地形和非绝热加热共同强迫的流型突变

本文用低谱模式,研究了地形和非绝热加热共同强迫的准地转气流的突变。在一定的参数集合,随着外部热力强迫参数的渐变,激发出了谱系数与西风急流位置的突变。 与单纯非绝热加热强迫的情况相比,地形引入后,改变了流型多平衡态的参数范围和流型突变的参数临界点的位置,并使平衡态流型的经向度加大。由夏至冬转换期间突然建立的南支急流的形态也更为接近观测事实。     

非绝热加热、湿斜压热动力耦合强迫激发辐合增长的一种机制

大气热力状态的非均匀分布及其与动力场的相互作用激发大气动力场发生变化,其事实已被许多数值模拟证实。散度作为度量大气运动辐散、辐合强度的物理量,其变化应受到大气热力状态分布及其与动力场相互作用的影响。然而,在传统的大气动力学理论框架中,并没有从理论上揭示大气热力场分布及其与动力场相互作用对散度场变化的影响。通过对热力学方程做变形处理,并与动力学方程结合,在替换平衡近似下(alternative balance),导出了显式包含非绝热加热分布与垂直风切变相互作用、斜压热动力耦合强迫影响的新型散度方程。利用该方程,重点分析了非绝热加热非均匀分布与垂直风切变相互作用对散度场变化的影响,从动力学理论上初步揭示了非绝热加热分布与垂直风切变相互作用激发大气散度场变化的物理机制。     

大气环流形态的分支现象

本文用二层准地转截断谱模式,研究了大气环流形态的分支现象.结果指出:依赖于热力强迫的不同取值,模式大气将显示出等温静止或Hadley环流两类定常流型,Hadley流型并具有基本解的属性.在热力强迫达到Hadley流型的分支点以前,Hadley流型是稳定的;到达分支点后,Hadley流型不稳定,激发出新的流型,显现出分支现象.绝热无地形时,分支出瞬变斜压波;绝热有地形时,在不同的分支点分别激发出地形驻波与地形瞬变波;非绝热时,分支出非绝热驻波或非绝热驻波与瞬变斜压波的混合波型,非绝热驻波失稳激发出二级分支,     

大尺度大气运动的浑沌形态

本文用一个确定的强迫耗散的八维非线性自治系统描述大尺度斜压大气的运动。在一定的热力强迫范围,流型演变显示出浑沌形态。随着热力强迫的渐变,发生了浑沌形态与周期形态之间的多次转换。在浑沌区内又清楚地显示出周期3和周期6的窗口。 流型演变的周期形态,相应于夏半年对流层上层副热带高压中心沿东西方向的周期的低频振荡。转为浑沌形态后,这种规则性的低频振荡即遭到破坏。这个副热带高压周期性东西振荡的建立与破坏的现象,与实际大气的观测事实十分相似。     

周期性热力外强迫和地形强迫共同激发的六月和十月的流型突变

本文用二层准地转模式,经低谱途径,数值地研究了周期性热力外强迫源和地形强迫源共同作用下斜压大气的流型突变问题。在一定的参数范围,季节转换期间的流型突变清楚地显示出来了,该突变恰好发生在摸式大气的6月和10月。     

非线性Rossby波动特征的进一步探讨

在一定的假定下,利用自然坐标系对正压非线性Rossby波的特征作了进一步探讨,指出控制Rossby波动过程的本质方程是摆型方程及类似于Sine—Gordon方程的关系式,得到了波动过程中复杂的质点振动轨迹和非线性振荡的周期;给出了定常Rossby波的几种复杂流型,证明了非线性Rossby波的位相传播和能量传播是沿着流线进行的。这些结果对深入认识大气内部动力过程和解释某些观测现象具有一定意义。     

局地环流与大气边界层湍流相互作用的数值研究

本文建立了一个二维定常大气中尺度数值模式，并用该模式讨论了由下垫面粗糙分布非均匀（局地动力强迫）和温度分布非均匀（局地热力强迫）而产生的局地大气环流与大气边界层湍流的相互作用。结果指出：下垫面存在温度分布非均匀时运动方程中的湍流交换项与水平气压梯项一样可促使局地热力环境的形成，由粗糙度分布非均匀强迫产生的局地环流与由下垫面温度分布非均匀强迫产和的局地热力环流间的非线笥相互作用是通过湍流交换实现的。     

