30—50 天振荡动能特性及其与平均气流正压不稳定能的转换

该文利用冬季500 hPa的欧洲中心（ECMWF）网格点逐日资料,分析了30—50天振荡的E矢量分布、动能特性及平均气流的正压不稳定能转换特征,从而得到：30—50天振荡的能量传播与西风急流的位置有密切的关系,在西风大风速区作纬向能量传播,在小风速区作指向赤道的经向传播;在急流的出口区有较强的正压能转换,低频振荡从基本流中获得能量,使这里的低频动能最大,并表现出较强的正压特性,与低纬度的斜压特性形成鲜明对照。     

有限区域湿有效能量的层结、斜压、正压分量及其收支方程

本文探讨了将湿有效能量分解成层结、斜压、正压分量的方法,并讨论了它们的收支方程式.其中层结分量与实际大气中对流不稳定有较好的对应关系,但在干有效位能中分解不出这一项.在斜压、正压分量的收支方程中,除附加项外都与干有效位能的方程类似.由三个分量的收支方程看出,湿有效能量向动能的转换只可能发生在层结不稳定或(与)有斜压存在的大气中.这与天气分析经验以及经典的马古列斯(M.Margules)模式是一致的.     

1978年10月份寒潮前后的正压能量转换与季节过渡

以1978年10月下旬东亚寒潮为例,研究寒潮前后一段时期的波谱域正压能量转换关系的变化。发现这次寒潮前后正压能量关系发生了突变。具体表现在:(1)盛行波动由长波转变为超长波;(2)正压能量转换关系由以长波段为主转变为以超长波为主;(3)2波,从作为其他波的动能源来看,发生了根本变化。这些突变具有季节性变化的特点。     

2007年夏季江淮强降水过程中10~30d低频变化及其与对流层上层波包活动的联系

利用NCEP/NCAR再分析和全国740站逐日降水资料,运用一点滞后相关等方法,对2007年夏季江淮流域强降水期间低频振荡的波动活动特征及其与降水低频变化的联系进行了分析。结果表明,在2007年夏季降水中,降水低频分量起着重要作用。降水的低频振荡主周期为10~30d,降水距平时间序列与10~30d低频分量具有较好的对应关系。低频扰动在对流层上层和低层都呈现波列状分布,且在降水活跃位相时,低频环流在高、低层具有斜压结构。在对流层上层,低频扰动有缓慢的东移倾向,相速度为每天2~3个经度。西风带中存在多次移动性波列向下游的传播,且在120°E以西以每天14经度的群速度向下游频散能量,表明10~30d低频波动具有明显的下游发展特征。在强降水开始5d前,低频波动与能量可起源于高纬的乌拉尔山附近,沿着西北-东南向的路径向下游传播。下游发展的低频波动为江淮流域带来了能量,为强降水的发生提供了条件。这些结果加深了人们对低频波动在江淮流域强降水过程中所起作用的认识,可为寻找江淮流域强降水过程预报线索提供科学依据。     

斜压大气波动准共振与中高纬低频振荡

从准地转两层模式出发,可能产生两种情况准共振：(1)纯正压波;(2)两个斜压波和一个正压波。本文对(2)求得准共振三波振幅的解析解和波能量变化周期的近似式。此近似式和数值计算结果两者都表明,斜压情况能量变化周期比正压情况更容易趋于准共振频率偏离Δω自身周期2πε/Δω,从物理上指出,波的位相与波振幅之间存在着一个反馈机制,由正压波与斜压波之间的慢变相差引起的动能与有效位能的互相转换,形成了正压波和斜压波交替增强和减弱2的低频振荡,其振荡周期与上述近似式一致。当Δω～(0.1-0.02).(ωj)时,平均能量周期为12-43d,而当Δω=0时,平均周期为366d.因此,频率偏离Δω出现仍可能是产生料压大气中高纬低频振荡的一种新的重要机制。     

《青藏高原大气低频变化的研究》获奖

由国家气象局高原气象科学基金资助的《青藏高原大气低频变化的研究》,经过5年研究,取得显著效益,获得1992年国家气象局科学技术进步三等奖。该成果在青藏高原30—50天周期大气低频振荡观测事实研究方面.发现青藏高原是大气低频振荡的活跃区和重要源地之一;在动力诊断研究方面,指出大气低频振荡的扰动动能,来自平均西风气流的正压能量转换;在数值试验研究方面,指出在夏季流型下,青藏高原大地形的动力作用可以激发出低频振荡,其传播具有准定常 Rossby 波的     

雷州半岛海风环流的数值研究

利用1984年N1月—1985年2月和1985年5—8月的欧洲中期预报中心格点资料,计算了北半球低频(40—50)天扰动动能和正压动能的分布以及时间平均气流与低频扰动的相互作用.发现高空低频扰动动能的高值区与西风急流的纬向非对称性和经向切变关系密切。在西风急流的出口区,特别是东亚西风急流的出口区,时间平均气流向低频扰动输送动能,这些地区的低频正压性也最强.亚洲季风区低频正压动能较小。提出可以用低频正压动能与低频动能的比值作为低频扰动的“结构参数”,“结构参数”为0.5的等值线可作为区分热带地区低频振荡与中高纬地区低频振荡的特征线.     

