Engineering topics

Sellers, Piers J.

Publications and source records attributed to Sellers, Piers J..

16 records

NASA NTRS2018

NASA's Carbon Cycle OSSE Initiative - Informing Future Space-Based Observing Strategies Through Advanced Modeling and Data Assimilation

Land and ocean carbon sinks absorb half of human CO2 emissions. The fate of these sinks in a changing world is unknown, introducing large uncertainties in climate projections. Satellite measurements of atmospheric CO2 are required to better understand the processes governing carbon uptake. Careful planning of future missions using Observing System Simulation Experiments (OSSEs) can help ensure that they meet the needs of the scientific and policy communities. NASA's Carbon Cycle OSSE Initiative brings together researchers from multiple universities and NASA centers to create model-derived data products in support of informed mission planning.

Ott, Lesley↗

NASA NTRS2012

The Race To Understand A Changing Planet

The Earth's climate is changing rapidly. In some respects, the rate of change is outpacing the predictions of only a few years ago. The challenge to Earth Science is to put forward credible projections of possible future climates so that the public and policy makers can make science-based decisions about energy development strategies. Models, observations and experiments all play strong roles in improving knowledge and increasing confidence in our predictions. The models have progressed from simple, coarse-resolution descriptions of atmospheric dynamics and physics only twenty years ago, to full-up Earth System models (ESMs) that include complete descriptions of the oceans and cryosphere. It has been convincingly argued that such complexity - the construction of realistic "toy" Earth's - is necessary to address the complex processes involved in climate change, including not only the physical atmosphere, oceans and cryosphere, but also the carbon cycle - both its natural and anthropogenic components - and the biosphere. Observations, particularly satellite observations, have more or less kept pace with the demands of the modelers, being able to observe progressively more and different facets of the Earth system, but the global satellite fleet is in need of an overhaul very soon. Lastly, field experiments and process studies confront the models with facts and allow us to develop more sophisticated and accurate satellite data algorithms. The challenges facing our relatively small Earth and planetary science communities are considerable and the stakes are significant. The stakeholders, now numbering 7 billion but soon to be 10 billion, will be relying on our results and capabilitie's to guide them into the future.

Sellers, Piers J.↗

NASA NTRS2012

Remote Sensing of Tropical Ecosystems: Atmospheric Correction and Cloud Masking Matter

Tropical rainforests are significant contributors to the global cycles of energy, water and carbon. As a result, monitoring of the vegetation status over regions such as Amazonia has been a long standing interest of Earth scientists trying to determine the effect of climate change and anthropogenic disturbance on the tropical ecosystems and its feedback on the Earth's climate. Satellite-based remote sensing is the only practical approach for observing the vegetation dynamics of regions like the Amazon over useful spatial and temporal scales, but recent years have seen much controversy over satellite-derived vegetation states in Amazônia, with studies predicting opposite feedbacks depending on data processing technique and interpretation. Recent results suggest that some of this uncertainty could stem from a lack of quality in atmospheric correction and cloud screening. In this paper, we assess these uncertainties by comparing the current standard surface reflectance products (MYD09, MYD09GA) and derived composites (MYD09A1, MCD43A4 and MYD13A2 - Vegetation Index) from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite to results obtained from the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm. MAIAC uses a new cloud screening technique, and novel aerosol retrieval and atmospheric correction procedures which are based on time-series and spatial analyses. Our results show considerable improvements of MAIAC processed surface reflectance compared to MYD09/MYD13 with noise levels reduced by a factor of up to 10. Uncertainties in the current MODIS surface reflectance product were mainly due to residual cloud and aerosol contamination which affected the Normalized Difference Vegetation Index (NDVI): During the wet season, with cloud cover ranging between 90 percent and 99 percent, conventionally processed NDVI was significantly depressed due to undetected clouds. A smaller reduction in NDVI due to increased aerosol levels was observed during the dry season, with an inverse dependence of NDVI on aerosol optical thickness (AOT). NDVI observations processed with MAIAC showed highly reproducible and stable inter-annual patterns with little or no dependence on cloud cover, and no significant dependence on AOT (p less than 0.05). In addition to a better detection of cloudy pixels, MAIAC obtained about 20-80 percent more cloud free pixels, depending on season, a considerable amount for land analysis given the very high cloud cover (75-99 percent) observed at any given time in the area. We conclude that a new generation of atmospheric correction algorithms, such as MAIAC, can help to dramatically improve vegetation estimates over tropical rain forest, ultimately leading to reduced uncertainties in satellite-derived vegetation products globally.

