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

<jats:title>Abstract</jats:title><jats:p>Turbulence 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 <jats:italic>W</jats:italic> (m s<jats:sup>−1</jats:sup>) is small and its standard deviation <jats:italic>W</jats:italic><jats:sub>sd</jats:sub> is large. These overestimated outliers can be efficiently removed by rejecting data with the criteria of <jats:italic>W</jats:italic><jats:sub>sd</jats:sub> > 0.2 <jats:italic>W</jats:italic> − 0.06, where <jats:italic>W</jats:italic> and <jats:italic>W</jats:italic><jats:sub>sd</jats:sub> are computed for a 1-s interval. After the data screening, thermal and energy dissipation, <jats:italic>χ</jats:italic> and <jats:italic>ε</jats:italic>, from CTD-attached and free-fall profilers are consistent within a factor of 3 in the ranges of 10<jats:sup>−10</jats:sup> < <jats:italic>χ</jats:italic> < 10<jats:sup>−7</jats:sup>°C<jats:sup>2</jats:sup> s<jats:sup>−1</jats:sup> and 10<jats:sup>−10</jats:sup> < <jats:italic>ε</jats:italic> < 10<jats:sup>−8</jats:sup> W kg<jats:sup>−1</jats:sup>, respectively, for 50-m depth-averaged data. Energy dissipation from the CTD-attached method tended to be underestimated in the higher turbulence range of <jats:italic>ε</jats:italic> > 10<jats:sup>−8</jats:sup>. This could be due to insufficient correction of the thermistor response for the faster fall rate (~1 m s<jats:sup>−1</jats:sup>) of CTD frames. Since <jats:italic>ε</jats:italic> < 10<jats:sup>−8</jats:sup> 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.</jats:p>