地形的动力作用与冷锋锋生研究

本文建立了一个考虑天气尺度水平变形场的二维、滞弹性、非静力平衡锋生模式，对冷锋过山时的锋生锋消机制进行了数值研究，试验结果表明:当无地形影响时，由于变形场的锋生作用与摩擦等因子的锋消作用相互抵消，冷锋最终达到其强度的准定常状态；当考虑地形影响时，地形强迫环流与锋面环流相互作用，使得锋面强度在山前和山后发生变化，在冷锋开始爬坡时锋生，爬过半山腰后开始锋消，过山顶后锋面强度变化不明显或者有微弱锋生，到达山脚后开始强烈锋生，并在地形下游其强度达到最大；总之，冷锋在翻越山脉或从高原移到平原之后，锋面发展的强度比冷锋不遇到地形时的更大。     

一次β-中尺度暴风雪的成因及动力热力结构

利用多普勒雷达、地面加密自动站监测资料、常规观测资料、4DVAR雷达风场反演和中尺度数值模式WRF模拟结果,对华北东部一次β-中尺度暴风雪的成因和影响天气系统的热力、动力结构及演变进行了分析。结果表明,(1)造成暴风雪的天气系统是回流形势下的冷锋锢囚锋,其在形成过程、生命史、尺度范围等方面均与温带气旋锢囚锋不同。(2)暴风雪过程分为东路弱冷锋降雪、锢囚降雪、西路强冷锋降雪3个阶段。降雪回波顶高在3～4km,回波强度均在35dBz以下。降雪量集中在锢囚降雪阶段。(3)风廓线(VWP)下层东风层和上层西南风的厚度在降雪各阶段不同,东风急流和西南风急流的变化和配置与降雪量关系密切。锢囚降雪阶段,地面和低层水平风场具有β-中尺度气旋性环流,是造成降雪回波旋转且长时间维持的动力。"人"字状回波的形成是因为风向或风速辐合线的作用。(4)锢囚锋的垂直结构和锢囚过程表明,西路冷锋比东路冷锋陡峭且势力强大,暖湿气团被东西两侧冷锋夹挤、抬升;锢囚时,两侧锋面形成冷式锢囚;锢囚消失时,大风首先在900hPa高度附近出现,进而下传到地面。     

地形上空边界层流中低层锋面结构的理论研究 I：冷锋、均匀地转流

文中利用一个包含地形、边界层摩擦作用的二层锋面简化模型 ,讨论了地形、边界层对冷锋锋面结构、环流的动力学影响作用问题。冷锋的倾斜主要与冷锋暖区的地转流分布、锋面移动速度、锋面与地形的相对位置有关。当冷锋位于迎风坡时 ,其坡度减小 ,位于背风坡时 ,其坡度增大。在静止冷锋存在两类不同的环流系 ,即位于大气低层、地面锋附近的反时针环流系和远离地面锋的顺时针环流系。静止冷锋位于背风坡 ,其冷域中的这支闭合环流增强 ,范围增大 ,而位于冷锋界面上的环流转向点沿锋面上移 ,暖域中沿锋面的暖空气运动范围变大。当静止冷锋位于迎风坡时 ,结果相反。冷锋移动较慢时 ,其冷域远离地面锋的这支顺时针环流主体可被地形完全阻塞在迎风坡。当静止冷锋移离地形时 ,由于地形作用可在锋面暖域、地形下游形成一个背风槽。地形对锋区的垂直运动影响主要通过地形与边界层共同作用 ,改变锋区流场辐合辐散的分布及地形强迫抬升两条途径实现。由于边界层摩擦的辐合作用 ,在地面冷锋的前缘可形成一支范围较窄、强度较大的上升运动带 ,当冷锋位于迎风坡时 ,其强度增强 ,当冷锋位于背风坡时 ,其强度减弱。当冷锋位于背风坡时 ,在暖区沿锋面上升的暖空气运动范围增大 ,可以出现沿锋面相间隔的多个上升区     

