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
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
"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
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
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."