The Effect of the Hydrostatic Approximation, Horizontal Resolution, and Timestep Size on Deep Convective Processes: A Model Intercomparison between COSMO and IFS

Abstract:

The increasing availability of computing power allows the use of kilometer-scale convection-permitting weather and climate models for operational forecasts. Next to horizontal resolution, other aspects such as the applied numerical methods, the use of the hydrostatic approximation, and timestep size are factors that influence a model’s ability of resolving convective processes.

In order to improve our understanding of the role of these factors, a model intercomparison between the nonhydrostatic COSMO model and the hydrostatic Integrated Forecast System (IFS) from ECMWF has been conducted. Both models have been run in NWP mode with different spatial resolutions up to about 2 km, on European and global domains, respectively. The simulations address two summer days over Europe with strong convection. The results are analyzed with focus on vertical wind speed and precipitation, and also compared to different observational precipitation datasets.

Results show that even at horizontal resolutions around 2 km the effect of the hydrostatic approximation is negligible for the weather type considered. However, a sufficiently small timestep size seems to be of importance in order to properly represent the very highest vertical velocities associated with deep convection. The study also indicates that the amount of horizontal diffusion might play a crucial role in the structure of convective cells and precipitation intensities. Furthermore, it is demonstrated that the parameterization of deep convection leads to much lower updraft and precipitation intensities as well as a shifted diurnal cycle with premature precipitation peaks for both models.