A New Rotating Axes Method for Processing High-Resolution Horizontal Velocity Measurements on EM-APEX floats
Journal
Journal of Atmospheric and Oceanic Technology
Journal Volume
41
Journal Issue
3
Start Page
319
End Page
339
Date Issued
2024-03-01
Author(s)
DOI
10.1175/JTECH-D-23-0014.1
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 DFoffset. The typical variations of estimated DFoffset do not affect the results of u significantly. Estimates of u are excluded when DFoffset fluctuates strongly in time and scatter significantly. RAM is applied to float measurements taken near Mien-Hua Canyon, Taiwan. Composite vertical shear spectra C computed using u from RAM exhibit a spectral slope of 21, 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.
Subjects
Algorithms
In situ oceanic observations
Measurements
Mixing
Oceanic profilers
Type
journal article