The new state-of-the-art CMIP6 climate model simulations will be the foundation for the upcoming IPCC AR6 and need rigorous evaluation. Major efforts have been made in the community in the past years to facilitate the CMIP climate model evaluation, such as the Obs4MIPs that collects, organizes and publishes satellite and reanalysis datasets and the COSP that develops instrument simulators. This session aims to bring together scientists who work in the Obs4MIPs, the COSP, and the CMIP6 climate models for their latest results. This session invites presentations in the following areas: (1) Description and analysis of Obs4MIPs and other observational data sets; (2) Global and regional evaluation of CMIP climate models (particularly CMIP6) in their mean state or variability at various time scales (e.g., diurnal, intraseasonal, interannual, and decadal) using observations and COSP; (3) Process-oriented modeling studies and diagnostics to identify and understand the nature and causes of systematic model errors.