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James A. Carton, Xianhe Cao, Benjamin S. Giese, and Arlindo M. Da Silva

Abstract

The mechanisms regulating interannual and decadal variations of sea surface temperature (SST) in the tropical Atlantic are examined. Observed variations of sea surface temperature are typically in the range of 0.3°–0.5°C and are linked to fluctuations in rainfall on both the African and South American continents. The authors use a numerical model to simulate the observed time series of sea surface temperature for the period 1960–1989. Based on the results, experiments are conducted to determine the relative importance of heat flux and momentum forcing. Two dominant timescales for variability of SST are identified: a decadal timescale that is controlled by latent heat flux anomalies and is primarily responsible for SST anomalies off the equator and an equatorial mode with a timescale of 2–5 years that is dominated by dynamical processes. The interhemispheric gradient of anomalous SST (the SST dipole) is primarily linked to the former process and thus results from the gradual strengthening and weakening of the trade wind system of the two hemispheres.

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S. C. Bloom, L. L. Takacs, A. M. da Silva, and D. Ledvina

Abstract

The IAU (incremental analysis updating) process incorporates analysis increments into a model integration in a gradual manner. It does this by using analysis increments as constant forcings in a model's prognostic equations over a 6-h period centered on an analysis time. A linear analysis of the IAU procedure shows it to have the attractive properties of a low-pass time filter. The IAU process affects the response of the model to the analysis increments, and it leaves the model state unaffected where there were no data to assimilate. This result is contrasted with a simple dynamical relaxation (or “nudging”) scheme, which is shown, in this linear analysis, to have less desirable response characteristics, both from the analysis increments and from the background state of the model.

The behavior of IAU in the context of the Goddard Earth Observing System (GEOS) Data Assimilation System is examined using a combination of large-scale diagnostics from month-long assimilations and detailed diagnostics from short assimilations. These studies indicate that IAU assimilations have improved observed-minus-forecast statistics and improved globally averaged precipitation by removing spinup effects. The detailed diagnostics of the behavior of the GEOS system with IAU corroborate the results of the linear analysis of the response behavior of IAU.

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Diana C. Garcia-Montiel, Michael T. Coe, Meyr P. Cruz, Joice N. Ferreira, Euzebio M. da Silva, and Eric A. Davidson

Abstract

Water distributed in deep soil reservoirs is an important factor determining the ecosystem structure of water-limited environments, such as the seasonal tropical savannas of South America. In this study a two-dimensional (2D) geoelectrical profiling technique was employed to estimate seasonal dynamics of soil water content to 10-m depth along transects of 275 m in savanna vegetation during the period between 2002 and 2006. Methods were developed to convert resistivity values along these 2D resistivity profiles into volumetric water content (VWC) by soil depth. The 2D resistivity profiles revealed the following soil and aquifer structure characterizing the underground environment: 0–4 m of permanently unsaturated and seasonally droughty soil, less severely dry unsaturated soil at about 4–7 m, nearly permanently saturated soil between 7 and 10 m, mostly impermeable saprolite interspaced with fresh bedrock of parent material at about 10–30 m, and a region of highly conductive water-saturated material at 30 m and below. Considerable spatial variation of these relative depths is clearly demonstrated along the transects. Temporal dynamics in VWC indicate that the active zone of water uptake is predominantly at 0–7 m, and follows the seasonal cycles of precipitation and evapotranspiration. Uptake from below 7 m may have been critical for a short period near the beginning of the rainy season, although the seasonal variations in VWC in the 7–10-m layer are relatively small and lag the surface water recharge for about 6 months. Calculations using a simple 1-box water balance model indicate that average total runoff was 15–25 mm month−1 in the wet season and about 6–9 mm month−1 in the dry season. Modeled ET was about 75–85 mm month−1 in the wet season and 20–25 mm month−1 in the dry season. Variation in basal area and tree density along one transect was positively correlated with VWC of the 0–3-m and 0–7-m soil depths, respectively, during the wettest months. These multitemporal measurements demonstrate that the along-transect spatial differences in soil moisture are quasi-permanent and influence vegetation structure at the scale of tens to hundreds of meters.

