This work is motivated by the identification of the land-atmosphere interactions as one of the key sources of uncertainty in climate change simulations. It documents new developments in related processes, namely, boundary layer/convection/clouds parameterizations and land surface parameterization in the Earth System Model of the Institut Pierre Simon Laplace (IPSL). Simulations forced by prescribed oceanic conditions are produced with different combinations of atmospheric and land surface parameterizations. They are used to explore the sensitivity to the atmospheric physics and/or soil physics of • major biases in the near surface variables over continents, • the energy and moisture coupling established at the soil/atmosphere interface in not too wet (energy limited) and not too dry (moisture limited) soil moisture regions also known as transition or "hot-spot" regions, • the river runoff at the outlet of major rivers. The package implemented in the IPSL-Climate Model for the Phase 6 of the Coupled Models Intercomparison Project (CMIP6) allows us to reduce several biases in the surface albedo, the snow cover, and the continental surface air temperature in summer as well as in the temperature profile in the surface layer of the polar regions. The interactions between soil moisture and atmosphere in hotspot regions are in better agreement with the observations. Rainfall is also significantly improved in volume and seasonality in several major river basins leading to an overall improvement in river discharge. However, the lack of consideration of floodplains and human influences in the model, for example, dams and irrigation, impacts the realism of simulated discharge. Plain Language Summary Land surface-atmosphere interactions play an essential role in the climate system. They strongly modulate the regional climates and have impacts on the global scale for instance through freshwater release into the oceans. Climate hazards (heat waves, droughts) and their impacts on populations also strongly depend on interactions between land and atmosphere and on their evolution with climate change. Climate models are precious tools to investigate how the Earth climate behaves. The sixth phase of the Climate Model Intercomparison Project (CMIP6) provides important tools to measure the progress and address the remaining open questions regarding the continental climate modeling. The representation of the land-atmosphere coupled system by the IPSL-Climate Model involved in CMIP6 is thoroughly evaluated against observations and compared with simulations using the CMIP5 version. Several biases concerning the temperature over land and over the ice sheets and with the snow cover are significantly reduced. Numerous improvements were made developping advanced parameterizations and tuning of the radiation and of the turbulent mixing in the atmospheric model. The realism of the seasonal

Abstract. We investigate how the representation of the boundary layer in a climate model impacts the representation of the near-surface wind and dust emission, with a focus on the Sahel/Sahara region. We show that the combination of vertical turbulent diffusion with a representation of the thermal cells of the convective boundary layer by a mass flux scheme leads to realistic representation of the diurnal cycle of wind in spring, with a maximum near-surface wind in the morning. This maximum occurs when the thermal plumes reach the low-level jet that forms during the night at a few hundred meters above surface. The horizontal momentum in the jet is transported downward to the surface by compensating subsidence around thermal plumes in typically less than 1 h. This leads to a rapid increase of wind speed at surface and therefore of dust emissions owing to the strong nonlinearity of emission laws. The numerical experiments are performed with a zoomed and nudged configuration of the LMDZ general circulation model coupled to the emission module of the CHIMERE chemistry transport model, in which winds are relaxed toward that of the ERA-Interim reanalyses. The new set of parameterizations leads to a strong improvement of the representation of the diurnal cycle of wind when compared to a previous version of LMDZ as well as to the reanalyses used for nudging themselves. It also generates dust emissions in better agreement with current estimates, but the aerosol optical thickness is still significantly underestimated.

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