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

Abstract

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 4-6 x 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.

Large eddy simulation of radiation fog: impact of dynamics on the fog life cycle

“…According to Skamarock (2004), kinetic energy (KE) spectra deduced from simulations allow the effective resolution to be set up as the scale at which the model starts to depart from the theoretical slope, which is −3 for vertical velocity spectra applied to stable turbulence. Mean KE spectra applied to the vertical wind component reveal an effective resolution of the order of 4-5 x for simulations with CEN4TH (DX2 and REF), in agreement with Ricard et al (2013), namely 8 and 20 m, respectively (Fig. 13).…”

“…This is shown on kinetic energy spectra applied to vertical velocity over the whole fog depth, computed according to Ricard et al (2013) and presented in Fig. 13.…”

A new downscaling method for sub-grid turbulence modeling

“…The Meso-NH spectra show instead gradual decreases, but the spatial resolution seems almost equivalent to the NCAR LES model. According to the NCAR LES cutoff frequency, the effective spatial frequency of the Meso-NH model is about 125 m. Thus, it leads to an effective resolution of 3. x, which is in good agreement with previous studies (Ricard et al, 2013).…”

Boundary-layer turbulent processes and mesoscale variability represented by numerical weather prediction models during the BLLAST campaign

“…This is consistent with the 20 min used to derive the TKE from surface point observations. We checked that none of the models directly resolved boundary-layer eddies -even the model with the finest resolution (due to its effective resolution of ∼ 9 x; see Ricard et al, 2013). The contribution of the mass-flux scheme in AROME was taken into account by adding the mass-flux contribution, estimated as 0.5·a up ·w 2 up , where a up is the coverage fraction of the thermals and w up the thermal vertical velocity, to the subgrid TKE.…”