北半球中低纬地区冬季平均垂直环流的某些特征

本文将继[1]之后,讨论北半球冬季中低纬地区的平均垂直环流.资料来源和计算方法与文献[2]相同,不过这里我们把计算区扩展到全北半球的中低纬地区.     

北半球中低纬地区春秋季平均垂直环流特征及其与气候的关系

春秋季是冬夏季之间的过渡季节。春季,亚洲南部西风急流的强度逐渐减弱,位置也北移。长江中下游地区常常出现连阴雨天气。秋季,中国大陆东部的一些地区出现了秋高气爽天气。北美大陆东部的一些地区也出现了所谓“Indian Summer”天气。 北半球中低纬不同地区春秋季这样一个气候分布特点,和这两个季节本身大气环流有着密切的联系。春、秋季的环流,过去专门研究得不多。对春、秋季平均垂直环流过去     

北半球夏季中低纬地区的平均垂直环流与大气中冷热源分布的关系

本文应用中央气象局出版的“北半球高空气候图集”中的风场资料,计算了45°N以南、北半球范围、7月份850 mb—100 mb各标准等压面层上的平均垂直运动。分析了不同区域经向和纬向垂直环流的特点,并将它与大气中冷热源进行对照。指出:夏季北半球低纬度平均是个Hadley环流,其北是个道环流。形成这种环流形势的主要贡献地区在两个大洋上,这主要与低纬度大气是个热源、共北是个冷源的分布有关。另外,由纬向环流看到,夏季北半球中低纬主要上升运动区和下沉运动区各有两个,这两对上升气流区和下沉气流区构成了夏季北半球中低纬地区垂直环流的大形势。此外,本文还指出夏季青藏高原上空的上升气流向四周流出后所影响的地理区域。     

北半球大气环流的纬向对称模态

运用自然正交分解方法（EOF）提取了北半球冬季热带外地区标准化纬向平均纬向风场的前两个模态。第1模态（方差贡献为34%）表现为中高纬带的西风（东风）异常和中纬带的东风（西风）异常,与之相联系的海平面气压场的异常表现为极区与中高纬度地区海平面气压场的反位相变化,即北极涛动（AO）;第2模态（方差贡献为21%）表现为中纬带的西风（东风）异常和中低纬带的东风（西风）异常,与之相联系的海平面气压场的异常表现为副极地地区与副热带地区海平面气压场的反位相变化,即副热带涛动STO（sub-tropical oscillation）。副热带涛动的水平结构表现出较强的纬向对称性,垂直方向上表现出相当正压性;与副热带涛动正位相相联系的纬向平均温度场表现为中高纬度地区负异常和副热带地区正异常,而且费雷尔环流加强南移。副热带涛动不仅在冬季有所表现,而且在整个冬半年（11月—翌年4月）都有所表现,但在夏半年则不存在。     

北半球冬季环流的特征纬向波数及其时空演变

应用三维数据的纬向空间小波变换方法，分析了北半球冬季环流的特征纬向波数及其与特征纬向波数相对应的北半球冬季环流的时空演变特征和规律。结果表明，北半球冬季环流的特征纬向波数分别是3，2和1。北半球冬季环流的3波活动主要在中纬度，2波活动主要在高纬度，1波活动则主要在低纬度。中纬度3波环流的槽脊线走向是东北-西南向，高纬度2波环流的槽脊线走向是西北-东南向，低纬度1波环流的槽脊线走向则又是东北-西南向。对应于各特征纬向波数，北半球的冬季环流从1949—1999年均表现出一种显著的系统东移特征，初步估计，北半球的冬季环流在过去50年中已大约向东移动了近5个经距。     

北半球中高纬月平均环流正压斜压动能的年变化特征

本文利用多年月平均资料计算了北半球中高纬月平均环流正压、斜压动能的年变化特征。结果表明无论正压、斜压动能都具有明显的年变化,而且两者的变化趋势也是一致的,但是正压动能要比斜压动能对总动能的贡献大得多。环流异常具有相当正压的垂直结构。有关结论可为长期数值预报模式的简化提供依据。     

