Turbulence closure modeling of estuarine stratification

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Abstract
Transient stratification in estuaries reflects competition between the stratifying influences of the vertical gravitational circulation and elastic straining of the longitudinal density gradient by vertical shear, set against the mixing influence of, principally, tidally generated turbulence. A one‐dimensional (vertical) numerical model of an estuary, using seven different parameterizations of vertical mass and momentum mixing coefficients from the literature, is used to make general predictions about the nature of the time‐dependent signal of stratification in an estuary. These predictions show good agreement with published predictions from simpler models and they demonstrate that, at higher frequencies (e.g., first and second species tides) stratification is produced primarily from elastic straining but that, at lower frequencies (due to the fortnightly spring‐neap modulation, for example) the estuarine gravitational circulation tends to dominate. The same model, with rotation included, is then used to hindcast a time series of stratification from Spencer Gulf, South Australia, in order to test parameterizations of mixing coefficients against real data. The character of the semidiurnal and diurnal stratification signal is better predicted by some schemes than others. Against qualitative and quantitative criteria, the Mellor and Yamada (1982) and Pacanowski and Philander (1981) schemes perform best. All schemes, however, underpredict the amplitude of the stratification signal at all timescales. At longer timescales, associated with the springs‐neaps cycle, this is believed to be due to a difference in the degree of geostrophic adjustment of the model and the observations, produced by the three‐dimensional topography of the observational site (Spencer Gulf, South Australia).

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