Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2)


NASA’s Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), is an atmospheric reanalysis designed to provide an intermediate dataset and bridge between the first MERRA reanalysis and the project’s long-term goal of producing a coupled Earth system reanalysis. MERRA-2 incorporates system changes and fundamental developments in modeling and data assimilation, including 1) assimilation of aerosol observations that can interact with atmospheric radiative processes; 2) constraining mass conservation even with the analysis of water vapor, allowing a global balance between evaporation and precipitation; 3) use of a cube sphere to reduce the effect of gridpoint singularities at the pole, allowing for improved polar circulation; 4) an updated radiative transfer model to permit the assimilation of data from many more instruments than could have been included in MERRA; and 5) inclusion of new observational forcing for the land model to provide more stable land feedback processes. This special collection documents the performance of MERRA-2 and addresses key research questions in large-scale climate and weather.

An overview article of the MERRA-2 project can be found here.

Collection organizer:
Dr. Michael G. Bosilovich, NASA Goddard Space Flight Center

Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2)

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Krzysztof Wargan and Lawrence Coy


The behavior of the tropopause inversion layer (TIL) during the 2009 sudden stratospheric warming (SSW) is analyzed using NASA’s Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), and short-term simulations with the MERRA-2 general circulation model. Consistent with previous studies, it is found that static stability in a shallow layer above the polar tropopause sharply increases following the SSW, leading to a strengthening of the high-latitude TIL. Simultaneously, the height of the thermal tropopause decreases by around 1 km. Similar behavior is also detected during other major SSW events between the years 2004 and 2013. Using an ensemble of general circulation model forecasts initialized from MERRA-2, it is demonstrated that the primary cause of the strengthening of the TIL is an increased convergence of the vertical component of the stratospheric residual circulation in response to an SSW-induced acceleration of the mean downward motion between 75° and 90°N. In addition, ~6% of the strengthening in 2009 is attributed to an enhanced anticyclonic circulation at the tropopause. A preliminary analysis indicates that during other recent SSW events there was a significant increase in the convergence of the vertical residual wind velocity throughout the middle and lower stratosphere. The static stability increase simulated by the model during the 2009 SSW is 60%–80% of that seen in MERRA-2. The underestimate is traced back to a tendency for the forecasts to underestimate the resolved planetary wave forcing on the stratosphere compared to the reanalysis.

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