Maurice A. Jarzembski

Laser-induced stimulated Raman scattering in the forward direction of a droplet - Comparison of Mie theory with geometrical optics

Comparison of Mie theory calculations of the internal electromagnetic source function for a 120-microm-diameter water droplet with geometrical optics suggests that the field enhancement located at the critical ring region encircling the axis in the forward direction of the droplet can support stimulated Raman scattering as found experimentally.

Comparison of calculated aerosol backscatter at 9.1- and 2.1-μm wavelengths

Calculated aerosol backscatter for three common atmospheric aerosol compositions is higher at 2.1 microm than at 9.1 microm for low backscatter conditions and almost comparable for high backscatter conditions.

Signal Processing and Calibration of Continuous-Wave Focused CO2 Doppler Lidars for Atmospheric Backscatter Measurement

Two continuous-wave (CW) focused CO(2) Doppler lidars (9.1 and 10.6 µm) were developed for airborne in situ aerosol backscatter measurements. The complex path of reliably calibrating these systems, with different signal processors, for accurate derivation of atmospheric backscatter coefficients is documented. Lidar calibration for absolute backscatter measurement for both lidars is based on range response over the lidar sample volume, not solely at focus. Both lidars were calibrated with a new technique using well-characterized aerosols as radiometric standard targets and related to conventional hard-target calibration. A digital signal processor (DSP), a surface acoustic wave spectrum analyzer, and manually tuned spectrum analyzer signal analyzers were used. The DSP signals were analyzed with an innovative method of correcting for systematic noise fluctuation; the noise statistics exhibit the chi-square distribution predicted by theory. System parametric studies and detailed calibration improved the accuracy of conversion from the measured signal-to-noise ratio to absolute backscatter. The minimum backscatter sensitivity is ~3 × 10(-12) m(-1) sr(-1) at 9.1 µm and ~9 × 10(-12) m(-1) sr(-1) at 10.6 µm. Sample measurements are shown for a flight over the remote Pacific Ocean in 1990 as part of the NASA Global Backscatter Experiment (GLOBE) survey missions, the first time to our knowledge that 9.1-10.6-µm lidar intercomparisons were made. Measurements at 9.1 µm, a potential wavelength for space-based lidar remote-sensing applications, are to our knowledge the first based on the rare isotope (12)C (18)O(2) gas.

LIDAR CALIBRATION TECHNIQUE USING LABORATORY-GENERATED AEROSOLS

A new calibration technique for continuous-wave Doppler lidars that uses an aerosol scattering target has been developed. Calibrations with both single- and many-particle scattering were performed at the same lidar operating conditions as in atmospheric measurements. The calibrating targets, simulating atmospheric aerosols, were laboratory-generated spherical silicone oil droplets with known complex refractive indices and sizes, hence with known single-particle backscatter cross sections as obtained from Mie theory. Measurements of lidar efficiency with the conventional hard target calibration method were consistently higher by a factor of ~2 than measurements with the aerosol calibration technique. This result may have important implications for lidar backscatter estimates both for aerosol modeling efforts and for optimal design of future lidar systems. The aerosol calibration method provides a validation of basic lidar theory for particle scattering for coherent detection.

High resolution remote sensing of sulfate aerosols from CO2 lidar backscatter

A high resolution technique for remotely sensing aerosol sulfate composition has been developed, based on the ratio of aerosol backscatter measured at 9.1 and 10.6 µm wavelengths with two continuous wave CO2 lidars. This is demontrated using data from the NASA GLObal Backscatter Experiment (GLOBE) over the Pacific Ocean in 1990. Results indicate changes from sulfuric acid with some ammoniation in clean conditions and presence of dust with ammoniated sulfates in continental plumes. Lidars provide good estimates of backscatter ratio with ∼5 second sample times (∼1 km spatial resolution) in aerosol concentrations as low as ∼10−2 µg/m³.

Vertical variability of aerosol backscatter from an airborne-focused continuous-wave CO 2 lidar at 9.1-µm wavelength

Atmospheric aerosol backscatter measurements taken with a continuous-wave focused Doppler lidar at 9.1-microm wavelength were obtained over western North America and the Pacific Ocean from 13 to 26 September 1995 as part of a NASA airborne mission. Backscatter variability was measured for approximately 52 flight hours, covering an equivalent horizontal distance of approximately 30,000 km in the troposphere. Some quasi-vertical backscatter profiles were also obtained during various ascents and descents at altitudes that ranged from approximately 0.1 to 12 km. Similarities and differences for aerosol loading over land and ocean were observed. A midtropospheric aerosol backscatter background mode near 3 x 10(-11) to 1 x 10(-10) m(-1) sr(-1) was obtained, which is consistent with those of previous airborne and ground-based data sets.

Comparison of Continuous-Wave CO2 Lidar Calibration by use of Earth-Surface Targets in Laboratory and Airborne Measurements

Backscatter of several Earth surfaces was characterized in the laboratory as a function of incidence angle with a focused continuous-wave 9.1-mum CO(2) Doppler lidar for use as possible calibration targets. Some targets showed negligible angular dependence, while others showed a slight increase with decreasing angle. The Earth-surface signal measured over the complex Californian terrain during a 1995 NASA airborne mission compared well with laboratory data. Distributions of the Earths surface signal shows that the lidar efficiency can be estimated with a fair degree of accuracy, preferably with uniform Earth-surface targets during flight for airborne or space-based lidar.

Spectral Analysis of Vibrational Harmonic Motion by use of a Continuous-Wave CO2 Doppler Lidar

Vibrational motion of a harmonic oscillator was investigated with a focused continuous-wave (cw) CO2 Doppler lidar at 9.1-microm wavelength. A continuum of frequencies along with many discrete, equally spaced, resonant frequency modes was observed. The frequency modes are similar in structure to the oscillatory longitudinal modes of a laser cavity and arise because of interference of the natural resonant frequency of the oscillator with specific frequencies within the continuum. Each consecutive resonant frequency mode occurred for a movement of the oscillator much less than the wavelength of incident lidar radiation. For vigorous vibration of the oscillator, the observed spectra may be indicating nonlinear motion.

Low pressure experimental simulation of electrical discharges above and inside a cloud

A low pressure laboratory experiment to generate sporadic electrical discharges in either a particulate dielectric or air, representing a competing path of preferred electrical breakdown, was investigated. At high pressures, discharges occurred inside the dielectric particulate; at low pressures, discharges occurred outside the dielectric particulate; at a transition pressure regime, which depends on conductivity of the dielectric particulate, discharges were simultaneously generated in both particulate dielectric and air. Unique use of a particulate dielectric was critical for sporadic discharges at lower pressures which were not identical in character to discharges without the particulate dielectric. Application of these experimental results to the field of atmospheric electricity and simulation of the above-cloud type discharges that have recently been documented, called jets and sprites, are discussed.