Engineering topics

Evans, Robert H.

Publications and source records attributed to Evans, Robert H..

11 records

NASA NTRS2013

Regional to Global Assessments of Phytoplankton Dynamics From The SeaWiFS Mission

Photosynthetic production of organic matter by microscopic oceanic phytoplankton fuels ocean ecosystems and contributes roughly half of the Earth's net primary production. For 13 years, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) mission provided the first consistent, synoptic observations of global ocean ecosystems. Changes in the surface chlorophyll concentration, the primary biological property retrieved from SeaWiFS, have traditionally been used as a metric for phytoplankton abundance and its distribution largely reflects patterns in vertical nutrient transport. On regional to global scales, chlorophyll concentrations covary with sea surface temperature (SST) because SST changes reflect light and nutrient conditions. However, the oceanmay be too complex to be well characterized using a single index such as the chlorophyll concentration. A semi-analytical bio-optical algorithm is used to help interpret regional to global SeaWiFS chlorophyll observations from using three independent, well-validated ocean color data products; the chlorophyll a concentration, absorption by CDM and particulate backscattering. First, we show that observed long-term, global-scale trends in standard chlorophyll retrievals are likely compromised by coincident changes in CDM. Second, we partition the chlorophyll signal into a component due to phytoplankton biomass changes and a component caused by physiological adjustments in intracellular chlorophyll concentrations to changes in mixed layer light levels. We show that biomass changes dominate chlorophyll signals for the high latitude seas and where persistent vertical upwelling is known to occur, while physiological processes dominate chlorophyll variability over much of the tropical and subtropical oceans. The SeaWiFS data set demonstrates complexity in the interpretation of changes in regional to global phytoplankton distributions and illustrates limitations for the assessment of phytoplankton dynamics using chlorophyll retrievals alone.

phytoplankton↗

NASA NTRS1995

Modis, SeaWIFS, and Pathfinder funded activities

MODIS (Moderate Resolution Imaging Spectrometer), SeaWIFS (Sea-viewing Wide Field Sensor), Pathfinder, and DSP (Digital Signal Processor) objectives are summarized. An overview of current progress is given for the automatic processing database, client/server status, matchup database, and DSP support.

Evans, Robert H.↗

NASA NTRS1995

SeaWiFS technical report series. Volume 28: SeaWiFS algorithms, part 1

This document provides five brief reports that address several algorithm investigations sponsored by the Calibration and Validation Team (CVT) within the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project. This volume, therefore, has been designated as the first in a series of algorithm volumes. Chapter 1 describes the initial suite of masks, used to prevent further processing of contaminated radiometric data, and flags, which are employed to mark data whose quality (due to a variety of factors) may be suspect. In addition to providing the mask and flag algorithms, this chapter also describes the initial strategy for their implementation. Chapter 2 evaluates various strategies for the detection of clouds and ice in high latitude (polar and sub-polar regions) using Coastal Zone Color Scanner (CZCS) data. Chapter 3 presents an algorithm designed for detecting and masking coccolithosphore blooms in the open ocean. Chapter 4 outlines a proposed scheme for correcting the out-of-band response when SeaWiFS is in orbit. Chapter 5 gives a detailed description of the algorithm designed to apply sensor calibration data during the processing of level-1b data.

Hooker, Stanford B.↗

NASA NTRS1994

SeaWiFS technical report series. Volume 17: Ocean color in the 21st century. A strategy for a 20-year time series

Beginning with the upcoming launch of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), there should be almost continuous measurements of ocean color for nearly 20 years if all of the presently planned national and international missions are implemented. This data set will present a unique opportunity to understand the coupling of physical and biological processes in the world ocean. The presence of multiple ocean color sensors will allow the eventual development of an ocean color observing system that is both cost effective and scientifically based. This report discusses the issues involved and makes recommendations intended to ensure the maximum scientific return from this unique set of planned ocean color missions. An executive summary is included with this document which briefly discusses the primary issues and suggested actions to be considered.

Abbott, Mark R.↗

NASA NTRS1994

Coastal zone color scanner 'system calibration': A retrospective examination

During its lifetime the Coastal Zone Color Scanner (CZCS) produced approximately 66,000 images. These have been placed in an archive of 'raw' radiance (sensor counts) in a subsampled format that is easily accessible. They have also been processed to form global fields, at reduced resolution, of normalized water-leaving radiance, phytoplankton pigments, and diffuse attenuation coefficient. Using this archive, we have tried to characterize some aspects of the 'system calibration' for the 8-year lifetime of CZCS. Specifically, we have assumed that the sensitivity of the red band decayed in a simple manner similar to the well-known long-term degradation of the shorter-wavelength bands, and we examined the sensitivity of the green and yellow bands by computing the globally averaged water-leaving radiance, over 10-day periods, for all of the imagery. The results provided evidence that in addition to the long-term degradation, short-term (2 weeks to 1 month) variations in the radiometric sensitivity of these bands started in early fall 1981 and continued for the rest of the mission. In contrast, the data suggested the absence of such variations prior to August 1981. It is reasonable to believe that the sensitivity of the blue (and probably the red) band underwent such variations as well; however our methodology cannot be used to study the other bands. Thus, after these fluctuations began, the actual values of CZCS - estimated pigment concentrations at a given location should be viewed with skepticism; however, the global patterns of derived pigment concentrations should be valid. Had an extensive set of surface measurements of water-leaving radiance, e.r., from moored buoyes or drifters, had been available during the CZCS mission, these fluctuations could have been removed from the data set, and this would have greatly increased its value. The lessons learned from CZCS that is, the requirement of good radiometric calibration and stability and the necessity of 'sea truth' stations to monitor the performance of the system (sensor plus algorithms), are being applied to the sea-viewing wide-field-of-view senso (Sea WiFS) scheduled for launch in August 1993.

