大地形对正压扰动演变影响

本文着重研究大地形对正压大气迭加在非均匀纬向基本气流上挑动演变的影响。用WKBJ方法推导了正压扰动的频率和频散关系以及波作用量所满足的方程。当基本流场定常且存在无限长的东西向山脉时,则有波作用量守恒。据此,获得了正压不稳定的必要条件:即在所考察的平面域内至少在某点或某些点上含地形效应的某一物理量等于零,扰动就可能得到发展。对波包动能的倚时变化的讨论得知,在无摩擦正压纬向非均匀基流的情形,地形对扰动动能并无直接影响,即导波和曳波分别在西风急流的南侧和北侧得到加强,并分别在西风急流的北侧和南侧强度减弱。扰动的发展由扰动的结构所决定。导波和曳波的波长缩短总伴同其轴线倾向于东西向相联系,而它们的波长伸长总是与其轴线倾向于南北向同时发生。还指出,正压扰动的加强(或减弱)与波长的伸长(或缩短)不是必然的联系,它的成立是有条件的。     

台风Haitang(0505)登陆过程地形影响的数值模拟试验

以2005年严重影响我国的登陆台风Haitang为研究对象,在用MM5模式成功模拟台风登陆前后路径、强度、降水以及结构特征的基础上,通过地形敏感性试验,分析了地形对台风的影响及其机理。结果表明:台风在经过台湾及其后在福建登陆期间的异常路径是地形诱生低压发生发展的结果;在降水强度上东部沿海特殊地形对台风北侧东南气流的辐合抬升使得暴雨增幅度1倍以上;地形对台风强度也有一定影响,约为4～5hPa。通过对有无地形影响时台风结构的比较发现地形影响主要在低层,使得高度场分裂两个中心,低层台风主体受山脉阻挡在位置上落后于高层。分析还发现台风登陆前夕移动方向西侧相当位温向西倾斜也可能与地形有关。     

EKMAN PUMPING OVER MOUNTAINS

The analytical results of the previous paper(Wu,1989)are used to study the properties of vertical veloc-ity distribution at the top of the boundary layer.The relative position of a jet like geostrophic flow and el-lipse-shaped orography will affect the characteristic feature of vertical motion at the top of the boundary layerwhen the contributions of friction and orography are taken into consideration simultaneously.     

冬季青藏高原对东移大槽动力影响的研究：Ⅲ.模拟试验结果的分析

具体分析了本文Ⅱ的模拟试验结果。结果表明，该试验较清楚地再现了青藏高原地形对西风气流的影响，也显示了不同地形、急流的有无、强度和位置以及初始槽强度和基本气流斜压性的强度对东移槽的影响。这些结果有的清楚地再现了过去熟知的或还不很清楚的地形影响事实，有的可为东移槽的预报提供着眼点。     

包络地形和重力波拖曳对区域气候模拟效果的影响

用NCAR区域气候模式(RegCM2)的数值试验，研究了包络地形和地形重力波拖曳作用对1991年5、6月份中国区域内气候模拟效果的影响。通过试验结果的对比分析发现，在模式中引入地形重力波拖曳作用可在一定程度上改善模式对气候系统和要素的模拟效果，使模拟结果更符合气候实况。包络地形对模拟结果也有一定的改善，但对于细网格的区域气候模式(RCM)来说，其效果不如重力波拖曳明显。不论是地形重力波拖曳还是包络地形，在模式积分的初期，它们的作用并不明显，随着积分时间的增长，它们对模拟结果的影响程度增大。     

Local meteorology in a northern Himalayan valley near Mount Everest and its response to seasonal transitions

