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Active Remote Sensing of the Atmosphere - Lidar -
Remote Sensing I
Lecture 9
Summer 2006
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LIDAR (Light Detection And Ranging)
• Idea: • Use of an active system that emits light pulses and measures the intensity of
the backscattered light (from air molecules, aerosols, thin clouds) as a function of time (optical Radar)
• Instrument: • a strong laser with short pulses• possibly several wavelengths emitted• a large telescope to collect the weak signal
• Measurement quantity:• time lag gives altitude information (z = 1/2 t c, with c speed of light)• signal intensity gives information on backscattering at given altitude and
extinction along the light path• measurements at different wavelengths provide information on absorbers and
aerosol types• polarisation measurements provide information on phase of scatterers
• => Very good vertical resolution can be achieved!
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Review: Scattering in the Atmosphere
Rayleigh Mie
Radius / Wavelength
r << r >>
Phase function P11() (1 + cos2 ) Highly variable, depending on = 2r / Strong forward peak
Asymmetry parameter
g = 0 g > 0
Polarization = 0, : LP = 0 = ± /2 : LP 1
Generally depolarizing,
but variable
Spectral depedence
R -4 M -m
m : Ångstrom exponent
(-1 < m < 4)
Summary Mie and Rayleigh Scattering
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Fig. from Liu, An introduction to atmospheric radiation
Comparison of Rayleigh and Mie phase functions
• The larger the size parameter, the larger the forward scattering peak
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LIDAR-Types and Target Quantities
• Applications:• altimeter• Rayleigh Lidar: temperature• DIAL (Differential Absorption)-Lidar: trace gases• multi wavelength aerosol Lidar: aerosol amount
and aerosol properties (size distribution, type)• Raman-Lidar: trace gases • Doppler-Lidar: particle velocities• Fluorescence-Lidar: temperature in the upper
atmosphere
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LIDAR: Instrument
• Laser:• short pulses (small dead range above instrument)• high pulse power (high backscattered signal)• typical lasers:
–solid state laser (e.g. Nd-YAG)–gas laser (e.g. XeCl)–dye lasers
• Detector:• excellent quantum efficiency needed (low signal)• low noise needed (low signal)• typical detectors
–Photomultiplier–Photodiodes–CCDs
• wavelength selective (use of filters)
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LIDAR: Example
G. Beyerle, PhD thesis, 1994
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LIDAR: Measurement Example
• two wavelengths (353 nm and 532 nm
• minimum altitude: 11 km• maximum altitude: 45 km• background signals of
calibration• exponential scale• signature of volcanic
aerosol• signature of PSCs
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Lidar equation
The detected intensity Pd(z,λ) is proportinal to
•Emitted intensity
•Backscatter coefficient
•Observed solid angle
(with A area of telescope)
•Transmission along the light path
•Sensitivity of the detector in this channel
(including geometric overlap):
EP
zzz ,,, AerosolRayleigh
2zA
z
ext zdzzT0
,2exp,
zC ,
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Lidar equation
Taking these factors together will give the so called
Lidar-Equation:
zzTzz
AzCPzP ED ,,,,
2
z
ext zdzzT0
,2exp, with
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DIAL LIDAR
• Idea:• two wavelengths are
emitted, one at an absorption line, the other one off the absorption but close enough to have small changes in scattering properties and absorption by other absorbers
• Application:• ozone profiles
• H2O profiles
http://www.etl.noaa.gov/et2/
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DIAL Lidar equation
Start from the Lidar-equation for two wavelength on/off:
zzTzz
AzCPzP ED ,,,, on/offon/off2on/off,on/off,
z
E
E
D
D zdzz
z
P
P
zP
zP
0
extoff
on
off,
on,
off,
on, ,2exp,
,
,
,
Forming the ratio between the received signals Pon and Poff:
z
E
E
D
D zdzz
z
P
P
zP
zP
0
extoff
on
off,
on,
off,
on, ,2,
,lnln
,
,ln
... And then the logarithm::
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DIAL Lidar equation
zz
z
dz
d
zP
zP
dz
d
D
D ,2,
,ln
,
,ln ext
off
on
off,
on,
z
E
E
D
D zdzz
z
P
P
zP
zP
0
extoff
on
off,
on,
off,
on, ,2,
,lnln
,
,ln
Differentiating wrt altitude z gives:
If the two wavelength are nearby, scattering properties will beSimilar, and we finally get:
zzP
zP
dz
d
D
D ,2,
,ln abs
off,
on,
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DIAL LIDAR: Examples
Stratospheric O3Tropospheric O3
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Aerosol LIDAR
• Idea:• Backscattering at different wavelengths is used to derive
information on aerosol properties
• for each wavelength, the backscattering coefficient βMie(z, λ) is computed from the Lidar equation using the Klett-algorithm:
–profiles of temperature and pressure as Input–use of reference height with known backscatter
coefficient (Rayleigh only)–Mie scattering ratio determined from model:
LMie(z, λ)= αMie(z, λ)/ βMie(z, λ)
• Measurement quantity is the backscattering ratio R.
