LIDAR: Introduction to selected topics Michael Gerding Leibniz-Institut für Atmosphärenphysik...

LIDAR: Introduction to selected topics


Michael GerdingLeibniz-Institut für AtmosphärenphysikSchlossstraße 6, 18225 Kühlungsborn

E-mail: [email protected]


LIDAR: What’s it all about?

• LIDAR = light detection and ranging (similar to radar, sodar)

– light: pulsed laser (nanosecond-

range)

– scatterer: air molecules and aerosols

– detector: telescope and photon

detectors

– ranging: time-resolved detection

– distinction between scatterers

by optical properties (e.g.

wavelength

and scattering cross section)


LIDAR: What’s the use of it?

• vertical distribution of scatterers

• aerosol particles from troposphere to mesosphere

– existence, phase, optical thickness (extinction)

– particle size, particle number ???

• trace gases in troposphere/stratosphere/mesosphere/

thermosphere

– concentration of pollutants: NO2, SO2, ..., H2O, O3, OH,

metal atoms (Fe, Na, K, Ca, ...), Ca+

• temperature of the

troposphere/stratosphere/mesosphere/...


1. Introduction and Overview

2. Lidar Basics2. Lidar Basics3. Lidar Application: Aerosols

4. Lidar Application: Temperature


Light Interaction with the Atmosphere

• Rayleigh scattering• elastic; atoms or molecules

• Mie (particle) scattering• elastic; aerosol particles

• Raman scattering• inelastic, molecules

• Fluorescence• inelastic, broadband emission; atoms

or molecules

• Resonance fluorescence• elastic at atomic transition; large cross section

• Absorption• attenuation in bands; molecules or particles

h

h

ground level

virtual level

h

h ’

ground level

virtual level

vibrationallyexcited level

h

h

ground level

excited level


Light Interactions with the Atmosphere (II)

process scheme species backscatter rel. factor

density dependence

Rayleigh scattering

atoms, molecules

5·10-28 cm2/sr 1

0 km: 2.5·1019 cm-3

25 km: 8·1017 cm-3

90 km: 6·1013 cm-3

Mie scattering aerosols 0.2 - 1018 20 km: < 100 cm-3

Raman scattering

molecules 1/1000 spectrum

fluorescence atoms, molecules

108 spectrum

resonance fluorescence

atoms (ions, molecules)

1014 - 1016 5 – 25000 cm-3 spectrum

absorption molecules, aerosols

spectrum

h

h

h h

h

h ’

h

h

h

h’

h


Light Interactions with the Atmosphere (III)

from: A. Behrendt, PhD thesis, University Hamburg, 2000

Rayleigh-Raman spectrum

wavelength [nm]

rela

tive

inte

nsit

y

Q branchtotal intensity of the rotational Raman bandsRayleigh/Mie line

pure rotation Raman bands

vibrational Raman scatter


Elastic Lidar Backscatter Signal

0

0

1

1

2

2

3

3

4

4

5

5

00

5050

100100

150150

log. photons / bin

[km

]

IAP Ca-Lidar 19./20.April 1998

19.04.98 21:38:56 UT - 19.04.98 21:41:08 UT

re s o n a n c e sc a tte r

R a y le ig h sc a tte r

M ie sc a tte r

b a c k g ro u n d s ig n a l

alti

tud

e

c h o p p e r

M. Gerding, PhD thesis, IAP Kühlungsborn, 2000


Basic Lidar Equation

intensity at the emitted wavelength received from altitude zi (z=c·t/2)

emitted intensity at the wavelength

solid angle of visible telescope aperture

transmission between ground and scattering altitude zi

detector sensitivity

total backscatter coefficient

geometric overlap between laser and telescope FOV

background

BzzzzTz

AIzI iii

ii )(),()(),(

4)(),( 2

20

bin width

ner-EquatioLambert-Be

dzzTz

0

')',(exp

ResMieRay


Lidar System: Schematic Drawing

laser

detection

telescope

Trigger

Diode

S H GTH G

Q -S /4 P P O -CO szilla tor

A m plifie r

IF

HV

PM T

electroniccounter

BW T

Trigger



2. Lidar Basics

3. Lidar Application: Aerosols3. Lidar Application: Aerosols3.1 Aerosol Determination by “Slope Method”3.1 Aerosol Determination by “Slope Method”

3.2 Aerosol Determination by “Klett Method”3.2 Aerosol Determination by “Klett Method”

3.3 Aerosol Determination by “Ansmann Method”3.3 Aerosol Determination by “Ansmann Method”



Aerosol in the Atmosphere

boundary layer and tropospheric aerosol

clouds

Junge layer and volcanic aerosol

polar stratospheric clouds

noctilucent clouds

aerosol free

0

10

20

30

40

50

60

70

80

90

alti

tud

e [k

m]

aerosol backscatter (out of scale)


