Jianan Chen, Xiaoming Shi

With increasing computational power, atmospheric simulations have approached the gray zone resolutions where energetic turbulent eddies are partly resolved. The representation of turbulence in the gray zone is challenging and sensitive to the choices of turbulence models and numerical advection schemes. Some numerical advection schemes are designed with numerical dissipation to suppress small-scale numerical oscillations. However, at gray zone resolutions, the numerical dissipation can dampen both numerical and physical oscillations. In this study, we first evaluate the impact of advection schemes on the simulation of an idealized squall line at two gray zone resolutions (1 and 4\,km). We found that at the 4-km resolution, the implicit numerical dissipation from advection schemes is excessive because it dampens convective cells across all scales and weakens the front-to-rear flow, producing insufficient convective precipitation and excessive stratiform precipitation. At the 1-km resolution, the numerical dissipation is desired, because without it, excessive spurious numerical oscillations develop into convections, weakening the large-scale front-to-rear flow through increased entrainment and mixing. The dynamic reconstruction model (DRM) of turbulence is designed to model both forward- and backscatter, having the potential to counter the effect of numerical dissipation from the advection schemes. Previous studies suggest superior performances of DRM in gray zone simulations for various atmospheric flows. Here, we show that although DRM can improve the squall line simulation at the 4-km resolution where the numerical damping is unwanted, it cannot improve squall line simulations at the 1-km resolution where numerical dissipation is needed.