夏季青藏高原热力场和环流场的诊断分析——Ⅱ:环流场的主要特征及其大型垂直环流场

本文使用经过青藏高原气象科学实验测站观测资料订正过的欧洲中心FGGE-Ⅲb资料,对1979年7月月平均进行分析,计算了垂直速度、散度、垂直剖面函数和速度势函数等物理量以及上升气流轨迹,给出了夏季高原主体地区环流场的主要特征和详细的高原地区不同经纬度剖面垂直环流场的特征和分布. 配合夏季高原高层强大稳定的反气旋高压带,高原主体地区为整层上升气流区,但ω场有东西两个上升中心,它们是两个对流活动上升中心,两部的中心位于狮泉河和改则之间偏北的地区,东部的位于那曲一带. 本文给出了高原地区三个主要的经向环流圈(南北两侧下沉的小环流圈、跨赤道的季风环流圈)的经度位置和高原地区与西太平洋之间发生遥相关的主要纬度位置,发现在跨赤道的季风环流圈中,在赤道以南的下沉气流主要来自高原与15°N之间,从高原上升的气流仅在对流层上部(200hPa左右)跨过赤道.从高原西部上升的气流往往从非洲至印度尼西亚一带跨过赤道,而从高原东部及其东侧我国大陆上升的气流往往下沉在太平洋和北大西洋地区.     

中高纬度垂直环流的低频振荡与华北地区夏季干旱初探

用国家气象中心１９９２年６月１０日至７月２３日Ｔ４２中期数值预报模式格点资料，分析了中高纬度夏季垂直环流的低频振荡。发现中高纬度纬向垂直环流与热带纬向环流不同存在着２０－４０天的局地低频振荡，在热带季风中断时，３５－４５°Ｎ范围内中高纬度纬向垂直环流主要表现为１２０°Ｅ附近区域内下沉气流加强。初步探讨了中高纬度地区经圈环流的低频振荡，所选范围为１００－１２０°Ｅ平均低频经向环流，热带季风中断时，下沉气流在３０－４０°Ｎ区域内加强。通过对中高纬度地区垂直环而的低频振荡的分析，探讨了中高纬度地区垂直环流的低频振荡与华北地区夏季干旱的可能联系。由于热带季风中断时，主要降雨带位于我国江淮一带，此时垂直环流表现为在对流层低层２８°Ｎ左右范围内为上升运动区，在３５－４５°Ｎ区域附近内下沉气流加强，即我国华北地区上空盛行下沉气流，对应于该地区干旱、少雨。     

青藏高原夏季热力状态和南半球大气环流的可能联系

本文通过对观测和再分析数据采用最大协方差分析以及回归、合成等分析方法,研究了青藏高原夏季地表气温与南半球大气环流之间的遥相关关系。结果表明,前期(4月)南半球极地—中高纬度大气环流呈现负位势高度异常、较低纬度印度洋—西太平洋区域呈现正位势高度异常时,高原中部和东部大部分区域夏季出现暖异常。在上述遥相关中,印度洋—西太平洋海温异常可能起到了重要的中间桥梁作用。在高原夏季温度偏高的年份,前期跨赤道的印度洋—西太平洋海温也持续偏暖,带来的海陆热力对比减小、经向跨赤道气流减弱有利于削弱夏季的季风环流,使得高原夏季降水偏少,有利于形成高原夏季的暖异常。在这一高原气温—南半球大气环流的遥相关关系中,4月南半球的大气位势高度场异常和与印度洋—西太平洋海温异常相关的异常高度场分布也十分相似。这一前期的跨赤道区域海温异常与南半球中高纬度位势高度场异常的因果关系仍有待进一步揭示。     

平流层剩余环流及其时间演变特征

平流层剩余环流是由剩余速度经向分量和垂直分量构成的平流层经向－垂直环流，它对于对流层－平流层相互作用和物质交换起着十分重要的作用。作者利用1979～2003年NCEP II再分析资料计算了剩余速度经向分量和垂直分量， 并与数值模拟结果进行了比较，再用计算的剩余环流讨论了它的季节变化、年际变化和长期变化趋势。计算结果表明，剩余环流的上升气流从低纬度赤道地区对流层顶上升到平流层下部，然后向极向下运动，在中纬度地区下沉， 进入对流层，这也就是Brewer－Dobson环流。计算结果同数值模拟结果比较一致。由此可见，可以利用NCEP资料得到比较清晰的剩余环流和Brewer－Dobson环流。剩余环流有明显的季节变化，上升气流的中心随着季节的变化在赤道地区南北移动，春秋季节其中心基本上位于赤道附近，南北半球大致呈对称分布，只是北半球副热带地区的下沉气流要比南半球强。在冬夏季节，上升气流的中心分别位于南北纬10°附近。北半球夏季的上升气流要比南半球夏季的上升气流强，同时冬半球的下沉气流比夏半球的下沉气流强。剩余环流还有年际变化和准两年周期振荡特征，在纬向风为西风位相时，赤道地区的上升气流比较弱；而在东风位相时，上升气流和水平方向的输送相对比较强。剩余环流的十年际变化表现为，1979～1983年、1990～1995年、2000～2003年较强，其他年份则较弱。在过去25年，就总的变化趋势而言，剩余环流的上升气流有所增强，平流层下部向中纬度地区的输送也有所增强，环流整体形势是增强的。     

