Development of a Coherent Transilient Turbulence and Convection (CTTC) parameterization scheme for use in numerical weather prediction models

• Applicant: Professor Dr. Andreas Bott
• Subject Area: Atmospheric Science
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 460816630

Usually, in numerical weather prediction (NWP) models vertical subgrid scale motions are either parameterized by a turbulence scheme or a convection scheme. The decision is often based on the expected vertical extent of the dynamics resulting from buoyancy or shear instability: If the motion is of short vertical range and/or confined to the planetary boundary layer (PBL), then it is treated as a mixing event by the turbulence scheme. In case of a more extended vertical range, the motion is treated by the convection scheme as a vertical transport with cloud physics, up- and downdrafts and possibly precipitation. Being aware that there are no sharp limits for such classifications in nature, the claim for a unified or coherent turbulence and convection parameterization arises.The proposed project aims at the development of a unified and seamless, i.e. a coherent turbulence and convection parameterization scheme. To solve this problem, we propose an approach that combines eddy diffusivity (K-approach) terms for local turbulence and mass flux terms for a variety of convective plumes for nonlocal turbulence and convection. As an intuitive formalism we will apply the transilient matrix method, in which each matrix element quantifies the transport between a certain source and target layer.In our project, the classical mass flux term will be replaced by a form allowing for net mass transport. A central question will be, when and to which extent the turbulent / convective circulation is resolved on the grid scale. In other words, when and how much net mass transport is to be parameterized. Sorting local turbulence, nonlocal turbulence, shallow and deep convection by their spatial scales, it becomes clear that there may be a more or less smooth transition from no to full net mass transport depending on the grid size of the hosting model. Furtheron, the involved physical processes such as thermodynamics, cloud formation and precipitation have to be controlled by physical arguments rather than model grid dependent decisions. Depending on the grid size, net mass transport will seamlessly taken into account in the spatial regimes of nonlocalturbulence / shallow convection.Because globally high-resolved NWP models are (and still will be) computationally too expensive for operational application, NWP has to rely either on a model chain or a multi scale model with online nesting. For both types of operational NWP models a coherent and scale adaptive turbulence and convection scheme will avoid the artificial switching between different schemes depending on the expected spatial scale of the dynamics and the current grid scale.In the scope of the proposed project, we will use the ICON model of DWD as the development environment but keep the parameterization scheme in a generalized form for implementation in other NWP models as well.

• DFG Programme: Research Grants