Interaction between a slowly moving planetary—scale dipole envelope rossby soliton and a wavenumber— two topography in a forced higher order nonlinear Schrödinger equation

A parametrically excited higher-order nonlinear Schr dinger (NLS) equation is derived to describe the interaction of a ,slowly moving planetary-scale envelope Rossby soliton for zonal wavenumber-two with a wavenumber-two topography under the LG-type dipole near-resonant condition. The numerical solution of this equation is made. It is found that in a weak background westerly wind satisfying the LG-type dipole near-resonance condition, when an incipient envelope Rossby soliton is located in the topographic trough and propagates slowly, it can be amplified through the near-resonant forcing of wavenumber-two topography and can exhibit an oscillation.However, this soliton can break up after a long time and excite a train of small amplitude waves that propagate westward. In addition, it is observed that in the soliton-topography interaction the topographically near-resonantly forced planetary-scale soliton has a slowly westward propagation, but a slowly eastward propagation after a certain time. The instantaneous total streamfunction fields of the topographically forced planetary-scale soliton are found to bear remarkable resemblance to the initiation, maintenance and decay of observed omega-type blocking high and dipole blocking. The soliton perturbation theory is used to examine the role of a wavenumber-two topography in near-resonantly forcing omega-type blocking high and dipole blocking. It can be shown that in the amplifying process of forced planetary-scale soliton, due to the inclusion of the higher order terms its group velocity gradually tends to be equal to its phase velocity so that the block envelope and carrier wave can be phase-locked at a certain time.This shows that the initiation of blocking is a transfer of amplified envelope soliton system from dispersion to nondispersion. However, there exists a reverse process during the decay of blocking. It appears that in the higher latitude regions, the planetary-scale envelope soliton-topography interaction could be regarded as a possible mechanism of the establishment of blocking.     

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。     

热岛效应强迫下的中尺度环流的动力特征及极限风速的一种解析表达

本文采用考虑热岛造成的局地热力扭动的Boussinesq二维垂直剖面的动力方程组,利用高截断谱方法,得到一类稳定的中尺度非线性平衡态和极限风速的解析表达,证明了局地中尺度环流是热岛造成的局地热力扰动和平流非线性共同作用的结果;并进一步讨论了热岛强迫下的这类局地环流的某些动力特征。     

Theoretical calculations of interactions between urban breezes and mountain slope winds in the presence of basic-state wind

Theoretical and Applied Climatology - Many big cities around the world are located near mountains. In city-mountain regions, thermally and topographically forced local winds are produced and they...     

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.     

阻塞动力学研究进展与展望

简介了大气阻塞动力学在多平衡态理论和外源强迫的共振理论、孤立子及偶极子理论、强迫耗散ＫＤＶ动力学以及天气扰动的激发等几方面理论研究的进展。并对阻塞动力学的研究进行了展望。     

Topographically Forced Rossby Wave Instability and the Development of Blocking in the Atmosphere

In this paper, the linear stability of disturbance superimposed on basic state Rossby wave forced by topography is investigated, and pointed out that when a certain criterion is satisfied by the basic flow and the height of topography for the subresonance, the small disturbance may be unstable. Furthermore, we also compare the evolution of the instability disturbance with the development of blocking in the Pacific, and we suggested that the topographically forced Rossby wave instability may provide a possible mechanism for the development of blocking in the Pacific.     

Numerical experiments on dynamic mechanism of blocking

By using a two-level quasi-geostrophic spectral model, the numerical experiments on blocking process affected by thermal forcing are conducted. The dynamic mechanisms of blocking, including the resonance between wave and forcing, the control effects of thermal forcing on the position and intensity of the highs and the instabilities of two kinds of equilibrium states are studied by controlled experiments.     

Investigation of Low-Frequency Perturbations Induced by a Steep Obstacle

Flow perturbation due to the orographic forcing at the top of a steep ridge has been investigated. Spectral and wavelet analyses of longitudinal and vertical wind velocity components highlight the presence of low-frequency perturbations produced by topography. Wavelet kurtosis reveals the intermittent character of these perturbations. The application of a multi-resolution data filter allows the detection of the topographically forced structures and the consequent statistical characterisation. The detected topographically forced structures remain active only for a 15% of the time on average. Nevertheless, they appear very efficient in the momentum transport, accounting for about 50% of the downward momentum flux produced in the low frequency range. Quadrant analysis highlights how topographical forcing produces the weakening of ejections and the strengthening of sweeps at the top of the ridge.     