NWP模式中纬向平均环流系统误差的动力诊断

证明了模式在预报时段的纬向平均环流误差，是由于在预报时段内模式大气和真实大气中西风加速过程的不同造成的。又由于原始方程的无加速定理中各强迫因子具有良好的线性迭加性，因此能够清楚地将造成模式系统误差的动力因子分解开来。通过对Ｔ４２Ｌ９模式的分析，说明原始方程的无加速定理可以作为分析模式系统误差的良好手段。动力诊断分析结果表明，模式对纬向平均环流分布具有较好的预报能力，但在各缔带也存在明显的误差。通过无加速定理的分解分析，本文研究了这些误差的成因及改善模式的可能途径。     

青藏高原及周围地区的平均垂直环流特征

为得到青藏高原及周围地区平均垂直环流的图像,更好地理解该地区的降水气候,利用美国NCEP/NCAR1961～1990年30年平均的ω、u、v、z等再分析格点资料,计算分析了该地区的平均垂直运动场和经、纬圈环流图,分析了它们的年变化特征和可能的原因.结果表明,该地区冬、夏半年的平均垂直运动场和经、纬圈环流有很大的年变化;揭示了夏季在西北干旱区和华北半干旱区分别存在干旱和半干旱经圈环流,夏季在高原325°N以南还存在巨大的向西纬圈环流等事实;也指出高原大气热源强度的年变化与随后高原及周边地区平均垂直环流的年变化联系密切.高原及周围地区的平均垂直环流特征也能较好地解释各地的降水气候.     

北半球月平均环流异常垂直结构的综合分析

本文用一种时空综合的经验正交函数分析方法,对北半球100、500和700hPa三层月平均高度距平场进行展开,得到了不仅能反映各层高度距平场本身的空间结构分布特征,而且能同时反映各层高层距平场之间有机联系的特征向量场.分析结果表明:北半球月平均环流异常在垂直方向上具有明显的相当正压性结构特征.     

夏季北半球中低纬地区热量的垂直输送

本文应用多年平均的风场、垂直速度场和温度场,研究了平均经纬向垂直环流和感热垂直输送的关系.并且讨论了北半球中低纬不同地理区域感热垂直输送的特征. 研究结果表明,夏季北半球中低纬地区感热垂直输送的方向基本上和平均垂直环流的方向相一致.大型定常涡旋对感热输送的高值区和涡旋位能向涡旋动能转化的高值区均在25°—45°N之间的中高空,不同的是后者中心比前者高. 850—200mb之间的对流层中,北半球中低纬有三个感热向上输送的高值区(亚洲大陆及其附近地区,中、东太平洋和大西洋大部分地区)和三个感热向下输送的高值区     

北半球中纬度地区地面气温的超级集合预报

基于TIGGE资料中的ECMWF、JMA、NCEP和UKMO四个中心2007年6月1日-8月31日北半球中纬度地区地面气温24～168 h集合预报资料,分别利用固定训练期超级集合(SUP, Superensemble)和滑动训练期超级集合(R-SUP, Running Training Period Superensemble )对2007年8月8-31日预报期24 d进行超级集合预报试验.采用均方根误差对预报结果进行检验评估,比较了两种超级集合方法与最好的单个中心模式预报、多模式集合平均的预报效果.结果表明,SUP预报有效降低了预报误差,24～144 h的预报效果优于多模式集合平均(EMN, Ensemble Mean)和最好的单个中心预报,168 h的预报效果略差于EMN.R-SUP预报进一步改善了预报效果.对于24～168 h的预报,R-SUP预报效果都要优于EMN.尤其对于168 h的预报,R-SUP改进了预报效果,优于EMN.     

Radiative effects of Pinatubo aerosols in the middle latitudes of the Northern Hemisphere

