Announcement: Significant change to MERRA-2 observing system

[GES DISC - jimacker]

This is a notification of a significant change to the Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA-2) observing system. As of June 16, 2025, MERRA-2 no longer assimilates AMSU-A radiances from NOAA-15, NOAA-18 , and NOAA-19, all of which have been decommissioned by NOAA. To ameliorate the impact of this loss, the following data types were added to the MERRA-2 assimilation on the same date:

• IASI, AMSU-A, and MHS radiances from Metop-C
• ATMS radiances from NOAA-20 and NOAA-21
• GNSS-RO (radio occultations) from COSMIC-2 and Metop-C

All these data types are closely related to ones previously assimilated in MERRA-2 whose impacts have been tested and are well understood. While there may be a small discernible discontinuity in the products from this modification, particularly in the middle stratosphere, our tests indicate that the discontinuity would have been greater without their addition. In a few weeks, the GMAO will post a more detailed summary of the change and the impact on MERRA-2 fields on our web site and on the MERRA-2 documentation pages.