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PANORAMA: 1997

Scientists use Lidar to Probe the Atmosphere
by Michael Boyd

RADAR has a sister called lidar. It is not as well known but has a wealth of applications in the important and fast-growing field of atmospheric science. What unites these two technologies is that they are concerned with both emitting and detecting radiation.

They are both now used in remote sensing that is important for a wide number of scientific disciplines. The University of Wales, Aberystwyth, is at the international forefront in using lidars for the detection of ozone and water vapour. In combination with the National Environment Research Council (NERC) atmospheric research radar, also at Aberystwyth, it is also obtaining further information on gravity waves, turbulence and temperature structure.

Such a combination of instruments is a powerful tool for atmospheric research and is available at very few sites worldwide. There are two basic types of lidar today: coherent infrared lidars, based on carbon dioxide lasers, and ultraviolet-visible lidars, based on a variety of gas and solid state lasers. Aberystwyth uses the latter type.

They have two systems that are based on Neodymium-YAG solid-state lasers. Such lasers are widely used in lidar applications since they are very powerful and stable, with excellent reliability. Their fundamental wavelength is 1,064 nanometres (nm), in the near infrared, but by using non-linear crystals as harmonic generators the frequency may be doubled, tripled or even quadrupled to give a range of visible and ultraviolet beams.

Although these beams are less powerful than the fundamental, this is more than offset by the much stronger atmospheric scattering at smaller wavelengths and the high quantum efficiency of photomultiplier detectors in the visible and ultraviolet. Thus, all the experiments conducted at Aberystwyth use one or other of the harmonic beams.

These laser beams are transmitted vertically into the atmosphere. The second harmonic at 532nm is in the visible part of the spectrum and appears as a spectacular shaft of green light reaching into the sky. This wavelength offers the best combination of laser power, scattering cross-section transmission through the atmosphere and detection efficiency. Water vapour and ozone are measured using the ultraviolet beams.

"Despite the power of the laser, the light scattered back from the atmosphere is very faint, especially from high altitudes," said Dr Geraint Vaughan, the project leader. "Very sensitive detection equipment is therefore required. The telescopes which collect the back-scattered radiation use paraboloid mirrors of about one metre diameter, with a very narrow field of view.

"Together with interference filters to isolate the required spectral band, this arrangement reduces background light contamination to a minimum. Light is detected by photomultipliers coupled to photon-counting electronic systems."

Above about 30 kilometres (km) the scattering of the laser beam is solely due to air molecules. The lidars at Aberystwyth can measure these signals up to about 105 km (65 miles). This profile can be inverted to produce an atmospheric temperature profile up to about 90km (56 miles), with a height resolution of 0.5 - 1km (0.3 - 0.6 miles) and time resolution of one hour.

Such measurements are essential for studies of gravity waves in the middle atmosphere. These disturbances propagating up from the troposphere represent a very important mechanism for dynamical coupling between different regions of the atmosphere. Because they cannot be resolved adequately by satellite measurements, the lidar is now the primary tool for their study.

The Aberystwyth lidars were used to monitor the aerosol clouds from the volcanic eruptions of Mount Pinatubo and el Chichon. This yielded important information on atmospheric mixing processes.

Since the laser beam is plane polarised, and scattering from spherical aerosol droplets preserves this polarisation, measurements of the polarisation of the backscattered signal can reveal information about non-spherical particles in clouds. Similar measurements have also been used to study cirrus clouds, to distinguish ice from water droplets.

Such is the brightness of scattering from cirrus clouds that they can be studied by lidar during daytime; indeed, spectacular reflections from ice plates is visible to the naked eye as sparklers in the laser beam.

In contrast to this, Raman scattering from molecules in the atmosphere is extremely faint, and can only be detected at night. The gas best suited to detection by this scattering is water vapour. Its profiles can be measured up to about seven kilometres from an hour's observation, while a whole night's data may be combined to stretch the vertical range up to the troposphere.

In the presence of aerosols, the signal returned at the laser wavelength is not a measure of density and cannot be used to derive a temperature profile. This restriction is overcome by exploiting rotational Raman scattering which, like its vibrational counterpart, is specific to molecules in the atmosphere.

Deriving atmospheric information from such an approach is a severe challenge, since a temperature profile must be measured with an accuracy of one degree or better to be scientifically useful. One of the major and unique achievements of the Aberystwyth team is in obtaining an absolute calibration of the lidar enabling it to be used in the studies of upper-level fronts and stratosphere-troposphere exchange.

The scientific value of these studies is enormously enhanced by measuring temperature and ozone profiles at the same time. Unlike Raman experiments, the ozone lidar is designed to exploit the effects of atmospheric absorption. This uses the DIAL (differential absorption) technique. In this method, signals are measured at two wavelengths, one strongly absorbed by the molecule of interest and the other of which is not absorbed or is absorbed weakly. For ozone, these are in the ultraviolet.

The Aberystwyth researchers use this method to obtain the ozone concentration. They can generate hourly ozone profiles from four to 12km with vertical resolution of 600 metres. The wavelengths used are solar-blind, so that in winter particularly the experiment may be conducted at any time of the day or night. The ability to record consecutive profiles is vital for the studies of stratosphere-troposphere exchange where features typically advect over the lidar site in a period of a few hours.

"Continued developments in laser and detector technology promise many more atmospheric applications for the lidar technique in the future," said Dr Vaughan. "With the support of NERC, the Aberystwyth group will continue at the forefront of these exciting new developments."

  

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