the observations. These may be partly explained by the topographic effect of the north slope of the Tibetan Plateau in a simple geostrophic theory.     

INTERACTION OF TOPOGRAPHY WITH SOLITARY ROSSBY WAVES IN A NEAR-RESONANT FLOW

An inhomogeneous KdV equation including topographic forcing is derived by usingperturbation expansions and stretching transforms of time and space.The generation of forcedsolitary Rossby waves by topography in a near-resonant flow and their interactions with freesolitary waves are discussed,and some interesting results are obtained.The numerical resultsshow that the topography has obvious effect on enhancing the amplitude of disturbances,and itmay explain to some degree the formation of blocking by localized topography.     

Parameterization of the thermal impacts of sub-grid orography on numerical modeling of the surface energy budget over East Asia

Summary A parameterization scheme for the thermal effects of subgrid scale orography is incorporated into a regional climate model (developed at Nanjing University) and its impact on modeling of the surface energy budget over East Asia is evaluated. This scheme includes the effect of terrain slope and orientation on the computation of solar and infrared radiation fluxes at the surface, as well as the surface sensible and latent heat fluxes. Calculations show that subgrid terrain parameters alter the diurnal cycle and horizontal distributions of surface energy budget components. This effect becomes more significant with increased terrain slope, especially in winter. Due to the inclusion of the subgrid topography, the surface area of a model grid box changes over complex terrain areas. Numerical experiments, with and without the subgrid scale topography scheme, show that the parameterization scheme of subgrid scale topography modifies the distribution of the surface energy budget and surface temperature around the Tibetan Plateau. Comparisons with observations indicate that the subgrid topography scheme, implemented in the climate model, reproduces the observed detailed spatial temperature structures at the eastern edge of the Tibetan Plateau and reduces the tendency to overestimate precipitation along the southern coastal areas of China in summer.     

THE EFFECTS OF THE PLATEAU'S TOPOGRAPHIC GRADIENT ON ROSSBY WAVES AND ITS NUMERICAL SIMULATION

By using barotropic model equations, this article analyzed the characteristics of Rossby waves, the propagation features of wave energy and the influence of dynamic and thermal effects of the Tibetan Plateau on Rossby waves, and the focus is on discussing the plateau's topographic gradient effects on atmospheric Rossby waves. Then based on the WRF3.2 and the NCEP/NCAR FNL reanalysis data, we devised comparative tests of changing the plateau's topographic gradient and simulated a process of persistent heavy rain that happened in May 2010 in South China. The results are shown as follows. The Tibetan Plateau’s topography is conducive to the formation of atmospheric Rossby waves. while the plateau's terrain, its friction and heating effects can all make the atmospheric Rossby waves develop into the planetary waves; The effects of plateau's north and south slopes on the Rossby wave’ phase velocity is opposite, and when the slope reached a certain value can the quasi-steady normal fluctuations be generated; Simultaneously, due to the plateau's topographic gradient, descending motion appears at the west side of the plateau while ascending motion appears at the east side, and the vertical movement increased with the amplification of topographic gradients. The plateau's topographic gradient also obviously amplified the precipitation in South China, and the rainfall area increased with the amplification of topographic gradients and gradually moved from south to north and from west to east, which is conducive to the occurrence and development of convective activities in the downstream areas of the Tibetan Plateau; Moreover, for the plateau’s dynamic and thermal effects, the Rossby wave’ propagation shows upstream effects of energy dispersion, so the plateau can then affect the weather in downstream areas. Moreover, the wave group velocity increased with the degree of topographic slope.     

A NUMERICAL EXPERIMENT OF DYNAMIC EFFECT OF QINGHAI-XIZANG PLATEAU ON SUMMER ATMOSPHERIC LOW FREQUENCY VARIATION

In this paper a numerical experiment is presented of the plateau's dynamic effect in terms of a two-layer spectralmodel, and the focus is put on the large-scale orography-excited ALFV with the summer flow pattern as background.together with its propagation and distribution of some features. The main results are as follows: (1) With the summerflow pattern as background the Plateau can excite ALFV, which is characterized by strong regionality. (2) ALFV of thiskind has a baroclinic (equivalent barotropic) structure on the south (north) side of the topography. (3) Its propagation ismarked by dispersion of quasi-stationary Rossby waves over a spherical surface.     

地形对涡旋自组织影响的初步研究

用一个带有地形项的f平面准地转正压涡度方程，实施5组积分时间长度为72h的试验，研究了中尺度地形对涡旋自组织的影响。结果指出：无地形时，准终态涡是一个带有螺旋带的类似台风的涡旋；有地形时，准终态涡是一个无螺旋带但有两个低涡量区的准圆形涡旋。有无地形两个准终态涡中心的位置可以相距100km以上。     

青藏高原地表分类及其东北部辐射通量的二维数值模拟

通过对青藏高原地区地表特征的格点分类及沿五道梁站的经向二维中尺度数值模拟,结果发现：青藏高原地区１３类地表特征都有,尽管其植被自西北向东南呈带状分布,但仍十分复杂;而其地表特征的这种格点分类为该地区的高分辨数值模式模拟提供了极其有用的下边界条件。地面净辐射通量及其诸分量的模拟结果与观测结果的相当接近与一致表明,该模式具有模拟青藏高原地区地面通量日变化的能力。模拟结果和观测分析还指出：夏天睛天条件下     

A study of the effect of topography on the merging of vortices

Eight sets of numerical experiments are performed in 48 hours of integtation by using a barotropic primitive equation model with a topographic term so as to investigate the effect of topography on the merging of vortices. It is pointed out that the introduction of topography may change the track of vortices,and it causes the low vortices and vorticity lumps to be detained on the southeast side of the topography,thus creating a favorable condition for the merging of the low vortex and vorticity lumps. It is also shown that the effect of topography may cause double mergers of vortices in a horizontally shearing basic flow,and it can strengthen the low vortex remarkably.     

