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NASA NTRS1992

Quantifying reflectance anisotropy of photosynthetically active radiation in grasslands

Quantifying the vegetative surface's reflectance anisotropy was an important part of the First ISLSCP Field Experiment, as its major objectives focused on retrieval of surface parameters from satellite-derived reflectances. The explicit remote measurements for approximating the bidirectional reflectance distribution function (BRDF) of photosynthetically active radiation had not been previously undertaken. In this paper the proper expression of reflectance for BRDFs for retrieval of canopy parameters is assessed.

Middleton, Elizabeth M.↗

NASA NTRS1992

Active and passive microwave measurements of soil moisture in FIFE

During the intensive field campaigns of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) in May-October of 1987, several nearly simultaneous measurements were made with low-altitude flights of the L-band radiometer and C- and X-band scatterometers over two transects in the Konza Prairie Natural Research Area, some 8 km south of Manhattan, Kansas. These measurements showed that although the scatterometers were sensitive to soil moisture variations in most regions under the flight path, the L-band radiometer lost most of its sensitivity in regions unburned for many years. The correlation coefficient derived from the regression between the radar backscattering coefficient and the soil moisture was found to improve with the increase in antenna incidence angle. This is attributed to a steeper falloff of the backscattering coefficient as a function of local incidence at angles near nadir than at angles greater than 30 deg.

Wang, J. R.↗

NASA NTRS1992

Soil moisture variability within remote sensing pixels

The effects of topography and the level of soil moisture on the variability of soil moisture within remote sensing pixels were assessed during the First ISLSCP Field Experiment (FIFE) during 1987 and 1989. Soil moisture data from flat, sloped, and valley-shaped pixels were obtained over a wide range of moisture conditions. Relative elevation data were obtained for each study area to create digital elevation models with which to quantify topographic variability. Within-pixel soil moisture variability was shown to increase with increased topographic heterogeneity. The flat pixel had significantly lower standard deviations and fewer outlier points than the slope and valley pixels. Most pixel means had a positive skewness, indicating that most pixels will have areas of markedly higher than average soil moisture.

Charpentier, Michael A.↗

NASA NTRS1992

A catchment scale water balance model for FIFE

A catchment scale water balance model is presented and used to predict evaporation from the King's Creek catchment at the First ISLSCP Field Experiment site on the Konza Prairie, Kansas. The model incorporates spatial variability in topography, soils, and precipitation to compute the land surface hydrologic fluxes. A network of 20 rain gages was employed to measure rainfall across the catchment in the summer of 1987. These data were spatially interpolated and used to drive the model during storm periods. During interstorm periods the model was driven by the estimated potential evaporation, which was calculated using net radiation data collected at site 2. Model-computed evaporation is compared to that observed, both at site 2 (grid location 1916-BRS) and the catchment scale, for the simulation period from June 1 to October 9, 1987.

Famiglietti, J. S.↗

NASA NTRS1991

Remote sensing of atmospheric chemistry; Proceedings of the Meeting, Orlando, FL, Apr. 1-3, 1991

The present volume on remote sensing of atmospheric chemistry discusses special remote sensing space observations and field experiments to study chemical change in the atmosphere, network monitoring for detection of stratospheric chemical change, stratospheric chemistry studies, and the combining of model, in situ, and remote sensing in atmospheric chemistry. Attention is given to the measurement of tropospheric carbon monoxide using gas filter radiometers, long-path differential absorption measurements of tropospheric molecules, air quality monitoring with the differential optical absorption spectrometer, and a characterization of tropospheric methane through space-based remote sensing. Topics addressed include microwave limb sounder experiments for UARS and EOS, an overview of the spectroscopy of the atmosphere using an FIR emission experiment, the detection of stratospheric ozone trends by ground-based microwave observations, and a FIR Fabry-Perot spectrometer for OH measurements.

Mcelroy, James L.↗

NASA NTRS1993

Estimation of surface heat and moisture fluxes over a prairie grassland. III - Design of a hybrid physical/remote sensing biosphere model

The design and formulation of an experimental biosphere model (Ex-BATS), based on the biosphere-atmosphere transfer scheme (BATS) of Dickinson (1983, 1984) and Dickinson et al. (1986) are described. The Ex-BATS was designed to incorporate both in situ measurements and satellite parameterizations of certain canopy variables which are slowly varying in the course of a growing season. The new model was validated using measurements obtained during the May 28 to August 21 First International Satellite and Surface Climatology Project Field Experiment, showing that the Ex-BATS model reproduced closely the diurnal behavior of the surface fluxes.