),(
),(),(),(
z
zzzR
Ray
MieRay
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Aerosol Lidar: Example PSC
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Aerosol Lidar: Example Cirrus Clouds
• airborne lidar measurements
• OLEX instrument (http://www.dlr.de/~flentje/olex.html )
• very good detection limit • high spatial and vertical
resolution• detection of cirrus
clouds, thin and even “subvisible“
• particle size from colour ratio
• particle phase from depolarisation
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LIDAR: Overview
Measurement Quantity Wavelength Measurement Principle
Ozone concentration (in Rayleigh atmosphere)
308 nm & 355 nm DIAL-technique
Ozone concentration (also in the presence of Mie scatterers)
332 nm & 387 nm Raman-DIAL-technique
Water vapour mixing ratios 387 nm & 408 nm H2O-Ramanlidar-technique
stratospheric temperature above 30 km height
355 nm Rayleigh integration method
tropospheric and stratospheric temperature (also in the presence of Mie scatterers)
530,85 & 529,35 nm Rotational Raman method
Backscatter ratio volume and particle Extinction coefficient, volume and particle Backscatter coefficient at three wavelengthsColour ratio
308 nm & 332 nm 355 nm & 387 nm 532 nm & 608 nm
combination of Raman scattering and elastic scattering (Raman lidar technique)
volume and particle Depolarisation 355 nm, 387 nm polarisation depending
Depolarisation lidar technique
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Lidar In-space Technology Experiment (LITE)
• Instrument:• flashlamp-pumped Nd:YAG laser • 1064 nm, 532 nm, and 355 nm • 1-meter diameter lightweight telescope • PMT for 355 nm and 532 nmavalanche photodiode (APD) for 1064 nm
• Mission Aims:• test and demonstrate lidar measurements from space• collect measurements on
–clouds–aerosols (stratospheric & tropospheric)–surface reflectance
• Operation:• on Discovery in September 1994
as part of the STS-64 mission• 53 hours operation
http://www-lite.larc.nasa.gov/index.html
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LITE: Example of Aerosol Measurements
Atlas mountains
Clouds (ITCZ)
complex aerosol layer
maritime aerosol layer
http://www-lite.larc.nasa.gov/index.html
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More LIDARS in space
• ICESat (January 12, 2003) • 532 nanometer lidar • ice sheet mass balance• aerosol and cloud heights• vegetation and land topography• http://icesat.gsfc.nasa.gov/
• CALIPSO (2005?)• 532 nm and 1064 nm) polarization-sensitive lidar • clouds and aerosols• http://www-calipso.larc.nasa.gov/
• WALES (2008?)• water vapour DIAL • high resolution water vapour profiles• http://www.esa.int/export/esaLP/ASE77YNW9SC_wales_0.html