Evans, Robert H.↗

NASA NTRS1992

Remote Earth Sciences data collection using ACTS

Given the focus on global change and the attendant scope of such research, we anticipate significant growth of requirements for investigator interaction, processing system capabilities, and availability of data sets. The increased complexity of global processes requires interdisciplinary teams to address them; the investigators will need to interact on a regular basis; however, it is unlikely that a single institution will house sufficient investigators with the required breadth of skills. The complexity of the computations may also require resources beyond those located within a single institution; this lack of sufficient computational resources leads to a distributed system located at geographically dispersed institutions. Finally the combination of long term data sets like the Pathfinder datasets and the data to be gathered by new generations of satellites such as SeaWiFS and MODIS-N yield extra-ordinarily large amounts of data. All of these factors combine to increase demands on the communications facilities available; the demands are generating requirements for highly flexible, high capacity networks. We have been examining the applicability of the Advanced Communications Technology Satellite (ACTS) to address the scientific, computational, and, primarily, communications questions resulting from global change research. As part of this effort three scenarios for oceanographic use of ACTS have been developed; a full discussion of this is contained in Appendix B.

Evans, Robert H.↗

NASA NTRS1991

On the seasonal phytoplankton concentration and sea surface temperature cycles of the Gulf of Mexico as determined by satellites

Multiyear series of coastal zone color scanner (CZCS) and AVHRR observations are presently used to derive monthly climatologies of near-surface phytoplankton pigment concentration and SST for the Gulf of Mexico; these, in combination with 1946-1987 SST data and NOAA hydrographic profile data covering 1914-1985, show that the most important single factor controlling seasonal cycle surface-pigment concentration is the depth of the mixed layer. The CZCS images indicate that seasonal variation seaward of the continental shelf is synchronous throughout the Gulf. The combination of ocean color and IR images allows year-round observation of surface circulation spatial structure in the Gulf, as well as of the dispersal pattern of the Mississippi River's plume.

Mueller-Karger, Frank E.↗

NASA NTRS1991

The annual cycle of satellite-derived sea surface temperature in the southwestern Atlantic Ocean

The annual cycle of sea surface temperature (SST) in the southwestern Atlantic Ocean was estimated using four years (July 1984-July 1988) of NOAA Advanced Very High Resolution Radiometer observations. High resolution satellite observations at 1-km space and daily time resolution were grided at 100-km space and 5-day time intervals to develop an analysis dataset for determination of low frequency SST variability. The integral time scale, a measure of serial correlation, was found to vary from 40 to 60 days in the domain of interest. The existence of superannual trends in the SST data was investigated, but conclusive results could not be obtained. The annual cycle (and, in particular, the annual harmonic) explains a large proportion of the SST variability. The estimated amplitude of the cycle ranges between 5 deg and 13 deg C throughout the study area, with minima in August-September and maxima in February. The resultant climatology is compared with an arbitrary 5-day satellite SST field, and with the COADS/ICE SST climatology. It was found that the higher resolution satellite-based SST climatology resolves boundary current structure and has significantly better structural agreement with the observed field.

Podesta, Guillermo P.↗

NASA NTRS1988

A semianalytic radiance model of ocean color

A semianalytical radiance model is developed which predicts the upwelled spectral radiance at the sea surface as a function of the phytoplankton pigment concentration for Morel Case 1 waters. The model is in good agreement with experimental measurements carried out in waters which were not included in the data base used to derive it. It suggests that the observed variability in the radiance is due to variations in the backscattering of plankton and the associated detrital material. The model is extended to include other material in the water, such as dissolved organic material, referred to as yellow substances, and detached coccoliths from coccolithophorids, e.g., Emiliana huxleyi. Potential applications include an improved biooptical algorithm for the retrieval of pigment concentrations from satellite imagery in the presence of interference from detached coccoliths and an improved atmospheric correction for satellite imagery. The model also serves to identify and to interpret deviations from Case 1 waters.

Gordon, Howard R.↗

NASA NTRS1988

Exact Rayleigh scattering calculations for use with the Nimbus-7 Coastal Zone Color Scanner

The radiance reflected from a plane-parallel atmosphere and flat sea surface in the absence of aerosols has been determined with an exact multiple scattering code to improve the analysis of Nimbus-7 CZCS imagery. It is shown that the single scattering approximation normally used to compute this radiance can result in errors of up to 5 percent for small and moderate solar zenith angles. A scheme to include the effect of variations in the surface pressure in the exact computation of the Rayleigh radiance is discussed. The results of an application of these computations to CZCS imagery suggest that accurate atmospheric corrections can be obtained for solar zenith angles at least as large as 65 deg.

Gordon, Howard R.↗

NASA NTRS1987

Multiplatform sampling (ship, aircraft, and satellite) of a Gulf Stream warm core ring

The purpose of this paper is to demonstrate the ability to meet the need to measure distributions of physical and biological properties of the ocean over large areas synoptically and over long time periods by means of remote sensing utilizing contemporaneous buoy, ship, aircraft, and satellite (i.e., multiplatform) sampling strategies. A mapping of sea surface temperature and chlorophyll fields in a Gulf Stream warm core ring using the multiplatform approach is described. Sampling capabilities of each sensing system are discussed as background for the data collected by means of these three dissimilar methods. Commensurate space/time sample sets from each sensing system are compared, and their relative accuracies in space and time are determined. The three-dimensional composite maps derived from the data set provide a synoptic perspective unobtainable from single platforms alone.

Smith, Raymond C.↗