R. E. Cupp

Optimizing a pulsed Doppler lidar.

NOAAs fieldable injection-seeded, pulsed, coherent CO(2) lidar was developed over a 5-yr period. Its performance and reliability are characterized. Techniques for calibration, alignment, collimation, and for improving detector performance and frequency stability are presented.

Doppler Lidar Measurement of Profiles of Turbulence and Momentum Flux

Abstract A short-pulse CO2 Doppler lidar with 150-m range resolution measured vertical profiles of turbulence and momentum flux. Example measurements are reported of a daytime mixed layer with strong mechanical mixing caused by a wind speed of 15 m s−1, which exceeded the speed above the capping inversion. The lidar adapted an azimuth scanning technique previously demonstrated by radar. Scans alternating between two elevation angles allow determination of 〈u2〉, 〈v2〉, and 〈w2〉. Expressions were derived to estimate the uncertainty in the turbulence parameters. A new processing method, partial Fourier decomposition, has less uncertainty than the filtering used earlier. Substantial improvements could be had with higher pulse rate, shorter pulses and wavelengths (to improve spatial resolution and minimum range by up to an order of magnitude), and operation from an aircraft.

Atmospheric Temperature Measurement Using Raman Backscatter

A ground-based method of measuring atmospheric vertical temperature profiles using Raman backscatter from N(2) is discussed. Experimental measurement of temperature fluctuations at tower heights is described.

Coherent differential Doppler measurements of transverse velocity at a remote point

We consider the analysis and results of a laser remote velocity sensing experiment in which both the longitudinal and transverse velocity components at one location were simultaneously measured from a range of 33 m. Fully coherent homodyne detection was used with a test target that simulates many aspects of distributed atmospheric aerosol targets.

CO(2) lidar backscatter profiles over Hawaii during fall 1988.

Aerosol and cloud backscatter data, obtained over a 24-day period in fall 1988 with the National Oceanic & Atmospheric Administrations Doppler lidar at 10.59-microm wavelength, are analyzed by using a new technique to lessen biases that are due to dropouts. Typical backscatter cross sections were significantly lower than those routinely observed over the continental United States, although episodic backscatter enhancements caused by cirrus and mineral dust also occurred. Implications of these data on the proposed Laser Atmospheric Wind Sounder wind profiling satellite sensor are discussed.

Measurement of Mie Scattering Intensities from Monodispersed Spherical Particles as a Function of Wavelength

Mie scattering intensities as a function of the size parameter are measured by use of a tunable dye laser and monodispersed spherical particles. The experimental results are compared with the Mie single scattering theory; a discrepancy in the exact position of the maxima and minima was detected. Agreement between experiment and theory was improved by applying a correction to the manufacturers index of refraction function for the particles.

Calibration of a cw infrared Doppler lidar.

A moving scattering target used as a transfer standard allows absolute calibration of the response of a cw Doppler lidar to an atmospheric target. The lidar in this study operated at a 10.6-microm wavelength. Consideration of the distribution of radiant energy density near the focus of the lidar transceiver permits measurement of a backscatter coefficient from a distributed array of scatterers, such as atmospheric aerosols, based on the diffuse reflectance of the surface of the transfer standard. The minimum detectable signal for our system with a 5-sec averaging time corresponds to a backscatter coefficient of 2.4 x 10(-12) m (-1) sr (-1) +/- 2.5 dB, which is ~ 9 dB greater than the theoretical threshold. Calibration shows that the lidar response is 5+/-1 dB less than the ideal limit for signal powers well above the minimum detectable signal.

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.

Offset local oscillator for cw laser Doppler anemometry

Using two CO(2) lasers with one frequency-locked to the other, we have developed a simple heterodyne cw Doppler backscatter anemometer for remote sensing. The configuration as tested in a lidar application requires no additional optics or detector over a homodyne setup. Spectra of the vertical wind show an example of the results available from a simple heterodyne Doppler lidar with a threshold of 0.25 m sec(-1) and an uncertainty of +/-10% in the velocity estimate.

Conservation properties of oil fog used as an atmospheric tracer

Abstract Oil fog used as a tracer of atmospheric motions is usually considered to be conserved in the sense of negligible evaporation loss. However, small drops can evaporate at a rapid pace. Lidar measurements of optical backscatter from a fog plume containing drops of a medium oil, which is typical of those suggested for battlefield obscuration, suffered significant losses in daytime. Backscatter from drops of a heavy oil with low volatility were nearly conserved. The physics of the evaporation process for oil drops in a diffusing plume are summarized. The rate and extent of evaporation depend on the molecular weight of the oil, on the temperature, and on the amount of plume dispersion. If a partially evaporating oil fog is to be detected optically, the signal is better conserved if drop sizes are large enough to be in the Mie rather than the Rayleigh scatter regime. Researchers should confirm the conservative property of an oil fog tracer if the analysis requires accurate measurements of concentration.