全球大气环流模式中平流层风场准两年振荡现象的模拟

发布时间：2018-08-25 15:20

【摘要】：热带地区平流层风场的准两年振荡现象(QBO)是中高层大气中重要的动力学过程。研究表明,包括开尔文波,混合罗斯贝-重力波在内的赤道特征波,惯性重力波和小尺度的对流重力波都是QBO现象所需动量通量的重要来源。这些波动具有不同的特征,但在赤道附近的活动都极为显著。QBO现象的影响非常广泛,通过不同的过程,QBO能对高纬地区的极涡产生一定影响,并与MLT区域的半年振荡(SAO)相互耦合,同时赤道附近的电离层状态也会受到QBO影响。QBO的观测已经经过了很长时间的数据积累,但由于探测手段的局限,缺乏能有效覆盖全区域范围的探测手段,因此QBO现象的模式模拟就具有了非常重要的意义。QBO已经可以被许多大气模式(GCMs)所模拟,并与观测具有较好的一致性。但是在全球大气环流模式(WACCM)中,现行WACCM4.0模式不能自行激发QBO现象,仍然使用强加的风场来作初始驱动。造成这种问题有几个可能的原因。一是模式中缺少必要的重力波参数化过程,QBO缺少必须的驱动力。(Xue et al.(2012)在之前的工作中通过向模式中添加理想化的惯性重力波谱成功实现了对QBO的模拟。这个理想化的重力波参数化建立在Lindzen的线性化理论基础之上并考虑了科里奥利力的影响,但有很多不足之处。在我们第一部分的工作中,我们通过分析赤道附近无线电探空仪数据为重力波的分布提供了观测依据。无线电探空仪数据中提取出的准单色重力波展现出一种沿着背景风正方向和负方向的"双高斯谱"叠加形态,我们以此作为新惯性重力波参数化的依据。之后我们进行了一系列的模拟,研究惯性重力波的谱参数对QBO风场,振幅,持续时间等特征的影响。所有的模拟都能在WACCM标准分辨率下产生近似QBO的风场振荡,QBO在周期,振幅,显著高度等方面有不同特征。重力波两个高斯分布峰值处的相速度实际上表征重力波相速度谱的宽度,两个峰值的大小与QBO风场振幅成正比,与QBO周期成反比。东向和西向的重力波对QBO振幅的影响是相对独立的,但共同作用于QBO周期;重力波谱两个峰值的高度表征重力波携带动量通量的大小,直接决定风场加速和减速的快慢,更大的重力波谱峰值会产生更短周期的近似QBO振荡,但动量通量的大小对纬向风场振幅影响较小。惯性重力波提供的动量通量能够影响QBO风场的显著范围,更强的动量通量会使东向QBO风场向下延伸到更低高度,反之亦然。此外,参数化的间歇性因子反映重力波活动在时间尺度上的频繁程度,对QBO周期也有显著的影响。而更高层的周年振荡,半年振荡也会与QBO活动有明显的耦合。在分析了这些因素之后,我们选取适合的参数进行了一个长时间的模拟,生成的QBO相比之前的工作在振荡周期,东相位的显著高度和参数化的真实性几个方面更加接近观测结果。二是WACCM模式中大于1千米的垂直分辨率对重力波上行的动力学和热学过程造成阻碍。上行重力波在到达临界层而停止传播之前,可能在每一层大气中通过饱和破碎作用而逐渐释放动量通量,模式粗糙的垂直分辨率无法还原出合适的大气分层结构,重力波与背景相互作用不足因而造成QBO驱动力不足。我们尝试通过不同程度地提高WACCM模式的垂直分辨率、改变对流参数化强度和考虑是否添加惯性重力波参数化,这三种方法对WACCM模式激发QBO的必要条件进行探讨。研究中我们认为,简单地提高对流参数化对QBO的激发没有显著作用;但提高模式垂直分辨率可以提高各种上行波动的动量通量传播效率,单独应用这种方法可以在模式中模拟出QBO振荡的雏形;WACCM模式中,QBO的驱动力有三个主要来源,分别是赤道特征波,对流重力波和惯性重力波。这些波动在QBO驱动过程中的作用彼此之间无法相互替代,缺一不可。三是WACCM现行版本采用有限体积元(FV)作为模式的基本单元,而CAM5模式中采用的光谱元素动力学单元(SE)与之相比,可以有效避免有限体积方法中的极区效应,并更好地承载大气动力学过程。由于暂时无法更换WACCM的模式单元,因此更深入的研究将在后续工作中进行。基于几十年对准两年振荡现象的观测和模拟工作,我们对QBO形成机制有了较为清楚的认识。针对WACCM模式在QBO激发过程中可能存在的缺陷,本文的核心目标有三个,分别是研究WACCM4.0中QBO特征与重力波参数之间的对应关系;设定合适的惯性重力波参数化,在之前工作基础上,模拟一个与观测较为一致的QBO;分析模式中多种重力波参数化过程的有无、模式分辨率高低对QBO的激发的必要性。
[Abstract]:The quasi-biennial oscillation (QBO) of the stratospheric wind field in the tropics is an important dynamical process in the upper and middle atmosphere. The results show that the equatorial characteristic waves, including Kelvin waves, mixed Rossby-gravity waves, inertial gravity waves and small-scale convective gravity waves are important sources of momentum flux required for the QBO phenomenon. QBO has a wide range of effects. Through different processes, QBO can affect the polar vortex in the high latitude region and couple with the semi-annual oscillation (SAO) in the MLT region. At the same time, the ionospheric state near the equator is also affected by QBO. QBO observations have gone through a long time. Because of the limitation of detection methods, QBO model simulation is very important. QBO has been simulated by many atmospheric models (GCMs) and is in good agreement with observations. However, the global atmospheric circulation model (WACCM) There are several possible reasons for this problem. First, the model lacks the necessary gravity wave parameterization process, and QBO lacks the necessary driving force. (Xue et al. (2012) In previous work, the model was idealized by adding to the model. Inertial gravity spectroscopy has successfully simulated QBO. This idealized gravity wave parameterization is based on Lindzen's linearization theory and considers the influence of Coriolis force, but there are many shortcomings. In the first part of our work, we analyze the gravity wave fraction of radiosonde data near the equator. The quasi-monochromatic gravity waves extracted from the radiosonde