Premium
Internal seiche modes and bottom boundary‐layer dissipation in a temperate lake from acoustic measurements
Author(s) -
Simpson J. H.,
Wiles P. J.,
Lincoln B. J.
Publication year - 2011
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2011.56.5.1893
Subject(s) - seiche , dissipation , internal wave , boundary layer , stratification (seeds) , turbulence kinetic energy , thermocline , atmospheric sciences , mechanics , kinetic energy , geology , turbulence , physics , climatology , oceanography , classical mechanics , seed dormancy , germination , botany , dormancy , biology , thermodynamics
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 ~ 10 ‐7 W kg ‐1 occurred during peak flow (~ 0.05 m s ‐1 ) 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 × 10 ‐9 W kg ‐1 ). 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.