Summary Using scattering coefficient profiles of the Pinatubo aerosols derived from the observation of skylight polarization and lidar backscattering ratio in Beijing, the radiative effect of Pinatubo aerosols in middle latitudes is assessed by a delta-four-stream radiative transfer model. It is shown that the Pinatubo aerosols significantly change the radiation field. Due to the presence of the volcanic aerosols, the downward short wave flux at the surface decreases with a maximum of 8 W/m² while the upward short wave flux at the top of the atmosphere increases with a maximum of 6.5 W/m². The volcanic aerosols are injected into the region bounded below by the tropopause and up by the 25 km level. The upward and downward radiative fluxes are changed in opposite directions at those two boundaries. Downward short wave fluxes below the tropopause are 7–9 W/m² less than background values and downward long wave fluxes below the tropopause are 2 W/m² more than background values. Upward short wave fluxes above 25 km level is 5–7 W/m² more and upward fluxes above there are about 3 W/m² less.The effects of the Pinatubo aerosols on heating rates are also significant. The maximum increase in the short wave heating rate can be as large as 0.2 K/day at 22 km. The increase in the long wave heating rate is less with a maximum amplitude of about 0.15 K/day. The maximum increase of the total heating rate is about 0.35 K/day, which is comparable with the heating rate caused by the ozone 9.6 µm band in this region.Results of this study are compared with studies of Lacis et al. (1992) and Russell et al. (1993) as well as ERBE measurements. The results generally agree well. Causes for the differences are analyzed.Based on the numerical study, it is also found that the LOWTRAN fresh volcanic model is not representative for the Pinatubo aerosols.With 9 Figures     

Solar activity and spatial variations of the middle troposphere at the middle and high latitudes of the Northern Hemisphere in winter

Long-term trends are observed in the spatial variability of atmospheric circulation in the 18th–23rd solar cycles in the middle troposphere at the middle and high latitudes of the Northern Hemisphere. The intensification of solar activity in the cycle chronologically coincides with the formation of the extensive anomalies of geopotential at the level of 500 hPa over the northern parts of the Atlantic and Pacific oceans. Upper-level troughs gradually move from middle to high latitudes and, having reached the Arctic Ocean, start moving back to the south. The certain synchronization in the manifestation of natural processes on the Sun and Earth enables considering the features of the spatial distribution of natural anomalies for several solar cycles as a kind of the “marker” of natural long-term fluctuations.     

中纬度平均经圈和纬圈环流维持的物理过程(一)

本文主要从分解计算的角度,讨论中纬度平均经圈环流和纬圈环流的维持,并把动量和感热的涡动输送联系起来,说明它们之间的内在联系和统一的物理机理. 大尺度涡动输送、加强和摩擦等强迫作用不断造成纬向平均环流的热成风不平衡,而热成风调整过程又不断使不平衡向平衡调整.平均经圈环流就是在这种强迫变化和调整过程的相互作用中产生的.我们引进了一个由强迫变化决定的温度场、流场的时间变率向量M.时间变率向量M的旋度在x方向的投影等于强迫作用所造成的非热成风产生率,它也等于平均经圈环流向量的旋度在x方向的投影. 当连续方程采用B     

中纬度平均经圈和纬圈环流维持的物理过程(二)

大尺度扰动的感热和动量输送是紧密联系的.在准地转运动中,大尺度涡动输送可分解为平衡输送和不平衡输送.在北半球冬季纬向风系和副热带高压带维持的过程中,中纬度不平衡涡动输送,以及与之相联系的平均经圈环流所造成的垂直输送,起着十分重要的作用.这时水平输送和垂直输送本身就处在内在联系之中.并且这种输送过程是和温度槽落后于流场槽的斜压扰动相联系的. 当把大气运动分解成垂直平均的正压动能和垂直切变的斜压动能时,可以更细致地了解中纬度大气环流能量转化的规律.我们进一步讨论和计算了中纬度北半球冬季能量转换的过程.     

Nonlinear stability analysis of the zonal flows at middle and high latitudes

An attempt has been made to apply Arnol’d type nonlinear stability criteria to the diagnostic study of the persistence (stability) or breakdown (instability) of the atmospheric flows. In the case of the blocking high, the cut-off low and the zonal flow, the relationships of the geostrophic stream function versus the po-tential vorticity of the observed atmosphere are analyzed, which indicates that Arnol'd second type nonlinear stability theorem is more relevant to the observed atmosphere than the first one. For both the sta-ble and unstable zonal flows, Arnol’d second type nonlinear stability criteria are applied to the diagnosis. The primary results show that our analyses correspond well to the evolution of the atmospheric motions. The synoptically stable zonal flows satisfy Arnol'd second type nonlinear stability criteria; while the synoptically unstable ones violate the nonlinear stability criteria.     

