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Numerical diffusion and turbulent mixing in convective self-aggregation
  • Lorenzo Silvestri,
  • Miriam Saraceni,
  • Paolina Bongioannini Cerlini
Lorenzo Silvestri
Department of Civil and Environmental Engineering, University of Perugia

Corresponding Author:[email protected]

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Miriam Saraceni
University of Perugia
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Paolina Bongioannini Cerlini
University of Perugia
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Spontaneous aggregation of deep convection is a common feature of idealized numerical simulations of the tropical atmosphere in Radiative-Convective Equilibrium (RCE). However, at coarse grid resolution where deep convection is not fully resolved, the occurrence of this phenomenon is extremely sensitive to subgrid-scale processes. This study focuses on the role played by mixing and entrainment, either provided by the turbulence model or the implicit numerical dissipation. We have analyzed the results of two different models, WRF and SAM, and compared different configurations by varying the turbulence models, the initial conditions and the horizontal spatial resolution. At coarse grid resolution (3 km), the removal of turbulent mixing prevents the occurrence of Convective Self-Aggregation (CSA) in models with low numerical diffusivity, while it is preserved in models with high numerical diffusivity. When the horizontal grid resolution is refined to 1 km (thus reducing the implicit numerical dissipation), CSA is only achieved by increasing the explicit turbulent mixing. In this case, CSA was found to occur even with a small amount of shallow clouds. Therefore, this study suggests that the sensitivity of CSA to horizontal grid resolution is not primarily due to the corresponding decrease in shallow clouds. Instead, it is found that turbulent mixing and dissipation at small scales regulate the amplitude of initial humidity perturbations introduced by convection in the free troposphere: the greater the dissipation at small scales, the greater the size and the strength of humidity perturbations in the free troposphere that can destabilize the RCE state.
09 Dec 2023Submitted to ESS Open Archive
10 Dec 2023Published in ESS Open Archive