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Richard I. Cullather and Sophie M. J. Nowicki

1. Introduction The Greenland Ice Sheet (GrIS) represents a significant store of freshwater that may be intermittently released into the global ocean. Recent studies using satellite-derived mass change have indicated the GrIS has made a substantial contribution to global sea level in the past decades ( Shepherd et al. 2012 ). Historically, it has been thought that this contribution has been equally divided between iceberg calving and surface processes (e.g., van den Broeke et al. 2009

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Laura M. Hinkelman

-of-atmosphere (TOA) radiative fluxes and surface temperature in the context of climate variability. Applications of MERRA data for polar studies include the work of La and Park (2016) on the effect of clouds on phytoplankton in Antarctica, as well as a paper by Letterly et al. (2016) concerning the impact of winter clouds on summer sea ice in the Arctic. Hundreds of other papers relying on MERRA data are in the literature. While use of MERRA continues, the number of studies based on MERRA-2 has grown

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Ronald Gelaro, Will McCarty, Max J. Suárez, Ricardo Todling, Andrea Molod, Lawrence Takacs, Cynthia A. Randles, Anton Darmenov, Michael G. Bosilovich, Rolf Reichle, Krzysztof Wargan, Lawrence Coy, Richard Cullather, Clara Draper, Santha Akella, Virginie Buchard, Austin Conaty, Arlindo M. da Silva, Wei Gu, Gi-Kong Kim, Randal Koster, Robert Lucchesi, Dagmar Merkova, Jon Eric Nielsen, Gary Partyka, Steven Pawson, William Putman, Michele Rienecker, Siegfried D. Schubert, Meta Sienkiewicz, and Bin Zhao

conservation and water balance ( section 2f ), the precipitation used to force the land surface and drive wet aerosol deposition ( section 2g ), the boundary conditions for sea surface temperature and sea ice concentration ( section 2h ), and reanalysis production ( section 2i ). a. Forecast model Since MERRA, the GEOS model has undergone changes to both its dynamical core and its physical parameterizations. Whereas in MERRA the horizontal discretization of the model was computed on a latitude

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Young-Kwon Lim, Robin M. Kovach, Steven Pawson, and Guillaume Vernieres

Tropical Rainfall Measuring Mission (TRMM) and Integrated Multisatellite Retrievals for GPM (IMERG; Huffman et al. 2007 , 2015 ) for precipitation. As the TRMM satellite mission was terminated in 2015, the transition from the TRMM data products to the Global Precipitation Measurement (GPM) mission products has begun to produce GPM-era IMERG datasets. We use IMERG precipitation data for 2015/16 and TRMM data for the 1997/98 El Niño. SST data are obtained from both the Hadley Centre Sea Ice and SST

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Kevin Hodges, Alison Cobb, and Pier Luigi Vidale

coupled atmosphere–ocean–land surface–sea ice reanalysis. NCEP-CFSR, MERRA, and MERRA-2 all use different versions of the 3D variational data assimilation (3D-Var) scheme: the Grid-point Statistical Interpolation (GSI) scheme ( Shao et al. 2016 ). For MERRA and MERRA-2 the Incremental Analysis Update (IAU; Bloom et al. 1996 ; Rienecker et al. 2011 ) system is also used. The data period used for all the reanalyses is 1979–2012, except for MERRA-2, which starts in 1980. A key difference between the

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Clara S. Draper, Rolf H. Reichle, and Randal D. Koster

but excludes inland seas. By contrast, the land estimates from MERRA-2, MERRA-Land, and MERRA apply to the area modeled by the land surface model, excluding land ice, lakes, and inland seas. The discrepancy due to the inclusion or exclusion of land ice is significant: land ice accounts for 10% of the continental area, with Antarctica making up 95% of this. NEWS provides energy budgets for each continent separately ( L’Ecuyer et al. 2015 ), and we use their (balance constrained) energy budget

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Michael G. Bosilovich, Franklin R. Robertson, Lawrence Takacs, Andrea Molod, and David Mocko

present. While several uncertainties exist, including greater uncertainty before routine microwave observations were included in late 1987 and also some variations when those become limited, the estimates are as reliable as we have for the entire globe. Remote Sensing Systems (2016) provides total precipitable water observations using SSM/I instruments over ice-free ocean areas. Mears and Wentz (2009) describe the RSS observations of the temperature of the lower troposphere, available globally

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Allison B. Marquardt Collow and Mark A. Miller

constraints that were heretofore neglected ( Reichle and Liu 2014 ; Molod et al. 2015 ; Takacs et al. 2016 ). An additional improvement to MERRA-2 is the assimilation of aerosol optical depth from MODIS and the Aerosol Robotic Network (AERONET) through the Goddard chemistry, aerosol, radiation, and transport model to allow for a simulation of dust, sea salt, black carbon, organic carbon, and sulfate ( Buchard et al. 2015 ). The Multiangle Imaging SpectroRadiometer (MISR) is also used to assimilate

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Franklin R. Robertson, Michael G. Bosilovich, and Jason B. Roberts

) satellite mission ( Tapley et al. 2004 ) and to recover a P − ET estimate with RO measurements from river and streamflow gauges. Although GRACE measurements are a unique resource for enabling this approach, those data exist only since 2003. Another source of information may come from reduced observation reanalyses that, in addition to SST, sea ice, and radiative constituents, also assimilate surface pressure observations ( Compo et al. 2011 ) and marine wind speeds ( Poli et al. 2015 ). While these

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Gloria L. Manney and Michaela I. Hegglin

.1007/s00382-011-1080-7 . 10.1007/s00382-011-1080-7 Screen , J. A. , and I. Simmonds , 2014 : Amplified mid-latitude planetary waves favour particular regional weather extremes . Nat. Climate Change , 4 , 704 – 709 , doi: 10.1038/nclimate2271 . 10.1038/nclimate2271 Screen , J. A. , I. Simmonds , C. Deser , and R. Tomas , 2013 : The atmospheric response to three decades of observed Arctic sea ice loss . J. Climate , 26 , 1230 – 1248 , doi: 10.1175/JCLI-D-12-00063.1 . 10

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