副热带季风环流圈的特征及其与东亚夏季环流的关系

本文利用ECMWF的2.5°×2.5°网格资料对1980—1983年夏季节12次西太平洋副高的中期进退过程进行合成分析。通过对合成场的诊断，论证了夏季东亚季风区存在着在大陆雨带中上升，副热带地区下沉的副热带季风环流圈。它是大陆季风雨带凝结加热驱动的上升辐散气流在向南运行过程中与南海ITCZ上空向北运行的上升辐散气流在副热带地区汇合下沉而构成的热力环流圈。这一环流圈的存在在东亚夏季环流中起着重要作用。     

大气热源和大地形对夏季印度季风和东亚季风环流形成作用的数值模拟

本文利用一个σ坐标的三层初始方程热带球圈数值模式,以全球夏季平均纬向风场为初始场,研究地形的纯动力作用和不同地区的大气热源、冷汇对亚洲夏季两支独立的季风环流系统形成的影响,针对它们对季风环流的主要成员如越赤道气流、季风槽、青藏高压等的相对重要性设计并进行了—系列试验,东亚季风环流和印度季风环流系统的成员被很好地模拟出来。     

夏季青藏高原热力场和环流场的诊断分析III:环流场稳定维持的物理机制

本文使用经过青藏高原气象科学实验测站观测资料订正过的欧洲中心FGGE-Ⅲb资料,对1979年夏季青藏高原地区进行了涡度方程诊断分析,研究了它们的月际变化、逐日变化和日变化,与高原上积云对流活动的强弱变化进行了比较,讨论了夏季高原稳定的环流场维持的物理机制;同时还对同时期热带赤道地区强对流活动区域进行了涡度方程诊断分析,将其涡旋环流场的维持机制与夏季高原地区进行了比较. 通过分析,发现夏季青藏高原月平均涡度方程平衡关系主要是次网格尺度项和散度项的平衡,水平平流项的耗散作用在高空较强,但不如次网格尺度项强,涡度方程其余各项均很小.从月际变化、逐日变化和日变化的比较,发现当积云对流活动发生强弱变化时,ω、D和涡度方程中的散度项、次网格尺度项均伴随很强的相应变化,对应关系很好,说明涡度方程中的次网格尺度项R(余差项)的主要部分来源于积云对流系统的活动,反映了夏季高原上存在的强盛频繁的积云对流活动对高原平均环流场的形成和维持具有重要的作用. 使夏季青藏高原高低层环流场加强的物理机制足高低层气流强大的辐散辐合,耗散机制是积云对流系统对高低层涡度的上下搅拌垂直输送作用和网格尺度水平平流项的非线性耗散作用,其中前者起主要的作用. 从涡旋能量维持的角度看,夏季青藏高原高低层环流场的维持大致是高原尺度环流系统的涡旋能量通过非线性过程,分别向高原区域以外更大尺度的系统和次网格尺度的对流系统输送,输送的损失由强大的高低层辐散辐合气流产生的涡旋能量补充,从而维持了高原地区环流场的稳定.赤道附近热带强对流活动区域环流场的维持机制与夏季高原地区的不同点,主要表现在其高层和低层的区域尺度环流场通过非线性作用都从更大尺度环流场得到涡旋能量,并把涡旋能量转送给次网格尺度积云对流系统,使自身维持稳定.     

高原低涡东移出高原的平均环流场分析

利用美国国家环境预测中心(NCEP)再分析资料,挑选出1998—2004年夏季高原涡移出高原多、少的年、月对它们的环流场进行对比分析。对比分析指出,6~8月是高原涡最易移出的月;当500hPa孟湾季风槽偏北,或西太副高明显西伸,高原东部有切变线活动;当200hPa南亚高压东伸明显,高原东部为南亚高压脊前西北气流控制时,有利于高原涡东移出高原。为高原低涡暴雨预报的气候背景提供了科学依据。     

一九八一年初夏东南亚地区大气热源结构及其对经向环流的影响

本文用热力学方程和水汽连续方程计算了东南亚地区1981年初夏的大气热源和水汽汇的分布情况。文章讨论了大气热源的演变与该地区经向垂直环流的相互关系,并讨论了大气热源和经向环流在南海和中南半岛的差异。分析结果表明,经向环流圈与大气冷热源的分布密切相关。东亚季风建立后的大气热源中心在南海北部,热汇区在赤道附近,相应的最大上升气流也出现在南海下沉区主要在赤道南侧,而且南海的经向环流比中南半岛的要强盛的多。大气热源对于经向环流圈的形成和维持有很大的作用。      

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.     

