Maintaining atmospheric mass and water balance in reanalyses

This study describes the modifications made to the Goddard Earth Observing System (GEOS) Atmospheric Data Assimilation System (ADAS) to conserve atmospheric dry‐air mass and to guarantee that the net source of water from precipitation and surface evaporation equals the change in total atmospheric water. The modifications involve changes to both the atmospheric model and the analysis procedure. In the model, sources and sinks of water are included in the continuity equation; in the analysis, constraints are imposed to penalize (and thus minimize) analysis increments of dry‐air mass. Finally, changes are also required to the Incremental Analysis Update (IAU) procedure. The effects of these modifications are evaluated separately in free‐running and assimilation experiments. Results are also presented from a multiyear reanalysis (Version 2 of the Modern Era Retrospective‐Analysis for Research and Applications: MERRA‐2) that uses the modified system. Many studies have shown that recent reanalyses (e.g., NASA's Global Modeling and Assimilation Office (GMAO) Modern Era Retrospective‐Analysis for Research and Applications: MERRA, and European Centre for Medium‐Range Weather Forecasts: ERA‐Interim) contain spurious trends in globally integrated dry‐air mass, as well as large variations in globally integrated total precipitation associated with changes in the satellite observing systems. In this study we examine modifications to GMAO's Goddard Earth Observing System‐Version 5 (GEOS‐5) Data Assimilation System (DAS) which preserves total dry‐air mass and maintains the natural global balance between total precipitation and surface evaporation. Results are shown from targeted historical periods, as well as from GMAO's 35‐year MERRA‐2 Reanalysis.