Contribution of high and low frequency internal waves to boundary
turbulence in a lake
Abstract
The interior of lakes is often quiescent and most of the mixing in a
lake occurs at the sloping boundaries, where wind-induced internal waves
create turbulence (which leads to mixing) through interactions with the
lakebed. To predict the occurrence and strength of turbulence in terms
of meteorological forcing and stratification, we investigated the
dependence of internal wave type, and their contribution to turbulence
on the slope, on the Lake number, which compares the stabilizing
tendency of stratification to the destabilizing tendency of the wind.
Three thermistor chains and a meteorological station were deployed in
West Okoboji Lake (length ~ 9 km, max. depth
~ 40 m) for two weeks. A wavelet analysis was conducted
to determine time periods when different wave frequencies were excited,
with particular focus on the first vertical mode seiche, the critical
frequency with respect to the stratification and slope, and high
frequency waves in the band of 1-10 times the buoyancy frequency. We
measured the velocities in the bottom boundary layer (BBL) with a high
resolution acoustic current profiler (2 MHz Nortek HR Aquadopp) and then
computed the turbulent dissipation rate using the structure function
method, which uses the spatial correlations of velocity along a beam to
estimate the dissipation. This generated a two week time series of
turbulent dissipation rate in the BBL which was then compared to the
wavelet amplitudes. During the deployment, a strong daily wind forced
near constant internal wave activity. The theoretical period of the
first vertical mode seiche was ~17 hours, but the
diurnal wind forcing interfered with free oscillation of this mode.
Although not an obvious natural frequency of the lake, waves of the
critical frequency (which had a period of ~11 hours)
were activated throughout the measurement period. High-frequency waves
were observed in the thermistor chain near the slope at the lowest Lake
number wind events. The turbulence observed on the boundary was highest
during these events, implying that the low frequency seiching was less
important than higher frequency motions in driving turbulence on the
slope.