Scaled temperature spectrum in the unstable oceanic surface layer

Abstract

[1] A nondimensional form of the temperature spectrum in a convective near-surface layer was derived empirically as a function of stability parameter x = z/L and surface wave parameter g = u * /(gz) 1/2 , under the assumption of horizontal isotropy, where z is the depth of the measurement, L is the Monin-Obukhov length scale, u * is the surface friction velocity, and g is the acceleration due to gravity. The wave number-weighted, onedimensional spectrum had +1 slope at low wave numbers and À2/3 slope at high wave numbers (characteristic of an inertial subrange). Spectral levels in the À2/3 range varied with g, and the spectrum width was a function of x. The wavelength of the spectral peak decreased as Àx (>0) increased. The variation of spectral level in the inertial subrange suggested that dissipation due to wave breaking was enhanced by a factor of 1.7 at 2-m depth for a wind speed of 10 m s À1 . Root-mean-square temperature fluctuations at 2-m depth versus Àx were in excellent agreement with atmospheric surface layer observations and agreed moderately well (within 30%) with the results of large-eddy simulation experiments. Root-mean-square fluctuations were proportional to (Àx) À1/3 for Àx > 0.4, consistent with the predictions of similarity theory. The skewness of temperature fluctuations varied with Àx, qualitatively similar to variation in the atmospheric surface layer. The skewness of the horizontal gradient of temperature at 2-m depth varied relative to the wind direction and was well described by a cosine function with amplitude 0.6.

Ocean Speed and Turbulence Measurements Using Pitot-Static Tubes on Moorings

“…Despite the disparate frequency ranges, the three estimates of « derived from these fits vary only from 1.0 3 10 26 to 1.3 3 10 -6 m 2 s -3 . For a frequency of 0.001 Hz, the equivalent advected length scale (at 0.26 m s -1 ) is 260 m. This suggests inhomogeneity of the turbulence compared to predictions from simple boundary layer scaling but is consistent with surface layer measurements in both the atmosphere (Peltier et al 1996) and the ocean (Wijesekera et al 2004).…”

Boils and Turbulence in a Weakly Stratified Shallow Tidal Sea

“…In the near-surface mixed layer the skewness of the horizontal spatial derivatives of temperature, or the skewness of the time derivatives for measurements made using sensors moving through the water, is of the order of unity and of a sign that depends on both the direction of traverse through the water relative to the wind and the direction of the heat flux through the water surface; the sign of the skewness is consistent with the presence of enhanced gradients or temperature ramps in braids within a billow structure transporting buoyancy vertically within a wind-driven shear (Thorpe 1985;Soloviev 1990; Thorpe et al 1991Thorpe et al , 2003Wijesekera et al 2004;Ozen et al 2006). …”