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Holben, B.

Publications and source records attributed to Holben, B..

30 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Comparison of Columnar Water Vapor Measurements During The Fall 1997 ARM Intensive Observation Period: Optical Methods

In the fall of 1997 the Atmospheric Radiation Measurement (ARM program conducted an intensive Observation Period (IOP) to study water vapor at its Southern Great Plains (SGP) site. Among the large number of instruments, four sun-tracking radiometers were present to measure the columnar water vapor (CWV). All four solar radiometers retrieve CWV by measuring solar transmittance in the 0.94-micrometer water vapor absorption band. As one of the steps in the CWV retrievals the aerosol component is subtracted from the total transmittance, in the 0.94-micrometer band. The aerosol optical depth comparisons among the same four radiometers are presented elsewhere. We have used three different methods to retrieve CWV. Without attempting to standardize on the same radiative transfer model and its underlying water vapor spectroscopy we found the CWV to agree within 0.13 cm (rms) for CWV values ranging from 1 to 5 cm. Preliminary results obtained when using the same updated radiative transfer model with updated spectroscopy for all instruments will also be shown. Comparisons to the microwave radiometer results will be included in the comparisons.

Schmid, Beat↗
NASA NTRS2000

Relative Contributions of the Saharan and Sahelian Sources to the Atmospheric Dust Load Over the North Atlantic

It has long been recognized that Saharan desert is the major source for long range transport of mineral dust over the Atlantic. The contribution from other natural sources to the dust load over the Atlantic has generally been ignored in previous model studies or been replaced by anthropogenically disturbed soil emissions. Recently, Prospero et.at. have identified the major dust sources over the Earth using TOMS aerosol index. They showed that these sources correspond to dry lakes with layers of sediment deposed in the late Holocene or Pleistocene. One of the most active of these sources seem to be the Bodele depression. Chiapello et al. have analyzed the mineralogical composition of dust on the West coast of Africa. They found that Sahelian dust events are the most intense but are less frequent than Saharan plumes. This suggests that the Bodele depression could contribute significantly to the dust load over the Atlantic. The relative contribution of the Sahel and Sahara dust sources is of importance for marine biogeochemistry or atmospheric radiation, because each source has a distinct mineralogical composition. We present here a model study of the relative contributions of Sahara and Sahel sources to the atmospheric dust aerosols over the North Atlantic. The Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model is used to simulate dust distribution in 1996-1997. Dust particles are labeled depending on their sources. In this presentation, we will present the comparison between the model results and observations from ground based measurements (dust concentration, optical thickness and size distribution) and satellite data (TOMS aerosol index). The relative contribution of each source will then be analyzed spatially and temporally.

Ginoux, Paul↗
NASA NTRS2000

Validation of MODIS Aerosol Retrievals during PRIDE

The Puerto Rico Dust Experiment (PRIDE) was held in Roosevelt Roads, Puerto Rico from June 26 to July 24, 2000. It was intended to study the radiative and microphysical properties of Saharan dust transported into Puerto Rico. PRIDE had the unique distinction of being the first major field experiment to allow direct comparison of aerosol retrievals from MODIS (MODerate Imaging Spectro-radiometer - aboard the Terra satellite) with data from a variety of ground, shipboard and air-based instruments. Over the ocean the MODIS algorithm retrieves optical depth as well as information about the aerosol's size. During PRIDE, MODIS passed over Roosevelt Roads approximately once per day during daylight hours. Due to sunglint and clouds over Puerto Rico, aerosol retrievals can be made from only about half the MODIS scenes. In this study we try to "validate" our aerosol retrievals by comparing to measurements taken by sun-photometers from multiple platforms, including: Cimel (AERONET) from the ground, Microtops (handheld) from ground and ship, and the NASA-Ames sunphotometer from the air.

Levy, R.↗
NASA NTRS1999

Aerosol Absorption Measurements from LANDSAT and CIMEL

Spectral remote observations of dust properties from space and from the ground create a powerful tool for determination of dust absorption of solar radiation with an unprecedented accuracy. Absorption is a key component in understanding dust impact on climate. We use Landsat space-borne measurements at 0.47 to 2.2 micrometer over Senegal with ground-based sunphotometers to find that Saharan dust absorption of solar radiation is two to four times smaller than in models. Though dust absorbs in the blue, almost no absorption was found for wavelengths greater than 0.6 micrometer. The new finding increases by 50% recent estimated solar radiative forcing by dust and decreases the estimated dust heating of the lower troposphere. Dust transported from Asia shows slightly higher absorption probably due to the presence of black carbon from populated regions. Large-scale application of this method to satellite data from the Earth Observing System can reduce significantly the uncertainty in the dust radiative effects.

Kaufman, Y. J.↗
NASA NTRS1988

Agriculture Multiband Experiment Radiometer

Agriculture Multiband Experiment Radiometer (Amber) was designed and built in 1979 to 1980 and deployed in 1981. Amber was designed to simultaneously measure sunlight reflected from vegetation in 15 optical bands. In 1982, program interest shifted from ground truth to satellite image reduction and Amber was retired. Early in 1987, the project scientists concluded that Amber, because of its 15 simple and independent optical systems, would be ideally suited to study polarized light. Changes were made to add polarimeter capability to the instrument. The changes are listed and briefly discussed.

