Search Results

You are looking at 1 - 10 of 10 items for

  • Author or Editor: Andrea M. Molod x
  • All content x
Clear All Modify Search
Andrea Molod, H. M. Helfand, and Lawrence L. Takacs

Abstract

The Goddard Earth Observing System (GEOS) General Circulation Model (GCM) is part of the GEOS Data Assimilation System (DAS), which is being developed at the Goddard Data Assimilation Office for the production of climate datasets. This study examines Version 1 of the GEOS CYCM by evaluating the quality of the fields that relate most closely to the GCM physical parameterizations and examines the impact of the GCM climate errors on the climate of the DAS assimilated fields.

The climate characteristics are evaluated using independent satellite and ground-based data for comparison. The GEOS-1 GCM shows reasonably good agreement with available observations in terms of general global distribution and seasonal cycles. The major biases or systematic errors are a tendency toward a dry tropical atmosphere and an inadequate cloud radiative impact in the extratropics. Other systematic errors are a generally wet subtropical atmosphere, slightly excess precipitation over the continents, and excess cloud radiative effects over the Tropics. There is also an underestimation of surface sensible and latent heat fluxes over the area of maximum flux.

The DAS climate characteristics, in general, show better agreement with available observations than the GCM. Four distinct ways that the GCM impacts the DAS have been identified, ranging from a DAS climate with little or no impact from the GCM bias to a DAS climate with a greater bias than the GCM due to a spurious feedback between the GCM and the input data.

Full access
Randal D. Koster, Siegfried D. Schubert, Anthony M. DeAngelis, Andrea M. Molod, and Sarith P. Mahanama

Abstract

Past studies have shown that accurate soil moisture initialization can contribute significant skill to near-surface air temperature (T2M) forecasts at subseasonal leads. The mechanisms by which soil moisture contributes such skill are examined here with a simple water balance–based model that captures the essence of soil moisture behavior in a state-of-the-art subseasonal-to-seasonal (S2S) forecasting system. The simple model successfully transforms initial soil moisture contents into average “forecast” evapotranspiration (ET) values at 16–30-day lead that agree well, during summer, with the values forecast by the full NASA GEOS S2S system, indicating that soil moisture initialization dominates over forecast meteorological conditions in determining ET fluxes at subseasonal leads. When the simple model’s ET anomalies are interpreted in terms of T2M anomalies, a similar conclusion is reached for T2M: soil moisture initialization explains much (about 50% in the eastern half of the continental United States) of the T2M anomaly values produced by the full GEOS S2S system at 16–30-day lead, and the T2M forecasts produced by the simple model capture about one-half of the skill attained by the full system. The simple model’s framework is particularly conducive to an analysis of uncertainty in forecasts. Drier soils are generally found to induce larger uncertainty in ET (and thus T2M) forecasts, a result linked to the functional form relating ET to soil moisture in the simple model and verified by an analysis of the ensemble spreads within the forecasts produced by the full GEOS S2S system.

Restricted access
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.

Full access
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.

Full access
Young-Kwon Lim, Siegfried D. Schubert, Yehui Chang, Andrea M. Molod, and Steven Pawson

Abstract

The factors impacting western U.S. winter precipitation during the 2015/16 El Niño are investigated using the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), data, and simulations with the Goddard Earth Observing System, version 5 (GEOS-5), atmospheric general circulation model forced with specified sea surface temperatures (SSTs). Results reveal that the simulated response to the tropical Pacific SST associated with the 2015/16 El Niño was to produce wetter than normal conditions over much of the North American west coast including California—a result at odds with the negative precipitation anomalies observed over much of the southwestern United States. It is shown that two factors acted to partly counter the canonical ENSO response in that region. First, a potentially predictable but modest response to the unusually strong and persistent warm SST in the northeastern Pacific decreased precipitation in the southwestern United States by increasing sea level pressure, driving anticyclonic circulation and atmospheric descent, and reducing moisture transport into that region. Second, large-scale unforced (by SST) components of atmospheric variability (consisting of the leading modes of unpredictable intraensemble variability) resembling the positive phase of the North Atlantic Oscillation and Arctic Oscillation are found to be an important contributor to the drying over the western United States. While a statistical reconstruction of the precipitation from our simulations that account for internal atmospheric variability does much to close the gap between the ensemble-mean and observed precipitation in the southwestern United States, some differences remain, indicating that model error is also playing a role.

Full access
Young-Kwon Lim, Siegfried D. Schubert, Oreste Reale, Andrea M. Molod, Max J. Suarez, and Benjamin M. Auer

Abstract

Interannual variations in seasonal tropical cyclone (TC) activity (e.g., genesis frequency and location, track pattern, and landfall) over the Atlantic are explored by employing observationally constrained simulations with the NASA Goddard Earth Observing System, version 5 (GEOS-5), atmospheric general circulation model. The climate modes investigated are El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Atlantic meridional mode (AMM).

The results show that the NAO and AMM can strongly modify and even oppose the well-known ENSO impacts, like in 2005, when a strong positive AMM (associated with warm SSTs and a negative SLP anomaly over the western tropical Atlantic) led to a very active TC season with enhanced TC genesis over the Caribbean Sea and a number of landfalls over North America, under a neutral ENSO condition. On the other end, the weak TC activity during 2013 (characterized by weak negative Niño index) appears caused by a NAO-induced positive SLP anomaly with enhanced vertical wind shear over the tropical North Atlantic. During 2010, the combined impact of the three modes produced positive SST anomalies across the entire low-latitudinal Atlantic and a weaker subtropical high, leading to more early recurvers and thus fewer landfalls despite enhanced TC genesis. The study provides evidence that TC number and track are very sensitive to the relative phases and intensities of these three modes and not just to ENSO alone. Examination of seasonal predictability reveals that the predictive skill of the three modes is limited over tropics to subtropics, with the AMM having the highest predictability over the North Atlantic, followed by ENSO and NAO.

