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NASA NTRS1984

Atmospheric aerosol backscatter measurements using a tunable coherent CO2 lidar

Measurements of atmospheric aerosol backscatter coefficients, using a coherent CO2 lidar at 9.25- and 10.6-micron wavelengths, are described. Vertical profiles of the volume backscatter coefficient beta have been measured to a 10-km altitude over the Pasadena, CA, region. These measurements indicate a wide range of variability in beta both in and above the local boundary layer. Certain profiles also indicate a significant enhancement in beta at the 9.25-micron wavelength compared with beta at the 10.6-micron wavelength, which possibly indicates a major contribution to the volume backscatter from ammonium sulfate aerosol particles.

Menzies, R. T.↗

NASA NTRS1985

Development of global model for atmospheric backscatter at CO2 wavelengths

The improvement of an understanding of the variation of the aerosol backscattering at 10.6 micron within the free troposphere and the development model to describe this was undertaken. The analysis combines theoretical modeling with the results contained within three independent data sets. The data sets are obtained by the SAGE I/SAM II satellite experiments, the GAMETAG flight series and by direct backscatter measurements. The theoretical work includes use of a bimodal, two component aerosol model, and the study of the microphysical and associated optical changes occurring within an aerosol plume. A consistent picture is obtained, which describes the variation of the aerosol backscattering function in the free troposphere with altitude, latitude, and season. Most data are available and greatest consistency is found inside the Northern Hemisphere.

Kent, G. S.↗

NASA NTRS1983

Determination of atmospheric backscatter at 10.6 microns

Profiles have been obtained for atmospheric aerosol backscatter coefficient versus altitude at four different geographical locations, by means of a 10.6-micron lidar that uses a CO2 laser. A Mach-Zehnder interferometer is used to separate the laser output beam into a measurement beam and a reference beam. The interferometer combines the reference beam with backscattered radiation collected by a telescope, and the combined beams are focused onto a cooled HgCdTe photodiode at the interferometer exit. Attention is given to the results of a backscatter study experiment flown aboard a NASA research aircraft.

Jones, W. D.↗

NASA NTRS1986

Development of a global model for atmospheric backscatter at CO2 wavelengths

The variation of the aerosol backscattering at 10.6 micrometers within the free troposphere was investigated and a model to describe this variation was developed. The analysis combines theoretical modeling with the results contained within three independent data sets. The data sets used were obtained by the SAGE I/SAM II satellite experiments, the GAMETAG flight series, and by direct backscatter measurements. The theoretical work includes use of a bimodal, two component aerosol model, and the study of the microphysical and associated optical changes occurring within an aerosol plume. A consistent picture is obtained that describes the variation of the aerosol backscattering function in the free troposphere with altitude, latitude, and season.

Kent, G. S.↗

NASA NTRS1986

Regression models for vegetation radar-backscattering and radiometric emission

Simple regression estimation of radar backscatter and radiometric emission from vegetative terrain is proposed, based on the exact radiative transfer models. A vegetative canopy is modeled as a Rayleigh scattering layer above an irregular Kirchhoff surface. The rms errors between the exact and the estimated ones are found to be less than 5 percent for emission, and 1 dB for the backscattering case, in most practical uses. The proposed formulas are useful in quickly estimating backscattering and emission from the vegetative terrain.

Eom, H. J.↗

NASA NTRS1986

The SIR-B observations of microwave backscatter dependence on soil moisture, surface roughness, and vegetation covers

An experiment was conducted from an L-band SAR aboard Space Shuttle Challenger in October 1984 to study the microwave backscatter dependence on soil moisture, surface roughness, and vegetation cover. The results based on the analyses of an image obtained at 21-deg incidence angle show a positive correlatlion between scattering coefficient and soil moisture content, with a sensitivity comparable to that derived from the ground radar measurements reported by Ulaby et al. (1978). The surface roughness strongly affects the microwave backscatter. A factor of two change in the standard deviation of surface roughness height gives a corresponding change of about 8 dB in the scattering coefficient. The microwave backscatter also depends on the vegetation types. Under the dry soil conditions, the scattering coefficient is observed to change from about -24 dB for an alfalfa or lettuce field to about -17 dB for a mature corn field. These results suggest that observations with a SAR system of multiple frequencies and polarizations are required to unravel the effects of soil moisture, surface roughness, and vegetation cover.