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Luís Gustavo N. Martins, Gervásio A. Degrazia, Otávio C. Acevedo, Franciano S. Puhales, Pablo E. S. de Oliveira, Claudio A. Teichrieb, and Samuel M. da Silva

Abstract

Turbulent wind data measured by sonic anemometers installed at various heights on a 140-m-tall micrometeorological tower located at a coastal site are used to obtain vertical profiles of the velocity standard deviations σ i, Lagrangian decorrelation local time scales TLi, and eddy diffusivities K α for distinct stability conditions. The novelty of the study lies in the use of turbulent data directly measured over the extension of the atmospheric surface layer at a coastal site for that purpose. Furthermore, the approach employs the Hilbert–Huang transform to determine the wind energy spectral peak frequencies. These are applied to the asymptotic spectral equation from Taylor statistical diffusion theory to obtain the turbulent dispersion parameters, which are shown to generally agree well with those provided by a classical autocorrelation approach. For neutral and stable situations the vertical profiles of momentum eddy diffusivities agree well with those derived from the spectral and autocorrelation method. Additionally, the turbulent integral time scales and eddy diffusivities determined by the method at a coastal location are found to overestimate those predicted from analytical expressions based on continental field observations. The turbulence parameters found are suitable to be employed in air pollution dispersion models.

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E. P. Nowottnick, P. R. Colarco, S. A. Braun, D. O. Barahona, A. da Silva, D. L. Hlavka, M. J. McGill, and J. R. Spackman

Abstract

During the 2012 deployment of the NASA Hurricane and Severe Storm Sentinel (HS3) field campaign, several flights were dedicated to investigating Hurricane Nadine. Hurricane Nadine developed in close proximity to the dust-laden Saharan air layer and is the fourth-longest-lived Atlantic hurricane on record, experiencing two strengthening and weakening periods during its 22-day total life cycle as a tropical cyclone. In this study, the NASA GEOS-5 atmospheric general circulation model and data assimilation system was used to simulate the impacts of dust during the first intensification and weakening phases of Hurricane Nadine using a series of GEOS-5 forecasts initialized during Nadine’s intensification phase (12 September 2012). The forecasts explore a hierarchy of aerosol interactions within the model: no aerosol interaction, aerosol–radiation interactions, and aerosol–radiation and aerosol–cloud interactions simultaneously, as well as variations in assumed dust optical properties. When only aerosol–radiation interactions are included, Nadine’s track exhibits sensitivity to dust shortwave absorption, as a more absorbing dust introduces a shortwave temperature perturbation that impacts Nadine’s structure and steering flow, leading to a northward track divergence after 5 days of simulation time. When aerosol–cloud interactions are added, the track exhibits little sensitivity to dust optical properties. This result is attributed to enhanced longwave atmospheric cooling from clouds that counters shortwave atmospheric warming by dust surrounding Nadine, suggesting that aerosol–cloud interactions are a more significant influence on Nadine’s track than aerosol–radiation interactions. These findings demonstrate that tropical systems, specifically their track, can be impacted by dust interaction with the atmosphere.

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Alexandrede S. Pinto, Mercedes M. C. Bustamante, Maria Regina S. S. da Silva, Keith W. Kisselle, Michel Brossard, Ricardo Kruger, Richard G. Zepp, and Roger A. Burke