Smith, Eric A.↗

NASA NTRS1993

Photosynthesis and stomatal conductance related to reflectance on the canopy scale

Field measurements of carbon dioxide and water vapor fluxes were analyzed in conjunction with reflectances obtained from a helicopter-mounted Modular Multiband Radiometer at a grassland study site during the First International Satellite Land Surface Climatology Project Field Experiment. These measurements are representative of the canopy scale and were made over a range of meteorological and soil moisture conditions during different stages in the annual life cycle of the prairie vegetation, and thus provide a good basis for investigating hpotheses/relationships potentially useful in remote sensing applications. We tested the hypothesis (Sellers, 1987) that the simple ratio vegetation index should be near-linearly related to the derivatives of the unstressed canopy stomatal conductance and the unstressed canopy photosynthesis with respect to photosynthetically active radiation. Even though there is some scatter in our data, the results seem to support this hypothesis.

Verma, S. B.↗

NASA NTRS1992

Measurement of volatile organic chemicals at selected sites in California

Urban air concentrations of 24 selected volatile organic chemicals that may be potentially hazardous to human health and environment were measured during field experiments conducted at two California locations, at Houston, and at Denver. Chemicals measured included chlorofluorocarbons, halomethanes, haloethanes, halopropanes, chloroethylenes, and aromatic hydrocarbons. With emphasis on California sites, data from these studies are analyzed and interpreted with respect to variabilities in ambient air concentrations, diurnal changes, relation to prevailing meteorology, sources and trends. Except in a few instances, mean concentrations are typically between 0 and 5 ppb. Significant variabilities in atmospheric concentrations associated with intense sources and adverse meteorological conditions are shown to exist. In addition to short-term variability, there is evidence of systematic diurnal and seasonal trends. In some instances it is possible to detect declining trends resulting from the effectiveness of control strategies.

Singh, Hanwant B.↗

NASA NTRS1992

An intercomparison of measured and modeled canopy parameters during the variable water-stressed period of FIFE 1989 using an hybrid biosphere-remote sensing model

The measured atmospheric parameters and the fluxes of latent and sensible heat across the biosphere-atmosphere interface collected during the water-stressed period of the First ISLSCP Field Experiment (FIFE) were compared with those calculated by an experimental version of the Biosphere-Atmosphere Transfer Scheme (Ex-BATS). It is shown that the brightness temperature (T(B)) values observed near the surface during FIFE 1987 are closely correlated to in-canopy temperatures calculated by Ex-BATS. The 1987 near-surface observations of T(B) are also well correlated to AVHHR channels 4 and 5 measurements. An inverted form of Ex-BATS was applied to determine the associated required in-canopy temperatures, T(icr), and regressions between T(icr) and T(B) found from the 1987 data were applied to the 1989 observed T(B) at a different site. The T(icr) so estimated showed excellent correlation to the 1989 model calculated T(icr).

Cooper, H. J.↗

NASA NTRS1993

Gravity waves generated by a tropical cyclone during the STEP tropical field program - A case study

Overflights of a tropical cyclone during the Australian winter monsoon field experiment of the Stratosphere-Troposphere Exchange Project (STEP) show the presence of two mesoscale phenomena: a vertically propagating gravity wave with a horizontal wavelength of about 110 km and a feature with a horizontal scale comparable to that of the cyclone's entire cloud shield. The larger feature is fairly steady, though its physical interpretation is ambiguous. The 110-km gravity wave is transient, having maximum amplitude early in the flight and decreasing in amplitude thereafter. Its scale is comparable to that of 100-to 150-km-diameter cells of low satellite brightness temperatures within the overall cyclone cloud shield; these cells have lifetimes of 4.5 to 6 hrs. These cells correspond to regions of enhanced convection, higher cloud altitude, and upwardly displaced potential temperature surfaces. The temporal and spatial distribution of meteorological variables associated with the 110-km gravity wave can be simulated by a slowly moving transient forcing at the anvil top having an amplitude of 400-600 m, a lifetime of 4.5-6 hrs, and a size comparable to the cells of low brightness temperature.

Pfister, L.↗

NASA NTRS1992

A data base of ASAS digital imagery

The Advanced Solid-State Array Spectroradiometer (ASAS) is an airborne, off-nadir tilting, imaging spectroradiometer that acquires digital image data for 29 spectral bands in the visible and near-infrared. The sensor is used principally for studies of the bidirectional distribution of solar radiation scattered by terrestial surfaces. ASAS has acquired data for a number of terrestial ecosystem field experiments and investigators have received over 170 radiometrically corrected, multiangle, digital image data sets. A database of ASAS digital imagery has been established in the Pilot Land Data System (PLDS) at the NASA/Goddard Space Flight Center to provide access to these data by the scientific community. ASAS, its processed data, and the PLDS are described, together with recent improvements to the sensor system.

Irons, James R.↗

NASA NTRS1992

Radiometric calibration and atmospheric correction of satellite and aircraft data for FIFE

The satellite and aircraft radiometric calibration and atmospheric correction work carried out as part of the first International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) are summarized. A large volume (120 Gbytes) of radiometric data were acquired and derived from a number of different instruments on a variety of platforms. The same basic procedure was applied to each instrument: derive the most recent calibration coefficients for converting sensor counts to reflective spectral radiances; correct the radiances for earth-sun distance variations and incident solar spectral irradiance within the bandpass of each respective instrument channel at the top of the atmosphere; characterize the atmosphere for aerosols and absorbing gases; and derive apparent surface reflectance by correcting the exoatmospheric values for atmospheric attenuation. The same basic approach was used for surface temperature derivation. The results of this processing were verified by surface measurements, and corroborated by sensor intercomparisons.