Wang, J. R.↗

NASA NTRS1985

Backscattered ultraviolet measurements of ozone 1970-2000 - An overview

The history of the Backscattered Ultraviolet Experiment (BUV) is recounted and related future efforts are discussed. Satellite measurement of backscattered UV radiation will be the major source of long-term global information about total ozone and ozone profiles for the rest of this century. These measurements started with the BUV experiment flown on Nimbus 4 from 1970 to 1977 and are presently being continued with the Solar and Backscattered Ultraviolet/Total Ozone Mapping Spectrometer (SBUV/TOMS) launched on Nimbus 7 in 1978. NOAA's improved version (SBUV/2) is to fly until sometime in the 1990s and NASA plans to fly an SBUV/2 on the Upper Atmosphere Research Satellite in 1989. Differences between these instruments and in the algorithms for deriving ozone amounts from these data sets are discussed.

Fleig, A. J.↗

NASA NTRS1986

Visibility related to backscatter at 1.54 micron

The lidar process was shown to have the necessary potential to fulfill the need for a remote measurement of visibility. Visibility can be inferred from a lidar return optical extinction. The wavelength 1.54 micron was chosen, being near the visible wavelength region and having a high eye safety threshol, 200,000 times higher than 1.06 micron; 1.54 is the erbium laser wavelength. This research utilized 105 measured height profiles of natural droplet size distributions data, taken in clouds, fog, and haze. These profiles were examined to determine the completeness of the droplet counting data. It was found that the particle spectrometer data were incomplete in the very light ford and haze so this portion of the data was eliminated. Utilizing the Mie theory, these droplet size distribution profiles were converted to backscatter at 1.54 micron and extinction in the visible region, 0.55 micron. Using Koschmeider's relationship, the extinction profiles were converted to visibility. The visibility and backscatter profiles were compared to develop a relationship between visibility and backscatter at 1.54 micron.

Barber, T. L.↗

NASA NTRS1986

Wavelength dependence of aerosol backscatter coefficients obtained by multiple wavelength Lidar measurements

Aerosols are often classified into several general types according to their origins and composition, such as maritime, continental, and stratospheric aerosols, and these aerosol types generally have different characteristics in chemical and physical properties. The present study aims at demonstrating the potential for distinguishing these aerosol types by the wavelength dependence of their backscatter coefficients obtained from quantitative analyses of multiple wavelength lidar signals. Data from the NASA Airborne Differential Abosrption lidar (DIAL) S ystems, which can measure aerosol backscatter profiles at wavelenghts of 300, 600, and 1064 nm and ozone profiles of backscatter coefficients for these three wavelength were derived from the observations of aerosols of different types. Observations were performed over the Atlantic Ocean, the Southwestern United States, and French Guyana.

Sasano, Y.↗

NASA NTRS1986

On the relationship between age of lava flows and radar backscattering

The observation that older lava flows have lower backscatter in radar images is assessed with multiwavelength/polarization scatterometer data with incidence angles from 15 to 50 deg. Backscatter decreases over time because surface roughness decreases due to infilling with dust and mechanical weathering of the rocks. Pahoehoe lavas in the Snake River Plain with ages of 2.1, 7,4, and 12.0 K yr are best separated with 2.25 cm wavelength data. Blocky obsidian flows at Medicine Lake Highland and Newberry Volcano with ages of 0.9, 1.1 and 1.4 K yr are best separated with 6.3 cm wavelength data. Two Pleistocene flows at the Snake River Plain are best separated with 19.0 cm wavelength data. Incidence angles from 20 to 35 deg are best. These data indicate it may be possible to separate lava flows into eruptive periods using calibrated multiwavelength radar backscatter data.

Blom, R. G.↗

NASA NTRS1986

A global-scale model of aerosol backscatter at CO2 wavelengths for satellite-based lidar sensors

The status of a global-scale model of background aerosol backscatter cross-sections at CO2 wavelengths is described. The model needs, strategy, concept, parameters, and capabilities are addressed, and the data base is discussed, concluding data selection, CO2 backscatter measurements, aerosol optical measurements, aerosol microphysical measurements, water vapor measurements, and data analysis. Strong evidence is reported for a 'universal' background tropospheric aerosol population. Typical background backscatter values at CO2 wavelengths appear to be about 3 x 10 to the -11th to 8 x 10 to the -11th/m/sr. Background signatures are evident in most aerosol data sets which have global-scale coverage in space or time.

Bowdle, David A.↗

NASA NTRS1987

Two-color short-pulse laser altimeter measurements of ocean surface backscatter

The timing and correlation properties of pulsed laser backscatter from the ocean surface have been measured with a two-color short-pulse laser altimeter. The Nd:YAG laser transmitted 70- and 35-ps wide pulses simultaneously at 532 and 355 nm at nadir, and the time-resolved returns were recorded by a receiver with 800-ps response time. The time-resolved backscatter measured at both 330-m and 1291-m altitudes showed little pulse broadening due to the submeter laser spot size. The differential delay of the 355-nm and 532-nm backscattered waveforms were measured with a rms error of about 75 ps. The change in aircraft altitudes also permitted the change in atmospheric pressure to be estimated by using the two-color technique.