tropical ecosystems↗

NASA NTRS2012

A Vision for the Exploration of Mars: Robotic Precursors Followed by Humans to Mars Orbit in 2033

The reformulation of the Mars program gives NASA a rare opportunity to deliver a credible vision in which humans, robots, and advancements in information technology combine to open the deep space frontier to Mars. There is a broad challenge in the reformulation of the Mars exploration program that truly sets the stage for: 'a strategic collaboration between the Science Mission Directorate (SMD), the Human Exploration and Operations Mission Directorate (HEOMD) and the Office of the Chief Technologist, for the next several decades of exploring Mars'.Any strategy that links all three challenge areas listed into a true long term strategic program necessitates discussion. NASA's SMD and HEOMD should accept the President's challenge and vision by developing an integrated program that will enable a human expedition to Mars orbit in 2033 with the goal of returning samples suitable for addressing the question of whether life exists or ever existed on Mars

Sellers, Piers J.↗

NASA NTRS2012

Data Assimilation of Photosynthetic Light-use Efficiency using Multi-angular Satellite Data: II Model Implementation and Validation

Spatially explicit and temporally continuous estimates of photosynthesis will be of great importance for increasing our understanding of and ultimately closing the terrestrial carbon cycle. Current capabilities to model photosynthesis, however, are limited by accurate enough representations of the complexity of the underlying biochemical processes and the numerous environmental constraints imposed upon plant primary production. A potentially powerful alternative to model photosynthesis through these indirect observations is the use of multi-angular satellite data to infer light-use efficiency (ε) directly from spectral reflectance properties in connection with canopy shadow fractions. Hall et al. (this issue) introduced a new approach for predicting gross ecosystem production that would allow the use of such observations in a data assimilation mode to obtain spatially explicit variations in ε from infrequent polar-orbiting satellite observations, while meteorological data are used to account for the more dynamic responses of ε to variations in environmental conditions caused by changes in weather and illumination. In this second part of the study we implement and validate the approach of Hall et al. (this issue) across an ecologically diverse array of eight flux-tower sites in North America using data acquired from the Compact High Resolution Imaging Spectroradiometer (CHRIS) and eddy-flux observations. Our results show significantly enhanced estimates of ε and therefore cumulative gross ecosystem production (GEP) over the course of one year at all examined sites. We also demonstrate that ε is greatly heterogeneous even across small study areas. Data assimilation and direct inference of GEP from space using a new, proposed sensor could therefore be a significant step towards closing the terrestrial carbon cycle.

Hilker, Thomas↗

NASA NTRS2012

Observations of the Climate System

Observations of the climate system are critical for model validation and initialization, and also for monitoring in case of "surprises." Presently. we are still benefiting from data provided by the international fleet of Earth Observing satellites launched from the late 1990's onwards as well as from the longer-term record provided hy the operational meteorological satellites. However, we could be facing some data gaps in the near term in some critical areas. In situ measurements continue to be vital and, while they may be augmented hy future satellite measurements, will continue to be irreplaceable.

Sellers, Piers J.↗

NASA NTRS1995

Partitioning of ocean and land uptake of CO2 as inferred by delta C-13 measurements from the NOAA Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network

