Pulsed Lidar Research Articles

The Pulsed Coherent Doppler Lidar: Observations of Frontal Structure and the Planetary Boundary Layer

The NOAA/WPL pulsed coherent Doppler lidar was used during the Texas Frontal Experiment in 1985 to study mesoscale preconvective atmospheric conditions. On 22 April 1985, the Doppler lidar, in conjunction with serial rawinsonde ascents and National Weather Service rawinsonde ascents, observed atmospheric features such as middle-tropospheric frontal and vertical wind shear layers and the planetary boundary layer. The lidar showed unique evidence of the downward transport of strong winds from an elevated vertical speed shear (frontal) layer into the planetary boundary layer. The lidar provided further evidence of atmospheric processes such as clear-air turbulence within frontal layers, and dry convection turbulence within the superadiabatic planetary boundary layer. As a result, high-technology remote sensing instruments such as the Doppler lidar show considerable promise for future studies of small-scale weather systems in a nonprecipitating atmosphere.

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.

Atmospheric correlation-time measurements and effects on coherent Doppler lidar

The time for which the backscatter from an ensemble of atmospheric aerosol particles remains coherent was studied by using a pulsed TEA CO2 lidar with coherent detection. Experimental results are compared with predictions by using model pulse shapes appropriate for TEA CO2 laser transmitters. The correlation time of the backscatter return signal is important in studies of atmospheric turbulence and its effects on optical propagation and backscatter. Techniques for its measurement are discussed and evaluated.

Ground-based measurements of atmospheric backscatter and absorption using coherent CO_2 lidar

In the present derivation of vertical profiles of atmospheric absorption and backscatter coefficients from coherent pulsed Doppler lidar by means of a slant path method, a strong seasonal variation of absorption and backscatter is evident throughout the lower troposphere. Measurements beyond the lower troposphere are hampered by modest pulse energy and lidar beam absorption, although small backscatter values are occasionally observed at midtropospheric levels during the winter months, when absorption is minimal. While a monomodal lognormal backscatter distribution is found within the lower boundary layer, evidence of a bimodal lognormal distribution is found at higher levels.

Lidar aerosol backscatter measurements: systematic, modeling, and calibration error considerations

Sources of systematic, modeling, and calibration errors that affect the interpretation and calibration of lidar aerosol backscatter data are discussed. The treatment pertains primarily to ground-based pulsed CO2 lidars that probe the troposphere and are calibrated using hard calibration targets. However, a large part of the analysis is relevant to other types of lidar system such as lidars operating at other wavelengths; cw focused lidars; airborne or earth-orbiting lidars; lidars measuring other regions of the atmosphere; lidars measuring nonaerosol elastic or inelastic backscatter; and lidars employing other calibration techniques.

Coherent DIAL measurement of range-resolved water vapor concentration

A pulsed coherent CO2 lidar was employed to measure water vapor profiles by the differential absorption lidar (DIAL) technique. Measurements were obtained to ranges of 10 km along horizontal paths and 6 km when the lidar beam angle was elevated. Comparisons with nearby rawinsonde soundings showed fairly good agreement, although a tendency for the lidar to overestimate relative to the sonde was observed. Uncertainties in the individual measurements were attributable primarily to speckle, quantum noise, and atmospheric nonstationarities. The DIAL data set was also used to obtain radial wind velocity measurements at ranges beyond the maximum range of the concentration measurement.

Evidence for speckle effects on pulsed CO_2 lidar signal returns from remote targets

A pulsed CO2 lidar was used to study statistical properties of signal returns from various rough surfaces at distances near 2 km. These included natural in situ topographic materials as well as man-made hard targets. Three lidar configurations were used: heterodyne detection with single temporal mode transmitter pulses, and direct detection with single and multiple temporal mode pulses. The significant differences in signal return statistics, due largely to speckle effects, are discussed.

Measuring atmospheric scattering and extinction at 10 μm using a CO_2 lidar

A pulsed CO2 lidar was used to measure atmospheric extinction and backscattering at 10 μm during different types of weather. Comparison was made between extinction evaluated from the atmospheric echo and from measurements against calibrated plates. Vertical soundings showed that the backscatter could drop a factor of 10 during the first kilometer. The implications of extinction and backscatter to the performance of single-ended CO2 lidars for atmospheric sensing are discussed.

Potential for a Lidar-Based, Portable, 1 km Meteorological Tower

Abstract Lidar measurements of wind, temperature and water vapor, using a variety of techniques that rely on the detection and analysis of laser light backscattered from the atmosphere, allow data to be obtained that are similar to those hypothetically available from a meteorologically instrumented lower extending to 1 km altitude (or more). This paper reviews these various recent accomplishments in lidar instrumentation without attempting historical completeness. Based on criteria of 1) altitude resolution to 50 m, 2) tower-like measurement geometry, 3) hardware commonality between techniques and 4) daytime as well as nighttime operation, the intercomparison results in recommended techniques to be combined for a compact, mobile lidar “tower.” For horizontal wind, recommendations include pulsed time-of-flight lidar, for vertical wind, pulsed direct Doppler lidar at visible or shorter wavelengths; for temperature, Cabannes-scattering linewidth or rotational Raman band shape; and for water vapor, vibrationa...

Detection of atmospheric aerosol flow using a transit-time lidar velocimeter

A cross-correlation analysis of lidar echoes from an artificial tracer transported through two closely spaced atmospheric scattering volumes provides a remote measurement of the wind speed along the line joining the two volumes. A pulsed nitrogen transit-time lidar has been developed and used to make these measurements in real-time intervals of approximately 4 sec. The wind speed measurements are compared with conventional anemometer data. An extension of this technique to natural aerosol concentrations that exploits the laser wavelength dependence of the system SNR is discussed.

Lidar study of atmospheric particulate over the city of London

Abstract The laser radar (lidar) is capable of detecting weak concentrations of particulate in the lower atmosphere at ranges up to several kilometres. Through this technique, models of thermal convection and theories of particulate diffusion may be studied by observing the spatial and temporal changes in particulate concentration. A pulsed ruby lidar has been assembled at this University, with scanning optics located at an elevated position overlooking the city. Criteria have been evolved here for deriving profiles of extinction coefficient, and particulate number and mass concentration from the lidar signatures. Sample profiles of particulate concentrations are given for comparison with the design prediction of lidar sensitivity.

Lidar Observations of Sierra-Wave Conditions

Early in 1967 a series of observations using pulsed ruby lidars were made near Independence, Calif.; the objective was to investigate the value of lidar for studying air motion in the Sierra wave, with special reference to indications of turbulence. Although no intense wave activity occurred, appreciable wave motions were observed, both in what appeared to the eye to be clear air and in air where the particulate matter was sufficiently concentrated as to be visible as clouds. Interruptions in the smooth laminar flow in the clear air were observed, and measurements were made of the length, amplitude and height of waves shown by clouds. With previously existing techniques, only limited observation of such phenomena have been possible. It is thus concluded that lidar observations are of considerable value in studying wave motion, even in the absence of visible clouds. There is also a possibility that lidar could indicate the presence of turbulence by revealing the breakdown of wave motion or the presence of rotors.