Development of a Coherent Transilient Turbulence and Convection (CTTC) parameterization scheme for use in numerical weather prediction models
Final Report Abstract
The proper representation of subgrid scale processes in numerical weather prediction (NWP) models is crucial for the forecast quality. As important subgrid scale processes, convection and turbulence are dynamically similar and, thus, imply a unified parameterization. Since for current NWP models the conceptional problems, when partially resolving convective transports on the grid scale, could successfully be solved by subgrid scale mass transport [convection scheme HYMACS], it appears reasonable to introduce this mass transport also for the turbulence scheme and to unify both schemes. We have chosen NLT3D as turbulence scheme, which had especially been developed for grid sizes including those with partially resolved processes. The new Coherent Transilient Turbulence and Convection (CTTC) scheme has been developed in three steps: First, the HYMACS scheme and especially its subgrid scale mass transport has been adapted to the new version of the ICON model of DWD which allows for online nesting for later checks with different grid sizes. Second, the NLT3D scheme, already successfully tested by means of case studies in the WRF model (NCAR/UCAR, USA) [Kuell and Bott, 2022], has been implemented in ICON. To make the validation of NLT3D more representative, the scheme has been extensively tested and compared to operational observations and classical turbulence schemes in a three month simulation during spring time with a variety of weather situations. Compared to the classical schemes, especially the forecast of precipitation benefits from NLT3D , and also the forecasted near-surface temperatures and moisture during nighttime could be improved, when a proper representation of the stable PBL is necessary. Small-scale relative structures tend to be simulated more realistically with NLT3D. However, the near-surface wind simulation, which is generally challenging, is still somewhat worse than with the classical schemes. As a third step both schemes have been unified: They have been coupled on the subgrid scale by passing convective buoyancy and shear to the turbulence scheme and, in return, turbulent triggers to the convection scheme. The NLT3D scheme has been extended by net turbulent mass transport expressed in a transilient matrix formulation similar to the tracer transport. This makes turbulent tracer transports and especially its horizontal spreading aloft more realistic. Net mass transport increases with the ratio of eddy size to vertical grid size resulting in a scale-adaptive behavior of the scheme.
