Inverse energy cascade in ocean macroscopic turbulence: Energy transfer rate ε and Richardson-Obhukov constant g from an surface drifter experiment in the Benguela upwelling system
- Universitaet Hamburg, Institute of Oceanography, Hamburg, Germany (julia.draeger-dietel@uni-hamburg.de)
We derive the energy transfer rate ε from the 3rd order relative (longitudinal) velocity structure function <Δul3>=(3/2)εs from ocean surface drifter trajectories in the turbulent mixed layer of the Benguela upwelling region off the coast of Namibia. Combination with the mean squared pair separation<s2(t)> =gεt3 reveals the Richardson-Obhukov constant g≅0.5, which is remarkably close to the one measured in controlled two-dimensional turbulent flows in laboratory. We verify the two coupled cascades of energy (upscale/inverse) and enstrophy (downwscale) by the theoretically predicted slope 1 for <Δul3> for inertial scales (above the injection scale) and slope 2 for the 2nd order structure function <Δul2> for non-local scales (below the injection scale) respectively. We detect additional 'ballistic contributions' in the central regime of the corresponding probability distribution P(st) of relative separations s for fixed time t, leading to an additional power law factor s-α with α ≅ 5/3. The algebraic decay with 1<α <2 revives to the relevance of Levy distributions in the stochastic description of the turbulent transport process in contrast to former claims. Our findings of a positively skewed probability distribution P(Δuls) of relative longitudinal velocity Δul for inertial scales s renews the question of intermittency in the inverse energy cascade.
How to cite: Draeger-Dietel, J. and Griesel, A.: Inverse energy cascade in ocean macroscopic turbulence: Energy transfer rate ε and Richardson-Obhukov constant g from an surface drifter experiment in the Benguela upwelling system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10184, https://doi.org/10.5194/egusphere-egu21-10184, 2021.