Boundary mixing in the thermocline of a large lake

Abstract

[1] High-resolution measurements of near-bottom temperature stratification and current velocity were performed on the sloping boundary of a large lake at the depth of the seasonal thermocline. The measurements cover nearly the entire stratified period and reveal the periodic occurrence of strong temperature and current velocity fluctuations, which can be attributed to shoaling high-frequency internal waves with periods between 5 and 20 min. Two different techniques are applied to obtain a long-term record of dissipation rates of turbulent kinetic energy from the current velocity measurements. Shoaling of high-frequency internal waves is associated with a strong and rapid increase of turbulent dissipation rates of up to four orders of magnitude. Since the occurrence of high-frequency internal waves on the slope is correlated with the passage of the dominant basin-scale internal Kelvin wave with a period of four days, energy dissipation rates on the slope vary with the same period. Diapycnal diffusivities, estimated by combining the dissipation estimates with simultaneously measured density stratification, follow a similar dynamics and a comparison with a basin-scale diffusivity estimate based on tracer measurements reveal the importance of boundary mixing, which, at this particular site, is mainly driven by the interaction of high-frequency internal waves with the sloping boundary.

Estimating turbulent diffusion in a benthic boundary layer

“…It is useful to divide the diffusivity models in our study (Table 1) (Fennel 1995;Gregg et al 1986) as well as in lakes (Lorke 2007), which involves the measurement of density gradients and kinetic energy dissipation. The second category of diffusivity models is only applicable for boundary flow, because these models depend on boundary dimensions.…”

“…Lake Constance is a large (536 km 2 ) monomictic lake that experiences a deep convective mixing during winter (December-March) and is thermally stratified during summer (June-October). The oscillatory current regime of Lake Constance is well described by Appt et al (2004) and Lorke (2007). A mooring was deployed in the northwestern sub-basin (Lake Überlingen) at 100 m depth.…”

“…Second, there is evidence that the log-law does not apply for conditions of low current velocities and stratified boundary layers (Brand et al 2008;Lorke et al 2002), which commonly occur in natural aquatic systems. Under these conditions, turbulent diffusivities as low as 10 -7 m 2 s -1 (Lorke 2007) have been measured that potentially control the solute flux between the sediment and the water column. It is, therefore, imperative to (i) derive a convenient method to measure the turbulent diffusivity independent from the log law and (ii) measure the turbulent diffusivity at critical conditions such as low current velocities and stratification.…”

Active and passive vertical motion of zooplankton in a lake

“…At high frequencies, zooplankton variance shows a local maximum at frequencies associated with a time period of about 10 min, which coincides with the upper frequency limit for internal wave motion at the buoyancy frequency (Boegman et al 2003;Lorke 2007). The strong and periodic vertical velocities associated with these waves result in identical vertical displacement of isotherms and zooplankton, which are passively transported (Fig.…”

“…3B,C), when the depth of the 6uC isotherm seems to limit the amplitudes of DVM. The depth of this isotherm fluctuates on timescales between 3 and 4 d with amplitudes of up to 20 m caused by a basin-scale internal Kelvin wave (Appt et al 2004;Lorke 2007). According to its long wavelength, this wave forms a shallow-water wave, and wave-induced current velocities are mainly restricted to the horizontal direction (Kundu and Cohen 2008).…”