Active Remote Sensing of the Atmosphere - Lidar -

Remote Sensing I

Lecture 9

Summer 2006

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!

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

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

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

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)

LIDAR: Example

G. Beyerle, PhD thesis, 1994

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

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 ,

Lidar equation

Taking these factors together will give the so called

Lidar-Equation:

zzTzz

AzCPzP ED ,,,,

2

z

ext zdzzT0

,2exp, with

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/

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::

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,

DIAL LIDAR: Examples

Stratospheric O3Tropospheric O3

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

Aerosol Lidar: Example PSC

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

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

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

LITE: Example of Aerosol Measurements

Atlas mountains

Clouds (ITCZ)

complex aerosol layer

maritime aerosol layer

http://www-lite.larc.nasa.gov/index.html

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