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Natalie P. Thomas, Michael G. Bosilovich, Allison B. Marquardt Collow, Randal D. Koster, Siegfried D. Schubert, Amin Dezfuli, and Sarith P. Mahanama

. Mon. Wea. Rev. , 96 , 833 – 850 , https://doi.org/10.1175/1520-0493(1968)096<0833:COTLLJ>2.0.CO;2 . 10.1175/1520-0493(1968)096<0833:COTLLJ>2.0.CO;2 Bosilovich , M. G. , and Coauthors , 2015 : MERRA-2: Initial evaluation of the climate. NASA Tech. Memo. NASA/TM-2015-104606/Vol. 43, 145 pp. , https://gmao.gsfc.nasa.gov/pubs/docs/Bosilovich803.pdf . Budikova , D. , T. W. Ford , and T. J. Ballinger , 2019 : United States heat wave frequency and Arctic ocean marginal sea ice

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Mohar Chattopadhyay, Will McCarty, and Isaac Moradi

1. Introduction Microwave temperature sounders provide key information on global temperature to the initial conditions of numerical weather prediction models via the assimilation of their radiance measurements. These observations are core to the global observing system, as they fill in key observing gaps that would otherwise exist. They are complementary to conventional observations of temperature, which are spatially sparse over much of the globe—particularly over oceans. The bulk of

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

during the Northern Hemisphere (NH) winter of 2015/16 ( Newman et al. 2016 ; Osprey et al. 2016 ), and several features of this singular disruption imply that a different mechanism may have been responsible for the disrupting accelerations than the vertically propagating waves responsible for the QBO. Most noticeably, anomalous easterly accelerations occurred in the center of the QBO westerlies, a region of weak vertical wind shear, rather than in the strong vertical wind shear regions as has been

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

on deep convection as part of the relaxed Arakawa–Schubert (RAS; Moorthi and Suarez 1992 ) convective scheme], turbulence, land and ocean surface, and gravity wave drag. In addition, the assimilation of interactive aerosols was implemented into the system, a feature of the Earth system absent from previous reanalyses ( Bosilovich et al. 2015 ). Further details of the upgrades made to the MERRA-2 system are described in Bosilovich et al. (2015) , Molod et al. (2015) , and Gelaro et al. (2017

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Allison B. Marquardt Collow, Michael G. Bosilovich, and Randal D. Koster

greatly influenced by geography, and the region is susceptible to different varieties of weather including hurricanes, extratropical cyclones, mesoscale systems, heat waves, and drought during the summer months. Mean climate features during JJA over the period 1980–2014 in MERRA-2 are shown in Fig. 1 . Temperature at 2 m tends to decrease with increasing latitude ( Fig. 1a ). Cooler temperatures over the ocean inhibit daytime warming along the coast and higher elevations associated with the

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Kevin Hodges, Alison Cobb, and Pier Luigi Vidale

introduce a dependency of the lifetime on the chosen criteria and how well TCs are represented in the reanalyses in terms of intensity and structure. The extended life cycles include pre-TC stages such as easterly waves and the stage after extratropical transition. Some of the reanalysis TCs can exist for longer than one month, in which time a precursor disturbance can travel across an ocean basin, develop into a TC, and recurve to high latitudes undergoing extratropical transition, whereas none of the

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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

–longitude grid, MERRA-2 uses a cubed-sphere grid. This allows relatively uniform grid spacing at all latitudes and mitigates the more severe grid spacing singularities that occur on a latitude–longitude grid. Upgrades to the physical parameterization schemes include increased reevaporation of frozen precipitation and cloud condensate, changes to the background gravity wave drag, and an improved relationship between the ocean surface roughness and ocean surface stress ( Molod et al. 2015 ). The MERRA-2 model

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Bin Guan, Duane E. Waliser, and F. Martin Ralph

time steps during which the grid cell is within any detected AR shape boundary. The overall pattern is characterized by enhanced AR frequencies in midlatitude ocean basins relative to inland and lower/higher-latitude regions, with multiple action centers located in the extratropical North Pacific/Atlantic, southeastern Pacific, South Atlantic, and southern Indian Ocean. The broad pattern is similar to Fig. 2b produced from tARget version 1, although AR frequency is notably increased based on the

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Gloria L. Manney and Michaela I. Hegglin

Screen 2015 , and references therein). However, many models predict a strengthening of upper-tropospheric temperature gradients, which would lead to a strengthening and poleward shift of the jets; lower- and upper-tropospheric jet responses may thus not be the same. Moreover, dynamical feedbacks resulting from the changing background winds (e.g., from changing waveguide conditions that affect wave activity, heat, and momentum fluxes) could play a role equal to or larger than that of changes in

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

generated by the AGCM within the cycling MERRA-2 system, hereinafter referred to as M2AGCM, and 2) the corrected precipitation that is seen by the MERRA-2 land surface and that modulates aerosol wet deposition over land and ocean, hereinafter referred to as M2CORR ( Table 1 ). Both are available in the published MERRA-2 data product ( Bosilovich et al. 2016 ) under the variable names PRECTOT and PRECTOTCORR, respectively ( Table 1 ). The remainder of this section is devoted to a detailed explanation of

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