Using delta C-13 measurements in atmospheric CO2 from a cooperative global air sampling network, we determined the partitioning of the net uptake of CO2 between ocean and land as a function of latitude and time. The majority of delta C-13 measurements were made at the Institute of Arctic and Alpine Research (INSTAAR) of the University of Colorado. We perform an inverse deconvolution of both CO2 and delta C-13 observations, using a two-dimensional model of atmospheric transport. Also, the discrimination against C-13 by plant photosynthesis, as a function of latitude and time, is calculated from global runs of the simple biosphere (SiB) model. Uncertainty due to the longitudinal structure of the data, which is not represented by the model, is studied through a bootstrap analysis by adding and omitting measurement sites. The resulting error estimates for our inferred sources and sinks are of the order of 1 GTC (1 GTC = 10(exp 15) gC). Such error bars do not reflect potential systematic errors arising from our estimates of the isotopic disequilibria between the atmosphere and the oceans and biosphere, which are estimated in a separate sensitivity analysis. With respect to global totals for 1992 we found that 3.2 GTC of carbon dissolved into the ocean and that 1.5 GTC were sequestered by land ecosystems. Northern hemisphere ocean gyres north of 15 deg N absorbed 2.7 GTC. The equatorial oceans between 10 deg S and 10 deg N were a net source to the atmosphere of 0.9 GTC. We obtained a sink of 1.6 GTC in southern ocean gyres south of 20 deg S, although the deconvolution is poorly constrained by sparse data coverage at high southern latitudes. The seasonal uptake of CO2 in the northern gyres appears to be correlated with a bloom of phytoplankton in surface waters. On land, northern temperate and boreal ecosystems between 35 deg N and 65 deg N were found to be a major sink of CO2 in 1992, as large as 3.5 GTC. Northern tropical ecosystems (equator-30 deg N) appear to be a net source to the source to the atmosphere of 2 GTC which could reflect biomass burning. A small sink, 0.3 GTC, was inferred for southern tropical ecosystems (30 deg S-equator).

Ciais, Philippe↗

NASA NTRS1995

A review of the US Global Change Research Program and NASA's Mission to Planet Earth/Earth Observing System

This report reflects the results of a ten-day workshop convened at the Scripps Institution of Oceanography July 19-28, 1995. The workshop was convened as the first phase of a two part review of the U.S. Global Change Research Program (USGCRP). The workshop was organized to provide a review of the scientific foundations and progress to date in the USGCRP and an assessment of the implications of new scientific insights for future USGCRP and Mission to Planet Earth/Earth Observing System (MTPE/EOS) activities; a review of the role of NASA's MTPE/EOS program in the USGCRP observational strategy; a review of the EOS Data and Information System (EOSDIS) as a component of USGCRP data management activities; and an assessment of whether recent developments in the following areas lead to a need to readjust MTPE/EOS plans. Specific consideration was given to: proposed convergence of U.S. environmental satellite systems and programs, evolving international plans for Earth observation systems, advances in technology, and potential expansion of the role of the private sector. The present report summarizes the findings and recommendations developed by the Committee on Global Change Research on the basis of the presentations, background materials, working group deliberations, and plenary discussions of the workshop. In addition, the appendices include summaries prepared by the six working groups convened in the course of the workshop.

Moore, Berrien, III↗

NASA NTRS1992

Relations between surface conductance and spectral vegetation indices at intermediate (100 m sq to 15 km sq) length scales

The relationship between surface conductance and spectral vegetation indices is investigated utilizing the FIFE data set, principally the surface flux station data and images from the TM instrument. It is found that the unstressed canopy conductance for a given site for a given day is near-linearly related to the incident PAR flux. Estimates of unstressed canopy conductance were acquired via a model inversion that separated the soil and vegetation contributions to evapotranspiration and made adjustments for the effects of vapor pressure deficit and soil moisture stress.

Sellers, Piers J.↗

NASA NTRS1992

Satellite remote sensing of surface energy balance - Success, failures, and unresolved issues in FIFE

Utilizing single-view-angle visible and NIR satellite observations, it was possible to make the necessary radiometric corrections to produce reliable estimates of the simple ratio and the normalized difference vegetation index as inputs to surface evaporation models. Agreement was achieved between helicopter and satellite measures of surface reflectance to within +/-1 percent absolute for both the visible and the NIR bands. A theoretical calculation utilizing radiative transfer models indicates that this translates into a +/-10 percent error in the estimation in Fpar.

Hall, Forrest G.↗

NASA NTRS1991

BOREAS (Boreal Ecosystem-Atmosphere Study): Global change and biosphere-atmosphere interactions in the boreal forest

The Boreal Ecosystems Atmosphere Study (BOREAS) is a cooperative field and analysis project involving elements of land surface climatology, tropospheric chemistry, and terrestrial ecology. The goal of the study is to understand the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. The study will be centered on two 20 by 20 km sites within the North American boreal forest region, located near the northern and southern limits of the biome. Studies based at these sites will be used to explore the roles of various environmental factors in controlling the extent and character of the biome. The sites will be the subject of surface, airborne, and satellite based observations which aim to improve understanding of the biological and physical processes and states which govern the exchanges of energy, water, carbon, and trace gases between boreal forest ecosystems and the atmosphere. Particular reference will be made to those processes and states that may be sensitive to global change. The study also aims to develop the use of remote sensing techniques to transfer understanding of the above process from local scales out to regional scales. The BOREAS project is being planned for 1992-1996, with a major field effort in 1994.

