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Krzysztof Wargan and Lawrence Coy

Abstract

The behavior of the tropopause inversion layer (TIL) during the 2009 sudden stratospheric warming (SSW) is analyzed using NASA’s Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), and short-term simulations with the MERRA-2 general circulation model. Consistent with previous studies, it is found that static stability in a shallow layer above the polar tropopause sharply increases following the SSW, leading to a strengthening of the high-latitude TIL. Simultaneously, the height of the thermal tropopause decreases by around 1 km. Similar behavior is also detected during other major SSW events between the years 2004 and 2013. Using an ensemble of general circulation model forecasts initialized from MERRA-2, it is demonstrated that the primary cause of the strengthening of the TIL is an increased convergence of the vertical component of the stratospheric residual circulation in response to an SSW-induced acceleration of the mean downward motion between 75° and 90°N. In addition, ~6% of the strengthening in 2009 is attributed to an enhanced anticyclonic circulation at the tropopause. A preliminary analysis indicates that during other recent SSW events there was a significant increase in the convergence of the vertical residual wind velocity throughout the middle and lower stratosphere. The static stability increase simulated by the model during the 2009 SSW is 60%–80% of that seen in MERRA-2. The underestimate is traced back to a tendency for the forecasts to underestimate the resolved planetary wave forcing on the stratosphere compared to the reanalysis.

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Krzysztof Wargan, Gordon Labow, Stacey Frith, Steven Pawson, Nathaniel Livesey, and Gary Partyka

Abstract

The assimilated ozone product from the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), produced at NASA’s Global Modeling and Assimilation Office (GMAO) spanning the time period from 1980 to the present is described herein, and its quality is assessed. MERRA-2 assimilates partial column ozone retrievals from a series of Solar Backscatter Ultraviolet Radiometer (SBUV) instruments on NASA and NOAA spacecraft between January 1980 and September 2004: starting in October 2004, retrieved ozone profiles from the Microwave Limb Sounder (MLS) and total column ozone from the Ozone Monitoring Instrument on NASA’s EOS Aura satellite are assimilated. The MERRA-2 ozone is compared with independent satellite and ozonesonde data, focusing on the representation of the spatial and temporal variability of stratospheric and upper-tropospheric ozone and on implications of the change in the observing system from SBUV to EOS Aura. The comparisons show agreement within 10% (standard deviation of the difference) between MERRA-2 profiles and independent satellite data in most of the stratosphere. The agreement improves after 2004, when EOS Aura data are assimilated. The standard deviation of the differences between the lower-stratospheric and upper-tropospheric MERRA-2 ozone and ozonesondes is 11.2% and 24.5%, respectively, with correlations of 0.8 and above, indicative of a realistic representation of the near-tropopause ozone variability in MERRA-2. The agreement improves significantly in the EOS Aura period; however, MERRA-2 is biased low in the upper troposphere with respect to the ozonesondes. Caution is recommended when using MERRA-2 ozone for decadal changes and trend studies.

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

Abstract

The structure, dynamics, and ozone signal of the quasi-biennial oscillation (QBO) produced by the 35-yr NASA Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), are examined based on monthly mean output. Along with the analysis of the QBO in assimilation winds and ozone, the QBO forcings created by assimilated observations, dynamics, parameterized gravity wave drag (GWD), and ozone chemistry parameterization are examined and compared with the original MERRA system. Results show that MERRA-2 produces a realistic QBO in the zonal winds, mean meridional circulation, and ozone over the 1980–2015 time period. In particular, the MERRA-2 zonal winds show improved representation of the QBO 50-hPa westerly phase amplitude at Singapore when compared to MERRA. The use of limb ozone observations creates improved vertical structure and realistic downward propagation of the ozone QBO signal during times when the MLS ozone limb observations are available (from October 2004 to present). The increased equatorial GWD in MERRA-2 has reduced the zonal wind data analysis contribution compared to MERRA so that the QBO mean meridional circulation can be expected to be more physically forced and therefore more physically consistent. This can be important for applications in which MERRA-2 winds are used to drive transport experiments.

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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

Abstract

The Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), is the latest atmospheric reanalysis of the modern satellite era produced by NASA’s Global Modeling and Assimilation Office (GMAO). MERRA-2 assimilates observation types not available to its predecessor, MERRA, and includes updates to the Goddard Earth Observing System (GEOS) model and analysis scheme so as to provide a viable ongoing climate analysis beyond MERRA’s terminus. While addressing known limitations of MERRA, MERRA-2 is also intended to be a development milestone for a future integrated Earth system analysis (IESA) currently under development at GMAO. This paper provides an overview of the MERRA-2 system and various performance metrics. Among the advances in MERRA-2 relevant to IESA are the assimilation of aerosol observations, several improvements to the representation of the stratosphere including ozone, and improved representations of cryospheric processes. Other improvements in the quality of MERRA-2 compared with MERRA include the reduction of some spurious trends and jumps related to changes in the observing system and reduced biases and imbalances in aspects of the water cycle. Remaining deficiencies are also identified. Production of MERRA-2 began in June 2014 in four processing streams and converged to a single near-real-time stream in mid-2015. MERRA-2 products are accessible online through the NASA Goddard Earth Sciences Data Information Services Center (GES DISC).

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