Revisiting Microstructure Sensor Responses with Implications for Double-Diffusive Fluxes

Tobias Sommer Eawag, Surface Waters Research and Management, Kastanienbaum, and Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Zurich, Switzerland

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Jeffrey R. Carpenter Eawag, Surface Waters Research and Management, Kastanienbaum, Switzerland, and Department of Geology and Geophysics, Yale University, New Haven, Connecticut

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Martin Schmid Eawag, Surface Waters Research and Management, Kastanienbaum, Switzerland

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Rolf G. Lueck Rockland Scientific International Inc., Victoria, British Columbia, Canada

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Alfred Wüest Eawag, Surface Waters Research and Management, Kastanienbaum, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Zurich, and Margaretha Kamprad Chair of Environmental Science and Limnology, Physics of Aquatic Systems Laboratory, ENAC, EPFL, Lausanne, Switzerland

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Abstract

Thin high-gradient interfaces that occur naturally within double-diffusive staircases are used to estimate the response characteristics of temperature and conductivity microstructure sensors. The knowledge of these responses is essential for resolving small-scale turbulence in natural water bodies and for determining double-diffusive fluxes of heat and salt. Here, the authors derive microstructure sensor responses from observed differences in the statistical distributions of interface thicknesses at various profiling speeds in Lake Kivu (central Africa). In contrast to the standard approach for determining sensor responses, this method is independent of any knowledge of the true in situ temperature and salinity structure. Assuming double-pole frequency response functions, the time constants for the Sea-Bird Electronics SBE-7 conductivity sensor and the Rockland Scientific International FP07 thermistor are estimated to be 2.2 and 10 ms, respectively. In contrast to previous assumptions, the frequency response for the SBE-7 is found to be substantial and dominates the wavenumber response for profiling speeds larger than 0.19 m s−1.

Corresponding author address: Tobias Sommer, Eawag, Seestrasse 79, CH-6047 Kastanienbaum, Switzerland. E-mail: tobias.sommer@eawag.ch

Abstract

Thin high-gradient interfaces that occur naturally within double-diffusive staircases are used to estimate the response characteristics of temperature and conductivity microstructure sensors. The knowledge of these responses is essential for resolving small-scale turbulence in natural water bodies and for determining double-diffusive fluxes of heat and salt. Here, the authors derive microstructure sensor responses from observed differences in the statistical distributions of interface thicknesses at various profiling speeds in Lake Kivu (central Africa). In contrast to the standard approach for determining sensor responses, this method is independent of any knowledge of the true in situ temperature and salinity structure. Assuming double-pole frequency response functions, the time constants for the Sea-Bird Electronics SBE-7 conductivity sensor and the Rockland Scientific International FP07 thermistor are estimated to be 2.2 and 10 ms, respectively. In contrast to previous assumptions, the frequency response for the SBE-7 is found to be substantial and dominates the wavenumber response for profiling speeds larger than 0.19 m s−1.

Corresponding author address: Tobias Sommer, Eawag, Seestrasse 79, CH-6047 Kastanienbaum, Switzerland. E-mail: tobias.sommer@eawag.ch
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