夏季中国降水型的年代际变化与大气内部动力过程及外强迫因子关系

根据中国160站月平均降水、NCEP/NCAR的再分析资料以及英国气象局哈德莱中心1900～1999年全球1°×1°格点月平均海表温度距平资料，利用物理量诊断、EOF分析等方法，探讨东亚季风区大气环流内部动力过程与大气外强迫因子（海温）年代际变化及对中国夏季降水型的影响。分析发现，中国东部地区、西北地区夏季降水型有各自不同的年代际变化趋势；中国东部夏季降水型及东亚夏季风环流年代际变化趋势与大气外强迫因子北太平洋中纬度海温年代际变化（PDO）关系密切；中国西北地区夏季降水年代际变化与大气内部动力过程中纬度西风带扰动动能年代际减弱有关。     

暴雨激发和维持的正、斜压强迫机制的理论研究

针对暴雨天气的发生、发展都伴随着对流层中低层有很强的气流与水汽辐合持续的天气事实，摒弃传统研究中把散度方程局限于水平动力方程约束的观念，从湿斜压原始方程出发，导出了包含热、动力耦合强迫作用的散度演化方程，揭示了正压大气非平衡强迫和湿斜压大气热、动力耦合强迫在暴雨激发和维持中的重要作用，阐明了持续性暴雨过程中对流层中低层强烈的气流与水汽辐合长时间维持的动力机制。     

热带大气平均环流的纬向不均匀性和Kelvin波的动力作用

本文利用一个包含Wave-CISK机制以及蒸发、感热与扰动风场的反馈机制，并引入实际的下垫面温度分布和海陆差异的热带大气模式，较好地模拟了低纬平均环流在风场、散度场及低层位势高度场上的纬向非均匀性。分析表明这种纬向非对称性是由于下垫面的热力非均匀性造成的强迫与大气内部的热力-动力反馈过程相互作用的结果。对热力强迫的响应形成了西太平洋暖水区上的西风和东太平洋上的东风，而对简化模式的解析分析说明，由于热力-动力反馈造成的Kelvin波的发展过程，可以解释平均东风强于西风的事实。     

A STUDY ON PHYSICAL MECHANISM OF INTERANNUAL VARIATIONS OF LARGE-SCALE FLOW PATTERNS*

The thermal forcings of annual and interannual periodic variations are introduced into the barotropic vorticity equation,by using low order spectral model of the equation,more than 40 numerical experiments whose integration time is larger than 100 model years are performed in order to investigate variations of large-scale flow patterns arising from both external interannual thermal forcing and internal dynamical processes.In certain parametric range,when the frequency of the forcing term with interannual period equals to the frequency which is created by the internal dynamical processes alone,the amplitude of interannual variations of flow patterns increases obviously,and the period becomes double.In other parametric range,the amplitude of interannual variations of flow patterns shows abrupt changes and other nonlinear behavior,along with gradual changes of interannual forcing parameters.     

Impact of land–surface processes on the interannual variability of tropical climate in the LMD GCM

Tropical monsoon circulations exhibit substantial interannual variability. Establishing clear links between this variability and the slowly varying boundary forcing (sea surface temperatures, SSTs, and land surface conditions) has proved difficult. For example, no clear relationships have been found between SST anomalies associated with El Nino/La Nina events and monsoon rainfall. Despite much research over the past 50 years, there are still questions regarding how different components of the land-atmosphere-ocean system contribute to tropical monsoon variability. This study examines the question of land-surface-atmosphere interactions in large-scale tropical convection and their role in rainfall interannual variability. The analysis method is based on a conceptual model of convection energetics applied every day of the simulation at the grid points within the region of interest. This allows for a distinction between the frequency and the characteristic energy and water cycle of these events. With two ensembles of five and three experiments in which different land-surface schemes are used, the relation between land-surface processes and variation of the frequency of convection is studied. It has been found in this modeling study that the formulation of land surface schemes may be important for both the simulation of mean tropical precipitation and its interannual variability by way of the frequency of convective events. Linked to this is an increased response of hydrological cycle over land to SSTAs. Numerous studies have suggested that large-scale factors, such as SST, are the dominant control. However the influence of surface processes depends on the areal extent and distance that separates the region from the ocean. The fact that differences between tropical regions decreases as convection intensifies strengthens this hypothesis. The conclusion is that it is inappropriate to separate the causes of interannual variability between SSTAs and land-surface anomalies to explain precipitation variations as land surface processes play a significant mediating role in the relationship between SSTs and monsoon strength. However there remains the possibility that a substantial portion of variability is due to dynamical processes internal to the atmosphere. Determining the relative roles of internal and lower boundary forcing processes in producing interannual variations in the tropical climate is a major objective of future research.     