Abshire, James B.↗

NASA NTRS1987

L-band radar backscatter modeling of forest stands

An L-band HH radar backscatter model of a coniferous forest stand is described and compared with SIR-B L-band image data of the Mount Shasta region of northern California. Being based upon an identification and implementation of the expected major components of forest backscattering, the model is simple in form and thus fast computationally, making possible extensive simulations of forest stands. A particularly important component in the model relates to representing the specular reflections expected from tree trunks to the ground and then back to the sensor. These are strong returns and are seen to be necessary to explain both the forest measurements made by the authors and the observations of others. Although the experimental data is limited in quantity and quality, agreement between available experimental and simulated values of forest backscatter is better than the residual uncertainty and relative calibration error of the experimental data, provided the model and experiment are matched initially at one set of parameter values.

Richards, John A.↗

NASA NTRS1988

A relationship between radar backscatter and aerodynamic roughness - Preliminary results

The ability of the wind to move particles and the flux of windblown sand are both dependent on the topographic roughness of the surface, as measured by the aerodynamic roughness, z(0). For most surfaces, topographic roughness controls many of the characteristics of the radar return, and the magnitude of the radar backscatter can be regarded as a measure of the surface roughness at or near the wavelength scale. Radar backscatter data may therefore be useful in obtaining a value of aerodynamic roughness which can be used to assess aeolian sediment transport via remote sensing. In this study, calibrated L(HH), C(HH) and Ku(VV) radar data were used to derive characteristic backscatter coefficients sigma(0) for three lava flow units, an alluvial fan, and a playa surface. Preliminary analyses show that values of sigma(0) and z(0) both increase with topographic roughness and that there is a good correlation between the two coefficients. This correlation suggests that it may be possible to assess aerodynamic roughness directly from radar data.

Greeley, Ronald↗

NASA NTRS1988

A composite L-band HH radar backscattering model for coniferous forest stands

The radar backscattering model developed by Richards et al. (1987), has been improved and further tested in this research. The trunk term may now be calculated from the exact solution to the electromagnetic wave equations instead of the corner reflector equation. Rough surface models have been introduced into the radar model, so that the forward reflectance and the backscattering from the ground surface are now calculated from the same model and, thus, are consistent. The number of trees in an individual pixel is assumed to be Poisson distributed, with tree height in a stand log-normally distributed. The simulated results show that the match of backscattering coefficients for eight forest stands between SIR-B image data and the simulated results are satisfying, and that the trunk term now seems to be convincingly established as the dominant term in the L-band HH radar return from coniferous forest stands.

Sun, Guoquing↗

NASA NTRS1988

Aerosol backscatter variability at 10.6 microns over Colorado high plains during JAWS experiment

This paper describes the aerosol backscatter profile measurements obtained by an airborne CW focused CO2 Doppler lidar (the Laser True Airspeed System) during the 1982 Joint Airport Weather Studies (JAWS) field experiment. The backscatter sounding flights were made in the lee of the front range of the Colorado Rocky Mountains, at about 1650 m above mean sea level. The qualitative relationships between the backscatter and the vertical distribution of temperature and water vapor are described.

Rothermel, Jeffry↗

NASA NTRS1988

Atmospheric backscatter vertical profiles at 9.2 and 10.6 microns - A comparative study

The paper reports a series of atmospheric aerosol backscatter measurements at two widely spaced CO2 laser wavelengths: 9.25 and 10.6 microns. Comparisons are made between backscatter coefficient profiles at these two wavelengths up to 20-km altitude. Measurements such as those reported here can be used to assess the feasibility of coherent CO2 lidar for wind measurements, and they also provide a partial test of backscatter model predictions.

Ancellet, Gerard M.↗

NASA NTRS1989

Australian aerosol backscatter survey

This paper describes measurements of the atmospheric backscatter coefficient in and around Australia during May and June 1986. One set of backscatter measurements was made with a CO2 lidar operating at 10.6 microns; the other set was obtained from calculations using measured aerosol parameters. Despite the two quite different data collection techniques, there is quite good agreement between the two methods. Backscatter values range from near 1 x 10 to the -8th/m per sr near the surface to 4 - 5 x 10 to the -11th/m per sr in the free troposphere at 5-7-km altitude. The values in the free troposphere are somewhat lower than those typically measured at the same height in the Northern Hemisphere.

Gras, John L.↗