地形上空边界层流中低层锋面结构的理论研究Ⅰ:冷锋、均匀地转流

文中利用一个包含地形、边界层摩擦作用的二层锋面简化模型,讨论了地形、边界层对冷锋锋面结构、环流的动力学影响作用问题.冷锋的倾斜主要与冷锋暖区的地转流分布、锋面移动速度、锋面与地形的相对位置有关.当冷锋位于迎风坡时,其坡度减小,位于背风坡时,其坡度增大.在静止冷锋存在两类不同的环流系,即位于大气低层、地面锋附近的反时针环流系和远离地面锋的顺时针环流系.静止冷锋位于背风坡,其冷域中的这支闭合环流增强,范围增大,而位于冷锋界面上的环流转向点沿锋面上移,暖域中沿锋面的暖空气运动范围变大.当静止冷锋位于迎风坡时,结果相反.冷锋移动较慢时,其冷域远离地面锋的这支顺时针环流主体可被地形完全阻塞在迎风坡.当静止冷锋移离地形时,由于地形作用可在锋面暖域、地形下游形成一个背风槽.地形对锋区的垂直运动影响主要通过地形与边界层共同作用,改变锋区流场辐合辐散的分布及地形强迫抬升两条途径实现.由于边界层摩擦的辐合作用,在地面冷锋的前缘可形成一支范围较窄、强度较大的上升运动带,当冷锋位于迎风坡时,其强度增强,当冷锋位于背风坡时,其强度减弱.当冷锋位于背风坡时,在暖区沿锋面上升的暖空气运动范围增大,可以出现沿锋面相间隔的多个上升区.     

对称和非对称地形对冷锋锋生过程的影响

本文利用半地转模式研究了对称和非对称地形以及双地形对冷锋锋生过程的影响。计算结果显示，地形坡度愈大，越容易产生背风气旋，并且其强度也愈大；背风气旋的强度主要由背风坡坡度决定。山对锋面垂直速度的增幅作用随地形坡度增大而增大，地形坡度越大．出现多次极大上升速度的可能性也越大。冷锋过双地形时，其锋面强度有两次减弱和加强的过程，上升速度得到明显加强，并有可能在锋前产生多条中尺度上升运动带，在合适的水汽条件下，冷锋过双地形有可能产生多重中尺度雨带，其间隔与锋面本身的尺度相当。     

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。     

锋面波发展演变与气旋形成的数值研究

本文将锋面作为密度的零级不连续面，对锋面上扰动的非线性发展演变进行了数值研究，所用的锋面模型是f平面上的浅水、两层模型，差分格式具有位涡拟能守恒和动能守恒的性质。锋面波的发展与初始扰动尺度有关，天气尺度的扰动可以维持较长的发展时间，形成较大振幅的波动，其结构类似于具有锢囚锋结构的气旋，相反，较小尺度扰动不易形成锢囚锋结构。同时，锋面波的发展演变受Richardson数(Ri)的影响，Ri越小，锋面波发展的时间越长，波动振幅越大；Ri越大时，锋面波发展的时间越短，波动容易衰减。锋面波的发展类似于气旋的形成过     

冷空气影响前广州局地异常增温的诊断

针对2015年1月6日冷空气到达广州前局地异常增温现象,利用逐时实测气温、MICAPS天气图分析天气背景发现,5日广州处于深厚的暖湿气流控制条件下,6日20:00后冷锋过境,则6日20:00前属于冷锋逼近时期。选用5日08:00—20:00和6日08:00—20:00时段的NCEPFNL再分析资料,借助热力学能量方程估算局地气温的平流项、垂直项大小,并由实测气温经过计算获得非绝热项的估算值。分析结果表明,冷锋逼近广州时非绝热项的贡献比例明显大于暖湿气流控制该地时非绝热项的贡献比例。通过对比分析这2个时段的非绝热项中天空状况、地形、下垫面性质、低空相对湿度和冷锋锋面移动5个影响因子的贡献情况,得出6日广州气温的异常增温与冷锋锋面的逼近密切相关。     

锋面波发展演变与气候形成的数值研究

西文将锋面作为密度的零级不连续面，对锋面上扰动的非线性发展演变进行了数值研究，所用的锋面模型ｆ平面上的浅水、两层模型，差分格式具有位涡拟能守恒和动能守恒的性质。锋面波的发展与初始扰动尺度有关，天气尺度的扰动可以维持较长的发展时间，形成较大振幅的波动，其结构类拟于具有锢囚锋结构的气旋，相反，较小尺度扰动不易形成锢囚锋结构。     

