Drifter measurements of turbulence and mixing in the Snohomish River Plume

Samuel Evan Kastner, University of Washington, Civil and Environmental Engineering, Seattle, WA, United States, Alexander Horner-Devine, University of Washington Seattle Campus, Department of Civil & Environmental Engineering, Seattle, WA, United States and James M Thomson, Applied Physics Laboratory University of Washington, Seattle, WA, United States
Abstract:
We present observations of stratification and turbulent eddy diffusivity in the Snohomish River plume. Measurements were taken using a new iteration of the Surface Wave Instrument Floats with Tracking (SWIFT) buoys, equipped with three CT sensors at depths of 0.1, 0.5, and 1.2 meters below the surface. An uplooking pulse coherent ADCP resolves near-surface turbulence and the addition of the CT sensors expands the capability of the SWIFTs to include stratification and mixing. This work presents a test and validation of this new capability in the lift-off region of the Snohomish plume. Pairs of SWIFTs were released upstream of the mouth of the river and allowed to drift seaward with the flow of the ebbing tide. CTD casts were made from a small boat drifting nearby. The SWIFTs drift with the plume water and observe salinity changes as a Lagrangian measurement, such that the control-volume method of McCabe et al (2008) can be applied to estimate entrainment and vertical salt flux. Combined with observations of stratification within the plume, these estimates are used to infer eddy diffusivity. We compare the control volume estimates to measurements of turbulence and stratification from the uplooking ADCP onboard the SWIFT. Comparison and cross-validation of these methods will improve quantification and understanding of plume mixing, especially in the hard to sample region near the water surface.
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