An update of shallow cloud parameterization in the AROME NWP model

Abstract. The representation of shallow clouds in numerical weather prediction models remains a challenge for the parameterizations of Atmospheric Boundary Layer (ABL). Previous evaluations of the AROME model have shown radiative budget weaknesses, which were later attributed to a lack of shallow clouds, especially stratocumulus and small cumulus. In this study, we investigate the difficulties of the AROME model to represent the ABL and the associated low clouds, and we provide consistent updates of the Eddy Diffusivity Mass Flux (EDMF) scheme (shallow convection plus turbulence schemes), the subgrid cloud scheme and the associated precipitation. For this purpose, we use the well-known Single Column Model (SCM) versus Large Eddy Simulation (LES) comparison approach to evaluate the modifications. Additionally, the semi-automatic HighTune Explorer tool helps us to explore the free parameter space associated with the modified parameterizations. The modifications are evaluated using several documented cases of boundary layer development, from stratocumulus to precipitating cumulus clouds, including a transition case. Although physical inconsistencies and questionable assumptions still remain in AROME and need to be clarified, the modifications improve the consistency between parameterizations and yield simulations in better agreement with LES, in particular for the diurnal cycle of clouds and the representation of non-local turbulence.