Patterns of low-frequency variability in a three-level quasi-geostrophic model

 To assess the extent to which atmospheric low-frequency variability can be ascribed to internal dynamical causes, two extended runs (1200 winter seasons) of a three level quasi-geostrophic model have been carried out. In the first experiment the model was forced by an average forcing field computed from nine winter seasons; in the second experiment we used a periodically variable forcing in order to simulate a seasonal cycle. The analysis has been focused on the leading Northern Hemisphere teleconnection patterns, namely the Pacific North American (PNA) and the North Atlantic Oscillation (NAO) patterns, and on blocking, both in the Euro-Atlantic and Pacific sectors. The NAO and PNA patterns are realistically simulated by the model; the main difference with observations is a westward shift of the centres of action of the NAO. Related to this, the region of maximum frequency of Atlantic blocking is shifted from the eastern boundary of the North Atlantic to its central part. Apart from this shift, the statistics of blocking frequency and duration compare favourably with their observed counterparts. In particular, the model exhibits a level of interannual and interdecadal variability in blocking frequency which is (at least) as large as the observed one, despite the absence of any variability in the atmospheric energy sources and boundary conditions on such time scales. Received: 30 January 1997 / Accepted: 17 June 1997     

ABRUPT CHANGE OF QUASI-GEOSTROPHIC FLOW FORCED BY OROGRAPHY AND DIABATIC HEATING

A low-order spectral model is used to study abrupt change of quasi-geostrophic current forced byboth orography and diabatic heating. In certain parametric combinations, along with the gradualchange of external thermal forcing, it is found that abrupt changes of both the spectral coefficient andthe locations of the westerly jet stream can be stimulated. Introducing the orography into the model results in a change of the parametric range in whichthe multiple equilibrium states in flow patterns exist and the shift of the parametric critical points atwhich the abrupt change of the locations of westerly jet streams is created, as well as increase of thewave components of the equilibrium flow patterns. The aspects of simulated "South Branch"jet streams are more similar to the observational fact in the presence of both the orography and thediabatic heating than those in the case of the heating forcing.     

Numerical experiments on NH subtropical upper tropospheric quasi-stationary vortices in summer

To reveal the possible factors affecting the maintenance and interannual variations of the subtropical planetary-scale vortices such as the south Asian anticyclone and the mid-oceanic troughs, a series of nu-merical experiments are conducted with a quasi -geostrophic low-resolution spectral model. Firstly a simu-lation experiment is performed in which the realistic thermal and topographic forcing are incorporated. The results of simulation show a certain similarity to the actual subtropical flow field in July. On the basis of simulation experiment a series of contrast experiments are performed. It is found that the topog-raphic boundary forcing is less important than the thermal forcing in the dynamics of these systems, and that the anomaly of heating field may cause significant change in position and intensity of the south Asian high and the other systems. It is speculated that the response of the subtropical large-scale systems to heat-ing anomaly is an important cause for the interannual variations of circulation, especially the monsoon circulation.     

局域热源强迫和大尺度流型的长期演化

积分时间达 7个模式月的 3组试验表明 :在一定的参数条件下 ,局域热源强迫的引进可以激发出大尺度流型的全局调整 ,即从一个两槽两脊的流型转换为一个一槽一脊的流型。局域热源的强度、范围和纬度位置 ,对大尺度流型长期演化特征 ,均具有重要影响。     

外强迫引起的夏季大气环流异常及其机制探讨

本文主要探讨了夏季模式大气对热带海温和高纬极冰异常的外强迫响应机制,结果表明,在大气环流模式长时间积分以后,不同的外强迫源均可在全球大气的一些关键性区域激发产生相同的环流异常型,即夏季大气主要异常型对外强迫源地理位置不敏感,很大程度上依赖于大气内部动力学过程.根据以上特点本文提出外强迫引起大气异常存在两类机制,其中外强迫和大气内部动力学过程相互作用机制是产生大气异常的重要动力学途径.     

QBO-like Oscillations Induced by Local Thermal Forcing

A zonal-vertical two-dimensional equatorial model is used to study the possibility that the long period oscillation of the zonal mean flow occurring in the lower equatorial stratosphere (QBO) is caused by local thermal ac-tivities at the tropical tropopause. The model successfully reproduces QBO-like oscillations of the zonal mean flow, suggesting that the local heating or cooling at the tropical tropopause is probably the main reason of QBO’s genera-tion. The analysis of the dependence of the oscillation on the wave fencing indicates that the oscillation is not sensible to the forcing scale. The model can reproduce QBO-like oscillations with any forcing scale if the fencing period and amplitude take appropriate values, proving that the internal gravity waves generated by local thermal source take much important roles in QBO.