COGNAC · Readdressing Convective-Surface Interaction in Global Climate Models

Readdressing Convective-Surface Interaction in Global Climate Models

A large part of our knowledge on Earth's climate is provided by Global Climate Models (GCMs), numerical models based on the time integration of the prognostic equations of the atmospheric and oceanic fluid-dynamics. Such models are able to produce simulations of the climate for both present and future scenarios. However, the skill and the predictive power of current state-of-the-art GCMs is limited by the low horizontal resolution and by the imprecise representation of several physical processes. Among them, the simulation of the coupling between the atmospheric circulation and the hydrological cycle, and more specifically the representation of clouds, is still to date one of the largest issues. Indeed, clouds controls the precipitation and the radiation budget of the Earth: COGNAC has been a fundamental project framed in this context. It aimed at re-addressing the representation of convection and clouds in GCMs. In this project we addressed specifically the physical processes leading to the formation and controlling the dynamics of stratocumulus clouds. Those clouds are ubiquitous over the globe and have a probably the most important impact on Earth’s radiation budget. The fractional coverage of those low-level clouds is controlled by different physical processes, including the dynamics of the boundary layer (henceforth STBL for Stratocumulus Topped Boundary Layer). The dynamics of the STBL are challenging for two main reasons: 1) the concurrence of several thermodynamical and turbulent processes in action and 2) the relative thin region over which the most of these processes take place. The STBL and the related stratocumulus cloud cover are in the climate community spotlight since they represent a large source of uncertainty in GCM simulations. Increasing in horizontal resolution, due to the augmented available computational power, will not alleviate the issue. Weather prediction simulations are now run at 9 km, and further grid refinement up to 5 km is expected before 2025: higher resolution would be needed to resolve the formation of stratocumulus clouds (hundreds of meters at least), preventing a numerical resolution of the STBL for a few decades at least. In order to provide a reasonable representation of the stratocumulus clouds dynamics in GCMs, an accurate parameterization of the STBL is mandatory. And in order to do so, a comprehensive knowledge of the dynamics and more specifically of the turbulent fluxes within the STBL must be achieved. To this day, no systematic study has investigated updrafts, downdrafts and entrainment in a unique framework and has evaluated each contribution to the overall turbulent transport of the STBL. Nonetheless, the identification of convective structures - including the role of entrainment - is of key importance to improve our knowledge and correctly parametrize stratocumulus clouds. COGNAC has been developed during these two years in this direction: aiming at assessing how the turbulent fluxes evolve and control the dynamics of the STBL, which is the key to govern stratocumulus cloud cover. This has been done in order to put the ground for the development of a new parametrization of STBL convection able to reduce the current bias and incertitude in the GCMs simulation of a such important element of the climate system.

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