Internal seiche modes and bottom boundary-layer dissipation in a temperate lake from acoustic measurements

Abstract

Measurements with Acoustic Doppler Current Profilers (ADCPs) and a thermistor string were used to investigate the seasonal variation of internal seiches in a 5-km-long temperate lake and the dissipation induced by the seiches in the bottom boundary layer. Velocity data, from a bottom-mounted 1.2-MHz ADCP were analyzed by cross-spectral analysis to determine the seiche modes. The first vertical mode was generally dominant but the second mode made a significant contribution at times and was present throughout the summer season. The modal periods, which were consistent with normal mode analysis, exhibited a marked seasonal change that correlated closely with the evolution of stratification. Measurements in the bottom boundary layer, by another ADCP, in pulse-pulse coherent-mode sampling at 4 Hz, allowed determination of the rate of turbulent kinetic energy dissipation via the Structure Function Method. Episodes of energy dissipation, 1027 W kg21 occurred during peak flow (, 0.05 m s21) in the seiche motion (period, 10 h) and coincided with times of significant Reynolds stress. Between these episodes, dissipation fell below the noise level (, 5 3 1029 W kg21). The ratio of internal wave energy to the boundary-layer dissipation rate implies a decay time of, 75 h, which is similar to that for energy decay in strong seiches, indicating that boundary-layer dissipation makes a substantial contribution to energy loss from seiche motions. A major mixing event in August caused by near-resonant wind forcing brought metalimnion water to the surface and induced substantial mixing. © 2011, by the American Society of Limnology and Oceanography, Inc.