Estimate of turbulent energy dissipation rate using free-fall and CTD-attached fast-response thermistors in weak ocean turbulence

Abstract

The measurement of turbulence is necessary to quantify the vertical, diapycnal transport of heat, water and substances influencing climate, nutrient supply and marine ecosystems. As specialist instrumentation and ship-time are required to conduct microstructure measurements to quantify turbulence intensity, there is a need for more inexpensive and easy measurement methods. This study demonstrated that the turbulent energy dissipation rate, ε, estimated from fast-response thermistors Fastip Probe model 07 (FP07) with the depth-average of a > 10 m depth interval well agreed with those from current shear probes to a range of 10^–11 W/kg (m²s⁻³) in the two casts of the most accurate and stable free-fall vertical microstructure profiler, VMP6000 in the Oyashio water. This range cannot be measured with velocity shear probes equipped in smaller profilers in which the lower limit of ε > O (10^–10) W/kg. These results extend turbulence measurements using the FP07 to 10^–11 W/kg. They may be especially useful for turbulence observations in deep oceans where ε is generally weak (< 10^–10 W/kg). As FP07 are much less sensitive to instrument vibrations than current shear and may be attached to various observational platforms such as temperature-conductivity-depth (CTD) profilers and floats. The CTD-attached FP07 observations near the VMP6000 profiles demonstrated their capabilities in the ε range of 10^–11–10^–8 W/kg by data screening using a ${W}_{\mathrm{sd}}>0.1(W-0.3)$ criterion (1 s mean lowering rate $W$ m/s and its standard deviation ${W}_{\mathrm{sd}}$) under rough conditions where the cast-mean ${W}_{\mathrm{sd}}>$ 0.07 m/s and the standard deviation of ${W}_{\mathrm{sd}}$ in each cast $\sigma$ >0.05 m/s.

Appendix

See Fig. 9.

Fig. 9

Example of wavenumber-temperature gradient spectra and $\varepsilon$ estimate from the observed spectrum (blue curve). This was undertaken by fitting the Kraichnan theoretical spectrum (red) and detecting the peak wavenumber, ${k}_{\mathrm{P}}$, (red vertical arrow) proportional to the Batchelor wavenumber, ${k}_{\mathrm{B}}$, to yield $\varepsilon (={k}_{\mathrm{B}}^{4}\nu {\kappa }^{2}$) through the estimate of the thermal dissipation rate, $\chi$, by integrating the observed spectrum in the wavenumber range (black arrow) determined by the noise spectrum (light-blue) (colour figure online)