地形与双冷锋的锋生过程

本文利用半地转模式，讨论了双冷锋的锋生过程及其越过山脉的演变。结果显示，在变形场作用下，双冷锋在移行过程中会发生上层锋合并的现象。在锋生过程中，下层前一条冷锋加强远大于后一条，在上层则是后一条冷锋的加强远大于前一条。地形对双冷锋有重大影响，山的迎风坡对冷锋移行有阻挡作用，并削弱锋的强度，背风坡则加速锋的移行，并加强锋的强度。山对双冷锋的上层锋的合并有促进作用，并造成近地面锋区呈现鼻状突出；地形促使锋区垂直速度大大增大，并导致其产生跳跃式变化；地形对锋的影响限于下层锋区。     

DISTORTION AND OCCLUSION OF COLD FRONTS UNDER THE INFLUENCE OF OROGRAPHY

Following classical theory,a two-layer frontal model is designed.It assumes that frontal surface is the zero-order discontinuity of density.In this paper,the model is used to study orographically distorted and occluded fronts.It is indicated that the distortion and occlusion of cold fronts are different under different conditions of orography.Frontal intensity also influences the distortion and occlusion.Some rules are revealed and compared with observations.In addition,we have analysed the vertical structure of cold front above mountains and found that frontal surface may be bulged under the influence of orography when it climes the mountain.The bulge of frontal surface is just like the bore as pointed out by Long(1972) when he analysed fluid over obstacles.     

地形上空边界层流中低层锋面结构的理论研究Ⅱ:暖锋、均匀地转流

利用一个有地形、边界层摩擦作用、简化的二层浅水锋面模型,在理论上研究了地形上空边界层流动中地面暖锋的结构及环流分布特征问题。暖锋的坡度主要取决于其暖域地转流、锋面移速,它随锋面移速增大而减小,这与冷锋特征相反。地形对暖锋坡度的影响作用较小。与无地形作用时相比,静止性暖锋冷域中,位于锋面界面附近的闭合正环流系,当暖锋位于地形上游,其伸展范围增大;当暖锋位于迎风坡时,其伸展范围缩小,中心位置上抬;锋面移至背风坡时,其伸展范围重新增大。对于冷域中远离地面暖锋的另一支正环流系来说,当暖锋位于地形上游或迎风坡时,它可被地形完全阻塞于背风侧,地形高度越高,地形阻塞作用越大。在暖锋锋区附近主要存在三支垂直上升运动带：（a）由于边界层摩擦辐合作用,导致在地面暖锋后缘暖区中形成一支水平尺度较小、强度较大的垂直运动带,它随着暖锋移速增大而减弱。该垂直运动带,当暖锋位于地形迎风侧,强度增加;暖锋位于地形背风侧,其强度减弱。（b）在锋区暖域沿锋面存在均匀的上升运动,（c）在冷域远离地面暖锋处,存在一支水平范围较宽,其中心位于边界层顶部附近的垂直运动带,当暖锋位于迎风坡时,这支垂直运动带可被地形阻塞于地形背风侧。     

The orographic deformation and retardation of a frontal zone

Summary A three-layer version of Davies' (1984) model is used to investigate orographic impact on fronts. Uniform potential vorticity is assumed for each layer. The uppermost layer represents the flow ahead and above a frontal zone approaching a mountain. The frontal zone is represented by the second layer and the cold air behind it by the third layer of the model. The upper layer induces weak frontogenesis when the frontal zone is moving towards the mountain. However, rapid frontogenesis sets in when the zone's leading edge is climbing up the slope while part of the frontal zone is still in the plain. When, finally, the whole frontal zone is moving up the slope, the flow evolution strongly depends on the structure of the frontal zone: frontolysis as well as frontogenesis can occur.With 5 Figures     

