A New Rotating Axes Method for Processing High-Resolution Horizontal Velocity Measurements on EM-APEX floats

Je-Yuan Hsu
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Abstract

A new rotating axes method (RAM) is developed to improve the vertical resolution of the horizontal current velocity measurements u at EM-APEX floats. Unlike the traditional harmonic fitting method (HFM), which yields u averaged in 50-s intervals, RAM decodes and interprets 1-Hz measurements of horizontal seawater velocity ũ, and averages ũ in 12-s windows for removing wind waves with a typical peak frequency ∼ 0.12 Hz. Estimates of u from RAM agree with those from HFM but with a higher vertical resolution of ∼1.5 m, 4 times better than HFM. Note that extracting float signals due to seawater motion needs to assume slow-varying voltage offset ΔΦoffset. The typical variations of estimated ΔΦoffset do not affect the results of u significantly. Estimates of u are excluded when ΔΦoffset fluctuates strongly in time and scatter significantly. RAM is applied to float measurements taken near Mien-Hua Canyon, Taiwan. Composite vertical shear spectra Ψ computed using u from RAM exhibit a spectral slope of −1, as expected for the saturated internal waves in the vertical fine scale range. The RAM provides EM-APEX float’s horizontal velocity measurements into fine vertical scales and will help improve our understanding of energy cascade from internal wave breaking and shear instability into turbulence mixing.
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处理 EM-APEX 浮筒上高分辨率水平速度测量数据的新旋转轴方法
为提高 EM-APEX 浮筒水平流速测量值 u 的垂直分辨率,开发了一种新的旋转轴法(RAM)。与传统的谐波拟合法(HFM)不同,RAM 对 1 赫兹的水平海水流速ũ 测量值进行解码和解释,并在 12 秒的窗口内平均ũ,以去除典型峰值频率∼ 0.12 赫兹的风浪。RAM 对 u 的估计值与 HFM 的估计值一致,但垂直分辨率更高,为 1.5 米,是 HFM 的 4 倍。需要注意的是,提取海水运动引起的浮标信号需要假定电压偏移 ΔΦoffset 变化缓慢。估计 ΔΦoffset 的典型变化对 u 的结果影响不大。当 ΔΦoffset 随时间剧烈波动且散差较大时,u 的估计值将被排除。RAM 应用于台湾面华峡谷附近的浮漂测量。使用 RAM 中的 u 计算出的综合垂直剪切谱Ψ 显示出谱斜率为-1,与垂直细尺度范围内饱和内波的预期一致。RAM提供了EM-APEX浮漂在垂直细尺度的水平速度测量,将有助于提高我们对内波破碎和剪切不稳定性到湍流混合的能量级联的理解。
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