3.1 Aerosol Determination by “Slope Method” (I)

ratior backscatte : )(

)()(

)(

)(

)(

)()(

altitude free aerosol : )(

)()(

)(

)(

0

0

02

00

2

Rzn

znzR

z

z

z

zz

zz

zz

zzI

zzI

Ray

Ray

Ray

MieRay

0Ray

MieRay

)()(

)()(

)(0

200

2

znzzI

znzzI

zR

zzzzTz

AIzI iii

ii )(),()(),(

4)(),( 2

20

n: molecule number density


3.1 Aerosol Determination by “Slope Method” (II)

lidar signaldensity profiledensity profile


Application of the “Slope Method”

Figure courtesy of M. Alpers


NLC photos

photo: P. Parviainen

photo: M Alpers


3.2 Aerosol Determination by “Klett Method” (I)

Equation Diff.oulli Bern2d

d

d

d

ratiolidar : const :assume

2d

d1

d

d

,'d)'(2)(lnconst)(

,ln2lnconst)(ln:

2

0

2

Lz

S

z

Lα(z)Lβ(z)

zz

S

zzzzS

TzzIS

AerMolz

MieRay

zzzzTz

AIzI iii

ii )(),()(),(

4)(),( 2

20


3.2 Aerosol Determination by “Klett Method” (II)

')'()'(exp2)(

)()(exp)( :Solution

known)()( :conditionBoundary

Equation Diff. Bernoulli 2d

d

d

d

01

0

0

00

2

0

dzzSzSLz

zSzSz

zz

Lz

S

z

z

z

Ray


Application of the “Klett Method”: Polar Stratospheric Clouds

100

101

102

103

104

10

15

20

25

30

35

40

532 par.

re f. d ensity N C E P532 pe rp .

signal [a.u.]

altit

ude

[km

]

0 1 2 3 4 510

15

20

25

30

35

40

backscatter ratio

altit

ude

[km

]

100

101

102

103

104

10

15

20

25

30

35

40

532 par.


signal [a.u.]

altit

ude

[km

]

0 1 2 3 4 510

15

20

25

30

35

40

backscatter ratio

altit

ude

[km

]

100

101

102

103

104

10

15

20

25

30

35

40

532 par.


signal [a.u.]

altit

ude

[km

]

0 1 2 3 4 510

15

20

25

30

35

40

backscatter ratio

altit

ude

[km

]Klett (unsmoothed)

Ny-

Åle

sund

, Jan

uary

20,

200

1, 0

:21-

2:10

UT

Slope (smoothed)


Polar Stratospheric Clouds

photo: M. Rex


3.3 Aerosol Determination by “Ansmann Method” (I)

R. Schumacher, PhD thesis,Alfred Wegener Institute, 2001

te lescope laser

frequency

inte

nsity

(var

iabl

e sc

ale

)

0 -

36

52 c

m-1

0-1

- 2

331

cm

0 -

15

56 c

m-1


3.3 Aerosol Determination by “Ansmann Method” (II)

),(),,()()(4

)(),,( 002

2000 zzTzzz

AIzI RamRRR

k

RMolMol

zIz

znzAer

RAer

RMol

R

z

z

RR

R

R

N

zzz

zzz

dzzzzT

0

02

0

1

),(),(ln),(

)',()',(),(

)',()',(exp),,(

0),,(

)(dd

0

,0,0,0

002

Aer(R)k Ångström-coefficient

),(

),(

)()()(

)()()(

),(),(),(),(

000

0

0000

zT

zT

zNzIzI

zNzIzI

zzzz

R

RRel

RelR

MolAerMolAer

No need of “L”!


Application of the “Ansmann Method”: Tropospheric Aerosol

altit

ud

e [k

m]

e x tinction coeff. [m ]-1

LR=25: sea salt

Kol

dew

ey A

eros

ol R

aman

Lid

ar K

AR

LM

arch

12,

200

2, 2

1:00

-23:

30 U

T


Methods for Aerosol Determination: A Comparison

Slope Method • requires only one

channel

• suitable for weak, noisy signals

• no extinction correction

only suitable for thin clouds

Klett Method • requires only one channel

• extinction considered

• needs assumption on extinction to backscatter ratio (lidar ratio)

Ansmann Method

• requires no assumption on lidar ratio

• requires additional Raman channel (small cross section) range limit



2. Lidar Basics

3. Lidar Application: Aerosols

4. Lidar Application: Temperature4. Lidar Application: Temperature4.1 Temperature Profile from Resonance Lidar4.1 Temperature Profile from Resonance Lidar

4.2 Temperature Profile from Rayleigh Lidar4.2 Temperature Profile from Rayleigh Lidar

4.3 Temperature Profile from Raman Lidar4.3 Temperature Profile from Raman Lidar


IAP Mobile Potassium Lidar

photo: J. Höffner

Potassium Temperature Lidar of Leibniz-Institute of Atmospheric Physics on thePlateauberget near Longyearbyen (78°N, 16°E)


4.1 Temperature Profile from Resonance Lidar

• atomic K exists in the mesopause region (like Si, Mg, Fe, Na, Ca ...)

