Understanding Turbulence Kinetic Energy Transfer in and between Atmospheric Synoptic and Mesoscales

Understanding the development of the atmospheric energy spectrum across scales is necessary to elucidate atmospheric predictability. We investigate energy transfer between synoptic and mesoscales using high Reynolds number direct numerical simulations (DNS) of Two-Dimensional (2D) turbulence transfer under forcing applied at different scales. First, DNS results forced by a single Kinetic Energy (KE) source at large scales show that the energy spectra slopes of the direct enstrophy cascade are steeper than the theoretically predicted -3 slope. Second, the synoptic feedback of the presence of two inertial ranges in 2D turbulence at intermediate scales is investigated by introducing a second energy source in the small mesoscales. The energy spectra for the DNS with two KE sources exhibit flatter slopes that are closer to -3, consistent with the observed KE spectra of horizontal winds in the atmosphere at synoptic scales. This finding suggests that atmospheric flows involve concurrent direct (downscale) enstrophy transfer in the synoptic scales and inverse (upscale) kinetic energy transfer from the meso- to the synoptic-scales, that is the existence of a mesoscale feedback on synoptic scale predictability. The results are compared against the real case studies over Andes Mountains. This comparison reveals the transient behavior of atmospheric energy spectra as a function of stability. In the stable conditions the spectra exhibit a -3 slope associated with the downscale KE transfer while in the unstable conditions the spectral slopes approach -5/3 associated with the upscale KE turbulence transfer.