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

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 W (m s(-1)) is small and its standard deviation W-sd is large. These overestimated outliers can be efficiently removed by rejecting data with the criteria of W-sd > 0.2 W - 0.06, where W and W-sd are computed for a 1-s interval. After the data screening, thermal and energy dissipation, chi and epsilon, from CTD-attached and free-fall profilers are consistent within a factor of 3 in the ranges of 10 (10)< chi < 10(-7)degrees C-2 s(-1) and 10(-10) < epsilon < 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 epsilon > 10(-8). This could be due to insufficient correction of the thermistor response for the faster fall rate (similar to 1 ms(-1)) of CTD frames. Since epsilon < 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.