Abstract

Planted pastures (mainly Brachiaria spp) are the most extensive land use in the cerrado (savannas of central Brazil) with an area of approximately 50 × 106 ha. The objective of the study was to assess the effects of pasture restoration on the N dynamics (net N mineralization/nitrification, available inorganic N and soil N oxide gas fluxes—NO and N2O), C dynamics (CO2 fluxes and microbial biomass carbon), and diversity of the soil bacterial community using denaturing gradient gel electrophoresis (DGGE) profiles. Sampling was done monthly on a farm in Planaltina, Goiás, Brazil (15°13′S, 47°42′W) from November 2001 to April 2002. Three areas of cerradão (dense cerrado) were converted to pasture (Brachiaria brizantha) in 1991, and after 8 years degradation was evident with the decreasing plant biomass production. Methods to restore these pastures were investigated for their sustainability, principally their effects on trace gas emissions. The pastures have been managed since 1999 as follows: 1) fertilized plot (N = 60 kg ha−1 yr−1, P = 12 kg ha−1 yr−1); 2) grass–legume plot, Brachiaria associated with a legume (Stylosanthes guianensis) with addition of P (12 kg ha−1 yr−1); and 3) a traditional plot without management. A fourth area of cerradão was converted to pasture in 1999 and was not managed (young pasture). Ammonium was the predominant inorganic N form in the soils (∼76 mg N kg−1) for all treatments throughout the study. In December 2001 a reduction in average soil N-NH4 + was observed (∼30 mg N kg−1) compared to November 2001, probably related to plant demand. All plots had high variability of soil N gases emissions, but during the wet season, the NO and N2O soil fluxes were near zero. The results of the water addition experiment made during the dry season (September 2002) indicated that the transition of dry to wet season is an important period for the production of N gases in the fertilized pasture and in the young pasture. Soil CO2 fluxes also increased after the water addition and the grass–legume plot had the highest increase in soil respiration (from ∼2 to 8.3 μmol m−2 s−1). The lowest values of soil respiration and microbial biomass carbon (∼320 mg C kg−1 soil) tended to be observed in the young pasture, because the superficial layer of the soil (0–10 cm) was removed during the conversion to pasture. Trace gas emissions measured after the water addition experiment corresponded to rapid changes in the soil bacterial community. The young pasture sample showed the lowest level of similarity in relation to the others, indicating that the bacterial community is also influenced by the time since conversion. This study indicates that the restoration technique of including Stylosanthes guianensis with B. brizantha increases plant productivity without the peaks of N oxide gas emissions that are often associated with the use of N fertilizers. Additionally, the soil bacterial community structure may be restored to one similar to that of native cerrado grasslands, suggesting that this restoration method may beneficially affect bacterially mediated processes.

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V. Buchard, C. A. Randles, A. M. da Silva, A. Darmenov, P. R. Colarco, R. Govindaraju, R. Ferrare, J. Hair, A. J. Beyersdorf, L. D. Ziemba, and H. Yu

Abstract

The Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), is NASA’s latest reanalysis for the satellite era (1980 onward) using the Goddard Earth Observing System, version 5 (GEOS-5), Earth system model. MERRA-2 provides several improvements over its predecessor (MERRA-1), including aerosol assimilation for the entire period. MERRA-2 assimilates bias-corrected aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer and the Advanced Very High Resolution Radiometer instruments. Additionally, MERRA-2 assimilates (non bias corrected) AOD from the Multiangle Imaging SpectroRadiometer over bright surfaces and AOD from Aerosol Robotic Network sunphotometer stations. This paper, the second of a pair, summarizes the efforts to assess the quality of the MERRA-2 aerosol products. First, MERRA-2 aerosols are evaluated using independent observations. It is shown that the MERRA-2 absorption aerosol optical depth (AAOD) and ultraviolet aerosol index (AI) compare well with Ozone Monitoring Instrument observations. Next, aerosol vertical structure and surface fine particulate matter (PM2.5) are evaluated using available satellite, aircraft, and ground-based observations. While MERRA-2 generally compares well to these observations, the assimilation cannot correct for all deficiencies in the model (e.g., missing emissions). Such deficiencies can explain many of the biases with observations. Finally, a focus is placed on several major aerosol events to illustrate successes and weaknesses of the AOD assimilation: the Mount Pinatubo eruption, a Saharan dust transport episode, the California Rim Fire, and an extreme pollution event over China. The article concludes with a summary that points to best practices for using the MERRA-2 aerosol reanalysis in future studies.

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C. A. Randles, A. M. da Silva, V. Buchard, P. R. Colarco, A. Darmenov, R. Govindaraju, A. Smirnov, B. Holben, R. Ferrare, J. Hair, Y. Shinozuka, and C. J. Flynn

Abstract

The Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), updates NASA’s previous satellite-era (1980 onward) reanalysis system to include additional observations and improvements to the Goddard Earth Observing System, version 5 (GEOS-5), Earth system model. As a major step toward a full Integrated Earth Systems Analysis (IESA), in addition to meteorological observations, MERRA-2 now includes assimilation of aerosol optical depth (AOD) from various ground- and space-based remote sensing platforms. Here, in the first of a pair of studies, the MERRA-2 aerosol assimilation is documented, including a description of the prognostic model (GEOS-5 coupled to the GOCART aerosol module), aerosol emissions, and the quality control of ingested observations. Initial validation and evaluation of the analyzed AOD fields are provided using independent observations from ground, aircraft, and shipborne instruments. The positive impact of the AOD assimilation on simulated aerosols is demonstrated by comparing MERRA-2 aerosol fields to an identical control simulation that does not include AOD assimilation. After showing the AOD evaluation, this paper takes a first look at aerosol–climate interactions by examining the shortwave, clear-sky aerosol direct radiative effect. The companion paper (Part II) evaluates and validates available MERRA-2 aerosol properties not directly impacted by the AOD assimilation (e.g., aerosol vertical distribution and absorption). Importantly, while highlighting the skill of the MERRA-2 aerosol assimilation products, both studies point out caveats that must be considered when using this new reanalysis product for future studies of aerosols and their interactions with weather and climate.