2011年江西汛期降水低频分量的延伸期预报试验

利用2011年2-8月逐日降水量序列及东亚地区850hPa经向风场资料建立多变量时滞回归(multivariable lagged regression, MLR)模型,对5-7月江西降水10-30d和50-70d低频分量分别进行延伸期逐日预报实验。结果表明：2011年江西降水存在显著的10-30d和50-70d的振荡周期。降水50-70d低频分量延伸期预报技巧明显优于10-30d低频分量延伸期预报技巧,平均预报技巧高达0.86。降水50-70d低频分量延伸期预报可准确预报降水低频位相的正负转换,能为江西延伸期强降水过程发生的时段预测提供预报信号。     

多变量时滞回归模型在江南地区初夏降水低频分量延伸期预报中的应用

利用江南地区77个台站的日降水资料及NCEP/NCAR再分析资料,基于不同时间尺度的江南地区降水低频分量和东亚地区850 h Pa低频经向风主成分,建立了多变量时滞回归(Multivariable Lagged Regression,MLR)模型,并对2011年5—7月江南降水低频分量进行延伸期逐日预报试验。结果表明,50~70 d时间尺度的江南低频降水的平均预报技巧高达0.92,可准确预报持续性强降水过程和降水低频位相的正负转换。对利用2001—2012年资料分别构建的MLR模型的历史回报预测试验表明,在50~70 d振荡较强和正常的年份,模型能提前30 d做出初夏江南低频降水分量预报。模型结果也表明,850 h Pa低频经向风的发展和演变是影响初夏江南低频降水未来30 d变化的显著信号,可作为延伸期强降水预报的关键因子。     

基于20—30d振荡的长江下游地区夏季低频降水延伸期预报方法研究

用长江下游降水低频分量和环流低频主成分,构造多变量时滞回归模型（MLR）和主成分复数自回归模型（PC-CAR）的混合预报模型（MLR/PC-CAR）,对长江下游降水低频分量进行延伸期逐日变化预报,延长预报时效。通过2011年6—8月预测试验表明,20—30 d时间尺度的长江下游低频降水预测时效可达50 d左右,采用南半球中高纬度地区850 hPa 低频经向风的主成分作为预测因子的模型的预测精度明显高于东亚地区低频经向风作为预测因子的模型。这表明在20—30 d时间尺度上,长江下游降水与南半球中纬度绕球遥相关（SCGT）型有关的主分量的时滞相关更加密切。进一步对于较强20—30 d振荡的多年资料构建的MLR/PC-CAR混合模型预测试验表明,SCGT是预测夏季长江下游低频降水未来50 d变化的显著信号。基于SCGT的发展和演变,对于把握类似长江下游地区2011 年6月初旱涝急转和7月中旬持续降水和强降水过程异常变化过程很有帮助,SCGT可以作为夏季长江下游20—30 d低频降水和强降水过程进行延伸期预报的主要可预报性来源之一。     

北半球冬季大气低频变化主模态的产生机制

最近的一系列研究工作指出,大气环流基本气流纬向非对称结构是产生大气低频变化的重要因子之一[1-4]。在低频时间尺度上,大气运动必然要受到外强迫因子的影响。但外因必须通过大气环流运动中的内因才能产生低频环流变化。      

One possible mechanism for the principal mode of atmospheric low— frequency variability in the northern hemisphere winter

With the specified basic flow in the Northern Hemisphere winter, a study is made of the structure characteristics and mechanism of the principal mode of atmospheric low-frequency variability in terms of a linear barotropic model. Statistical and dynamical analyses of the model results indicate that the mode and the related dominant-forcing excitation zone are featured by evident spatial distribution and that the mechanism responsible for the mode bears re-lation to the zonal asymmetry of the basic flow and the associated barotropic energy conversion.     