Orographical modification and large scale forcing of a cold front

Summary The development of a cold front influenced by orography and large scale forcing is examined with a two-dimensional meso-scale model. The model is based on the primitive equations and uses the hydrostatic and anelastic approximations. Gradients of the basic flow and temperature field in the third dimension are taken into account during the simulations. Low diffusive numerical schemes and radiation boundary conditions reduce the numerical errors to an acceptable minimum for a two day simulation and avoid reflections at the upper and lateral boundaries. Frontogenetical forcing is included in the simulations by specifying either a vertically sheared or horizontally convergent basic zonal flow field. Model runs with an idealized cold front were carried out over flat terrain and in the presence of a bell shaped mountain ridge.The simulations show a weakening of the cold front on the windward side of the mountain ridge and a strong reintensification on the leeward side relative to the control runs without topography. Analysis of frontogenesis terms demonstrates the importance of convergence in the ageostrophic circulation and of along-front temperature advection for the development of the cold front. The strong intensification of the cold front on the leeward side of the mountain ridge can only partly be explained by superposition with the mountain induced wave. It is mainly caused by ageostrophic deformation forcing in the strong downward flow of this wave.The results also show that the cold front passage over the mountain ridge is not a continuous process. The formation of a new frontal structure on the leeward side of the mountain ridge, well separated from the primary one, is observed while the initial cold front still exists in the upslope region. Generally nonlinear interactions between the mountain wave and the cold front are the important mechanisms to explain these phenomena.With 18 Figures     

A NUMERICAL INVESTIGATION OF THE IMPACT OF MOIST PROCESSES UPON THE STRUCTURE OF ACTIVE COLD FRONTS:APPLICATION OF A 3D MESOSCALE MODEL IN FRONTAL SITUATIONS

A 3D mesoscale model is applied in a marine cold front case in order to investigate the impact oflatent heating on the structure of cold fronts.Results of the moist and dry simulations are comparedto stress the effects of moist processes.It is found that both the temperature gradient and the cyclonicvorticity across the frontal zone considerably increase due to the latent heating,especially in the lowertroposphere.A thermally direct cross-front ageostrophic circulation.forced by frontogenesis,is foundbetter developed and organized in the moist case than in the dry case.This cross-front directageostrophic circulation,particularly its ascending branch,is considerably enhanced by latent heatingdue to increased frontogenetical forcing and reduced effective static stability in the rising motion regionwhere condensation occurs.One important feature in most observed cold fronts is the presence of anintense rising cell just above their leading edge.This intense rising cell is well simulated in the moistcase but is less clear and much weaker in the dry case,indicating the important contribution by the la-tent heat release to the formation of this intense updraft above the surface cold front.     

Asynchronous responses of East Asian and Indian summer monsoons to mountain uplift shown by regional climate modelling experiments

It has been demonstrated in climate models that both the Indian and East Asian summer monsoons (ISM and EASM) are strengthened by the uplift of the entire Asian orography or Tibetan Plateau (TP) (i.e. bulk mountain uplift). Such an effect is widely perceived as the major mechanism contributing to the evolution of Asian summer monsoons in the Neogene. However, geological evidence suggests more diachronous growth of the Asian orography (i.e. regional mountain uplift) than bulk mountain uplift. This demands a re-evaluation of the relation between mountain uplift and the Asian monsoon in the geological periods. In this study, sensitivity experiments considering the diachronous growth of different parts of the Asian orography are performed using the regional climate model COSMO-CLM to investigate their effects on the Asian summer monsoons. The results show that, different from the bulk mountain uplift, the regional mountain uplift can lead to an asynchronous development of the ISM and EASM. While the ISM is primarily intensified by the thermal insulation (mechanical blocking) effect of the southern TP (Zagros Mountains), the EASM is mainly enhanced by the surface sensible heating of the central, northern and eastern TP. Such elevated surface heating can induce a low-level cyclonic anomaly around the TP that reduces the ISM by suppressing the lower tropospheric monsoon vorticity, but promotes the EASM by strengthening the warm advection from the south of the TP that sustains the monsoon convection. Our findings provide new insights to the evolution of the Asian summer monsoons and their interaction with the tectonic changes in the Neogene.     

Interaction of Orographic Disturbance with Front

1.IntroductionTheimportanceoforographiceffectsonfrontwasrecognizedintheearly20thcentury.Butforthecomplexityofthisproblem,theinvestigationoforographiceffectsonfrontfromdynamicalviewpointisnottakenuntilthe1980s.Bannon(1983)derivedanalyticalsolutionsforthequasi--geostrophicfrontforcedbyahorizontalwinddeformationfieldthatmovesoveratwo--dimensionalmountainridge.Thesolutionsshowthatasacoldfrontapproachestheridge,itweakens,relativetotheflat--bottomsolution,andthefrontstrengthensasitmovesdowntothelees…