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Measuring vertical velocities with ADCPs in low-energy ocean

Vertical velocities play a key role in the export of CO2 as well as organic and mineral matter. New research proposes a sampling platform with a measurement sensitivity able to measure vertical velocities on the order of several 10-3 to 10-2 m s-1.

Quantification of vertical velocities is essential to study finescale dynamics in the surfacelayers of the ocean and to understand their impact on biological production mechanisms. However, vertical velocities have long been neglected, simply parameterized, or considered as not measurable, mainly because of their order of magnitude (< mm s-1 up to cm s-1) which is generally much smaller than that of horizontal velocities (cm s-1 to dm s-1).

In the paper “Measuring vertical velocities with ADCPs in low-energy ocean” Comby et al. present an upgraded method for direct in situ measurement of vertical velocities using data from different Acoustic Doppler Current Profilers (ADCPs) associated with CTD probes, and perform a comparative analysis of the results obtained by this method.
Datasets were collected during the FUMSECK cruise using three Acoustic Doppler Current Profilers (ADCP): two classic Workhorse (one lowered on a carousel and the other deployed in free-fall mode) and one Sentinel V (next generation ADCP providing two types of vertical velocity measurements) which was also lowered on a carousel. The three datasets were then compared.

The analyses provide profiles of vertical velocities on the order of mm s-1, as expected, with standard deviations of a few mm s-1. While the fifth beam of the Sentinel V exhibits a better accuracy than conventional ADCPs, the free-fall technique provides a more accurate measurement compared to the carousel technique.

Results from this study open up the possibility to perform simple and direct in situ measurements of vertical velocities, coupling the free-fall technique with a five-beam ADCP. 

The SWOT mission (Surface and Water Ocean Topography) will enable very high spatio-temporal resolution altimetry measurements, opening up the possibility of evaluating vertical velocity by satellite. These SWOT-derived vertical velocities will have to be compared with in situ measurements.

The research was supported by the BioSWOT project and the FUMSECK-vv LEFE project.

FOR MORE INFORMATION Comby, C., Barrillon, S., Fuda, J., Doglioli, A. M., Tzortzis, R., Grégori, G., Thyssen, M., and Petrenko, A. A. (2022). Measuring vertical velocities with ADCPs in low-energy oceanJournal of Atmospheric and Oceanic Technology (published online ahead of print 2022). https://doi.org/10.1175/JTECH-D-21-0180.1