Kinetic energy spectra characteristics of two convection-permitting limited-area models AROME and Meso-NH

Kinetic energy (KE) spectra are applied to evaluate two convection-permitting models: the AROME numerical weather prediction operational model and the Meso-NH research model, that share the same physics and differ only in the dynamics (semi-Lagrangian semi-implicit versus Eulerian explicit schemes). A first analysis of AROME spectra for winter and summer seasons shows that the model-derived spectra match the observational spectra well, including the transition between k(-3) and k(-5/3) regimes. The vertical distribution of the spectra is coherent with previous observational and numerical studies and the diurnal cycle has a strong impact on the amount of KE in the mesoscale during summer. A comparative analysis of KE spectra for both models is then performed on a real case of individual convective cells that developed over plains, during the afternoon of 11 April 2007, characterized by a strong cold air outflow in the low levels. AROME spectra are characterized by a coarser effective resolution than Meso-NH, even without explicit diffusion, revealing the impact of the implicit diffusion of the semi-implicit semi-Lagrangian scheme used in AROME. For large time steps, the damping increases and can be attributed preferentially to the SI part of the SISL (semi-implicit semi-Lagrangian) formulation. Adiabatic runs show that the transition to a shallow mesoscale regime is still apparent even if the mesoscale KE variance strongly depends on the presence of the physical processes. Effective resolution of Meso-NH remains around 4-6x for horizontal grid spacings between 2.5 km and 250 m. The effects of subgrid mixing schemes are also investigated with Meso-NH at 500 m horizontal grid spacing in the grey zone for turbulence, illustrating the difficulty in finding a good equilibrium between resolved and subgrid mixing at this scale.