Assimilating Earth System Observations at NASA:
MERRA and Beyond
Siegfried Schubert, Michael Bosilovich, Michele Rienecker, Max Suarez, Ron Gelaro, RandyKoster, Julio Bacmeister, Ricardo Todling, Larry Takacs, Emily Liu, Gi-Kong Kim, Rob
Lucchesi, Man-Li Wu, Phil Pegion, Myong-In Lee, Junye Chen, Steve Bloom, Rolf Reichle,
Steven Pawson, Ivanka Stajner, Arlindo da Silva, Christian Keppenne, Watson Gregg, andothers in the GMAO
Global Modeling and Assimilation Office, NASA/GSFC, USA
Correspondence: Siegfried.D.Schubert@nasa.gov

INTRODUCTION
This report provides an overview of the status and progress of the NASA/Global Modeling andAssimilation Office (GMAO) atmospheric global reanalysis project called the Modern Era
Retrospective-Analysis for Research and Applications (MERRA). The report also includes a brief
overview of NASA’s emerging capabilities for assimilating a variety of other Earth Scienceobservations of the land, ocean, and atmospheric constituents.
MERRA supports NASA Earth science by synthesizing the current suite of research satellite
observations in a climate data context (covering the period 1979-present), and by providing thescience and applications communities with of a broad range of weather and climate data with anemphasis on improved estimates of the hydrological cycle.
MERRA
MERRA is based on a major new version of the Goddard Earth Observing System DataAssimilation System (GEOS-5), that includes the Earth System Modeling Framework(ESMF)-based GEOS-5 AGCM (e.g., Collins et al., 2005) and the new NCEP unified grid-point
statistical interpolation (GSI) analysis scheme (e.g., Derber et al., 2003) developed as acollaborative effort between NCEP and the GMAO. The MERRA system employs an incrementalanalysis update (IAU) procedure (Bloom et al., 1996) that eliminates the shocks that are otherwise
associated with the insertion of the observations (see Fig. 1). The GEOS-5 AGCM includes afinite-volume dynamical core (Lin, 2004), model physics as described in Bacmeister et al. (2006) a
new catchment land surface model (e.g., Koster et al., 2000), prescribed aerosols, and interactiveozone. The GSI assimilates radiances directly using the JCSDA community radiative transfer
model, and includes a variational bias correction to the radiances. The native resolution of MERRA
is 2⁄3 degree longitude by 1⁄2 degree latitude with 72 levels extending to 0.01 hPa. Further details
are provided by Bosilovich et al. (2006).
MERRA products consist of a host of prognostic and diagnostic fields including comprehensive setsof cloud, radiation, hydrological cycle, ozone, and land surface diagnostics. A special collectionof data files are designed to facilitate off-line forcing of chemistry/aerosol models. Analysis statesand 2-dimensional diagnostics are made available hourly and at the native resolution, while many of
the three-dimensional diagnostics are available 3 hourly on a coarser 1.25° latitude ×1.25° longitudegrid. The MERRA data collections will be available from an online data server, the Modeling andAssimilation Data and Information Services Center (MDISC) at
http://daac.gsfc.nasa.gov/MDISC/index.shtml.
MERRA processing consists of three streams that begin in 1979, 1989, and 1998. Each stream
includes a spin-up period that starts three years prior to the start date, consisting of a two year
coarse (2°) resolution spin-up period, followed by a one-year ½° spin-up period. Prior to the start ofproduction, we have conducted a preliminary validation of the system by conducting analyses for2004 and for some other selected periods to investigate issues associated with the changing
observation data base. The 2° system is continuing as a data sweep to identify any issues
associated with the staging of the input data stream. An initial pass through the data at the coarse
resolution has also been used to initialize the satellite bias estimates for the ½° system. In thefollowing, we show a few results from the ½° spin-up runs and from our 2004 validation runs.
Figure 2 compares the globally-averaged precipitation for the observations and the various
reanalysis products. While the observations are fairly constant throughout the period, the analyses
all show some tendency for a trend with increasing precipitation during the last decade. All therecent products (ERA40, NCEP/DOE R2 and JRA25) have global values considerably larger thanthe observed. MERRA is an exception, producing values very close to the observed products,
although there is also an indication of a trend or shift towards larger values during the recent twodecades. Figure 3 shows the seasonal evolution of the tropical precipitation during 2004. Acomparison with the GPCP product shows that MERRA captures the seasonal evolution quite well,including such fine structure as the split in the ITCZ during Northern spring, and changing structureof the anomalies in the Pacific warm pool.
The above results are a small sample of an extensive validation effort that was carried out within theGMAO to ensure that the MERRA system was ready for production. These results, together withan evaluation carried out by an external user’s group, showed that the MERRA product will be (inmany aspects) a substantial improvement over existing products. The strengths of the MERRA
product compared to the existing global reanalysis products include: increased horizontal resolution,a well-resolved stratosphere, improved precipitation and hydrological cycle, and exact (closed)
vertically integrated budgets. Known problems include possible impacts of changes in theobserving system (e.g., artificial trends in the precipitation), and deficiencies in the phasing of thediurnal cycle.
It is anticipated that the MERRA production will be completed in the middle of 2009.
information about MERRA can be found at: http://gmao.gsfc.nasa.gov/research/merra/

Further

EARTH SYSTEM ANALYSISIn addition to MERRA, the GMAO has systems for ocean and land surface assimilation and isdeveloping new capabilities in aerosol and constituent assimilation, and ocean biology assimilation(see Fig. 4). This includes the development of an assimilation capability for tropospheric air quality

monitoring and prediction, the development of a carbon-cycle modeling and assimilation system,
and an ocean data assimilation system for use in coupled short-term climate forecasting. The
long-term goal of the GMAO is the development of a fully Integrated Earth System Analysis
capability.

Figure 1: The incremental analysis update (IAU) procedure used in the GEOS-5 Data Assimilation
System.
--- GPCP, ---CMAP, --- ERA40, --- NCEP R1, --- NCEP R2, and --- JRA25.

Figure 2: The globally-averaged precipitation from MERRA spin-up and validation runs (blacksquares), observations (GPCP, CMAP) and other reanalyses. Units: mm/day.

Figure 3: Distribution of the 2004 monthly mean tropical precipitation from the MERRAvalidation runs (left panel), and GPCP (right panel). The red (blue) numbers are the spatial
correlations (rms) between the two products. The black numbers are the tropical mean values.
Units: mm/day.

Jan 04 Precipitation in MERRA

Assimilation of AMSR - E soil moistureretrievals

Assimilation of AURA/MLS and OMI Ozone

Assimilation of TOPEX/Jason Altimeter

Aerosol Transport

Assimilated Chlorophyll Apr 1 2001

Data

Figure 4: Moving towards an Integrated Earth System analysis at the GMAO. The individualpanels highlight some of the other efforts within the GMAO (besides MERRA) to assimilate soil
moisture, various constituents including ozone, aerosols, altimeter data, and chlorophyll.

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