Jones, W. H.↗
NASA NTRS1986

Directional reflectance response in AVHRR red and near-IR bands for three cover types and varying atmospheric conditions

Surface directional red and near-infrared reflectances of bare soil, orchard grass, and fescue were extracted from a multitemporal data set to correspond to NOAA-7 Advanced Very High Resolution Radiometer (AVHRR) scanning and illumination geometry at 30 deg latitude. Radiances were simulated at satellite altitude and a ratio vegetation index was calculated. The results show that off-nadir directional reflectance measured at the surface in the red and near-IR portions of the spectrum are approximately maintained with AVHRR viewing and illumination characteristics. The two-channel reflectance response is such that the ratio vegetation index is more constant with scan angle and atmospheric conditions than individual channels. It is shown that inclusion of atmospheric and surface reflectance data can greatly improve interpretation of AVHRR data, when knowledge of the range of atmospheric conditions and approximate directional reflectances of major cover types are known.

Holben, B.↗
NASA NTRS1986

Identifying deforestation in Brazil using multiresolution satellite data

The use of multiresolution satellite data to monitor deforestation on a continental/subcontinental scale is examined. MSS, local area coverage (LAC), global area coverage (GAC), and GOES data were applied to the study of deforestation in Rondonia, Brazil; the characteristics of these sensors are described. The probability thresholding and vegetation-index thresholding procedures used to process the data in order to differentiate between forest from nonforest are analyzed. The LAC, GAC, and GOES data are compared to MSS data. It is observed that GOES data is not useful for monitoring colonization projects due to excessive noise in the data; the GAC data is only applicable in large areas of contiguous forest clearing; the MSS data when available is applicable as a ground data reference source for differentiating cleared areas from primary; the LAC data are capable of delineating colonization clearings; and the probability thresholding procedure differentiates forest from nonforest more accurately than the vegetation-index procedure. The data reveal that the LAC data combined with the probability threshold procedure provide the best data-source/classification-procedure combination.

Nelson, R.↗
NASA NTRS1984

Monitoring and mapping global vegetation cover using data from meteorological satellites

The role of coarse resolution meteorological satellite data for monitoring and mapping of vegetation for global, continental and regional scales is outlined. In the NOAA products used the effects of cloud cover are reduced by the generation of temporal composites of images of the normalized difference vegetation index. Different land cover types are shown to have characteristic spectral phenological curves. Such data have the disadvantage of effectively increasing the apparent areal extent of small areas of green vegetation.

Townshend, J. R. G.↗
NASA NTRS1984

Red and near-infrared sensor response to off-nadir viewing

It has first been intended to employ the Advanced Very High Resolution Radiometer (AVHRR) as a meteorological instrument to aid in detecting clouds and atmospheric temperature stratifications over large areas. However, more recently the satellite data have been used for monitoring large areas of vegetation with emphasis on the visible (VIS) and near-infrared (NIR) bands. The present investigation is concerned with the objective to model ranges in AVHRR VIS and NIR radiances and normalized differences expected under typical AVHRR viewing and illumination conditions. Atmospheric data sets reported by Dave (1978) and Dave and Braslau (1975) were used. It is shown that sensor response to atmospheric path length can be substantial for the AVHRR VIS and NIR channels and normalized difference (ND) values. However, errors can be minimized by high sun and clear atmosphere viewing.

Holben, B.↗
NASA NTRS1981

An examination of spectral band ratioing to reduce the topographic effect on remotely sensed data

Spectral-band ratioing of radiance data is examined as a means of reducing the topographic effect in multispectral data. A ground-based nadir-pointing two-channel radiometer filtered for the red and photographic IR portions of the spectrum was used to measure the topographic effect associated with a uniform surface inclined from horizontal to 60 deg at 16 compass points and for several solar elevations. It is found that ratioing reduced the topographic effect in the field-measured radiance data by an average of 83%, that the remaining topographic effect could be further reduced by subtracting the scattered-light component of the global irradiance before ratioing, and that ratioing was not effective in reducing the topographic effect on shaded surfaces illuminated solely by scattered light. It is concluded that additional variations in ratios can be expected for Landsat data owing to sensor calibration and quantization.

Holben, B.↗
NASA NTRS1980

The contribution of the diffuse light component to the topographic effect on remotely sensed data

The topographic effect is measured by the difference between the global radiance from inclined surfaces as a function of their orientation relative to the sensor position and light source. The short wave radiant energy incident on a surface is composed of direct sunlight, scattered skylight, and light reflected from surrounding terrain. The latter two components are commonly known as the diffuse component. The contribution of the diffuse light component to the topographic effect was examined and the significance of this diffuse component with respect to two direct radiance models was assessed. Diffuse and global spectral radiances were measured for a series of slopes and aspects of a uniform and surface in the red and photographic infrared parts of the spectrum, using a nadir pointing two channel handheld radiometer. The diffuse light was found to produce a topographic effect which varied from the topographic effect for direct light. The topographic effect caused by diffuse light was found to increase slightly with solar elevation and wavelength for the channels examined. The correlations between data derived from two simple direct radiance simulation models and the field data were not significantly affected when the diffuse component was removed from the radiances. Radiances from a 60 percent reflective surface, assuming no atmospheric path radiance, the diffuse light topographic effect contributed a maximum range of 3 pixel values in simulated LANDSAT data from all aspects with slopes up to 30 degrees.

Justice, C.↗