Lidars
Description
Lidars measure the radiation that is
returned either from molecules and particles
in the atmosphere or from Earth’s surface
when illuminated by a laser source. Compared
with radar, the shorter wavelengths used in
a lidar allow greater detail to be observed.
On the other hand, the light cannot
penetrate optically thick layers such as
clouds.
There are different types of lidar
instrument:
— Backscatter lidar, in which the laser beam
is backscattered, reflected or re-radiated
by the target, gives information on the
scattering and extinction coefficients of
the various atmospheric layers being
probed;
— Differential absorption lidar analyses the
returns from a tuneable laser at different
wavelengths to determine densities of
specific atmospheric constituents, as well
as water vapour and temperature profiles;
— Doppler lidar measures the Doppler shift
of the light backscattered from particles or
molecules moving with the wind, thereby
allowing the determination of wind
velocity;
— Ranging and altimeter lidar provides
accurate measurements of the distance from a
reference height to precise locations on
Earth’s surface.
The first satellite-borne ranging and
altimeter lidar, ALISSA, was flown on the
manned Mir station in 1996 and provided
valuable information for more than a year on
cloud-top heights and multi-layer cloud
structure. Later, the GLAS ranging and
altimeter lidar was flown on the NASA ICESat
mission (launched in January 2003 and
completed September 2010) to study
variations of ice topography, as well as
cloud and atmospheric properties. Launched
in April 2006, the CALIOP backscatter lidar,
flying on the NASA–CNES Calipso platform,
has completed seven years of successful
operations and continues measurements of
cloud and aerosol properties. ESA is
currently implementing two laser missions,
ADM-Aeolus and EarthCARE. The ALADIN high
spectral resolution Doppler wind lidar on
ADM-Aeolus will measure profiles of
line-of-sight winds globally. The ATLID high
spectral resolution lidar on EarthCARE will
measure cloud and aerosol optical
properties.
|
|
|
|
|
|
|
Current & planned
instruments
|
|
ALADIN
|
|
CO2 LIDAR (ASCENDS)
|
|
|
|
|
ATLAS
|
HDWL (3D Winds)
|
|
|
|
|
ATLID
|
Laser altimeter (LIST)
|
|
|
|
|
CALIOP
|
|
|
|
|
Applications
The different types of lidars may be used to
measure a diverse range of parameters. Ranging
and altimeter lidars may be used to provide
surface topography information, for example,
on ice-sheet height and land altitude.
Missions planned within the next few years
will undertake to determine the mass balance
of the polar ice sheets and their
contributions to global sea-level change;
others will focus on study of the vegetation
canopy structure and provide unique datasets,
including estimations of global biomass and
carbon stocks, and fractional forest cover.
Multifrequency and high spectral resolution
ranging lidars with probe wavelengths in the
UV, visible and near-IR will be used to
measure aerosol height distributions, heights
of clouds and their vertical profiles.
Differential absorption and backscatter lidar
may be used to measure aerosol and cloud
properties as well as atmospheric composition.
Doppler lidars may be used to measure wind
profiles in clear air (i.e. in the absence of
clouds or winds above clouds) and within
optically thin layers. The capability of
measuring clear air winds is of particular
importance since it will correct a major
deficiency in wind-profiling of the current
global meteorological observing systems.
Instruments such as ESA’s ALADIN on ADM-Aeolus
will provide wind profile measurements to
establish significant advances in atmospheric
prediction and analysis.
|
|
