LIDAR: Introduction to selected topics
LIDAR: Introduction to selected topics
Michael GerdingLeibniz-Institut für AtmosphärenphysikSchlossstraße 6, 18225 Kühlungsborn
E-mail: [email protected]
LIDAR: Introduction to selected topics
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: Introduction to selected topics
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/...
LIDAR: Introduction to selected topics
1. Introduction and Overview
2. Lidar Basics2. Lidar Basics3. Lidar Application: Aerosols
4. Lidar Application: Temperature
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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: Introduction to selected topics
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
LIDAR: Introduction to selected topics
1. Introduction and Overview
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”
4. Lidar Application: Temperature
LIDAR: Introduction to selected topics
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)
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
3.1 Aerosol Determination by “Slope Method” (II)
lidar signaldensity profiledensity profile
LIDAR: Introduction to selected topics
Application of the “Slope Method”
Figure courtesy of M. Alpers
LIDAR: Introduction to selected topics
NLC photos
photo: P. Parviainen
photo: M Alpers
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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.
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.
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
]Klett (unsmoothed)
Ny-
Åle
sund
, Jan
uary
20,
200
1, 0
:21-
2:10
UT
Slope (smoothed)
LIDAR: Introduction to selected topics
Polar Stratospheric Clouds
photo: M. Rex
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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”!
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
1. Introduction and Overview
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
LIDAR: Introduction to selected topics
IAP Mobile Potassium Lidar
photo: J. Höffner
Potassium Temperature Lidar of Leibniz-Institute of Atmospheric Physics on thePlateauberget near Longyearbyen (78°N, 16°E)
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
80-105 km: K resonance lidar
-2pm -1pm 0pm 1pm 2pm
wavelength offset from 769.898 nm
0
100000
200000
300000
400000
coun
trat
e/ch
anne
l (20
0m)
measurementfit to data
-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
Measured and fitted shape of the resonance line
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
Temperature profile from air density profile
temperature
air density
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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
LIDAR: Introduction to selected topics
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
• hydrostatic equilibrium assumed
Resonance-Doppler Mesopause region
(80-105 km) • limited to atomic metal
layer
Brilloiun-Doppler lower troposphere • very weak signal
• hydrostatic equilibrium assumed
Rotation-Raman Tropo- and Stratosphere • weak signal
LIDAR: Introduction to selected topics
1. Introduction and Overview
2. Lidar Basics
3. Lidar Application: Aerosols
4. Lidar Application: Temperature
5. … add on’s5. … add on’s
LIDAR: Introduction to selected topics
LIDAR: Introduction to selected topics
LIDAR: Introduction to selected topics
LIDAR: Introduction to selected topics
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