Connecting large-scale velocity and temperature bursts with small-scale intermittency in stratified turbulence

¹ Laboratoire de Mécanique des Fluides et d'Acoustique, CNRS, École Centrale de Lyon, Université Claude Bernard Lyon 1, INSA de Lyon - F-69134 Écully, France
² Dipartimento di Fisica, Università della Calabria - I-87036 Rende, Italy
³ Université de Lyon, ENS de Lyon, Université Claude Bernard, CNRS, Laboratoire de Physique F-69342 Lyon, France
⁴ Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, and IFIBA, CONICET - Buenos Aires 1428, Argentina
⁵ Louisville, CO 80027, USA
⁶ Laboratory for Atmospheric and Space Physics, University of Colorado - Boulder, CO 80309, USA
⁷ Università degli Studi dell'Aquila - L'Aquila, Italy
⁸ CIRES, University of Colorado - Boulder, CO 80309, USA
⁹ Centrum Wiskunde & Informatica - 1098 XG Amsterdam, The Netherlands
¹⁰ Department of Astrophysical Sciences, Princeton University - 4 Ivy Lane, Princeton, NJ 08544, USA
¹¹ Leibniz Institute of Atmospheric Physics, University of Rostock - Kühlungsborn, Germany

^(a) raffaele.marino@ec-lyon.fr (corresponding author)

Received: 1 February 2021
Accepted: 5 July 2021

Abstract

Non-Gaussian statistics of large-scale fields are routinely observed in data from atmospheric and oceanic campaigns and global models. Recent direct numerical simulations (DNSs) showed that large-scale intermittency in stably stratified flows is due to the emergence of sporadic, extreme events in the form of bursts in the vertical velocity and the temperature. This phenomenon results from the interplay between waves and turbulent motions, affecting mixing. We provide evidence of the enhancement of the classical small-scale (or internal) intermittency due to the emergence of large-scale drafts, connecting large- and small-scale bursts. To this aim we analyze a large set of DNSs of the stably stratified Boussinesq equations over a wide range of values of the Froude number –1). The variation of the buoyancy field kurtosis with Fr is similar to (though with smaller values than) the kurtosis of the vertical velocity, both showing a non-monotonic trend. We present a mechanism for the generation of extreme vertical drafts and vorticity enhancements which follows from the exact equations for field gradients.