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William M. Putman, David M. Legler, and James J. O’Brien

Abstract

A technique is applied to seamlessly blend height-adjusted Florida State University (FSU) surface wind pseudostress with National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis-based pseudostress over the Pacific Ocean. The FSU pseudostress is shown to be of higher quality in the equatorial Pacific and thus dominates the analysis in that region, while the NCEP–NCAR reanalysis-based pseudostress is used outside the equatorial region. The blending technique is based on a direct minimization approach. The functional to minimize consists of five constraints; each constraint is given a weight that determines its influence on the solution. The first two constraints are misfits for the FSU and NCEP–NCAR reanalysis datasets. A spatially dependent weighting that highlights the regional strengths of each dataset is designed for these misfit constraints. Climatological structure information is used as a weak smoothing constraint on the solution through Laplacian and kinematic (divergence and curl) constraints. The weights for the smoothing constraints are selected using a sensitivity analysis and evaluation of solution fields. The resulting 37 yr of monthly pseudostress fields are suitable for use in a variety of modeling and climate variability studies.

The monthly mean analyses are produced for 1961 through 1997, over the domain 40°S–40°N, 125°E–70°W. NCEP–NCAR reanalysis data, from 40° to 60°N, are added to the minimization solution fields, and the monthly mean climatologies, based on the solution fields, are removed from the combined fields. The resulting pseudostress anomalies are filtered with an 18-month low-pass filter to focus on interannual and ENSO timescales, and a complex empirical orthogonal function (CEOF) analysis is performed on the filtered anomalies. The CEOF analysis reveals tropical and extratropical linkages, for example, the presence of a strengthening of the Aleutian low in the North Pacific, coincident with the anomalous westerlies along the equator associated with El Niño events. The analysis also reveals a weakening of the Aleutian low during the winter–spring preceding the El Niño events of 1973 and 1983, and during the peak period of El Viejo, the cold phase of ENSO. A change in the nature of the tropical and extratropical linkages is observed from the warm events of the 1960s to those of the 1980s. These linkages are not found using NCEP–NCAR reanalysis data alone.

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Oreste Reale, Deepthi Achuthavarier, Marangelly Fuentes, William M. Putman, and Gary Partyka

Abstract

The National Aeronautics and Space Administration (NASA) nature run (NR), released for use in observing system simulation experiments (OSSEs), is a 2-yr-long global nonhydrostatic free-running simulation at a horizontal resolution of 7 km, forced by observed sea surface temperatures (SSTs) and sea ice, and inclusive of interactive aerosols and trace gases. This article evaluates the NR with respect to tropical cyclone (TC) activity. It is emphasized that to serve as an NR, a long-term simulation must be able to produce realistic TCs, which arise out of realistic large-scale forcings. The presence in the NR of the relevant dynamical features over the African monsoon region and the tropical Atlantic is confirmed, along with realistic African easterly wave activity. The NR Atlantic TC seasons, produced with 2005 and 2006 SSTs, show interannual variability consistent with observations, with much stronger activity in 2005. An investigation of TC activity over all the other basins (eastern and western North Pacific Ocean, north and south Indian Ocean, and Australian region), together with important elements of the atmospheric circulation, such as the Somali jet and westerly bursts, reveals that the model captures the fundamental aspects of TC seasons in every basin, producing a realistic number of TCs with realistic tracks, life spans, and structures. This confirms that the NASA NR is a very suitable tool for OSSEs targeting TCs and represents an improvement with respect to previous long simulations that have served the global atmospheric OSSE community.

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Laura A. Holt, M. Joan Alexander, Lawrence Coy, Andrea Molod, William Putman, and Steven Pawson

Abstract

This study investigates tropical waves and their role in driving a quasi-biennial oscillation (QBO)-like signal in stratospheric winds in a global 7-km-horizontal-resolution atmospheric general circulation model. The Nature Run (NR) is a 2-yr global mesoscale simulation of the Goddard Earth Observing System Model, version 5 (GEOS-5). In the tropics, there is evidence that the NR supports a broad range of convectively generated waves. The NR precipitation spectrum resembles the observed spectrum in many aspects, including the preference for westward-propagating waves. However, even with very high horizontal resolution and a healthy population of resolved waves, the zonal force provided by the resolved waves is still too low in the QBO region and parameterized gravity wave drag is the main driver of the NR QBO-like oscillation (NR-QBO). The authors suggest that causes include coarse vertical resolution and excessive dissipation. Nevertheless, the very-high-resolution NR provides an opportunity to analyze the resolved wave forcing of the NR-QBO. In agreement with previous studies, large-scale Kelvin and small-scale waves contribute to the NR-QBO driving in eastward shear zones and small-scale waves dominate the NR-QBO driving in westward shear zones. Waves with zonal wavelength < 1000 km account for up to half of the small-scale (<3300 km) resolved wave forcing in eastward shear zones and up to 70% of the small-scale resolved wave forcing in westward shear zones of the NR-QBO.

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Baijun Tian, Huikyo Lee, Duane E. Waliser, Robert Ferraro, Jinwon Kim, Jonathan Case, Takamichi Iguchi, Eric Kemp, Di Wu, William Putman, and Weile Wang

Abstract

Several dynamically downscaled climate simulations with various spatial resolutions (24, 12, and 4 km) and spectral nudging strengths (0, 600, and 2000 km) have been run over the contiguous United States from 2000 to 2009 using the high-resolution NASA Unified Weather and Research Forecasting (NU-WRF) regional model initialized and constrained by the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). This paper summarizes the authors’ efforts on the development of a model performance metric and its application to assess summer precipitation over the U.S. Great Plains (USGP) in these downscaled climate simulations. A new model performance metric T was first developed that uses both the linear correlation coefficient and mean square error and is consistent with other commonly used metrics, but gives a bigger separation between good and bad simulations. This metric T was then applied to the summer mean precipitation spatial pattern, diurnal Hovmöller diagram, and diurnal spatial pattern over the USGP from the simulations focusing on the summer precipitation diurnal cycle related to mesoscale convective systems (MCSs). The metric T skill scores increase significantly from the control simulation to the nudged simulations and from the nudged simulations with shorter wavelengths to the nudged simulations with longer wavelengths, but do not change much from MERRA-2 to the downscaled simulations or between the various downscaled simulations with different spatial resolutions. Thus, there is some credibility, but no significant value added compared to MERRA-2, of the downscaled climate simulations of the summer precipitation over the USGP.

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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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