阻塞形势动力机制的数值试验

采用一个准地转两层波谱模式,进行了在热力强迫作用下阻塞的非常定演变过程的数值试验,试验结果说明了实际阻塞形成、维持及崩溃过程的一些典型特点。本文并用大量的控制性试验,探讨了波动与强迫源的非线性共振、强迫源对阻塞位相及强度的控制作用、各类平衡态稳定性等阻塞形势动力机制的问题。     

青藏高原东北侧干旱的数值试验

用谱方法（Ｔ４２）求解半球球面无辐散正压涡度方程,采用实际的干旱环流资料,分别在有地形和无地形的情况下,求出其对应的干旱环流型的强迫场,模拟了在强迫场的作用下干旱环流的形成、维持情况及在强迫场消失后干旱环流型的崩溃情况。结果表明：（１）强迫场在干旱环流型的形成、维持及崩溃过程中起重要作用;（２）青藏高原的存在使其东北侧干旱形成和崩溃均加快。     

不同海温强迫的月动力延伸集合预报试验

利用全球谱模式T106L19和增长模繁殖法（BGM）建立了月动力延伸集合预报系统,基于气候海表面温度（SST）和预测海表面温度,设计了三组集合预报试验,一组为气候SST作为模式下边界条件的集合预报试验（CSST试验）,另一组为预测SST作为模式的下边界条件的集合预报试验（FSST试验）,第三组为前两组试验的集合预报结果之和（AVE30试验）,对两种海温强迫分别进行了48个月的试验,并对预报结果进行了检验和分析。结果表明：相对于单一的控制预报,不管是CSST试验还是FSST试验,利用BGM方法制作的初值集合预报能显著提高月平均环流的预报技巧,集合预报对PNA区域的预报技巧改进显著,特别是预测SST强迫有正的贡献;同时考虑初值和边值不确定性影响的集合预报试验（AVE30试验）,其全球预报技巧不仅高于控制预报,也分别高于FSST试验和CSST试验,这说明要提高月延伸预报技巧,必须同时考虑初值和边值的影响;大气对SST强迫的响应在模式积分10天开始显著,SST对第二旬和第三旬的作用直接影响月平均环流的预报效果,而SST对第二旬和第三旬预报的影响不仅与SST本身变化有关,还与初值有关,不同的初值其作用不同;集合预报对我国夏季月平均温度分布具有较强预报能力,采用预报海温强迫的预报结果,总体上优于气候海温强迫的结果。     

Mathematical analysis of a chaotic model in relevance to monsoon ISO

Mathematical analysis of a predominantly bimodal chaotic attractor, Lu system, was carried out to investigate a possible application of the model as a prototype of monsoon intra-seasonal oscillation (ISO). Bifurcation structures of the system are explored as the system parameter c and the forcing parameter F are varied. Stability criteria of equilibrium points of the forced Lu system are also explored in detail. A sensitivity study is carried out, by changing forcing parameter F, to explore relationships between some of the derived variables of the model and, based on such relationships, an empirical rule is used for extended range prediction. Analogous variables are also derived from the ISO data and prediction results are compared. Application of the prediction rule of regime transition to the observed ISO and chaotic model data is purely based on the bimodal characteristics of ISO and neglects some of the intricate mechanisms therein. We have found that a forced Lu system can be a good prototype in the prediction of peak anomaly of the monsoon ISO when growth rates around a threshold value are taken as predictors.     

太阳活动11年周期对气候系统中准两年振荡的影响(英)

用一个有外强迫的、简单的动力系统研究气候系统中的准两年振荡(平均周期长度比两年稍长或稍短的准周期振荡)。结果显示,准两年周期性源于该系统对于受11年周期调制的季节强迫的非线性响应。当系统的非线性固定时,准两年震荡的周期长度和振幅随季节变化的强度和太阳活动11年周期变化的强度而变化。这可能是造成气候中准两年震荡的性质有时空变化的原因之一。     

Field Evidence for the Upwind Velocity Shift at the Crest of Low Dunes

Flow that is topographically forced by hills and sand dunes accelerates on the upwind (stoss) slopes and reduces on the downwind (lee) slopes. This secondary wind regime, however, possesses a subtle effect, reported here for the first time from field measurements of near-surface wind velocity over a low dune: the wind velocity close to the surface reaches its maximum upwind of the crest. Our field measurements show that this upwind phase shift of velocity with respect to topography is found to be in quantitative agreement with the prediction of hydrodynamical linear analysis for turbulent flows with first-order closures. This effect, together with sand transport spatial relaxation, is at the origin of the mechanisms of dune initiation, instability and growth.     

A STUDY ON THE EXCITATION, ESTABLISHMENT AND TRANSITION OF MULTIPLE EQUILIBRIUM STATES PRODUCED BY NEARLY RESONANT THERMAL FORCING---- PART I: ASYMPTOTIC SOLUTIONS OF MULTIPLE EQUILIBRIUM STATES

A two-layer quasi-geostrophic baroclinic model in a narrow, longitudinally periodic channel on a β-plane is used, which involves near-resonant thermal forcing, frictional dissipation and a uniformly sheared basic current. By means of the multi-scale technique, a system of simplified differential equations or disturbances due to the steady thermal-forcing waves projected on the X-T plane is derived, which contains the nonlinear interactions between forced waves and free waves. The asymptotic solutions of the equilibrium states of these equations are analytically obtained by a singular perturbation method. The results show that these mul-tiple equilibrium state (MES) solutions can exist in a wide range of the parameters used.     

