The Nonhydrostatic ICosahedral Atmospheric Model for CMIP6 HighResMIP simulations (NICAM16-S): experimental design, model description, and impacts of model updates

Abstract

Abstract. The Nonhydrostatic ICosahedral Atmospheric Model\n(NICAM), a global model with an icosahedral grid system, has been under\ndevelopment for nearly two decades. This paper describes NICAM16-S, the\nlatest stable version of NICAM (NICAM.16), modified for the Coupled Model\nIntercomparison Project Phase 6, High Resolution Model Intercomparison\nProject (HighResMIP). Major updates of NICAM.12, a previous version used\nfor climate simulations, included updates of the cloud microphysics scheme\nand land surface model, introduction of natural and anthropogenic aerosols\nand a subgrid-scale orographic gravity wave drag scheme, and improvement of\nthe coupling between the cloud microphysics and the radiation schemes.\nExternal forcings were updated to follow the protocol of the HighResMIP. A\nseries of short-term sensitivity experiments were performed to determine and\nunderstand the impacts of these various model updates on the simulated mean\nstates. The NICAM16-S simulations demonstrated improvements in the ice water\ncontent, high cloud amount, surface air temperature over the Arctic region,\nlocation and strength of zonal mean subtropical jet, and shortwave radiation\nover Africa and South Asia. Some long-standing biases, such as the double\nintertropical convergence zone and smaller low cloud amount, still exist or\nare even worse in some cases, suggesting further necessity for understanding\ntheir mechanisms, upgrading schemes and parameter settings, and\nenhancing horizontal and vertical resolutions.