MF radar observations of seasonal variability of semidiurnal motions in the mesosphere at high northern and southern latitudes
MF radar observations of seasonal variability of semidiurnal motions in the mesosphere at high northern and southern latitudes
复制标题
DOI：
10.1016/s1364-6826(02)00340-1
复制
发表时间：
2003-03
影响因子：
1.9
通讯作者：
D. Riggin;C. Meyer;D. Fritts;M. Jarvis;Y. Murayama;W. Singer;R. Vincent;D. Murphy
中科院分区：
文献类型：
--
作者：
D. Riggin;C. Meyer;D. Fritts;M. Jarvis;Y. Murayama;W. Singer;R. Vincent;D. Murphy
通过比较分析在扑克平原（65°N，147°W），安德尼斯（69°N，16°E），戴维斯（69°S，78°E）和罗瑟拉（68°S，69°W）测量的水平风来研究半日潮（SDT）。在北方半球站点，SDT在秋分前后最大。1999-2001年的Poker Flat和Andenes结果表明，在秋分前后，SDT振幅有明显的可重复增强，最大值位于86 km左右的高度。在南半球，1997-1998年SDT的季节依赖性更为复杂，秋季增强不太明显。许多竞争机制可能有助于所观察到的季节依赖性的SDT，但本研究的重点是在平均纬向风和温度梯度剪切的折射效应。折射影响的主要证据是SDT振幅的季节性增强伴随着波的垂直尺度的急剧缩短。垂直尺度的缩短与SDT能量折射到水平风分量中是一致的。简化的线性潮汐理论方程被用来估计预期的折射效应的大小，使用风和温度场观测到的Andenes，挪威。预测的折射效应被证明是潜在的显着性和定性与观察一致。除了季节性的依赖性，在所有的雷达站点获得的SDT振幅表现出深的振幅调制的时间尺度上的行星波的特性。这种调制通常归因于潮汐和行星波之间的非线性相互作用。我们认为，折射可能会产生，或至少有助于，观察到的调制。虽然行星波的振幅很弱（<5 m s−1），但SDT（特别是最严重的S（2，2）模）在高纬度地区的传播非常有限。因此，对背景的小扰动足以周期性地抑制SDT向更高水平的传播。
The semidiurnal tide (SDT) is investigated through comparative analysis of horizontal winds measured at Poker Flat (65°N, 147°W), Andenes (69°N, 16°E), Davis (69°S, 78°E), and Rothera (68°S, 69°W). At the northern hemisphere sites the SDT maximizes around the autumn equinox. Poker Flat and Andenes results from 1999–2001 are used to demonstrate that there is a clear repeatable enhancement in SDT amplitudes around the autumn equinox, and that the maximum is localized in height around 86 km . In the southern hemisphere seasonal dependence of the SDT during 1997–1998 is more complicated, and the autumn enhancement is less pronounced. Many competing mechanisms might contribute to the observed seasonal dependence of the SDT, but this study focuses on the refractive effects of shears in the mean zonal wind and gradients in temperature. The main evidence for a refractive influence is that the seasonal enhancement in the SDT amplitude is accompanied by a dramatic shortening in the wave's vertical scale. This shortening of the vertical scale is consistent with refraction of the SDT energy into the horizontal wind component. Simplified linear tidal theory equations are used to estimate the expected magnitude of the refractive effects using wind and temperature fields observed over Andenes, Norway. The predicted refractive effects are shown to be potentially significant and qualitatively consistent with the observations. In addition to a seasonal dependence, the SDT amplitudes obtained at all the radar sites exhibit a deep amplitude modulation on a time scale characteristic of planetary waves. This sort of modulation has most often been attributed to nonlinear interactions between the tides and planetary waves. We suggest that refraction might instead produce, or at least contribute to, the observed modulation. Although the planetary waves are of weak (<5 m s−1) amplitude, the SDT (particularly the gravest S(2,2) mode) is only marginally propagating at high latitudes. Thus, small perturbations to the background are enough to periodically inhibit propagation of the SDT to higher levels.