Similarity scaling of turbulence in a temperate lake during fall cooling

Turbulence, quantified as the rate of dissipation of turbulent kinetic energy (epsilon), was measured with 1400 temperature-gradient microstructure profiles obtained concurrently with time series measurements of temperature and current profiles, meteorology, and lake-atmosphere fluxes using eddy covariance in a 4 km(2) temperate lake during fall cooling. Winds varied from near calm to 5 m s(-1) but reached 10 m s(-1) during three storm events. Near-surface values of e were typically on the order of 10(-8) to 1027 m(2) s(-3) and reached 10(-5) m(2) s(-3) during windy periods. Above a depth equal to vertical bar L-MO vertical bar, the Monin-Obukhov length scale, turbulence was dominated by wind shear and dissipation followed neutral law of the wall scaling augmented by buoyancy flux during cooling. During cooling, epsilon(z) = 0.56 u(*W)(3)/kz + 0.77 J(BO) and during heating epsilon(z) = 0.6 u(*w)(3)/kz, where u(*w) is the water friction velocity computed from wind shear stress, k is von Karman's constant, z is depth, and J(BO) is surface buoyancy flux. Below a depth equal to vertical bar L-MO vertical bar during cooling, dissipation was uniform with depth and controlled by buoyancy flux. Departures from similarity scaling enabled identification of additional processes that moderate near-surface turbulence including mixed layer deepening at the onset of cooling, high-frequency internal waves when the diurnal thermocline was adjacent to the air-water interface, and horizontal advection caused by differential cooling. The similarity scaling enables prediction of near-surface e as required for estimating the gas transfer coefficient using the surface renewal model and for understanding controls on scalar transport.