Entropy production and coarse graining of the climate fields in a general circulation model

We extend the analysis of the thermodynamics of the climate system by investigating the role played by processes taking place at various spatial and temporal scales through a procedure of coarse graining. We show that the coarser is the graining of the climatic fields, the lower is the resulting estimate of the material entropy production. In other terms, all the spatial and temporal scales of variability of the thermodynamic fields provide a positive contribution to the material entropy production. This may be interpreted also as that, at all scales, the temperature fields and the heating fields resulting from the convergence of turbulent fluxes have a negative correlation, while the opposite holds between the temperature fields and the radiative heating fields. Moreover, we obtain that the latter correlations are stronger, which confirms that radiation acts as primary driver for the climatic processes, while the material fluxes dampen the resulting fluctuations through dissipative processes. We also show, using specific coarse-graining procedures, how one can separate the various contributions to the material entropy production coming from the dissipation of kinetic energy, the vertical sensible and latent heat fluxes, and the large scale horizontal fluxes, without resorting to the full three-dimensional time dependent fields. We find that most of the entropy production is associated to irreversible exchanges occurring along the vertical direction, and that neglecting the horizontal and time variability of the fields has a relatively small impact on the estimate of the material entropy production. The approach presented here seems promising for testing climate models, for assessing the impact of changing their parametrizations and their resolution, as well as for investigating the atmosphere of exoplanets, because it allows for evaluating the error in the estimate of their thermodynamical properties due to the lack of high-resolution data. The findings on the impact of coarse graining on the thermodynamic fields on the estimate of the material entropy production deserve to be explored in a more general context, because they provide a way for understanding the relationship between forced fluctuations and dissipative processes in continuum systems.     

Telecorrelation of the 500 hPa polar circulation and El Nino/SO with the temperature fields in China

By using the monthly data from 1951 through 1984, empirical orthogonal expansion is performed for the 500 hPa geopotential height north of 65oN and the canonical fields are clustered by fuzzy classification. It is noted that both the mean monthly polar vortex fields and the large-scale anomaly fields fall into three regimes, with those of the January mean field and the April anomaly field having characteristic features. In addition, the relationship between the time weigthing coefficients of the canonical fields and El Nino / SO is examined, showing significant anomalies in the large-scale polar anomaly fields during April and October of the year when El Nino occurs. These polar circulation anomalies have considerably influenced the temperature fields in China during April and October. Thus, we may con-clude that this is one of the most important reasons for a relatively cool April and a warm October in China during the El Nino year.     

近30 a影响皖北东部降水的台风环流场研究

分析1981—2014年7—9月影响皖北东部地区的台风特征,统计不同台风路径和降雨量之间的关系,研究不同路径下皖北东部出现暴雨的环流场特征,结果表明:(1)西北行、陆面转向、海面转向和其他类路径的台风都可以影响皖北东部,并产生降水。其中产生降水个例最多的台风路径为陆面转向类,次多为西北行类。(2)台风影响时无降水、非暴雨和暴雨3类降水个例的500 hPa平均环流场对比分析表明:当有台风影响皖北东部时,配合西风槽或副高阻挡更有利于产生强降水。(3)西北行类台风暴雨的环流场特征是华北到河套存在西南-东北走向高压带阻挡使台风停滞少动;海面转向类台风暴雨环流场特征是河套东部存在低槽与台风相互作用;陆面转向类台风暴雨环流场可以分为3类:贝湖两槽一脊型、贝湖单槽型和副热带高压阻挡型。     

Evaluation of wind fields for storm events in the black sea region using a numerical regional atmospheric circulation model

A method of the NCEP/NCAR reanalysis wind data recalculation using a regional atmospheric circulation model is described. A comparison of model results and satellite measurements is performed. The results of wind wave computation using the reanalysis and model winds are presented and their comparison is made. It is shown that the NCEP/NCAR reanalysis wind recalculation using a regional atmospheric circulation model removes the underestimation in the reanalysis winds, provides a better representation of the wind regime, and, consequently, gives a possibility to create the reanalysis of wind regime in the Black Sea region to be used in climate estimates.     

