Freeman F. Hall

Feasibility studies for a global wind measuring satellite system (Windsat): analysis of simulated performance.

A detailed computer simulation of the Windsat global wind measuring process has been developed and used to establish error limits as a function of design parameters. Studies were conducted for a Windsat research system in a 300- and an 800-km orbit. Wind measuring errors were <2 m sec−1 in the troposphere for the recommended set of parameters. Our study results indicate the feasibility of measuring global winds from a space platform using a coherent laser radar.

Aerosol backscattering profiles at λ = 10.6 μm

A systematic program of observing atmospheric backscatter beta profiles from 4 to 16-km above sea level is described. Initial monthly averages indicating a lognormal distribution are presented. Cirrus prevalence, volcanic layers in the stratosphere, diurnal effects, convection, calibration, and absorption effects are discussed as well.

Visibility reductions from soil dust in the Western U.S.

Abstract Soil dust can at times contribute significantly to atmospheric turbidity in the western U.S.A. An examination of meteorological surface observations from three sites in the west shows an increase in the number of cases of blowing dust (BD) over the past 30 years, while the mean wind speed associated with BD has decreased in growing, urban areas. Dust is reported more often in developing areas (such as Denver and Tucson) than in less disturbed areas (Winslow, AZ). Dust turbidity models, whose development was stimulated by the 1930s great plains dust bowl catastrophe are applicable to western soils as well and help explain visibility reductions caused by dust. Vehicle generated dust from unpaved roads can limit visibilities to values of one-third or less than the molecular scattering limit. The upper limits to the amounts of dust and visibility reduction to be expected in the west for differing meteorological conditions are calculated. It is estimated that dust devils may occasionally contribute 247 μg m −3 of dust to the lower atmosphere in the west and a possible correlation between dust devils and human activity is noted.

Calibration of coherent lidar targets

We determine the backscatter reflectance (more properly the bidirectional reflectance distribution function at a 45 degrees angle of incidence and observation) for circularly polarized radiation of lambda = 10.6 microm for 120-grit aluminum oxide sandcloth (1.5 x 10(-2) sr(-1)) and 400-grit silicon carbide sandpaper (1.1 x 10(-2) sr(-1)) with respect to sublimed flowers of sulfur (1.8 x 10(-1) sr(-1)). The effect of range and the atmospheric effect of turbulence-induced beam wander are discussed for both rotating disks and linearly translated belts. The advantages of large slowly rotating disks for field calibrations are presented.

The Effect of Cirrus Clouds on 8–13-μ Infrared Sky Radiance

An experimental investigation of ir sky radiance and radiance fluctuations in the 8-13-micro atmospheric window is reported. Measurements were made with ground-based, filtered bolometer detector radiometers under clear sky and cirrus overcast conditions. Sky radiance was measured very close to the limb of the sun to permit detection of the solar aureole caused by forward scattering by cirrus ice crystals. Polarized sky radiance was found at large zenith angles and is attributed to scattering by cirrus of thermal emission from the earth. The radiance due to tropospheric water vapor is predicted by means of a radiation chart. Measurements of clear sky radiance exceeded that predicted by the chart in all but one case. The radiance of visible cirrus greatly exceeds the radiation chart prediction. Diffraction about cirrus cloud particles leads to a prediction of a solar aureole of a size that corresponds to the measured aureole. It is concluded that even a cirrus haze, which is quite difficult for an unaided, observer to detect, can cause an excess zenith radiance of 0.1 mW cm(-2)sr(-1), which increases to twice this value at a zenith angle of 60 degrees . Even thin but visible cirrus clouds can easily produce an excess zenith radiance of 1 mW cm(-2)sr(-1), which increases by a factor 1.4 at a zenith angle of 60 degrees .

Depolarization, backscatter, and attenuation of CO2 lidar by cirrus clouds.

A coherent CO2 lidar system has been modified to record various states of polarization of the backscattered radiation. Methods for measuring the degree of polarization in the backscattered radiation as well as the optical thickness of clouds are described and demonstrated successfully.

A Physical Model of Cirrus 8–13-μ Infrared Radiance

A simplified physical model of cirrus cloud 8-13-micro radiance is derived, in an attempt to explain measured cloud radiance characteristics. Thermal emission and scattering of irradiance from the earth are considered separately. The scattering diagram for a 50-micro radius ice sphere, which is opaque at these wavelengths, is computed from diffraction theory and specular surface reflection. Thermal emission is found to exceed scattered radiance significantly for all zenith angles, and computed radiance values are found to match measured cloud radiance closely.

Cirrus cloud transmittance and backscatter in the infrared measured with a CO 2 lidar

Two independent methods of measuring the transmittance of cirrus clouds are compared. Both used a CO(2) pulsed Doppler lidar at a wavelength of 10.59 microm. The first method used backscatter from the calibration target El Chichon stratospheric cloud that was present over Boulder in 1982 and 1983. The second method used conical lidar scans at different zenith angles when uniform cirrus decks were present. Extinction coefficients measured from both methods average 0.1 km(-1) for tenuous cirrus 1.0 km thick to 0.78 km(-1) for cirrus several kilometers thick. There is a wide standard deviation in extinction values. Extinction-tobackscatter ratios S vary from <1000 sr for tenuous clouds to 2600 sr for dense clouds. Mie scattering and extinction calculations for spherical ice particles of 10-50 microm in radius lead to ratios S > 2000 sr, so long as the ice absorption is entered into the calculations. The backscattering ratio for ice cylinders is 1 order of magnitude lower than for spheres. Backscatter in the IR may, therefore, be reasonably well modeled by some combination of spheres and cylinders. Cloud thickness statistics from lidar returns show that cirrus decks average ~500 m thick. Clouds thinner than 300 m were often overlooked by the unaided surface-based observer. These preliminary results are in rather close agreement with the LOWTRAN 6 cirrus cloud model predictions.

Ground-Based Coherent Lidar Measurement Of Tropospheric And Stratospheric Parameters

During 1982 we used the NOAA pulsed Doppler lidar as part of experimental programs to measure precisely lidar system performance and extend the instruments demonstrated atmospheric monitoring capabilities. Key system characteristics such as pulse shape, chirp, alignment stability, telescope efficiency, and pulse-to-pulse variability were studied and their effect on measurement accuracy quantified. The field experiments also demonstrated the systems capabilities and limitations in monitoring winds, backscatter, turbulence, and moisture. By scanning the wind field at low scan elevation angles, we have observed such small-scale meteorological events as thunderstorm gust fronts, downbursts, and cold front passage. Scans at higher elevation angles enable us to monitor upper level winds, such as those in the vicinity of the polar-front jet. We also monitor backscatter coefficient (β) over both the short and long term. Daily observations show a noticeable decrease in tropospheric β during the winter months. The stratospheric aerosol layer resulting from the El Chichon volcano eruption was easily observed during fall 1982.

Heterodyne quantum efficiency of a HgCdTe infrared Doppler detector

Quantum efficiency of a HgCdTe photodiode was measured for both direct and heterodyne detection. Both measurements employed a blackbody radiation source; the receiver bandwidth for the heterodyne measurements was 50 MHz to approximate a Doppler lidar receiver. The quantum efficiency of the detector in the heterodyne mode was ~0.3, nearly 40% lower than the direct detection efficiency.