Improvements of shallow cloud and boundary layer parameterizations in the AROME-France model.

Shallow convection affects the shape of the Planetary Boundary Layer (PBL) and has a significant impact on low cloud formation, and thus on model computed mean tendencies and radiation budget. Recently, observational studies have been used to compare AROME (the kilometer-scale operational Numerical Weather Prediction model used at Météo-France) versus in situ observations of the radiation budget components. Comparisons with several measurement sites (Meteopole Flux, Sirta) have shown biases of solar radiations in AROME. Extended investigations have attributed a part of these biases to shallow convection scheme representing the PBL with rather poor performance, in particular stratocumulus clouds. The present work aims to investigate and improve the parameterizations used in AROME and Meso-NH models (both models share the same 1D physics package). Inconsistencies and bugs have been corrected, while new parameterization components and updates of the Eddy Diffusivity Mass Flux scheme (EDMF) have been added from the literature. This includes changes in the shallow convection, turbulence, subgrid cloud and microphysics schemes, from the updraft hypothesis to shallow cumulus subgrid precipitation. We use the Single Column Model (SCM) version of AROME (MUSC) to evaluate the adjustments brought to the model on well-known idealized cases of cloud development like cumulus over land (ARMCu) or ocean (RICO), stratocumulus (FIRE), transition cases (SANDU) and dry convection (IHOP, AYOTTE) as well. The Meso-NH research model is used to perform Large Eddy Simulations (LES), in which conditional sampling methods and diagnostics are applied to retrieve resolved quantities that serve as a reference for evaluating parameterizations. In addition, the semi-automatic tool High-Tune Explorer helps us to explore the n-dimensional space of free parameters of the parameterizations, and thus closes the EDMF scheme and then gives us hints on the sensibility over the set of relevant parameters. This complete update of AROME has shown significant improvements especially for stratocumulus cases, triggered precipitation and cloud formation on most of the cases.