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Gerhard Theurich, C. DeLuca, T. Campbell, F. Liu, K. Saint, M. Vertenstein, J. Chen, R. Oehmke, J. Doyle, T. Whitcomb, A. Wallcraft, M. Iredell, T. Black, A. M. Da Silva, T. Clune, R. Ferraro, P. Li, M. Kelley, I. Aleinov, V. Balaji, N. Zadeh, R. Jacob, B. Kirtman, F. Giraldo, D. McCarren, S. Sandgathe, S. Peckham, and R. Dunlap IV

Abstract

The Earth System Prediction Suite (ESPS) is a collection of flagship U.S. weather and climate models and model components that are being instrumented to conform to interoperability conventions, documented to follow metadata standards, and made available either under open-source terms or to credentialed users.

The ESPS represents a culmination of efforts to create a common Earth system model architecture, and the advent of increasingly coordinated model development activities in the United States. ESPS component interfaces are based on the Earth System Modeling Framework (ESMF), community-developed software for building and coupling models, and the National Unified Operational Prediction Capability (NUOPC) Layer, a set of ESMF-based component templates and interoperability conventions. This shared infrastructure simplifies the process of model coupling by guaranteeing that components conform to a set of technical and semantic behaviors. The ESPS encourages distributed, multiagency development of coupled modeling systems; controlled experimentation and testing; and exploration of novel model configurations, such as those motivated by research involving managed and interactive ensembles. ESPS codes include the Navy Global Environmental Model (NAVGEM), the Hybrid Coordinate Ocean Model (HYCOM), and the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS); the NOAA Environmental Modeling System (NEMS) and the Modular Ocean Model (MOM); the Community Earth System Model (CESM); and the NASA ModelE climate model and the Goddard Earth Observing System Model, version 5 (GEOS-5), atmospheric general circulation model.

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Christopher J. Anderson, Raymond W. Arritt, Zaitao Pan, Eugene S. Takle, William J. Gutowski Jr., Francis O. Otieno, Renato da Silva, Daniel Caya, Jens H. Christensen, Daniel Lüthi, Miguel A. Gaertner, Clemente Gallardo, Filippo Giorgi, René Laprise, Song-You Hong, Colin Jones, H-M. H. Juang, J. J. Katzfey, John L. McGregor, William M. Lapenta, Jay W. Larson, John A. Taylor, Glen E. Liston, Roger A. Pielke Sr., and John O. Roads

Abstract

Thirteen regional climate model (RCM) simulations of June–July 1993 were compared with each other and observations. Water vapor conservation and precipitation characteristics in each RCM were examined for a 10° × 10° subregion of the upper Mississippi River basin, containing the region of maximum 60-day accumulated precipitation in all RCMs and station reports.

All RCMs produced positive precipitation minus evapotranspiration (PE > 0), though most RCMs produced PE below the observed range. RCM recycling ratios were within the range estimated from observations. No evidence of common errors of E was found. In contrast, common dry bias of P was found in the simulations.

Daily cycles of terms in the water vapor conservation equation were qualitatively similar in most RCMs. Nocturnal maximums of P and C (convergence) occurred in 9 of 13 RCMs, consistent with observations. Three of the four driest simulations failed to couple P and C overnight, producing afternoon maximum P. Further, dry simulations tended to produce a larger fraction of their 60-day accumulated precipitation from low 3-h totals.

In station reports, accumulation from high (low) 3-h totals had a nocturnal (early morning) maximum. This time lag occurred, in part, because many mesoscale convective systems had reached peak intensity overnight and had declined in intensity by early morning. None of the RCMs contained such a time lag. It is recommended that short-period experiments be performed to examine the ability of RCMs to simulate mesoscale convective systems prior to generating long-period simulations for hydroclimatology.

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