On the climatology of persistent circulation anomalies in the atmosphere and in a general circulation model

Abstract

The climatologies of upper‐air persistent circulation anomalies found in observations of the Northern Hemisphere and in a General Circulation Model (GCM) integration are compared with each other and with those found in previous studies. The model simulation is that of the Canadian Climate Centre GCM run at resolution T20. The objective criteria that define the persistent events differ from those of some earlier investigations in that the anomalies are not required to be as nearly stationary. It is found that the GCM generates persistent circulation anomalies downstream of the synoptic‐scale storm tracks, in very nearly the correct geographical locations, but that the frequency of occurrence is too low in the model. A kinetic energy and streamfunction variance analysis is presented for both dataseis to clarify the differences between the observed and simulated distributions of circulation anomalies. It is evident that, apart from the mean annual cycle, the middle‐latitude transient eddies of the model are too weak.     

A diagnostic study of the southern hemisphere summer circulation of the CCC general circulation model

Abstract

The medium‐scale wave regime, consisting largely of zonal wavenumbers 5–7, frequently dominates the summer Southern Hemisphere tropospheric circulation. We perform a diagnostic study of this circulation as simulated by the Canadian Climate Centre (CCC) general circulation model (GCM). The analysis of Hövmöller diagrams, space‐time and zonal wavenumber spectra shows that the CCC GCM is able to simulate the observed medium‐scale wave regime.

The zonally averaged meridional eddy heat and momentum transports and the associated baroclinic and barotropic energy conversions are also examined. The distributions of the transports on the vertical plane agree well with the observations. After comparison with the observed December‐January‐February 1979 distributions, some quantitative differences remain: the heat transport is too weak aloft and too large near the surface, whereas the momentum transport tends to be too weak. The baroclinic and barotropic conversions show a maximum in the medium‐scale waves. The time evolution of the Richardson number of the mean flow suggests that the medium‐scale wave is due to a baroclinic instability.     

Response of the South Atlantic circulation to an abrupt collapse of the Atlantic meridional overturning circulation

The South Atlantic response to a collapse of the North Atlantic meridional overturning circulation (AMOC) is investigated in the ECHAM5/MPI-OM climate model. A reduced Agulhas leakage (about 3.1?Sv; 1?Sv?=?10⁶?m³?s^?1) is found to be associated with a weaker Southern Hemisphere (SH) supergyre and Indonesian throughflow. These changes are due to reduced wind stress curl over the SH supergyre, associated with a weaker Hadley circulation and a weaker SH subtropical jet. The northward cross-equatorial transport of thermocline and intermediate waters is much more strongly reduced than Agulhas leakage in relation with an AMOC collapse. A cross-equatorial gyre develops due to an anomalous wind stress curl over the tropics that results from the anomalous sea surface temperature gradient associated with reduced ocean heat transport. This cross-equatorial gyre completely blocks the transport of thermocline waters from the South to the North Atlantic. The waters originating from Agulhas leakage flow somewhat deeper and most of it recirculates in the South Atlantic subtropical gyre, leading to a gyre intensification. This intensification is consistent with the anomalous surface cooling over the South Atlantic. Most changes in South Atlantic circulation due to global warming, featuring a reduced AMOC, are qualitatively similar to the response to an AMOC collapse, but smaller in amplitude. However, the increased northward cross-equatorial transport of intermediate water relative to thermocline water is a strong fingerprint of an AMOC collapse.     

Winter season stratospheric circulation in the SAMIL/LASG general circulation model

In this paper, we examine the performance of the 26-level version of the SAMIL/LASG GCM (R42/L26) in simulating the seasonal cycle and perpetual winter mean stratospheric circulation as well as its variability by comparing them with the NCEP/NCAR reanalysis. The results show that the model is capable of reproducing many key features of the climatology and seasonal variation of the stratospheric circulation despite that the model’s mean polar vortex is stronger and more zonally symmetric compared to the obse...     

Strengthening of the Walker circulation under globalwarming in an aqua-planet general circulation model simulation

Most climate models project a weakening of the Walker circulation under global warming scenarios. It is argued, based on a global averaged moisture budget, that this weakening can be attributed to a slower rate of rainfall increase compared to that of moisture increase, which leads to a decrease in ascending motion. Through an idealized aqua-planet simulation in which a zonal wavenumber-1 SST distribution is prescribed along the equator, we find that the Walker circulation is strengthened under a uniform 2-K SST warming, even though the global mean rainfall–moisture relationship remains the same. Further diagnosis shows that the ascending branch of the Walker cell is enhanced in the upper troposphere but weakened in the lower troposphere. As a result, a "double-cell" circulation change pattern with a clockwise(anti-clockwise) circulation anomaly in the upper(lower) troposphere forms, and the upper tropospheric circulation change dominates. The mechanism for the formation of the "double cell" circulation pattern is attributed to a larger(smaller) rate of increase of diabatic heating than static stability in the upper(lower) troposphere. The result indicates that the future change of the Walker circulation cannot simply be interpreted based on a global mean moisture budget argument.