Full access
Chaim I. Garfinkel, Luke D. Oman, Elizabeth A. Barnes, Darryn W. Waugh, Margaret H. Hurwitz, and Andrea M. Molod

Abstract

A robust connection between the drag on surface-layer winds and the stratospheric circulation is demonstrated in NASA's Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM). Specifically, an updated parameterization of roughness at the air–sea interface, in which surface roughness is increased for moderate wind speeds (4–20 m s−1), leads to a decrease in model biases in Southern Hemispheric ozone, polar cap temperature, stationary wave heat flux, and springtime vortex breakup. A dynamical mechanism is proposed whereby increased surface roughness leads to improved stationary waves. Increased surface roughness leads to anomalous eddy momentum flux convergence primarily in the Indian Ocean sector (where eddies are strongest climatologically) in September and October. The localization of the eddy momentum flux convergence anomaly in the Indian Ocean sector leads to a zonally asymmetric reduction in zonal wind and, by geostrophy, to a wavenumber-1 stationary wave pattern. This tropospheric stationary wave pattern leads to enhanced upward wave activity entering the stratosphere. The net effect is an improved Southern Hemisphere vortex: the vortex breaks up earlier in spring (i.e., the spring late-breakup bias is partially ameliorated) yet is no weaker in midwinter. More than half of the stratospheric biases appear to be related to the surface wind speed biases. As many other chemistry–climate models use a similar scheme for their surface-layer momentum exchange and have similar biases in the stratosphere, the authors expect that results from GEOSCCM may be relevant for other climate models.

Full access
Young-Kwon Lim, Siegfried D. Schubert, Oreste Reale, Myong-In Lee, Andrea M. Molod, and Max J. Suarez

Abstract

The sensitivity of tropical cyclones (TCs) to changes in parameterized convection is investigated to improve the simulation of TCs in the North Atlantic. Specifically, the impact of reducing the influence of the Relaxed Arakawa–Schubert (RAS) scheme-based parameterized convection is explored using the Goddard Earth Observing System version 5 (GEOS-5) model at 0.25° horizontal grid spacing. The years 2005 and 2006, characterized by very active and inactive hurricane seasons, respectively, are selected for simulation.

A reduction in parameterized deep convection results in an increase in TC activity (e.g., TC number and longer life cycle) to more realistic levels compared to the baseline control configuration. The vertical and horizontal structure of the strongest simulated hurricane shows the maximum wind speed greater than 60 m s−1 and the minimum sea level pressure reaching ~940 mb, which are never achieved by the control configuration. The radius of the maximum wind of ~50 km, the location of the warm core exceeding 10°C, and the horizontal compactness of the hurricane center are all quite realistic without any negatively affecting the atmospheric mean state.

This study reveals that an increase in the threshold of minimum entrainment suppresses parameterized deep convection by entraining more dry air into the typical plume. This leads to cooling and drying at the mid to upper troposphere, along with the positive latent heat flux and moistening in the lower troposphere. The resulting increase in conditional instability provides an environment that is more conducive to TC vortex development and upward moisture flux convergence by dynamically resolved moist convection, thereby increasing TC activity.

Full access
Michele M. Rienecker, Max J. Suarez, Ronald Gelaro, Ricardo Todling, Julio Bacmeister, Emily Liu, Michael G. Bosilovich, Siegfried D. Schubert, Lawrence Takacs, Gi-Kong Kim, Stephen Bloom, Junye Chen, Douglas Collins, Austin Conaty, Arlindo da Silva, Wei Gu, Joanna Joiner, Randal D. Koster, Robert Lucchesi, Andrea Molod, Tommy Owens, Steven Pawson, Philip Pegion, Christopher R. Redder, Rolf Reichle, Franklin R. Robertson, Albert G. Ruddick, Meta Sienkiewicz, and Jack Woollen

Abstract

The Modern-Era Retrospective Analysis for Research and Applications (MERRA) was undertaken by NASA’s Global Modeling and Assimilation Office with two primary objectives: to place observations from NASA’s Earth Observing System satellites into a climate context and to improve upon the hydrologic cycle represented in earlier generations of reanalyses. Focusing on the satellite era, from 1979 to the present, MERRA has achieved its goals with significant improvements in precipitation and water vapor climatology. Here, a brief overview of the system and some aspects of its performance, including quality assessment diagnostics from innovation and residual statistics, is given.

By comparing MERRA with other updated reanalyses [the interim version of the next ECMWF Re-Analysis (ERA-Interim) and the Climate Forecast System Reanalysis (CFSR)], advances made in this new generation of reanalyses, as well as remaining deficiencies, are identified. Although there is little difference between the new reanalyses in many aspects of climate variability, substantial differences remain in poorly constrained quantities such as precipitation and surface fluxes. These differences, due to variations both in the models and in the analysis techniques, are an important measure of the uncertainty in reanalysis products. It is also found that all reanalyses are still quite sensitive to observing system changes. Dealing with this sensitivity remains the most pressing challenge for the next generation of reanalyses.

Production has now caught up to the current period and MERRA is being continued as a near-real-time climate analysis. The output is available online through the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC).

Full access
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).

Full access