Convection under internal waves in an alpine lake
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Convection under internal waves in an alpine lake Hans van Haren1 · Henk A. Dijkstra2 Received: 31 March 2020 / Accepted: 2 November 2020 © Springer Nature B.V. 2020
Abstract Turbulent mixing processes in deep alpine Lake Garda (I) have not extensively been observed. Knowledge about drivers of turbulent fluxes are important for insights in the transport of matter, nutrients and pollutants, in the lake and in natural water bodies in general. In this paper, the occurrence of internal wave induced turbulent convection, termed ’internally forced convection’, is addressed as opposed to the more common shear-induced turbulence in a density stratified environment. Observations are analyzed from a dedicated yearlong mooring holding 100 high-resolution temperature sensors at 1.5 m intervals under a single current meter in the deeper half of the 344 m deep lake-center. Episodically, the weakly density stratified waters in the lower 50 m above the lake-floor show spectral slope and coherence evidence of short-term (15–30 min) convective motions under internal waves that are supported by the stronger stratified waters above. The near-homogeneous conditions are not attributable to frictional Ekman dynamics, but to large-scale internal wave crests. Keywords Deep-sea turbulence · Convection · Internal waves · High-resolution moored temperature observations · Lake Garda
1 Introduction ‘Natural (free)’ turbulent convection is generally considered to occur during a period of gravitational instability when denser (relatively cool) fluid is over less dense (warmer) fluid [1, 2]. It is an efficient mixing process [3] and therefore important for fluxes of nutrients and suspended particles in geophysical environments like the atmosphere and ocean. Natural convection commonly occurs in the atmosphere due to solar radiation reflecting from the Earth surface below. In a bounded fluid, convection is classically studied in a laboratory oil-filled pan over a heat source as Rayleigh-Bénard convection [e.g., 4]. This convection starts at sufficiently high Rayleigh number Ra = gαΔTh3/(νκ) > Rac > 1000, where the * Hans van Haren [email protected] 1
Royal Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, the Netherlands
2
Department of Physics, Institute for Marine and Atmospheric research Utrecht, Utrecht University, Utrecht, the Netherlands
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Environmental Fluid Mechanics
critical Rac depends on boundary conditions, g denotes the acceleration of gravity, α the thermal expansion coefficient, ΔT the vertical temperature difference driving the positive buoyancy, h the layer depth, and ν and κ the kinematic viscosity and thermal diffusivity, respectively. It is characterized by up- and down-ward cells of motion that have an aspect ratio (of vertical over horizontal dimensions) of approximately unity [5]. In natural waters like oceans and lakes, Ra = O(108) and the convective, full turbulence has more than one (overturning-)cell-size, in fact many from the large (> 100 m) energy con
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