TY - JOUR

T1 - The vertical structure of turbulent dissipation in shelf seas

AU - Simpson, John

AU - Crawford, Bill

AU - Rippeth, Tom

AU - Campbell, Andrew

AU - Cheok, Joe

PY - 1996/8/1

Y1 - 1996/8/1

N2 - The free-fall FLY profiler has been used to determine the variation in energy dissipation epsilon in the water column over a tidal cycle at mixed and stratified sites in the Irish Sea. It was found that epsilon exhibits a strong M(4) variation with a pronounced phase lag that increases with height above the bed. In mixed conditions this M(4) signal, which extends throughout the water column, is reasonably well reproduced by turbulent closure models of the vertical exchange, In the summer stratified situation, the M(4) signal in epsilon is confined to about 40 m above the seabed with phase delays of more than 4 h relative to the seabed. The lowest levels of dissipation (similar to-10(-5) W m(-3)), measured in the pycnocline, are significantly above the system noise level and much higher than predicted by a model using the Mellor-Yamada level 2 closure scheme (MY2.0). However, when allowance is made for the diffusion of TKE, the model (MY2.2) simulates the depth-time distribution of dissipation in the stratified case satisfactorily if the diffusivity K-q = 0.2ql. With K-q set equal to vertical eddy viscosity N-z, which depends on the Richardson number Ri, the model underestimates dissipation in the pycnocline by two decades, which would imply the possibility of a midwater source of TKE. The observed depth-integrated dissipation is found to be consistent with estimates based on the energy lost from the tidal wave when adjustment is made for the unsampled high energy region close to the bed.

AB - The free-fall FLY profiler has been used to determine the variation in energy dissipation epsilon in the water column over a tidal cycle at mixed and stratified sites in the Irish Sea. It was found that epsilon exhibits a strong M(4) variation with a pronounced phase lag that increases with height above the bed. In mixed conditions this M(4) signal, which extends throughout the water column, is reasonably well reproduced by turbulent closure models of the vertical exchange, In the summer stratified situation, the M(4) signal in epsilon is confined to about 40 m above the seabed with phase delays of more than 4 h relative to the seabed. The lowest levels of dissipation (similar to-10(-5) W m(-3)), measured in the pycnocline, are significantly above the system noise level and much higher than predicted by a model using the Mellor-Yamada level 2 closure scheme (MY2.0). However, when allowance is made for the diffusion of TKE, the model (MY2.2) simulates the depth-time distribution of dissipation in the stratified case satisfactorily if the diffusivity K-q = 0.2ql. With K-q set equal to vertical eddy viscosity N-z, which depends on the Richardson number Ri, the model underestimates dissipation in the pycnocline by two decades, which would imply the possibility of a midwater source of TKE. The observed depth-integrated dissipation is found to be consistent with estimates based on the energy lost from the tidal wave when adjustment is made for the unsampled high energy region close to the bed.

U2 - 10.1175/1520-0485

DO - 10.1175/1520-0485

M3 - Article

VL - 26

SP - 1579

EP - 1590

JO - Journal of Physical Oceanography

JF - Journal of Physical Oceanography

SN - 0022-3670

IS - 8

ER -