Megie, G.

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LIDflR nEftSUREHEMTS OF 5TRRTOSPHERIC TEOPERflTURE
G. HEGIE
Service d'Aeronoale da C.N.R.S.
B.P. 3 - 91371 Verrl$res-2e-Bu!sson CEDEX - France
The radiative budget of the stratosphere and mesosphere is
mainly influenced by ozone and carbon dioxide and thus directly sensi-
tive to the variations of these trace species which can be induced by
anthropogenic activities. Temperature distribution m i l l thus vary at
various temporal and spatial scales related to dynamical processes such
as planetary uaves activity or radiative effects induced by trace
species or aerosols fluctuations. Long term variations or trends either
related to solar variability or to anthropogenic perturbations have not
yet been detected and w i l l constitute part of the rationale to implement
a monitoring programme. Furthermore, systematic observations of the
temperature profile w i l l provide a data base of particular value for the
establisment of Reference Model of the Middle Atmosphere. Use of ground
based systems which can provide a high accuracy and vertical resolution
is also important for the validation of satellite data including tempe-
rature measurements and trace species concentration retrievals from IR
and microwave passive sensors.
Rayleigh backscattering of a laser beam by atmospheric molecules
has been observed by most of the first lidar researchers but the first
tentative determination of stratospheric temperature was only performed
in the late 1970s. In 1980 a lidar system devoted to that type of measu-
rements has been set up at the Observatoire de Haute Provence and
operated on a quasiroutine basis from this time (Hauchecorne and Chanin.
1983). Several systems are presently under development in various
countries CRFA, USA, UK, France....) which might provide the basis of a
future global network.
When a monochromatic laser beam, at a wavelength which does not
coincide with any absorption or resonant feature of atmospheric species,
is sent in the atmosphere, two processes can provide a bacXscattered
signal : Rayleigh scattering by atmospheric molecules and Mie scattering
by aerosol particles. At the height above 30 to 35 km where the aerosol
content is very low, if any, the atmosphere can be considered as purely
molecular and the lidar backscattered signal is proportional to the
Uorkshop on Barly Detection of Changes In Stratospheric Structure
NASA, NOAA, UNO, CHA
Boulder. March 1906
https://ntrs.nasa.gov/search.jsp?R=19910073968 2020-03-17T14:53:04+00:00Z
atmospheric density. The temperature profile can then be computed from
the density profile assuming that the atmosphere obeys the perfect gas
law and is in hydrostatic equilibrium. This second assumption im p l i e s
that atmospheric turbulence does not affect the mean air density, uhich
is the case considering the temporal and spatial resolutions of the
lidar data. The constant mixing ratio of the major atmospheric consti-
tuents CN2, 02 and Ar) and the negligible value of the H20 mixing ratio
justify the choice of a constant value for the air mean molecular
weight. Details of the analysis are given in Annex. It should be pointed
out that the only assumption to be made to compute the temperature
profile is that of a pressure boundary at the top of the atmosphere, the
accuracy of uhich can be evaluated to be of the order of 15 %. Its
contribution to the temperature uncertainty decreases rapidly with
altitude and is smaller than 2 % at 15 km from the top, and smaller
than 1 fc, 5 km lower. It is important to note that only the ratio of
experimental density values is taken into account and that consequent-
ly any constant of normalization in the return signal disappears. The
temperature determination can then be considered as absolute as soon as
one can neglect the term due to the pressure at the top, even though the
density is only measured on a relative uay.
The characteristics of the "Rayleigh" lidar in operational use
at the Observatoire de Haute Provence are given in the Appendix. Such a
system w i l l include a powerful hd Yag laser emitting at the second
harmonic wavelength C532 nm). The choice of the emitter is obviously a
function of the state of the art of laser development and particular
attention should be brought to the new sources such as exciplex lasers
CXeF) at 355 nm. The receiver is a large diameter C- 1 m) telescope
required for high altitude measurements. Due to the large dynamics of
the signal two acquisition modes are used, i.e. analogical time sampling
for the lower attitude ranges and photon counting in the upper levels.
All the experiment is computer controlled and can be run efficiently for
many hours without an operator assistance. Typical accuracy on the
measurements can be quoted as follows for the OHP system :
- fl T = ; 1 K at 40 km for a 1 hour integration time and 1 km height
resolution
- A T = ; 5 K at 70 km for a 3 hours integration and a 0.5 km height
resolution.
Uith the performances already available, the advantages of a
Rayleigh lidar, in terns of atmospheric needs, are mainly due to three
characeristies :
- The possibility of making absolute temperature determination without
requiring an external calibration, and this with an accuracy better
than a few K in the stratosphere.
- The availability of good vertical resolution C- 1 km)
- The continuity of the survey in altitude, from 30 to 80 km, and in
time. The observations are only limited by meteorological conditions
which implies a good choice of the site and at the present time
restricted to nighttime.
These three characteristics maVe the Rayleigh lidar a powerful
tool mostly for two types o? studies based either on the knowledge of
the absolute temperature and its eventual evolution, or on the descrip-
tion of the temporal and spatial fine structure of both density and
temperature. For both approaches it is a necessary complement, of
satellite observations.
CHARACTERISTICS OF THE RfWLEIGH LIDAR
IN OPERATIONAL USE AT THE OBSERUATOIRE DE HAUTE PROVEMCE
Laser
type
Energy per pulse
Repetition rate
Pulse width
Divergence
Divergence
Receiver
Telescope diameter
Telescope area
Field of view
Band pass fi Iter
P.P. interferometer
Gate width
Neodymium - Vag CQuantel Model 408)
400 mj at 532 nm
C150 mj at 355 nm)
10 Hz
15 ns
4.10" rad
10"* rad (with a beam expander)
80 cm
0.5 m2
10"3 to 10*« rad
0.8 nm (FWHM)
20 pm CFWHM)
4 urn (0.6 km)
REFERENCES
HAUCHECORNE. A. and M.L. CHAN1N (1983) : Mid latitude lidar observations
of planetary waves in the middle atmosphere during the winter of
1981-1988
J. Oeophys. Res. 88, 3843-3849.
CHAN1N M.L. and A. HAUCHECORNE (1984) : Lidar studies of temperature and
density using Rayleigh scattering
MAP Handbook n° 13, 87-99.


Lidar measurements of stratospheric temperature

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