Goetz, Scott J.↗

NASA NTRS1992

Comparison of measurements and theory for backscatter from vegetation-covered soil on the Konza prairie

Radar backscatter measurements over the Konza Prairie were obtained by means of C- and X-band scatterometers as a part of the first ISLSCP Field Experiment (FIFE) to determine soil moisture. Nearly simultaneous radar and radiometer data sets were collected along two transects that coincided with direct soil-moisture measurements. The results show that radars can be used for soil-moisture estimation over the complete transect, whereas radiometer sensitivity to soil moisture is drastically reduced over regions left unburned for many years. A combined rough-surface/volume scatter model was formulated. Calculated and measured scattering data are compared to determine the sensitivity of the scattering coefficient to different surface treatments.

Gogineni, S.↗

NASA NTRS1992

Estimating reflected and emitted components of the radiation balance using remotely-sensed spectral data from KUREX-91

The estimation of incoming and outgoing radiation streams using bidirectional spectral reflectances and bidirectional thermal emittances is described. Good agreement between measured and modeled estimates of the radiation balance is obtained. The data used were collected over selected grassland sites on the Streletskaya Steppe Reserve in Russia in July 1991. Results from this study compare well with results obtained during the first ISLSCP field experiment (FIFE) study in Kansas in 1987-89.

Blad, B. L.↗

NASA NTRS1992

Spectral bidirectional and hemispherical reflectance characteristics of selected sites in the Streletskaya steppe

Measurements of plant canopy bidirectional reflectance made by the PARABOLA (portable apparatus for rapid acquisition of bidirectional observations of the land and atmosphere) instrument in three spectral bands are analyzed for steppe grassland sites of differing productivity levels. The variation of spectral reflectance and the normalized difference vegetation index in the solar principal plane is presented. Comparisons are made with PARABOLA measurements from selected first ISLSCP field experiment (FIFE) grassland sites in the Konza prairie, Kansas. The Streletskaya steppe sites showed no strong hot spot reflectance, while this effect was present in some FIFE sites but absent in others. The hot spot effect seems to be dependent on canopy geometry and background reflectance characteristics of these sites. Spectral hemispherical reflectance was computed from the angular integration of the bidirectional measurements for the steppe sites. Total shortwave albedo was estimated from these hemispherical reflectance measurements and compared to albedo measured by pyranometers. The albedo estimates from PARABOLA were found to be approximately 12-17 percent higher than the pyranometer measurements.

Eck, Thomas F.↗

NASA NTRS1992

Passive microwave signatures of simulated pancake ice and young pressure ridges

Surface-based passive microwave observations of simulated pancake ice and of a first-year pressure ridge are described. The observed emissivity spectra are very similar to those observed on several field experiments in the polar regions. Certain spectral features such as a maximum emissivity at 37 GHz for pancake ice and a polarization ratio very close to zero for ridged ice are distinct from those of new, young, and first-year ice. They suggest possible signatures which may help to characterize these ice types using passive microwave techniques.

Grenfell, T. C.↗

NASA NTRS1993

A stepped-frequency Delta-K microwave radar for oceanographic studies

A stepped-frequency Delta-K (SFDK) radar was developed to remotely sense ocean surface characteristics. This C-band radar uses frequency diversity and real-time signal processing for improved Delta-K measurements. In this paper, we show how frequency diversity can be implemented to enhance the energy of the resonant peak of the cross-product spectrum relative to the background energy. This feature is essential for making real-time measurements of resonant peak frequency over extended time periods. The SFDK was used in a month-long field experiment at N. Truro (Massachusetts). The results presented show that the phase velocity of ocean surface waves could be precisely measured 87 percent of the time.

Popstefanija, Ivan↗

NASA NTRS1993

Comparison of land surface temperatures derived from satellite observations with ground truth during FIFE

Surface temperatures of the FIFE (First ISLSCP Field Experiment) experimental area derived from thermal infrared radiances recorded from different satellite platforms at different scales were compared with reference observations by means of infrared thermometers at ground stations distributed over the area. FIFE was conducted during late spring, summer and fall over an area of 15 km by 15 km in a hilly tall-grass prairie region in northeastern Kansas. The data available for this purpose were produced by AVHRR and TOVS instruments aboard NOAA-9 and NOAA-10, the TM instrument aboard Landsat-5, and VISSR instrument aboard GOES-7. The scales covered by these instruments span a wide range, namely between hundreds of meters (Landsat TM) and hundreds of kilometers (TOVS). The data are analyzed both with and without the application of an atmospheric correction.

Sugita, M.↗