Dynamical Forcing of Stratospheric Planetary Waves by Tropospheric Baroclinic Eddies.
Dynamical Forcing of Stratospheric Planetary Waves by Tropospheric Baroclinic Eddies.
复制标题
对流层斜压涡流对平流层行星波的动力强迫。
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发表时间：
1998
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通讯作者：
P. Haynes
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作者：
J. Scinocca;P. Haynes
本文考虑对流层天气尺度斜压涡旋对平流层行星波变率的强迫作用。简单的强迫耗散数值试验在原始方程模式中使用深半球模式域进行。流动是热松弛向纬向对称假想冬季条件。没有应用纬向不对称的热力或地形强迫。所有行星尺度的纬向不对称性都是由对流层中斜压涡旋的非线性波-波相互作用引起的。数值实验表明，现实的平流层行星波的振幅和变化，在南半球观察到的，可以迫使通过这种机制。在这些模拟中，没有发现平流层原位不稳定引起行星尺度扰动的证据。进一步研究了数值模拟中对流层非线性强迫机制，用一个线性模式再现了平流层行星波响应，该线性模式是由非线性涡动强迫作用于对流层的。强迫线性模式试验表明：（i）正如预期的那样，涡度强迫和涡度温度强迫都需要考虑行星波响应，（ii）只有低频分量的非线性强迫是重要的，（iii）涡度强迫的垂直结构相当于对流层顶附近的一个致密源，平流层行星波响应的变化主要来自对流层非线性涡动强迫的变化，而不是来自与基本状态纬向平均流相关的波传播特性的变化。涡度涡度和涡度温度强迫场被合并成一个单一的表达式，通过引入一个变换的方程，管理的傅立叶分解偏离纬向平均流，称为变换傅立叶分解（TFD）。TFD变换本质上是变换欧拉平均形式主义中所使用的变换的推广。分析了总涡动强迫的时空特征。模拟结果中对流层的斜压涡旋表现出较强的波包结构，其振幅以2波结构为主。有一个强大的，高频，非线性波2强迫与这些包。然而，在模拟中的背景流的传播特性不允许向上传播的波-2扰动与相应的频率，并有很少的相关信号在平流层。线性模型的实验，应用相同的非线性强迫，表明有背景的纬向流，与逼真的速度场，允许向上传播的这种扰动。因此，斜压波包可能是强迫在真实的南半球平流层观测到的高频波2扰动的一个重要机制。在非线性模拟中获得的低频平流层扰动似乎与斜压波包的更微妙的方面，如它们的空间和时间的变化。
The forcing of planetary wave variability in the stratosphere by synoptic-scale baroclinic eddies in the troposphere is considered. Simple forced‐dissipative numerical experiments are performed in a primitive equation model using a deep hemispheric model domain. The flow is thermally relaxed toward zonally symmetric notional wintertime conditions. No zonally asymmetric thermal or topographic forcing is applied. All planetary-scale zonal asymmetry arises solely through the nonlinear wave‐wave interaction of the baroclinic eddies in the troposphere. The numerical experiments indicate that realistic stratospheric planetary wave amplitudes and variability, comparable to those observed in the Southern Hemisphere, can be forced through this mechanism. No evidence is found in these simulations for planetary-scale disturbances arising through in situ instability in the stratosphere. The nonlinear tropospheric forcing mechanism in the numerical simulations is further investigated by reproducing the stratospheric planetary wave response with a linear model that is forced by the nonlinear eddy forcing that acted in the troposphere of the nonlinear simulation. The forced linear model experiments indicate that (i) as anticipated, both the eddy vorticity forcing and the eddy temperature forcing are required to account for the planetary wave response, (ii) only the low-frequency component of the nonlinear forcing is important, (iii) the vertical structure of the eddy forcing is equivalent to a compact source near tropopause level, and (iv) the variability of the planetary wave response in the stratosphere arises primarily from the variability of the nonlinear eddy forcing in the troposphere, rather than from the variability of the wave propagation characteristics associated with the basic-state zonally averaged flow. The eddy vorticity and eddy temperature forcing fields are combined into a single expression by introducing a transformation of the equations that govern the Fourier decomposition of deviations away from the zonally averaged flow, referred to as the transformed Fourier decomposition (TFD). The TFD transformation is essentially a generalization of that used in the transformed Eulerian mean formalism. The spatial and temporal characteristics of the total eddy forcing are then analyzed. The baroclinic eddies in the troposphere of the full simulation show strong organization into wave packets with a dominant wave-2 structure in amplitude. There is a strong, high-frequency, nonlinear wave-2 forcing associated with these packets. However, the propagation characteristics of the background flow in the simulation do not allow upward propagation of wave-2 disturbances with the corresponding frequency and there is little associated signal in the stratosphere. Experiments with a linear model, applying the same nonlinear forcing, show that there are background zonal flows, with plausibly realistic velocity fields, that allow upward propagation of such disturbances. It is therefore suggested that baroclinic wave packets may be an important mechanism for forcing higher-frequency wave-2 disturbances observed in the real Southern Hemisphere stratosphere. The lowfrequency stratospheric disturbances obtained in the nonlinear simulations appear to be associated with more subtle aspects of the baroclinic wave packets such as their spatial and temporal variability.