Turbulence Estimation Using Fast-Response Thermistors Attached to a Free- Fall Vertical Microstructure Profiler

Estimation of turbulence intensity with a fast-response thermistor is examined by comparing the energy dissipation rate epsilon(T) from a Fastip Probe, model 07 (FP07), thermistor with epsilon(S) from a shear probe, both of which are attached to a free-fall microstructure profiler with the fall rate of 0.6-0.7 ms(-1). Temperature gradient spectra corrected with previously introduced frequency response functions represented by a single-pole low-pass filter yields epsilon(T) with a bias that strongly depends on turbulence intensity. Meanwhile, the correction with the form of a double-pole low-pass filter derives less bias than of single-pole low-pass filter. The rate epsilon(T) is compatible with epsilon(S) when the double-pole correction with the time constant of 3 x 10(-3) s is applied, and 68% of epsilon(T) data are within a factor of 2.8 of epsilon(S) in the wide range of epsilon(S) = 10(-10) -3 x 10(-7) W kg(-1). The rate epsilon(T) is still compatible with epsilon(S) even in the anisotropy range, where the buoyancyReynolds number I = epsilon/(nu.N-2) is20-100. Turbulence estimation from the fast-response thermistor is thus confirmed to be valid in this range by applying the appropriate correction to temperature gradient spectra. Measurements with fast-response thermistors, which have not been common because of their poor frequency response, are less sensitive to the vibration of profilers than thosewith shear probes. Hence, measurements could be availablewhen a fast-response thermistor is attached to a CTD frame or a float, which extends the possibility of obtaining much more turbulence data in deep and wide oceans.