Comparison of Turbulence Intensity from CTD-Attached and Free-Fall Microstructure Profilers

Abstract

AbstractTurbulence intensity estimated from fast-response thermistors is compared between conductivity?temperature?depth (CTD)-attached and free-fall microstructure profilers, conducted at the same location within 2 h. The agreement is generally good but anomalously overestimated values, deviating from a lognormal distribution, appear sporadically in the CTD-attached method. These overestimated outliers are evident as spiky patches in the raw temperature gradient profiles. They often occur when the fall rate of the CTD frame W (m s?1) is small and its standard deviation Wsd is large. These overestimated outliers can be efficiently removed by rejecting data with the criteria of Wsd > 0.2 W ? 0.06, where W and Wsd are computed for a 1-s interval. After the data screening, thermal and energy dissipation, ? and ε, from CTD-attached and free-fall profilers are consistent within a factor of 3 in the ranges of 10?10 < ? < 10?7°C2 s?1 and 10?10 < ε < 10?8 W kg?1, respectively, for 50-m depth-averaged data. Energy dissipation from the CTD-attached method tended to be underestimated in the higher turbulence range of ε > 10?8. This could be due to insufficient correction of the thermistor response for the faster fall rate (~1 m s?1) of CTD frames. Since ε < 10?8 in most parts of the intermediate and deep ocean, use of the CTD-attached fast-response thermistors provides an efficient way to expand the presently sparse turbulence observations.