关于西风急流在强迫扰动中作用的数值试验

本文利用非线性水波方程谱模式模拟了不同基本气流对低纬和中纬地区涡源扰动的强迫响应，进而讨论西风急流在大尺度流场强迫扰动中的作用。试验分析表明，中纬度西风急流对来自低纬的扰动波列有增哟的阻 阻滞的作用，并能影响强迫扰动的波数和路径。在存在西风急流的情况下，基本气流对低纬扰源的响应比对中纬扰源的响应要强得多。     

On tidal resonance in the global ocean and the back‐effect of coastal tides upon open‐ocean tides

Abstract

The resonance of semi‐diurnal tidal elevations is investigated with a forward numerical forced damped global tide model and an analytical model of forced‐damped tides in a deep ocean basin coupled to a shelf. The analytical model contains the classical half‐wavelength and quarter‐wavelength resonances in the deep ocean and shelf, respectively, as well as a forcing‐scale dependence which depends on the ratio of the phase speed of open‐ocean gravity waves to that of the astronomical forcing. In the analytical model, when the deep ocean and shelf resonate separately at the same frequency, the resonance in the coupled system shifts to frequencies slightly higher and lower than the original frequency, such that a ‘double bump’ is seen in plots of elevation amplitude versus frequency. The addition of a shelf to a resonant open ocean tends to reduce open‐ocean tides, especially when the shelf is also near resonance. The magnitude of this ‘back‐effect’ is controlled by shelf friction. A weakly damped resonant shelf has a larger back‐effect on the open‐ocean tide than does a strongly damped shelf. Numerical simulations largely bear out the analytical model predictions, at least qualitatively. Idealized simulations show that continents enhance tides by enabling the half‐wavelength resonance. Simulations with realistic geometry and topography but varying longitudinal structure in the astronomical forcing display an influence of the forcing scale on tidal amplitudes somewhat similar to that seen in the analytical model. A frequency sweep in the semi‐diurnal band in experiments with realistic geometry and topography reveals weakly resonant peaks in the amplitudes of several shelf regions and in the globally averaged open‐ocean amplitudes. Finally, the back‐effect of the shelf upon the open ocean is seen in simulations in which locations of resonant coastal tides are blocked out and open‐ocean tidal elevations are significantly altered (increased, generally) as a result.     

Diurnal Evolution and Annual Variability of Boundary-Layer Height and Its Correlation to Other Meteorological Variables in California’s Central Valley

One year of observations from a network of five 915-MHz boundary-layer radar wind profilers equipped with radio acoustic sounding systems located in California’s Central Valley are used to investigate the annual variability of convective boundary-layer depth and its correlation to meteorological parameters and conditions. Results from the analysis show that at four of the sites, the boundary-layer height reaches its maximum in the late-spring months then surprisingly decreases during the summer months, with mean July depths almost identical to those for December. The temporal decrease in boundary-layer depth, as well as its spatial variation, is found to be consistent with the nocturnal low-level lapse rate observed at each site. Multiple forcing mechanisms that could explain the unexpected seasonal behaviour of boundary-layer depth are investigated, including solar radiation, precipitation, boundary-layer mesoscale convergence, low-level cold-air advection, local surface characteristics and irrigation patterns and synoptic-scale subsidence. Variations in solar radiation, precipitation and synoptic-scale subsidence do not explain the shallow summertime convective boundary-layer depths observed. Topographically forced cold-air advection and local land-use characteristics can help explain the shallow CBL depths at the four sites, while topographically forced low-level convergence helps maintain larger CBL depths at the fifth site near the southern end of the valley.     

Chaos of a simple coupled system generated by interaction and external forcing

Summary ?This paper is concerned with the chaotic behavior of a coupled system consisting of two components, one representing the atmosphere and the other representing the ocean. The system is expressed as a highly truncated spectral model and for each component, the spectral model is similar to that of Lorenz (1963). Interactions between the two components are permitted, which lead to the temporal variation of surface temperature and hence that of a critical model parameter (the Rayleigh number). The emphasis of the paper is placed upon the chaotic behavior arising from the interactions between the two components and from periodic external forcing. Numerical tests are carried out to show that through interactions, the chaotic behavior of one component may result in chaos of the other even if the latter is otherwise stationary or periodic. It is shown that chaos may also occur if the system is forced periodically at certain frequencies. This study indicates that a new mechanism for chaos exists for coupled systems which are subject not only to internal fluid dynamic nonlinear interactions, but also to interactions between different components and external forcing. Received July 24, 2001; revised March 25, 2002