An automatic weather station(AWS) has been installed at the Qomolangma Station of the China Academy of Sciences(QOMS) since 2005, in a northern Himalayan valley near Mount Everest, with an altitude of 4,270 m a.s.l.. Nine years of meteorological records(2006–2014) from the automatic weather station(AWS) were analyzed in this study, aiming to understand the response of local weather to the seasonal transition on the northern slopes of Mount Everest, with consideration of the movement of the subtropical jet(STJ) and the onset of the Indian Summer Monsoon(ISM). We found:(1) Both the synoptic circulation and the orography have a profound influence on the local weather, especially the local circulation.(2) Southwesterly(SW) and southeasterly(SE) winds prevail alternately at QOMS in the afternoon through the year. The SW wind was driven by the STJ during the non-monsoon months, while the SE was induced by the trans-Himalayan flow through the Arun Valley, a major valley to the east of Mount Everest, under a background of weak westerly winds aloft.(3) The response of air temperature(T) and specific humidity(q) to the monsoon onset is not as marked as that of the nearsurface winds. The q increases gradually and reaches a maximum in July when the rainy period begins.(4) The alternation between the SW wind at QOMS and the afternoon SE wind in the pre-monsoon season signals the northward shift of the STJ and imminent monsoon onset. The average interval between these two events is 14 days.     

台湾岛地形对台风移动路径的作用

本文用一个β平面准地转正压模式实施了九组试验，研究台湾岛地形对台风移动路径的作用。初始场系根据SPECTRUM和TCM-90期间加强观测资料确定。结果表明:台湾岛地形的引进会使台风路径向右偏移；地形使台风环流的最大风速区向台风中心方向推移，造成台风环流空间尺度减小，这个结构的变化是路径右偏的一个可能原因。     

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 inEkman 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 arederived.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 boun-dary layer:(1)vorticity distribution in the boundary layer,which is directly related to the divergence andconvergence of air flow caused by friction,(2)the upslope or downslope motion of flow over the mountainslopes,and(3)the mutual effect of orography and friction induced by the ageostrophic component climbing up-ward or downward in the boundary layer over mountain regions.     

Hemispheric-scale climate response to Northern Eurasia land surface characteristics and snow anomalies

This paper presents a synopsis of recently published studies by the co-authors, which show that several land surface characteristics unique to Northern Eurasia are responsible for facilitating a causal relationship between autumn snow anomalies in this region and subsequent hemispheric winter climate patterns. The large size and extratropical location of the contiguous Eurasian land mass results in broad, continental-scale interannual snow cover extent and depth variations throughout autumn and winter, and corresponding diabatic heating anomalies. These surface anomalies occur in the presence of a large region of stationary wave activity, produced in part by the orographic barriers that separate northern/central Eurasia from southern/eastern Eurasia. This co-location of snow-forced anomalies and ambient wave energy is unique to Northern Eurasia, and initiates a teleconnection pathway involving stationary wave–mean flow interaction throughout the troposphere and stratosphere, ultimately resulting in a modulation of the winter Arctic Oscillation (AO). Complementary new results are also presented which show that partial snow cover extent or snow depth only anomalies in Northern Eurasia are insufficient to initiate the teleconnection pathway and produce a winter AO signal. This synopsis provides a useful interpretation of the earlier studies in the specific context of Northern Eurasia regional climate and environmental change.     

INFLUENCE OF TIBETAN PLATEAU ON AIR-SEA INTERACTION OVER TROPICAL PACIFIC IN OCEAN-ATMOSPHERIC COUPLED SYSTEM*

A hybrid coupled ocean-atmosphere model is designed,which consists of a global atmospheric general circulation model(L9R15 AGCM)and a simple ocean model(ZC ocean model over tropical Pacific).Using the model,experimental predictions are performed for the 1986/87 El Nino event and the 1988/89 La Nina event with and without the Tibetan Plateau respectively(called TP FORC and NTP FORC hereinafter).It is found as follows:(1)The coupled system can successfully predict the El Nino or La Nina event even if the Tibetan Plateau orography is not included in the model.The patterns of SSTA and wind anomalies in the model without the Tibetan Plateau are similar to those with the Tibetan Plateau,which further verifies the fact that ENSO process is mainly caused by the air-sea interaction in tropical Pacific.(2)However.the existence of the Tibetan Plateau exerts its influences on the intensity and duration of El Nino(or La Nina).It is unfavorable to the development and maintenance of westerly anomalies,so to some extent,restrains the development of El Nino,but favors the development of La Nina.(3)Effects of the Tibetan Plateau orography on the wind anomalies in the coupled system are different from those inuncoupled AGCM simulation.