Continental radiative-convective equilibrium experiments in a single column model (LMDZ5B GCM)

The radiative-convective instability results both from (i) the average net cooling experienced by the Earth's atmosphere (~ 110 W/m?) and (ii) from the equivalent warming of the Earth's surface. Ultimately, this drives the Earth atmosphere to a radiative-convective equilibrium (RCE) state, in a sense that, at the global scale, surface fluxes and radiative cooling compensate each other. Since the convection time scale (i.e. some hours) is much shorter than the radiation one (i.e. about 40 days), the resulting global temperature lapse rate is generally closer to the moist adiabat than to the dry adiabat. This is especially true over the tropics, where moist convection is in near-equilibrium. The RCE is then often used as a common approximation of the tropical mean state. It has been extensively used over oceans in SCMs (Single Column Models), as well as in CRMs (Cloud Resolving Models), to investigate the tropical moist convection sensitivity (i) to boundary conditions (e.g. SST, surface wind, drag coefficient, etc...) and (ii) to atmospheric conditions (e.g. radiative cooling, wind shear, tropospheric humidity, etc...). Nevertheless, to our knowledge the present study is the first one investigating the RCE over a continental surface. Indeed, in the present study, the single column version of the LMDZ GCM (LMDZ5B, from the Laboratoire de Meteorologie Dynamique) is ran to RCE, with a coupled land surface both in terms of temperature and moisture. This continental RCE demonstrates very different sensitivity compared to its oceanic counterpart in particular because of the large- amplitude heat flux diurnal cycle, which is shown to strongly impact the equilibrium state. Sensitivity studies (i) to solar forcing (latitude) (ii) to total water content, and (iii) to the initial conditions are performed to study the different equilibrium states, with a particular focus on the role of clouds. We also performed a bifurcation diagram. Low-level clouds and fog are shown to be key for the equilibrium since they strongly modify the diurnal course of heat at the land surface. Total water content is shown to mostly impact the soil water budget but, interestingly, the surface energy budget and atmospheric moisture is not too much sensitive to the initial total water content; that is, for a given latitude, the atmospheric precipitable water tends toward a single attractor.