Distribution of blockings in the middle troposphere of the Northern Hemisphere

Distribution of pressure systems in the Northern Hemisphere is analyzed in the latitudinal range of 40°–60° from the data of three sets of reanalysis. It is shown that the latitude-mean value of the 500-hPa isobaric surface tends to grow during last decades. By means of analysis of tropospheric frontal zone characteristics, the cases are revealed with blocking deformation of the pressure field. Occurrence frequency of blockings is analyzed.     

The interaction of a mechanically driven three-wave zonal background flow with model mountains in the Northern Hemisphere

Summary Laboratory experiments concerning the interaction of a mechanically driven, uniform shear, zonal, westerly flow superimposed on a three-wave pattern, including cut-off lows, with models of Greenland, the Rocky Mountains and Tibet are conducted. The modeling criteria considered are the matching of the Rossby, Burger and Ekman numbers as well as a number of geometrical parameters. The -effect has not been modeled.The model demonstrates the variation in strength of cut-off lows as they advect over and/or around the model mountains. For example, the low upwind of the Rocky Mountains weakens on approaching the mountains, transforming to a trough in passing over the model and then strengthening again in the lee. The model also shows the influence of the mountains in blocking the lows upstream and then having them accelerate as they advect over the topography.The experiments show how mother cut-off lows approaching the Rocky Mountains can develop daughter troughs/cut-off lows in the lee and further how these mother/daughters coalesce again in the lee. Finally the experiments demonstrate the effects of a cut-off low being advected from west to east past and to the north of the Tibetan Plateau. The process first develops a closed anticyclone in the southeastern region of the Tibetan Plateau. This anticyclone is later replaced by a trough and then a cut-off low. The experiments, generally, demonstrate a number of phenomena observed in the real atmosphere.With 12 Figures     

The signature of ENSO in the Northern Hemisphere midlatitude seasonal mean flow and high-frequency intraseasonal variability

Summary The impact of pronounced positive and negative sea surface temperature (STT) anomalies in the tropical Pacific associated with the El Niño/Southern Oscillation (ENSO) phenomenon on the atmospheric circulation in the Northern Hemisphere extratropics during the boreal winter season is investigated. This includes both the impact on the seasonal mean flow and on the intraseasonal variability on synoptic time scales. Moreover, the interaction between the transient fluctuations on these times scales and the mean circulation is examined. Both data from an ensemble of five simulations with the ECHAM3 atmospheric general circulation model at a horizontal resolution of T42 each covering the period from 1979 through 1992 and operational analyses from ECMWF for the corresponding period are examined. In each of the simulations observed SSTs for the period of investigation are given as lower boundary forcing, but different atmospheric initial conditions are prescribed.The simulations with ECHAM3 reveal a distinct impact of the pronounced SST-anomalies in the tropical Pacific on the atmospheric circulation in the Northern Hemisphere extratropics during El Niño as well as during La Niña events. These changes in the atmospheric circulation, which are found to be highly significant in the Pacific/North American as well as in the Atlantic/European region, are consistent with the essential results obtained from the analyses. The pronounced SST-anomalies in the tropical Pacific lead to changes in the mean circulation, which are characterized by typical circulation patterns. These changes in the mean circulation are accompanied by marked variations of the activity of the transient fluctuations on synoptic time scales, that are changes in both the kinetic energy on these time scales and the atmospheric transports of momentum and heat accomplished by the short baroclinic waves. The synoptic disturbances, on the other hand, play also an important role in controlling the changes in the mean circulation associated with the ENSO phenomenon. They maintain these typical circulation patterns via barotropic, but counteract them via baroclinic processes.The hypothesis of an impact of the ENSO phenomenon in the Atlantic/European region can be supported. As the determining factor the intensification (reduction) of the Aleutian low and the simultaneous reduction (intensification) of the Icelandic low during El Niño and during La Niña events respectively, is identified. The changes in the intensity of the Aleutian low during the ENSO-events are accompanied by an alteration of the transport of momentum caused by the short baroclinic waves over the North American continent in such a way that the changes in the intensity of the Icelandic low during El Niño as well as during La Niña events are maintained.With 16 Figures