Mesoscale Energy Transfers in Regional domains: Spectral and Physical Space diagnostics.

Observations and numerical simulations consistently show that the horizontal kinetic energy spectrum follows a -5/3 slope at mesoscales from the troposphere to the lower stratosphere. Various fundamentally different theories have been proposed to explain this mesoscale spectral slope, including gravity waves, stratified turbulence, and wave-vortex interactions. To investigate the underlying mesoscale mechanism, we implement a combined diagnostic framework consisting of two complementary approaches: a non-hydrostatic, Fourier-based spectral energy budget and a scale-dependent energy transfer in physical space, used to diagnose the instantaneous, local structure of energy transfers in regional atmospheric domains, with particular emphasis on the mesosphere and lower thermosphere (MLT).

The methodology is validated using idealized mountain-wave simulations, where the dominant dynamical mechanisms are well understood. The framework is then applied to high-resolution nested UA-ICON simulations from the NASA Vorticity Experiment (VortEx) over Andøya, Norway, a dynamically active region. The results reveal pronounced spatial and scale-dependent variability in energy transfers that is not captured by domain-averaged spectral diagnostics alone. The scale-dependent energy transfers are consistent with independent turbulence indicators, including the Richardson number and parameterized turbulent kinetic energy (TKE). Regions characterized by low Richardson numbers and elevated TKE exhibit significantly stronger downscale energy cascades than those in more stable, high Richardson number regimes. This study provides insight into mesoscale dynamics by extending energy transfer analyses into the MLT and offers a robust framework for investigating energy transfer across different atmospheric regimes.