Frequency spectra of vertical velocity from Flatland VHF radar data
Frequency spectra of vertical velocity from Flatland VHF radar data
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Flatland VHF 雷达数据的垂直速度频谱
DOI：
10.1029/90jd02220
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发表时间：
1991
影响因子：
--
通讯作者：
J. Green
中科院分区：
文献类型：
--
作者：
T. Vanzandt;G. Nastrom;J. Green
在伊利诺伊州香槟-厄巴纳附近的平地雷达在对流层和平流层下部每153秒观测一次非常平坦地形上的垂直风速。从1987年3月到5月，从所有公认的6小时时间序列中计算出数百个频谱。通过对谱的分层，发现：（1）在对流层和平流层下部，谱与高度无关，但在两个区域谱的振幅和频率不同：（2）在一定高度上，谱与风切变dz/dz、浮力频率N和16 km以下的最大风速无关;（3）谱形和振幅随背景风速的增加而变化的幅度比近山站小得多。谱的方差等于每单位质量垂直动能的两倍，大约是每单位质量垂直动能增加10 m s-1时的两倍;（4）频谱与重力波频谱一致，如在小的λ时N附近的谱振幅的急剧下降所指示的，以及观察到的形状随着λ的增加而变化的事实与模型多普勒的形状变化是完全一致的，位移重力波谱;（5）观测谱与模型谱的比较结果表明，重力波谱与内禀重力波谱是一致的，内禀重力波谱不随δ、dz/dz等的变化而变化。与重力波理论的预期相反;（6）结果对重力波能量的方位角分布不敏感，只要该分布相对于平均流大致对称;（7）由此产生的固有频谱的特征水平相速度c* 在对流层和平流层都约为6 m s−1。相应的特征垂直波长分别约为3300米和1800米，与以前的估计一致。
The vertical wind velocity over very flat terrain was observed every 153 s in the troposphere and lower stratosphere by the Flatland radar, near Champaign-Urbana, Illinois. Several hundred frequency spectra were calculated from all accepted 6-hour time series from March through May 1987. By stratifying the spectra in various ways we find the following: (1) The spectra were independent of altitude within the troposphere or lower stratosphere, but the spectra in the two regions differed in amplitude and frequency; (2) At a given altitude the spectra were independent of the wind shear dū/dz, the buoyancy frequency N, and the maximum wind speed below 16 km; (3) The change of spectral shape and amplitude with increasing background wind speed ū was much less than at stations near mountains. The variance of the spectra, equal to twice the vertical kinetic energy per unit mass, roughly doubled as ū increased by 10 m s−1; (4) The spectra were consistent with being due to a spectrum of gravity waves, as indicated by the sharp drop in spectral amplitude near N at small ū and by the fact that the observed change of shape with increasing ū was quite consistent with the change of shape of model Doppler-shifted gravity wave spectra; (5) The results of comparison between the observed and model spectra are consistent with an intrinsic gravity wave spectrum that is invariant with ū, dū/dz, etc., contrary to expectations from gravity wave theory; (6) The results are insensitive to the azimuthal distribution of gravity wave energy, as long as the distribution is roughly symmetrical relative to the mean flow; (7) The resulting characteristic horizontal phase velocity c* of the intrinsic frequency spectrum was about 6 m s−1 in both the troposphere and the stratosphere. The corresponding characteristic vertical wavelengths were about 3300 and 1800 m, respectively, consistent with previous estimates.