• investigation by resonance lidar

• alkali metals show hyperfine structure of electronic transitions

• temperature dependent Doppler broadening of resonant backscatter can be detected by narrowband laser

altit

ude

[km

]

photons/bin (4000 pulses)

IAP Potassium Lidar

Figure courtesy of J. Höffner


above 80 km: K resonance lidar

-2 pm 0 pm 2 pm

wavelength offset from 769.898 nm

0.0

0.2

0.4

0.6

0.8

1.0

ba

ck

sc

att

er

cro

ss

se

cti

on

[1

e-1

6 s

qm

/st]

125 K

200 K

275 K

39K

41K41

K

1:106

Hyperfinestructure and Doppler broadening of a K resonance line

von Zahn and Höffner, 1996


80-105 km: K resonance lidar

-2pm -1pm 0pm 1pm 2pm


0

100000

200000

300000

400000

coun

trat

e/ch

anne

l (20

0m)

measurementfit to data

-2 pm 0 pm 2 pm


0.0

0.2

0.4

0.6

0.8

1.0

ba

ck

sc

att

er

cro

ss

se

cti

on

[1

e-1

6 s

qm

/st]

125 K

200 K

275 K

39K

41K41

K

1:106

Hyperfinestructure and Doppler broadening of a K resonance line

Measured and fitted shape of the resonance line


22-90 km: Rayleigh lidar

hydrostatic equation

ideal gas law

)(zgdz

dp

TknVp

z

z

toptop

top

dzzn

znzg

k

mzT

zn

znzT '

)(

)'()'()(

)()(

relative density profile required - derived from

(aerosol free) lidar backscatter signal


Temperature profile from air density profile

temperature

air density


1-30 km: Rotation-Raman lidar

Rotation-Raman spectrum of air for excitation at 532.05 nm

w a ve le n g th [n m ]528 529 530 531 532 533 534 535 5360

0.2

0.4

0.6

0.8

1.0

1.2

1.4

po

we

r[a

rb.

un

its]

N itro g enO xyg en

IF 529 nm

IF 530 nm

tem perature = 250 Klaser wavelength = 532.05 nmratio(529/530) = 0 .5463

Alpers et al., 2004


4.3 Temperature Profile from Raman Lidar

• Rotation-Raman spectrum depends on temperature

• Intensity of transitions to high J-numbers increase with temperature, intensity of transitions to low J-numbers decrease

• Intensity ratio between two different wavelengths depends on temperature

• For lidar choose narrow fractions of the spectrum

wavelength [nm]

high-J filterlow-J filter

A. Behrendt, Uni Hamburg, 2000


5.3 Temperature Profile from Raman Lidar (II)

• Backscatter signal at the different wavelengths depend on temperature, but also on the filter characteristic, the transmission of the detection system, atmospheric extinction

temperature dependence of the signal can (hardly) be calculated

or lidar can be calibrated withrespect to temperature response(comparison with other methods likeradiosondes)

00.20.40.60.81.015020025030035000.20.40.60.81.0

150200250300350

signal ratio(529nm/530nm)temperature[K]0 = 532.05 nm


Comparison of temperature sounding principles

range complexity limits

Rayleigh-Integration Strato- and Mesosphere • aerosol inhibits sounding

• hydrostatic equilibrium assumed

Raman-Integration (Troposphere) Stratosphere • aerosol disturbs sounding


Resonance-Doppler Mesopause region

(80-105 km) • limited to atomic metal

layer

Brilloiun-Doppler lower troposphere • very weak signal


Rotation-Raman Tropo- and Stratosphere • weak signal



2. Lidar Basics

3. Lidar Application: Aerosols


5. … add on’s5. … add on’s


P M T

HS

PM

T

532 nm(stratosphere)

532 nm(m esosphere)

ch op p e r

H O R am an2

407 nm

387 nm

N R am an2

N R am an2

608 nm

355 nm

308 nm

1064 nm

529 nm

530 nm

R=50%

rot. Ram an

legendquartz fibre

lens

IF filter

motorised filter whee land

neutral density filter

dichroicbeam splitte r

beam splitte r

m irror

photon counter

additionalligh t shie ld ing

tele

sco

pe

telescope

Ch

op

pe

r

K 770 nm

Detector of the IAP T lidars

LIDAR: Introduction to selected topics Michael Gerding Leibniz-Institut für Atmosphärenphysik...

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Transcript of LIDAR: Introduction to selected topics Michael Gerding Leibniz-Institut für Atmosphärenphysik...