Sellers, Piers J.↗

NASA NTRS1990

The FIFE information system

A description is given of the FIFE (First ISLSCP Field Experiment) information system, which was developed to serve FIFE investigators as a tool for designing the experiment and for organizing and manipulating the complex data set. Fulfilling these functions on an experiment-driven timeline led to abandoning the classical sequential development paradigm of software engineering in favor of a more responsive and broadly based approach. The design, development, and operation of the information system supporting the experiment had to be flexible and under direct day-to-day control of scientist/users. Because of the organization around scientific requirements, the system was able to incorporate diverse data types in a systematic way as they became available, to add scientific rigor by identifying data gaps at an early stage, and to provide real-time quality assurance. These factors are important for designing and building future databases and long-term information systems to support interdisciplinary scientific research.

Strebel, Donald E.↗

NASA NTRS1989

Calibrating the simple biosphere model for Amazonian tropical forest using field and remote sensing data. I - Average calibration with field data

Using meteorological and hydrological measurements taken in and above the central-Amazon-basin tropical forest, calibration of the Sellers et al. (1986) simple biosphere (SiB) model are described. The SiB model is a one-dimensional soil-vegetation-atmosphere model designed for use within GCMs models, representing the vegetation cover by analogy with processes operating within a single representative plant. The experimental systems and the procedures used to obtain field data are described, together with the specification of the physiological parameterization required to provide an average description of data. It was found that some of the existing literature on stomatal behavior for tropical species is inconsistent with the observed behavior of the complete canopy in Amazonia, and that the rainfall interception store of the canopy is considerably smaller than originally specified in the SiB model.

Sellers, Piers J.↗

NASA NTRS1988

Large-scale experimental technology with remote sensing in land surface hydrology and meteorology

Two field experiments to study atmospheric and land surface processes and their interactions are summarized. The Hydrologic-Atmospheric Pilot Experiment, which tested techniques for measuring evaporation, soil moisture storage, and runoff at scales of about 100 km, was conducted over a 100 X 100 km area in France from mid-1985 to early 1987. The first International Satellite Land Surface Climatology Program field experiment was conducted in 1987 to develop and use relationships between current satellite measurements and hydrologic, climatic, and biophysical variables at the earth's surface and to validate these relationships with ground truth. This experiment also validated surface parameterization methods for simulation models that describe surface processes from the scale of vegetation leaves up to scales appropriate to satellite remote sensing.

Brutsaert, Wilfried↗

NASA NTRS1987

Albedos of homogeneous semi-infinite canopies - Comparison of two-stream analytic and numerical solutions

The albedo of plant canopies is treated as a problem in radiative transfer. Albedos calcualted from an iterative multistream numerical model are compared with those calculated with an analytic two-stream solution. With the assumption of a randomly homogeneous distribution of leaf positions and orientations and isotropic scattering by individual leaves, the single-scattering albedo of the canopy can be found analytically. This single-scattering solution is incorporated into the two-stream solution and used to benchmark the multistream numerical model in the single-scattering limit. Relative errors so established in the multistream model are O(0.3 percent) or less. The two-stream model is also found to be remarkably accurate, with the error in multiply scattered radiation O(5 percent) or less, corresponding to absolute errors in visible albedo of less than 0.001 and near-infrared albedo of less than or equal to 0.01. Thus the two-stream model should be adequate for many purposes, such as climate modeling, provided the assumptions of homogeneous canopy and isotropic scattering are not too unrealistic.

Dickinson, Robert E.↗

NASA NTRS1987

The First ISLSCP Field Experiment (FIFE)

In 1987, the International Satellite Land Surface Climatology Project (ISLSCP) will conduct a major field experiment aimed at the development of superior methods for relating satellite remote sensing to biological and physical parameter and exchange process data obtained during simultaneous ground truth measurements. This, the First ISLSCP Field Experiment, will attempt to arrive at a better understanding of the role of the land surface in the behavior of the global climate system, as required by next-generation global circulation models and general biospheric models. The field experiment will be conducted in the Konza Prairie of Kansas.

Murphy, Robert E.↗