Dresden 2020 – scientific programme

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DY: Fachverband Dynamik und Statistische Physik

DY 55: Poster: Turbulence; Complex Fluids; Microfluidics; Droplets and Wetting

DY 55.8: Poster

Thursday, March 19, 2020, 15:00–18:00, P1A

Two-point velocity statistics from ocean surface drifter observations in the Benguela upwelling system: In search of the inverse energy cascade — •Julia Draeger-Dietel¹, Alexa Griesel¹, and Dhruv Balwada² — ¹Institut für Meereskunde, Universität Hamburg, Hamburg, Germany — ²Courant Institute of Mathematical Sciences, New York University, New York, NY, USA

Lagrangian trajectories from surface drifters in the ocean constitute a time evolving, highly non-uniform spatial grid. The corresponding drifter velocity observations can be treated as scattered point Eulerian measurements, which can help in deducing the turbulent properties of the flow. Here we examine the probability distribution P(Δ u_ℓ |s) of relative longitudinal velocity Δ u_ℓ, as a function of spatial separation s, from surface drifters deployed in the upwelling region off Namibia. We find the P(Δ u_ℓ |s) to be positively skewed for relative separations s of 10 km - 80 km. The rescaled 3rd order structure function, ⟨Δ u_ℓ³⟩/s, reveals a (positive) plateau, supporting an inverse energy cascade with energy transfer rate є≃ 2.4 · 10⁻⁸ m³/s². The combination with former findings of Richardson scaling of pair separations, ⟨s²(t) ⟩= gє t³, yield a reduced Obhukov constant g close to the finding in laboratory experiments. The reduced Obhukov constant can be traced back to an algebraic decay s^−α with α ≃ −5/3 in the central regime of the corresponding probability distribution P(s|t) of relative separations s for fixed time t. An algebraic decay with 1<α<2 points to the relevance of Levy walk models in the stochastic description of the turbulent transport process in the ocean.