The development of the polarization lidar technique is reviewed, and the current capabilities and limitations of the technique for the cloud research are discussed. At present, polarization lidar is a key component of climate-research programs designed to characterize the properties of cirrus clouds and is an integral part of multiple remote-sensor studies of mixed-phase cloud systems such as winter mountain storms, making it possible to discriminate between cloud phases and to identify some particle types and orientations. Recent theoretical approaches involving ice crystal ray-tracing and cloud microphysical-model simulations are expected to increase the utility of the polarization lidar technique.

The Polarization Lidar Technique for Cloud Research: A Review and Current Assessment

The laser generation of sound in liquids and gases is reviewed. The sound‐generating mechanisms of laser interaction with matter are discussed with emphasis on the thermoelastic process. The studies on strongly absorbing liquids include detailed theoretical considerations of the thermoelastic sound generation with pulsed lasers. Acoustic waveforms for H2O and D2O are calculated analytically on the basis of a model laser‐pulse shape. Both free and rigid boundaries on the surface of the liquid are considered. Good agreement between theory and experiments with respect to waveforms and amplitudes is obtained. The experiments are performed with a hybrid CO2 laser and piezoelectric or optical detection of the acoustic transients. In view of a present controversy, special emphasis is put on the temperature dependence of the acoustic amplitudes in H2O, D2O, and in aqueous MgSO4 solutions. Good agreement is found between experimental data and a new, pure thermal model which takes heat conduction into account. The distortion of the acoustic waveform during the propagation through the liquid is treated in terms of sound absorption, diffraction, and nonlinear acoustics. A simple experimental method for the determination of Beyer’s nonlinearity parameter B/A is presented. In the last section some characteristics of photoacoustic spectroscopy (PAS) in gaseous media are reviewed. This method has been demonstrated to be highly sensitive. The measurement of absorption coefficients as low as 10− 8 cm− 1 is possible. PA studies on H2O vapor are discussed with new results on line and continuum absorption in the 9–11‐μm wavelength range. Finally, the impact of PAS on trace gas analysis is demonstrated. With PAS the detection of gas concentrations in the ppb range is feasible. The operational characteristics of a stationary CO laser and a mobile CO2 laser‐PAS system are presented, including first results on continuous i n s i t u air pollution monitoring.

Laser generation of acoustic waves in liquids and gases

The subject of air pollution monitoring is reviewed. First, we discuss the main conventional techniques currently applied to the detection of gaseous air pollutants, as well as the state of the art of novel detection schemes. We distinguish between nonspectroscopic and spectroscopic methods. Spectroscopic techniques are of primary interest since they offer several advantages, e.g., the simultaneous monitoring of numerous substances. Photoacoustic (PA) spectroscopy represents a promising spectroscopic technique due to its intrinsically high sensitivity, the large dynamic range, and the comparatively simple experimental arrangement. Emphasis is put on detection selectivity which often may restrict the range of applications of the technique for pollution monitoring due to the lack of powerful, continuously tunable IR laser sources. The theoretical aspects of PA spectroscopy with respect to trace gas detection and multicomponent analysis are thus discussed in detail. Some characteristics of a portable PA system built by us are outlined in order to illustrate the usefulness of such systems.

Atmospheric pollution monitoring using CO2‐laser photoacoustic spectroscopy and other techniques

Different definitions for estimating the degree of changes in signal polarization measured by lidar measurements are used both to detect the presence of nonspherical aerosol particles and to estimate their shape and density. Our aim is to provide a tool for calculation and interpretation of changes in polarization that are due to aerosol backscatter measured by the lidar technique. An overview of several techniques used to calculate linear depolarization from two-channel lidar measurements is given. Advantages and disadvantages of each method are analyzed when we apply them on a lidar vertical profile. Systematic errors are also discussed. First, an overview of different estimations of polarizability of atmospheric molecules is given. The presence of signal with orthogonal polarization in each channel (cross talk) is a source of error in depolarization estimation. It is calculated at various degrees of contamination, and the total uncertainty on depolarization definition is retrieved.

Comparison of various linear depolarization parameters measured by lidar.

Detailed knowledge of the wind resource is necessary in the developmental and operational stages of a wind farm site. As wind turbines continue to grow in size, masts for mounting cup anemometers—the accepted standard for resource assessment—have necessarily become much taller, and much more expensive. This limitation has driven the commercialization of two remote sensing (RS) tools for the wind energy industry: The LIDAR and the SODAR, Doppler effect instruments using light and sound, respectively. They are ground-based and can work over hundreds of meters, sufficient for the tallest turbines in, or planned for, production. This study compares wind measurements from two commercial RS instruments against an instrumented mast, in upland (semi-complex) terrain typical of where many wind farms are now being installed worldwide. With appropriate filtering, regression analyses suggest a good correlation between the RS instruments and mast instruments: The RS instruments generally recorded lower wind speeds than the cup anemometers, with the LIDAR more accurate and the SODAR more precise.

https://www.mdpi.com/2072-4292/3/9/1871/pdf

LIDAR and SODAR Measurements of Wind Speed and Direction in Upland Terrain for Wind Energy Purposes

Lidar systems are now widely used for diagnostics of the atmosphere. These systems employ the backscattering of laser radiation to provide information on atmospheric parameters via a number of scat...

Lidar measurements of the atmosphere

OPTICAL RESONATOR EFFECTS ON THE POPULATION DISTRIBUTION IN He–Ne GAS LASERS DETERMINED FROM SIDE LIGHT MEASUREMENTS

Off-axis feedback modes in a three-mirror laser system

This paper describes experimental studies on the scattering of microwaves from bodies whose sizes are much smaller than one wavelength. A focused-beam microwave system operating at a frequency of 8.68 Gc/sec is employed to measure backscattering cross sections as small as 5 × 10−4λ2. An effort is made to investigate the dependence of the radar cross section of small spheres and flat-ended cylinders on such variables as the volume of the scatterer, the length-to-width ratio, and the polarization of the incident wave. Measurements with systems containing more than one small scatterer are also described.

Allan I. Carswell

Papers

Lidar measurements of the atmosphere

OPTICAL RESONATOR EFFECTS ON THE POPULATION DISTRIBUTION IN He–Ne GAS LASERS DETERMINED FROM SIDE LIGHT MEASUREMENTS

Off-axis feedback modes in a three-mirror laser system

Microwave scattering measurements in the rayleigh region using a focused-beam system