data exhibit a superposition pattern of "double Gaussian spectra" along the positive and negative directions of the background wind, which we use as the basis for parameterization of the new inertial gravity waves. The phase velocity at the two peaks of the Gaussian distribution of the gravity wave actually represents the width of the phase velocity spectrum of the gravity wave, the magnitude of the two peaks and the Q. The amplitude of the BO wind field is proportional to the QBO period, and inversely proportional to the QBO period. The influence of the gravity waves in the East and west directions on the QBO amplitude is relatively independent, but they act on the QBO period together. The height of the two peaks of the gravity spectrum indicates the magnitude of the momentum flux carried by the gravity wave, which directly determines the speed of acceleration and deceleration of the wind field. The momentum flux provided by the inertial gravity wave can affect the significant range of the QBO wind field, and the stronger momentum flux can make the eastward QBO wind field extend downward to a lower height, and vice versa. Frequency on the time scale also has a significant effect on the QBO period, and the higher level annual oscillation, the half-year oscillation will also have a significant coupling with the QBO activity. Two is that the vertical resolution greater than 1 km in the WACCM model hampers the dynamics and thermal processes of the gravity wave traveling upward. The upward gravity wave may gradually break through saturation in each layer of the atmosphere before reaching the critical layer and stopping propagating. With the release of momentum flux, the rough vertical resolution of the model can not restore the proper stratified structure of the atmosphere, and the insufficient interaction between gravity waves and background results in the insufficient QBO driving force. In this study, we consider that simply increasing the convection parameterization has no significant effect on QBO excitation, but increasing the vertical resolution of the model can improve the momentum flux propagation efficiency of various upstream fluctuations, which can be simulated in the WACCM model by using this method alone. In the WACCM model, there are three main driving forces of QBO, namely, equatorial characteristic wave, convective gravity wave and inertial gravity wave. These waves can not be replaced by each other in the QBO driving process. Third, the current version of WACCM uses finite volume element (FV) as the basic unit of the model, while CAM5 mode. The spectral element dynamics unit (SE) used in the formula can effectively avoid the polar effect in the finite volume method and better support the atmospheric dynamics process. Since the WACCM model unit can not be replaced for the time being, further research will be carried out in the follow-up work. In view of the possible shortcomings of WACCM mode in QBO excitation process, three core objectives of this paper are to study the corresponding relationship between the QBO characteristics and the parameters of gravity waves in WACCM 4.0, and to set up appropriate inertial gravity wave parameterization. Simulate a QBO which is more consistent with the observation; analyze the parameterization process of various gravity waves in the model, and the necessity of the excitation of QBO by the resolution of the model.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P433

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