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Lawrence Coy, Paul A. Newman, Steven Pawson, and Leslie R. Lait

involves some combination of stratospheric wave breaking and direct tropospheric forcing that we plan to investigate in future modeling studies. Figure 13 shows the evolution of this feature in EPV on the 530-K potential temperature surface at 5-day intervals. The winter polar vortex (red shading) displayed a strong wavenumber 2 pattern on 31 January 2016 ( Fig. 13a ) that interacted with the tropical EPV (green shading) near 90°E. This produced an intrusion of subtropical air (transparent shading

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Lawrence Coy, Krzysztof Wargan, Andrea M. Molod, William R. McCarty, and Steven Pawson

. Using two reanalyses provides additional insight into how modeling and assimilation development can affect the balance between physical forcing mechanisms and the observations in producing the QBO. The plan of the paper is as follows: First, section 2 introduces the MERRA-2 system and the methods used to isolate the QBO signal in the reanalysis. Next, in section 3 , the QBO circulations are shown, followed in section 4 by the results showing the QBO signal as seen in the zonal momentum and

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Krzysztof Wargan and Lawrence Coy

Hemisphere in 2004, 2006, 2009 (discussed in detail in previous subsections), 2010, and 2013. The choice of these recent events is motivated by the fact that MERRA-2 starts assimilating AIRS radiance data in late 2002 and there are no hyperspectral radiance observations available before then. In addition, the GPS radio occultation data are not assimilated until the summer of 2004. In the future, we plan to expand our analysis over the entire MERRA-2 period (1980–present), but first a careful evaluation

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Rolf H. Reichle, Clara S. Draper, Q. Liu, Manuela Girotto, Sarith P. P. Mahanama, Randal D. Koster, and Gabrielle J. M. De Lannoy

-00170.1 Dirmeyer , P. , and T. Oki , 2002 : The Second Global Soil Wetness Project (GSWP-2) science and implementation plan. IGPO Publ. 37, 64 pp. Dorigo , W. A. , and Coauthors , 2011 : The International Soil Moisture Network: A data hosting facility for global in situ soil moisture measurements . Hydrol. Earth Syst. Sci. , 15 , 1675 – 1698 , doi: 10.5194/hess-15-1675-2011 . 10.5194/hess-15-1675-2011 Entekhabi , D. , R. H. Reichle , R. D. Koster , and W. T. Crow , 2010 : Performance

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

.1029/2004EO190002 Guenther , B. , and Coauthors , 1996 : MODIS calibration: A brief review of the strategy for the at-launch calibration approach . J. Atmos. Oceanic Technol. , 13 , 274 – 285 , doi: 10.1175/1520-0426(1996)013<0274:MCABRO>2.0.CO;2 . 10.1175/1520-0426(1996)013<0274:MCABRO>2.0.CO;2 Hayes , S. P. , and Coauthors , 1986 : The Equatorial Pacific Ocean Climate Studies (EPOCS) Plans: 1986–1988 . Eos, Trans. Amer. Geophys. Union , 67 , 442 – 444 , doi: 10.1029/EO067i018p00442 . 10

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Rolf H. Reichle, Q. Liu, Randal D. Koster, Clara S. Draper, Sarith P. P. Mahanama, and Gary S. Partyka

model background through 2014 and the GEOS-5 FP-IT product version 5.9.1 ( Lucchesi 2015 ) thereafter. (The current plan is to use FP-IT version 5.12.4 starting in 2017.) For a given day (or pentad) and 0.5° (or 2.5°) grid cell, the correction factor used in the downscaling is the ratio of the observed to the (aggregated) background precipitation. This factor is then applied uniformly to all background (hourly) time steps and grid cells that make up the given day (or pentad) and 0.5° (or 2.5°) grid

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Kevin Hodges, Alison Cobb, and Pier Luigi Vidale

Shao , H. , and Coauthors , 2016 : Bridging research to operations transitions: Status and plans of community GSI . Bull. Amer. Meteor. Soc. , 97 , 1427 – 1440 , doi: 10.1175/BAMS-D-13-00245.1 . 10.1175/BAMS-D-13-00245.1 Strachan , J. , P. L. Vidale , K. Hodges , M. Roberts , and M.-E. Demory , 2013 : Investigating global tropical cyclone activity with a hierarchy of AGCMs: The role of model resolution . J. Climate , 26 , 133 – 152 , doi: 10.1175/JCLI-D-12-00012.1 . 10

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

:// . 10.5194/hess-17-1661-2013 NEWS Science Integration Team , 2007 : A NASA Earth science implementation plan for energy and water cycle research: Predicting energy and water cycle consequences of Earth system variability and change. NASA Energy and Water Cycle Study Rep., 89 pp., . Reichle , R. , and Q. Liu , 2014 : Observation-corrected precipitation estimates in GEOS-5. NASA Tech. Memo. NASA/TM-2014-104606, Vol

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Gloria L. Manney and Michaela I. Hegglin

), and there are additional uncertainties in the asymmetry between the hemispheres and the seasonality of the expansion rates (e.g., Lucas et al. 2014 ). Several studies suggest strong regional variations in tropical width, including regions of narrowing rather than widening (e.g., Lucas et al. 2012 ; Peña-Ortiz et al. 2013 ; Lucas and Nguyen 2015 ). Robust information on regional variations and long-term changes is crucial for planning and climate change adaptation. The annual and/or zonal

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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

also capable of assimilating microwave and hyperspectral infrared radiances from planned future satellites, including MetOp-C and JPSS. The temporary spike in the number of QuikSCAT data assimilated in MERRA-2 in late 2000 is due to an error in preprocessing, which led to observations beyond the midswath “sweet spot” being used in the analysis. This has no discernible impact on the quality of the analyzed fields or on the use of other observations in the assimilation system. Fig . 1. Observations

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