Turbulence comes in bursts in stably stratified flows.
There is a clear distinction between simple laminar and complex turbulent fluids; however, in some cases, as for the nocturnal planetary boundary layer, a stable and well-ordered flow can develop intense and sporadic bursts of turbulent activity that disappear slowly in time. This phenomenon is ill understood and poorly modeled and yet it is central to our understanding of weather and climate dynamics. We present here data from direct numerical simulations of stratified turbulence on grids of 20483 points that display the somewhat paradoxical result of measurably stronger events for more stable flows, not only in the temperature and vertical velocity derivatives as commonplace in turbulence, but also in the amplitude of the fields themselves, contrary to what happens for homogenous isotropic turbulent flows. A flow visualization suggests that the extreme values take place in Kelvin-Helmoltz overturning events and fronts that develop in the field variables. These results are confirmed by the analysis of a simple model that we present. The model takes into consideration only the vertical velocity and temperature fluctuations and their vertical derivatives. It indicates that in stably stratified turbulence, the stronger bursts can occur when the flow is expected to be more stable. The bursts are generated by a rapid nonlinear amplification of energy stored in waves and are associated with energetic interchanges between vertical velocity and temperature (or density) fluctuations in a range of parameters corresponding to the well-known saturation regime of stratified turbulence.
Nordita, Roslagstullsbacken 23, 106 91 Stockholm, Sweden and ICTP, Strada Costiera 11, 34151 Trieste, Italy.
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Rorai
Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, C1053ABJ Buenos Aires, Argentina and IFIBA, CONICET, Ciudad Universitaria, 1428 Buenos Aires, Argentina.
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Mininni
Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado 80309, USA.
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Pouquet