The Modern-Era Retrospective analysis for Research and Applications for the early 21st century (MERRA-21C) is NASA’s global atmospheric reanalysis beginning in 1998, building on MERRA-2 with approximately 25-km horizontal resolution, hybrid four-dimensional ensemble-variational data assimilation, and updated atmospheric and aerosol modeling, while providing hourly products and specialized collections for planetary boundary layer (PBL) studies, the Fire Weather Index, and ensemble-based uncertainty estimates for selected variables.