Understanding turbulent mixing in strong stratification is a longstanding fundamental problem in fluid dynamics. The objectives of the proposed study are to determine the mechanisms responsible for estuarine mixing at large gradient Richardson number and to test turbulence parameterizations in this regime. Most high-quality data in this regime have resulted from laboratory measurements and direct numerical simulations. Field measurements of turbulent fluxes and dissipation rates are essential to understand turbulence at geophysically relevant scales. Estuaries are excellent laboratories for stratified turbulence because they combine strong stratification with strong tidal forcing. The measurements, simulations, and analyses will elucidate both stratified turbulence and estuarine dynamics. Measurements are conducted in two estuaries where the mixing is expected to be driven by distinctly different mechanisms. The Mobile Array for Sensing Turbulence (MAST) measurements will be complemented by three-dimensional Reynolds-averaged numerical simulations, which will place the measurements in context with the larger scale estuarine dynamics and permit evaluations of the turbulence parameterizations. The MAST is a new field instrument platform that can resolve key turbulent quantities with unprecedented spatial and temporal resolution. The objectives will be examined with MAST measurements of turbulent fluxes, dissipation rates, and length scales with high spatial and temporal resolution in fixed and underway modes, thus permitting both long time-series measurements and quantification of spatial variability. Three-dimensional numerical models play an increasingly important role as management tools in estuarine and coastal environments. The broader also impacts will include an explicit component to communicate the implications of this study to improve the turbulence-closure parameterizations in estuarine and coastal circulation models for interdisciplinary science and management applications.
