Experimental and numerical studies of an eastward jet over topography
Experimental and numerical studies of an eastward jet over topography
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地形上向东射流的实验和数值研究
DOI：
10.1017/s0022112001004372
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发表时间：
2001
影响因子：
3.7
通讯作者：
H. Swinney
中科院分区：
文献类型：
--
作者：
Yudong Tian;E. Weeks;K. Ide;J. Urbach;C. Baroud;M. Ghil;H. Swinney
受地球大气中阻塞流和纬向流现象的影响，我们进行了实验室实验和数值模拟，以研究波数为2的地形上的东向急流的动力学。实验室实验研究了正压旋转环中流动的动力学行为与实验Rossby数和Ekman数的函数关系。两种不同的流动模式，类似于阻塞和纬向流在大气中，观察到持续很长一段时间。早期的模型研究表明，大气中通常向上传播的Rossby波可以共振地锁定到底层地形上，这种地形共振将纬向流与阻塞流分开。在环形空间中，纬向流确实具有超共振平均纬向速度，而阻塞流出现亚共振。低频的变化，周期性或不规则的，是存在于测得的方位角速度的时间序列中的阻塞制度，在6-25环旋转的范围内的主导周期性。在带状状态下也检测到振荡，具有较小的振幅和相似的频率。此外，在一个大的区域的参数空间的两个流动状态表现出自发的，间歇性的过渡从一个到另一个。我们在一个准地转正压模式中数值模拟了相似参数范围内的实验室流动几何。两种流态，阻塞和纬向，在模拟中再现，具有相似的空间和时间特性，包括与阻塞流相关的低频振荡。阻塞和纬向流模式是目前在很大范围内的强迫，地形高度，和底部摩擦。对于参数空间的重要部分，两个模型流都是稳定的。取决于初始状态，无论是阻塞或带状流获得和无限期地持续下去，表明存在多个平衡。
Motivated by the phenomena of blocked and zonal flows in Earth's atmosphere, we conducted laboratory experiments and numerical simulations to study the dynamics of an eastward jet flowing over wavenumber-two topography. The laboratory experiments studied the dynamical behaviour of the flow in a barotropic rotating annulus as a function of the experimental Rossby and Ekman numbers. Two distinct flow patterns, resembling blocked and zonal flows in the atmosphere, were observed to persist for long time intervals. Earlier model studies had suggested that the atmosphere's normally upstream- propagating Rossby waves can resonantly lock to the underlying topography, and that this topographic resonance separates zonal from blocked flows. In the annulus, the zonal flows did indeed have super-resonant mean zonal velocities, while the blocked flows appear subresonant. Low-frequency variability, periodic or irregular, was present in the measured time series of azimuthal velocity in the blocked regime, with dominant periodicities in the range of 6–25 annulus rotations. Oscillations have also been detected in zonal states, with smaller amplitude and similar frequency. In addition, over a large region of parameter space the two flow states exhibited spontaneous, intermittent transitions from the one to the other. We numerically simulated the laboratory flow geometry in a quasi-geostrophic barotropic model over a similar range of parameters. Both flow regimes, blocked and zonal, were reproduced in the simulations, with similar spatial and temporal characteristics, including the low-frequency oscillations associated with the blocked flow. The blocked and zonal flow patterns are present over wide ranges of forcing, topographic height, and bottom friction. For a significant portion of parameter space, both model flows are stable. Depending on the initial state, either the blocked or the zonal flow is obtained and persists indefinitely, showing the existence of multiple equilibria.