Norman B. Nielsen

Rayleigh lidar system for middle atmosphere research in the arctic

A Rayleigh/Mie lidar system deployed at the Sondrestrom At- mospheric Research Facility located on the west coast of Greenland near the town of Kangerlussuaq (67.0 deg N, 50.9 deg W) has been in operation since 1993 making unique observations of the arctic middle atmosphere. The vertically directed lidar samples the elastically back- scattered laser energy from molecules (Rayleigh) and aerosols (Mie) over the altitude range from 15 to 90 km at high spatial resolution. The limited amount of arctic observations of the middle atmosphere currently available emphasizes the importance and utility of a permanent Rayleigh lidar system in Greenland. The lidar system consists of a frequency- doubled, 17-W Nd:YAG laser at 532 nm, a 92 cm Newtonian telescope, and a two-channel photon counting receiver. The principal objective of the lidar project is to contribute to studies concerned with the climatology and phenomenology of the arctic middle atmosphere. To this end, we describe the lidar system in detail, evaluate system performance, de- scribe data analysis, and discuss the system capabilities in determining the density, temperature, and the presence of aerosols in the arctic middle atmosphere. Particular emphasis is placed on the derivation of temperature from the lidar measurement and on the impact of signal- induced noise on this analysis. Also, we develop a statistical filter based on a Bayesian approach to optimally smooth the lidar profile in range. This filter preserves the short-term fluctuations in the low-altitude data consisting of relatively high SNR, whereas more smoothing is applied to the high-altitude data as the SNR decreases.

Noctilucent cloud observations over Greenland by a Rayleigh lidar

We report on the detection of noctilucent clouds (NLCs) by a Rayleigh lidar system located on the west coast of Greenland during operations in July and August 1994. The lidar detected NLCs in the zenith on three nights (July 28–29, July 31–August 1, and August 4). The lidar measurements are presented at temporal (1 minute) and spatial (192 meter) resolutions exceeding previously reported lidar measurements of NLCs and reveal significantly more structure in the clouds. Unique cloud features are observed for each night NLCs were detected. Because of the large backscatter ratio associated with NLCs, the volume backscatter coefficient for the cloud particles can be determined more accurately from the lidar signal than the backscatter ratio as the upper mesospheric molecular number density is negligible in the volume backscatter coefficient calculation. The volume backscatter coefficient values determined from the three nights of NLC observations would be classified by current models as moderate to extreme. The extreme backscatter from NLC measurements on August 4 tests the assumptions used in existing models.

Airborne lidar measurements of smoke plume distribution, vertical transmission, and particle size.

Observations were made of a dense smoke plume downwind from a forest fire using the ALPHA-1 two-wavelength downward-looking airborne lidar system. Facsimile displays derived from lidar signatures depict plume dimensions, boundary layer height, and underlying terrain elevation. Surface returns are interpreted in terms of vertical transmission as function of cross-plume distance. Results show significantly greater plume attenuation at 0.53-microm wavelength than at 1.06-microm, indicating ~0.1-microm mean particle diameters or the presence of gaseous constituents that absorb the visible radiation. These results demonstrate the potential of multiple-wavelength airborne lidar for quantitative analysis of atmospheric particulate and gaseous constituents.

Airborne Lidar Plume and Haze Analyzer (ALPHA-1)

A new two-wavelength airborne lidar system has been constructed and field-tested. The system was designed to observe the distribution of particle concentrations over large regional areas. During a ...

Airborne Lidar Mapping of Ozone Concentrations During the Lake Michigan Ozone Study

An airborne differential absorption lidar (DIAL) was used during the 1991 Lake Michigan Ozone Study (LMOS) to map the vertical distribution of ozone concentrations across Lake Michigan downwind of urban and industrial areas of the south lake region. The DIAL, which was designed as a compact instrument for installation on relatively small-sized twin-engine aircraft typically used on regional air quality studies, is based on an excimer laser and Raman cell to generate multiple-wavelength ultraviolet energy appropriate for remote measurement of tropospheric ozone. Collected DIAL data are displayed in terms of contour analyses of vertical ozone concentration distributions across Lake Michigan. During times of southwesterly winds, large-scale urban ozone plumes were observed (mostly over the lake and eastern shoreline) that increased in concentration during the day. Small-scale elevated ozone minima, caused by subsidence of clean air aloft and by chemical destruction of ozone by industrial reactive-gas plumes ...

Airborne scanning lidar observations of aircraft contrails and cirrus clouds during SUCCESS

An angular scanning backscatter lidar was deployed on the NASA DC-8 research aircraft as part of SUCCESS. The lidar viewing direction could be continuously scanned from vertically upward to forward to vertically downward. Real-time pictorial displays generated from lidar signatures were used to locate clouds and contrails above, ahead of, and below the DC-8; to depict their spatial structure; and to help select DC-8 altitudes for achieving optimum sampling by onboard in situ sensors. The lidar data are being analyzed to establish their value in the interpretation and extension of the in situ sensor databases. Data examples are presented that illustrate (1) correlation with particulate, gas, and radiometric measurements made by onboard sensors, (2) discrimination and identification between contrails observed by the onboard sensors, (3) a 13.1 km altitude layer that exhibits greatly enhanced vertical backscatter relative to off-vertical backscatter, and (4) mapping of vertical distributions of individual precipitating ice crystals and their capture by cloud layers.

NASA DC-8 Airborne Scanning Lidar System

A scanning lidar system is being developed for installation on the NASA DC-8 atmospheric research aircraft to support and extend the domain of in-situ aerosol and gas measurements. Design and objectives of the DC-8 scanning lidar are presented.