Monday, 9 June 2008: 4:15 PM
Aula Magna (Aula Magna)
Analysis of the atmospheric turbulence of the order of inertial subrange scales in the stable boundary layer (SBL) is presented. Measurements of atmospheric turbulence made during the Surface Heat Budget of the Arctic Ocean Experiment (SHEBA) over the Arctic pack ice are used to study small-scale turbulence. Turbulent fluxes and mean meteorological data were continuously measured and reported hourly at five levels on a 20-m main tower for 11 months. According to the SHEBA data, energy spectra of wind speed components and temperature for weakly and moderately stable conditions have a wide inertial subrange, which obeys the Kolmogorov power law with slope close to -2/3 at high frequencies. Although stability increases with increasing height, the surface-generated wind shear is large enough to maintain turbulence at all sonic levels in this regime. In contrast to the weakly and moderately stable conditions, the low-frequency disturbances in spectra for very stable conditions are well pronounced. However, the low-frequency part is separated by a spectral gap from high-frequency turbulence. In this regime, atmospheric turbulence at lower levels is usually associated with the subcritical regime (gradient Richardson number is below its critical value), and the inertial subrange is still identifiable. The Richardson number increases with increasing height and at some level reaches its critical value. SHEBA data shows that small-scale turbulence in the supercritical regime in both wind velocity and the temperature spectra is obviously collapsed, and the inertial subrange associated with a Kolmogorov cascade dies out. Furthermore, spectral densities in the high-frequency range increase with increasing frequency with a slope close to unity that is associated with white noise of the sensor. However according to the SHEBA data, some turbulence, basically in the lower frequency range, persists even in the supercritical regime, and there is no evidence that suppression of turbulence by stable stratification causes a transition to laminar flow in the atmospheric SBL. Thus, one may define a critical Richardson number as a value beyond which the Kolmogorov cascade fails.
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