Quasi-40-day oscillation and its teleconnection structure together with the possible dependence on conversion of barotropic unstable energy of temporal mean flow

A study is made of the distribution of the diagnostic quantity vector E and the teleconnection structure of 30-50 (quasi-40) day oscillation, together with the dependence on the conversion of barotropic unstable energy of mean flow in terms of ECWMF daily 500 hPa grid data in winter, indicating that the energy transportation is closely associated with the westerly jet position, with zonal (meridional) propagation in the strong (weak) wind region, that considerable conversion of barotropic energy occurs at the jet exit region where low-frequency oscillation gains energy from the mean flow, leading to maximum kinetic energy for the oscillation observed there, which is marked by evident barotropy in striking contrast to the baroclinicity at low latitudes and that the teleconnection core is related to the center of action in the atmosphere and bound up with the pattern of the west wind.     

1981—1982年纬向非对称基本流正压不稳定对30—50天振荡作用的诊断分析

本文运用Hoskins和Simmons的波流相互作用原理,分析1981—1982年冬、夏季对流层高层200hPa上纬向非对称基本流(时间平均流)的正压不稳定和30—50天振荡的关系。指出西风急流的出口区是30—50天振荡动能和基本流以及低频振荡相互作用的最强的区域,且低频振荡从基本流的正压不稳定中获得能量,并在一周内得到充分的发展。 另外还表明,由于基本流的正压不稳定作用,使40°N的亚洲大陆和沿海及45°N的大西洋中部附近成为夏季30—50天振荡的主要能源区,使30°N的中太平洋和40°N的北美东部附近成为冬季30—50天振荡的主要能源区。      

Low—frequency waves forced by large—scale topography in the barotropic model

A barotropic model containing large-scale topography and zonal mean flow is established to discuss the effects of large-scale topography on the low-frequency waves. The results show that what affects low-frequency waves mostly is maximal height of topography and topographic slope. The former makes frequency of topographic Rossby waves decrease, the latter makes Rossby waves instable. Moreover, when topographic slope is appropriate, it can also make Rossby waves turn into low-frequency waves.     

Transformed eddy-PV flux and positive synoptic eddy feedback onto low-frequency flow

Interaction between synoptic eddy and low-frequency flow (SELF) has been the subject of many studies. In this study, we further examine the interaction by introducing a transformed eddy-potential-vorticity (TEPV) flux that is obtained from eddy-potential-vorticity flux through a quasi-geostrophic potential-vorticity inversion. The main advantage of using the TEPV flux is that it combines the effects of the eddy-vorticity and heat fluxes into the net acceleration of the low-frequency flow in such a way that the TEPV flux tends to be analogous to the eddy-vorticity fluxes in the barotropic framework. We show that the anomalous TEPV fluxes are preferentially directed to the left-hand side of the low-frequency flow in all vertical levels throughout the troposphere for monthly flow anomalies and for climate modes such as the Arctic Oscillation (AO). Furthermore, this left-hand preference of the TEPV flux direction is a convenient three-dimensional indicator of the positive reinforcement of the low-frequency flow by net eddy-induced acceleration. By projecting the eddy-induced net accelerations onto the low-frequency flow anomalies, we estimate the eddy-induced growth rates for the low frequency flow anomalies. This positive eddy-induced growth rate is larger (smaller) in the lower (upper) troposphere. The stronger positive eddy feedback in the lower troposphere may play an important role in maintaining an equivalent barotropic structure of the low-frequency atmospheric flow by balancing some of the strong damping effect of surface friction.     

Anemobaric low-frequency waves in the Chukchi Sea

The characteristics of currents and sea level wave perturbations of synoptic scale in the Chukchi Sea are compared with well-known dispersion relations of low-frequency waves of different types. This comparison allowed identifying the currents and sea level wave perturbations as internal Kelvin waves and barotropic and baroclinic topographic waves. Assessments of statistical relations between wave perturbations of currents and different meteorological characteristics showed that the energy supply of low-frequency waves is provided sporadically by various components of anemobaric (wind-induced) forces in the local areas of the Chukchi Sea and the Bering Strait.     

Quasi-two weeks oscillation in the tropical atmosphere

Summary Based on analysis of ECMWF data (1981–1987) and numerical simulations using a general circulation model (GCM), a quasi-two-week (10–20 day) oscillation in the tropical atmosphere is studied in this paper. It is shown that the kinetic energy of the quasi-two-week oscillation is larger than that of the intraseasonal oscillation, and is another important low-frequency system in the tropical atmosphere. By comparing it with the intraseasonal oscillation, some obvious differences can be found. For example, the zonal scale of the quasi-two-week oscillation is dominated by perturbations with wavenumber 2–4; its vertical structure mainly shows barotropic features; the zonal propagation is basically westward; and its meridional and zonal components the same size.With 10 Figures