Boundary mixing in lakes: 2. Combined effects of shear- and convectively induced turbulence on basin-scale mixing

Abstract

[1] A detailed comparison of results from a numerical three-dimensional hydrostatic lake model with high-resolution observations of the vertical structure of the turbulent bottom boundary layer (BBL) in a medium-size lake (Lake Alpnach, Switzerland) is provided. The focus of this study is on the shear-induced generation and destruction of stratification in the BBL that may ultimately lead to unstable layers (convection). The model was shown to provide a reliable description of the internal seiching dynamics, as well as the local BBL properties, including the generation of shear-induced convection in two data sets from 2003 and 2007. Basin-scale mixing parameters, inferred from the simulations, are closely connected to the seiching motions, with the hypolimnetic mixing reacting almost immediately to the variable wind-forcing and seiching activity. During upslope flow, the BBL becomes convectively turbulent, causing low mixing efficiency on a basin-scale, whereas during downslope flow, the BBL is restratifying and shear-induced turbulence is weak but leads to a higher mixing efficiency. The overall deep-water mixing efficiency varied in the range of 5 to 10% in this system dominated by turbulent boundary processes.

The Interaction of Large Amplitude Internal Seiches with a Shallow Sloping Lakebed: Observations of Benthic Turbulence in Lake Simcoe, Ontario, Canada

“…Observations of shear-driven convection in lakes stem from two different single point observations in Lake Alpnach [17] and Lake Constance [4]. The results from these point measurements are consistent with analysis of numerical simulations of shear-driven convection [18], [19], [23]. Nonetheless, to date there are only a few field observations in lakes leaving it unclear as to the relative importance of shear-induced convection versus breaking of NLIW in energizing benthic BBLs.…”

Vertical propagation of lakewide internal waves

“…Therefore, for diurnal forcing with s = (24 hours) À1 , small vertical wavelengths are favored in short (low L), strongly-stratified lakes. Previous observations show a tendency for short vertical wavelengths to be observed in small or medium lakes [LaZerte, 1980;Weigand and Chamberlain, 1987;Münnich et al, 1992;Vidal et al, 2005Vidal et al, , 2008Vidal and Casamitjana, 2007;Pannard et al, 2011;Lorrai et al, 2011].…”

“…Seiche dissipation generates turbulence, which is responsible for mixing heat [Lorrai et al, 2011], sediments [Pierson and Weyhenmeyer, 1994], chemicals [MacIntyre and Jellison, 2001], organisms [Serra et al, 2007], and pollutants [Sorensen et al, 2004].…”

The internal seiche field in the changing South Aral Sea (2006–2013)

“…Several authors have shown the effect of changing modes on the spatial variability of turbulence. Lorrai et al (2011) have shown how the mixing rates present variability at twice the seiching frequency. Sakai and Redekopp (2011) have also shown that, in a lake where super-inertial rotatory waves dominated, the superposition of higher modes enhanced particle transport.…”

“…For this work, a 5-day spin-up period of the model was found to be enough for the standing waves to develop, although accurately settling the circulation of the lake might require a larger spin-up period (Korotenko et al, 2010). Lorrai et al (2011) also used a spin-up period of 5 days of the 3-D hydrostatic Boussinesq model to study the seiching dynamics of a medium-size lake. Note that if the POM model had not reproduced the modes studied in this paper, the surface elevation filtered around the periods of the observed oscillations would not present a coherent structure over the lake that is repeated for every period, as presented in the results section.…”