LMDZ6A: The Atmospheric Component of the IPSL Climate Model With Improved and Better Tuned Physics

This study presents the version of the LMDZ global atmospheric model used as the atmospheric component of the Institut Pierre Simon Laplace coupled model (IPSL-CM6A-LR) to contribute to the 6th phase of the international Coupled Model Intercomparison Project (CMIP6). This LMDZ6A version includes original convective parameterizations that define the LMDZ "New Physics": a mass flux parameterization of the organized structures of the convective boundary layer, the "thermal plume model," and a parameterization of the cold pools created by reevaporation of convective rainfall. The vertical velocity associated with thermal plumes and gust fronts of cold pools are used to control the triggering and intensity of deep convection. Because of several shortcomings, the early version 5B of this New Physics was worse than the previous "Standard Physics" version 5A regarding several classical climate metrics. To overcome these deficiencies, version 6A includes new developments: a stochastic triggering of deep convection, a modification of the thermal plume model that allows the representation of stratocumulus and cumulus clouds in a unified framework, an improved parameterization of very stable boundary layers, and the modification of the gravity waves scheme targeting the quasi-biennal oscillation in the stratosphere. These improvements to the physical content and a more well-defined tuning strategy led to major improvements in the LMDZ6A version model climatology. Beyond the presentation of this particular model version and documentation of its climatology, the present paper underlines possible methodological pathways toward model improvement that can be shared across modeling groups. Plain Language Summary The improvement of global numerical models is essential for the anticipation of future climate changes. We present significant advances in the physical content of a particular atmospheric model which contributes to the simulations of the Coupled Model Intercomparison project CMIP that feed reports from the IPCC. We document in particular the improvements of the representation through "parameterizations" of convective and cloudy processes. The article emphasizes the importance of strengthening the formalization of the methodology of development and tuning of models, so that new physical ideas can be translated into effective improvement of the climate representation.