Global Downscaling of the 50-km MERRA2 Reanalysis to 12.5-km with GEOS-5: Methodology

Abstract:

The GEOS-5 system has a "Replay" capability (akin to nudging) that adds a forcing term to the model equations to constrain it to follow a specified trajectory. This capability is adapted here to produce a downscaled version of an existing, relatively low-resolution reanalysis. The presentation will describe the Replay methodology in detail, particularly how it differs from nudging and how it relates to the way the underlying MERRA-2 reanalysis was generated. The adaptation made for downscaling consists of modifying Replay to constrain only the longer scales of the flow, while allowing the model to produce its own mesoscale. This adaptation will be described and our choice for the shortest constrained length scale will be justified.