On the importance of dry and cloudy boundary layer convection and of its parameterization in climate models

The intrinsic failure of eddy-diffusion parameterizations in representing upward transport of heat in the convective boundary layer, recognized since the 70s, has lead to various propositions of parameterizations like counter-gradient terms and third order closures to account for the asymmetry of the vertical transport. An approach that is now well recognized consists in combining a mass flux parameterization of the organized structures of the convective boundary layer with a local TKE closure for small scale turbulence. The idea traces back to a proposition by Chatfield and Brost (1987) and is since often referred to as the Eddy Diffusion Mass Flux (EDMF) approach. The “thermal plume model” developed for LMDZ was the first EDMF scheme published and tested in a climate model (Hourdin et al., 2002). It was first introduced in the LMDZ5B atmospheric component of the IPSL model for CMIP5. However, this first version suffered from youth problems. It is only for CMIP6A, about 20 years after the development of the parameterization, that a first satisfactory version of the model was delivered. Through years, and more often with this last version, the key role of the representation of shallow convection on many component of the system has been realized: 1) the ventilation of air by the subsiding air around thermal plumes dries the surface, reinforcing the near surface evaporation. Representing this convection correctly both over trade winds and subsiding regions in the tropics, together with the associated cumulus and stratocumulus clouds, is one of the key for the reduction of the East Tropical Ocean warm bias; 2) the preconditioning of the deep convection by a phase of shallow convection is a key for a correct representation of the phasing of the diurnal cycle of convective rainfall over continents; 3) the strong diurnal cycle of the convective boundary layer in desert areas is essential to well represent the maximum of near surface wind in the morning, responsible for a maximum of dust emission, when the momentum of the nocturnal low level jet is brought suddenly back toward the surface when reached by the developing dry convection. 4) the thermal plume model being active about on half of the globe all the time, it controls the transport of all trace elements, with some non linear effects when the emissions themselves show a diurnal cycle. In this presentation, we review these lessons learned with LMDZ, identify the issues which should require further developments, and expose how new machine assisted techniques allow to reconcile improvement of parameterizations at process scale and climate model improvement.

Chatfield, R. B., & Brost, R. A. (1987). A two-stream model of the vertical transport of trace species in the convective boundary layer. Journal of Geophysical Research, 92, 13,263–13,276

Hourdin, F., Couvreux, F., & Menut, L. (2002). Parameterisation of the dry convective boundary layer based on a mass flux representation of thermals. Journal of the Atmospheric Sciences, 59, 1105–1123