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Introduction to MERRA

Retrospective-analyses (or reanalyses) have been a critical tool in studying weather and climate variability for the last 15 years. Reanalyses blend the continuity and breadth of output data of a numerical model with the constraint of vast quantities of observational data. The result is a long-term continuous data record. The Modern Era Retrospective-analysis for Research and Applications was developed to support NASA's Earth science objectives, by applying the state-of-the-art GMAO data assimilation system that includes many modern observing systems (such as EOS) in a climate framework.

The MERRA time period will cover the modern era of remotely sensed data, from 1979 through the present, and the special focus of the atmospheric assimilation will be the hydrological cycle. Previous long-term reanalyses of the Earth's climate had high levels of uncertainty in precipitation and inter-annual variability. The GEOS5 data assimilation system to be used for MERRA implements Incremental Analysis Updates (IAU) to slowly adjust the model states toward the observed state. The water cycle benefits as unrealistic spin down is minimized. In addition, the model physical parameterizations have been tested and evaluated with data assimilation present which also reduces the shock of adjusting the model system. Along with the Catchment hydrology land surface model, MERRA is anticipated to improve upon the representation of the water cycle in reanalyses.

In addition, the 72 vertical levels in GEOS5 extend through the stratosphere. A special data product, developed in conjunction with the chemistry community will support chemistry transport modeling. The disseminated data products will provide much improved initial conditions for predicting weather and other subseasonal variability that is strongly linked to tropical moisture. Studies of climate variability rely on reanalysis data sets. Limited domain models use reanalyses to provide the boundary forcing and initial conditions for mesoscale and regional climate simulations.

MERRA output data will resemble existing reanalyses, with several key advances. The use of IAU also allows output at higher frequencies than the 6 hourly analyses. Two dimensional diagnostics (surface fluxes, single level meteorology, vertical integrals and land states) will be produced at 1 hour intervals. These data products and the 6 hourly three dimensional atmospheric analyses are also available at the full spatial resolution (1/2 degrees Latitude × 2/3 degrees Longitude). Extensive three dimensional 3 hourly atmospheric diagnostics on 42 pressure levels will also be available, but at the coarse (1.25 degree) resolution. The Goddard Earth Sciences Data Information Services Center (GES DISC) is developing the utilities for users to access and subset the MERRA data products.

MERRA is funded by the NASA Modeling Analysis and Prediction (MAP) program and by a NASA Cooperative Agreement Notice (CAN): Earth Science REASoN - Research, Education, and Applications Solutions Network.

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