近共振地形强迫Rossby孤立波

利用扰动展开和时空伸长变换导得了包括地形强迫的非齐次ＫｄＶ方程，讨论了近共振地形强迫Ｒｏｓｓｂｙｙ孤立波的产生，及其与自由孤立波的相互作用，得到了一些有意义的结果，显示出地形对扰动具有明显的增幅作用，在某种程度上可以解释局地地形在阻塞高压形成中的作用。     

MM4模式积云参数化方案的改进和检验Ⅱ：改进模式的检验试验

为了检验用Fritsch-Chappell积云参数化方案改进的MM4模式^[1]对梅雨锋暴雨系统的模拟能力，初步确定该模型的稳定性，可靠性，用不同的侧边界条件，地形条件，行星边界层参数化方法进行了模拟试验，并将模拟结果与采用Kuo-Anthes积云参数化方案的模式模拟结果分别进行了比较，结果表明，改进模式对高度场和降水的预报均有改善，并能预报出一些细致特征，此外，改进模式对侧边界条件和地形极为敏感，而对行星边界层参数化方法的敏感性较弱，因此在模式侧边界条件选取和地形处理方面应十分谨慎。     

地形强迫对偶极型阻塞形势的影响分析

本研究运用多尺度变换和摄动法简化具有地形强迫的正压准地转涡度方程,得到带有扰动项的非线性Shr?dinger方程,在此基础上,采用孤立子直接微扰理论研究地形强迫对偶极型阻塞结构的作用。研究表明:地形强迫对偶极型阻塞发展具有增强和持续作用,其增长率与大地形山体的斜率成正比。      

Mesoscale numerical modeling of meteorological events in a strong topographic gradient in the northeastern part of Mexico

A series of numerical experiments were carried out to study the effect of meteorological events such as warm and cold air masses on climatic features and variability of a understudied region with strong topographic gradients in the northeastern part of Mexico. We applied the mesoscale model MM5. We investigated the influence of soil moisture availability in the performance of the model under two representative events for winter and summer. The results showed that a better resolution in land use cover improved the agreement among observed and calculated data. The topography induces atmospheric circulation patterns that determine the spatial distribution of climate and seasonal behavior. The numerical experiments reveal regions favorable to forced convection on the eastern side of the mountain chains Eastern Sierra Madre and Sierra de Alvarez. These processes affect the vertical and horizontal structure of the meteorological variables along the topographic gradient.     

An Observational and Numerical Study on the Topographic Influence on Dust Transport in East Asia

Based on observations and numerical simulations, the topographic impacts on dust transport in East Asia were studied. Two regions frequently attacked by dust storms have been confirmed： one is the western part of Inner Mongolia and the southern Mongolia （namely the Mongolia Plateau）, and the other is the Tarim Basin. The most frequent dust storm occurrence area within the first region appears in its hinterland while that of the second one lies in its southern boundary. Moreover, the region from the northeastern edge of the Tibetan Plateau （TP） to the Loess Plateau is attacked by dust storms second frequently. The dust storms frequently occurring over the Mongolia Plateau axe related not only to the abundant sand and dust sources, but also to the special topographic conditions of East Asia. The most significant factor that influences the dust storms forming in the hinterland of the Mongolia Plateau is the canyon low level jet （CLLJ）, which dominates around the southern areas of the Altay-Sayan Mountains with an east-west direction in the beginning of its formation, and is accompanied by significantly enhanced surface wind afterwards. Due to the obstructive effects of the CLLJ, a lot of dust particles carried by the southward down-slope cold air mass would pile up over the southern slope of the Sayan Mountains. Meanwhile, uneven surface conditions are favorable for the dust particles to go up into the upper atmosphere. With the dust particles piling up continuously, a dust layer is formed in the troposphere and can be recognized as a ＇!dust accumulating container＂, which provides abundant dust particles to be transported later to the downstream areas. Additionally, the topographic features of East Asia also exert a great influence on dust transport. Generally, the easterly CLLJ enhances the easterly dust transport. The down-slope air current over the southern Sayan Mountains and the air flow surrounding the TP near its northeastern edge enhance the southward dust transport. Lastly, weather system influences are also examined. The weathers associated with cold fronts frequently appear over the areas of Mongolia and North China in springtime. The cold front system, in general, carries the sand and dust southwards. Among all topographic influencing elements, the rounding effect of the TP is the strongest. Under the combined influences of the cold front and the rounding effect of topography, most sand and dust particles are transported and then deposited over the Loess Plateau.     

青藏高原大地形作用下的Rossby波

在对青藏高原大地形的动力和热力作用物理分析的基础上,着重分析了青藏高原地形坡度和非绝热和非绝热加热对二维Ｒｏｓｓｂｙ波的影响。研究指出：高原地开坡度对Ｒｏｓｓｂｙ波的作用与Ｒｏｓｓｂｙ参数的作用相当,而且其相速度和群速度之间存在一个圆的关系：（ｃｇｘ－ｃｘ）＾２＋ｃｇｙ＾２＝Ｒ＾２;高原加热作用与水平辐合作用相当。地形坡度和非绝热加热都有使二维Ｒｏｓｓｂｙ波向低频发展的的趋向。