The sensitivity of convective precipitation in South Africa to horizontal turbulent exchange in the km-scale regional climate model REMO-NH

This study addresses the challenges of simulating precipitation in South Africa using the convection-permitting climate model REMO-NH. In the WaRisCo project, which focuses on hydroclimatic extremes under a changing climate, a realistic representation of precipitation is essential for providing suitable forcing data for hydrological modelling. Traditional regional climate models (RCMs) with resolutions of about 11km have the limitation of not accurately reproducing extreme precipitation events such as thunderstorms. Convection-permitting RCMs (CP-RCMs) represent an alternative that offers a higher resolution and explicit simulation of convection.

For the study, the non-hydrostatic climate model REMO-NH is adopted with a resolution of about 3 km and driven by ERA5 using the double nesting technique. It enables explicit simulation of deep cumulus clouds with high vertical velocities. As entrainment of ambient air strongly influences precipitation, its representation depends critically on horizontal turbulent transfer in the model. In the standard model setup, second-order horizontal diffusion (DIFF2) takes care of this transfer. However, excessively high precipitation occurs in the autumn and winter seasons in comparison to the CHIRPS precipitation data.

A simulation with fourth order horizontal diffusion (DIFF4) reveals an even stronger precipitation bias. As an alternative to artificial diffusion, a 3D turbulence scheme has been implemented. A simulation with this scheme (TURB3D) removes this bias. Further evaluation of the results shows that the bias appears mainly for intermediate values in the frequency distribution and that the boundary layer moisture and, therefore, CAPE (convective available potential energy), are higher in the simulations with artificial horizontal diffusion. These results demonstrate that accurate treatment of 3D turbulent exchange is essential for improving convection-permitting simulations, and it will, therefore, be used for the km-scale climate projections within the WaRisCo project, which is part of the “Water Security in Africa – WASA” program.