A new technique for the retrieval of near-surface vertical current shear from marine X-band radar images

B. Lund, Hans C Graber, H. Tamura, C. O. Collins, S. M. Varlamov

Research output: Contribution to journalArticle

29 Citations (Scopus)

Abstract

A new method to determine near-surface vertical current shear from noncoherent marine X-band radar (MR) data is introduced. A three-dimensional fast Fourier transform is employed to obtain the wave number-frequency spectrum of a MR image sequence. Near-surface currents are estimated from the Doppler-shifted surface gravity wave signal within the spectrum. They represent a weighted mean of the upper ocean flow. The longer the ocean waves on which the current estimates are based, the greater their effective depth. The novelty lies in the wave number-dependent retrieval method, yielding ∼100 independent current estimates at effective depths from ∼2 to 8 m per ∼12 min measurement period. First, MR near-surface vertical current shear measurements are presented using data collected from R/V Roger Revelle during the 2010 Impact of Typhoons on the Ocean in the Pacific experiment in the Philippine Sea. Shipboard acoustic Doppler current profiler (ADCP) and anemometer measurements as well as WAVEWATCH III (WW3) model results are used to demonstrate that results are in accord with physical expectations. The wind and wave-driven Ekman flow is obtained by subtracting ADCP-based background currents from the radar measurements. At ∼2 m, it is on average ∼1.6% of the wind speed and ∼38.9 to the right of the wind. With increasing effective depth, the speed factor decreases and the deflection angle increases. Based on WW3 results, the MR-sensed Stokes drift speed is ∼50% of the Ekman flow at ∼2 m and ∼25% at ∼8 m. These findings are consistent with previous observations and Ekman theory.

Original languageEnglish (US)
Pages (from-to)8466-8486
Number of pages21
JournalJournal of Geophysical Research C: Oceans
Volume120
Issue number12
DOIs
StatePublished - Dec 1 2015

Fingerprint

superhigh frequencies
retrieval
radar
Radar
Acoustic Doppler Current Profiler
shear
Acoustics
Radar measurement
Gravity waves
anemometer
ocean wave
Anemometers
Water waves
upper ocean
deflection
gravity wave
Fast Fourier transforms
Surface waves
surface wave
oceans

Keywords

  • Ekman flow
  • marine X-band radar
  • near-surface current shear
  • Stokes drift

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Oceanography

Cite this

A new technique for the retrieval of near-surface vertical current shear from marine X-band radar images. / Lund, B.; Graber, Hans C; Tamura, H.; Collins, C. O.; Varlamov, S. M.

In: Journal of Geophysical Research C: Oceans, Vol. 120, No. 12, 01.12.2015, p. 8466-8486.

Research output: Contribution to journalArticle

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