Sampling the Structure of Convective Turbulence and Implications for Grey-Zone Parametrizations
Honnert, Rachel ; Couvreux, Fleur ; Masson, Valéry ; Lancz, Dávid
The grey zone of dry convection is the range of scales in which boundary-layer thermals are partly explicitly resolved by numerical weather prediction (NWP) models and partly parametrized. We seek to determine how thermals are divided into subgrid and resolved scales in the grey zone of convective boundary-layer thermals. Reference data for grid-scale and subgrid-scale fields at these resolutions are constructed by filtering 62.5-m large-eddy simulation data. A conditional sampling is adapted to detect subgrid thermals, and is used to characterize the subgrid thermals at several grid spacings in the grey zone. A mass-flux parametrization used in NWP models is compared with the subgrid thermal field. The analysis demonstrates that, although the mass-flux framework is suitable in the grey zone, some assumptions of the mass-flux schemes, usually used at the mesoscale, cannot be made in the grey zone. In particular, the thermal fraction is not small, the resolved vertical velocity is not negligible, the entrainment and detrainment rates depend on the horizontal resolution, the triggering and the closure at the surface are moreover random.
Accès à la notice sur le site du portail documentaire de Météo-France