Turbulence generation by large-scale extreme vertical drafts in stratified geophysical flows

Recent studies based on direct numerical simulations (DNS) have shown that stably stratified geophysical flows can develop sporadic, extreme events in the form of bursts in the temperature and in the vertical velocity (Feraco et al. 2021), the latter having been observed also in the Mesosphere and Lower Thermosphere (MLT) (Chau et al. 2021). These events result from the interplay of gravity waves and turbulent motions, through a mechanism producing the amplification of the vertical velocity in a range of the governing parameters of geophysical interest (Feraco et al. 2018), such as those typical for the MLT. In this work we show how the extreme velocity drafts developing in DNS of stratified flows - here detected through the kurtosis of the vertical velocity (Kw) - are responsible for the local generation of turbulence and enhanced (kinetic V and potential P) energy dissipation - see figure (Marino et al. 2021) - providing an explanation based on fundamental fluid dynamics for the variability of the local dissipation and the modulation of its probability distribution function in geophysical fluids, as observed in global models (see Pearson & Fox-Kemper, 2018, for the ocean). Here we demonstrate how the presence of extreme vertical drafts is needed for stably stratified turbulent flows to achieve a local dissipation efficiency comparable to that of homogeneous isotropic turbulence (without waves). In particular, our study shows that roughly 10% of the domain volume is sufficient to account for up to 50% of the total volume kinetic energy dissipation in DNS with moderate/strong stratification (corresponding to values of the Froude number <0.1). The way energy is dissipated in geophysical flows remains an important open problem; our study indicates that in a certain region of the parameter space, powerful vertical drafts and the associated steepening of gravity waves, can ultimately lead to an inadequacy of the description of these systems in terms of weakly interacting waves. Pearson & Fox-Kemper, PRL, 2018, Vol. 120, p.094501 Feraco et al., EPL, 2018, Vol. 123 (4), p.44002 Feraco et al., EPL, 2021, in press, https://arxiv.org/pdf/2106.07574.pdf Chau et al., GRL, 2021, in press, https://www.essoar.org/pdfjs/10.1002/essoar.10507388.1 Marino et al. PRF, 2021, submitted, https://arxiv.org/pdf/2106.15219.pdf