A Low-cost Differential Absorption Lidar For
Measuring Tropospheric Water Vapour Concentration
K-H. Bae, M.W. Hamilton and R.A. Vincent
Physics and Mathematical Physics   ,
Adelaide University   ,   Adelaide, SA5005
We are developing a lidar for measuring the profile of water
vapour concentration in the lowest 4km of the atmosphere.
This will provide data on the atmospheric gas that is most responsible
for the greenhouse effect and the weather.
- It will be a low cost device, based on laser diodes,
- installed at many locations, giving greater geographical
coverage than is now possible.
The lack of accurate and reliable data on water vapour concentration
in the lower troposphere hampers both scientific and applied studies
of the atmosphere. The data gathered by current techniques is too sparse
and infrequent for weather and climate models.
Semi-real-time measurements of the variability will help greatly in weather forecasting, prediction of fog and hydrology.
At present, such measurements are mainly made by balloon sonde, which has high recurrent costs and is limited in
geographical and temporal coverage.
DIfferential Absorption Lidar (DIAL).
How it works ...
(assumes: scattering is incoherent, elastic and fast, no multiple scattering)
- Pulses of laser light are produced at 2 wavelengths with equal
- One wavelength is tuned to water vapour absorption resonance ...
the ther is tuned a small way from this where there is no absorption.
- Light scatters from molecules and aerosols in the atmosphere
with more or less equal probability for each wavelength
- Scattered light at one wavelength inherently less intense than the
other because that wavelength has also been absorbed by water in
the beam path.
- From the ratio of back-scattered power detected at the two
wavelengths, water vapour density can be inferred.
- Wavelength range around 830nm (3n + d polyad).
- (Other suitable wavelengths are around 720nm but there are no diode
lasers there. )
- Pulse length ~500ns (initially) giving range resolution ~200m.
- Expected sensitivity at 4km range with 0.5W pulses is about 0.5g/kg
(water vapour mixing ratio)
- Optical package (excl. telescope) should fit into 20 litre volume.
A diagram showing the optical aspects of our planned lidar.
Optical issues to be dealt with:
- Slave laser is to be pulsed synchronously with master pulses - will
chirping be a problem?
- Would a tapered amplifier be better? too much amplified spontaneous
emission? too expensive?
- How is laser frequency difference (~10GHz) to be maintained?
- GPS: Analysis of signals from Global Positioning System satellites
can provide data on integrated column densities of water vapour.
- Spaceborne Lidar: The
collaboration aims to build a spaceborne
system (lambda = 940nm) to measure stratospheric water vapour densities.
- Other ground-based lidars have been built, but are based on Ti:sapph
lasers (expensive! but give greqter range)
- One other known diode based system under construction at NOAA
(see Machol et al. 1998 - reference below)
Machol, J. L., R. M. Hardesty, J. B. Abshire, M. A. Krainik, G. W. Switzer,
D. M. Cornwell, and M. A. Randall,
Development of a miniature water vapor DIAL system,
in Proceedings of
Fourth International Symposium on Tropospheric Profiling: Needs and
Technologies, Snowmass, Colorado, September 21-25, 212-214, 1998.
... and a useful general reference
Wulfmeyer, V., and J. Bosenberg,
Ground-based differential absorption lidar for water-vapor profiling: Assessment of
accuracy, resolution and meteorological applications,
Applied Optics, 1999.
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last update 12/12/01