Lecture 9: Raman lidar

Water vapor mixing ratio measured by the SRL during the dryline event. Temporal resolution is 3 minutes, vertical smoothing varied between 90 meters at 0.5 km to 330 meters above 4 km.The calibration of the data was determined with respect to SuomiNet GPS mounted on the SRL trailer.

NASA/GSFC Raman Airborne Spectroscopic Lidar (RASL)

RASL

RASL Specifications

Laser Continuum 9050 Nd:YAG (355 nm), 350 mj/pulse, 50 Hz

Telescope Custom 24” athermal, manufactured by DFM Engineering

Data acquisition 250 Mhz photon counting and 20 Mhz analog detection

Range resolution 7.5 meter

Measurements [Molecule/

Wavelength (nm)/Bandpass (nm)]

water vapor/407.5/0.25

liquid water/403.2/6.0

nitrogen/386.7/0.3

oxygen/375/0.3 or CO2/371.6/0.3

elastic unpolarized/354.7/0.3

elastic parallel polarized/354.7/0.3

elastic perpendicular polarized/354.7/0.3

Detectors Hamamatsu R1924 (Raman) and R7400 (aerosol) PMTs

Field of View 0.25 mrad

NASA/GSFC Raman Airborne Spectroscopic Lidar (RASL)

Raman lidar system for the measurements of tropospheric

water vapor

0.625mCassegraintelescope

PMT

407.8nm

Amplifier

PMT Amplifier

PMT Amplifier

386.7nm

355nm

T

D

R

LASER

MCS1

MCS2

MCS3

355nm PD

SynchronizationControl

Electronic gatecontrol signal

Trigger signal

ControlComputer

Ocular

FieldStop

F1

F2

F3

407.8nm386.7nm355nm

Laser control signal

Raman Lidar System

Main technical parameters of Raman Lidar System Laser Nd:YAG Wavelength (nm) 355 Pulse energy (mJ) 80 Pulse width (ns) 20 Beam divergence (mrad) ≤ 1

Pulse repetition (Hz) 10 Receiving telescope Cassegrain Diameter (mm) 625 Filed of view (mrad) 3 Interference filters Central wavelength (nm) 407.8 386.7 355 Bandwidth (nm) 4.7 4.3 1 Transmission (%) 55 60 40 PMT(EMI) 9214QB×2 9817B

Preamplifier(EG&G) VT120×3

Gain 200 Bandwidth (MHz) 350 Multi-channel Scaler EG&G 914P×3

Maximum count rate 150MHz

Characteristics of three dichronic beam splitter

Beam Splitter 407.8nm 386.7nm 355nm

T >95%(R) 15%(R) 85%(T)

>85%(T)

D 65 %(R) 35%(T)

>85%(T)

R >95%(R) >85%(R)

350 360 370 380 390 400 410 4200

10

20

30

40

50

60

70

80

90

100

Trichronic

Tran

smiss

ion

(%)

Wavelength (nm)350 360 370 380 390 400 410 4200

10

20

30

40

50

60

70

80

90

100

DichronicTr

ansm

issio

n (%

)

Wavelength (nm)350 360 370 380 390 400 410 4200

10

20

30

40

50

60

70

80

90

100

Reflector

Tran

smiss

ion

(%)

Wavelength (nm)

Raman Water Vapor

Raman Nitrogen

Rayleigh-Mie

Central wavelength (nm) 407.8 386.7 355 Bandwidth (nm) 4.7 4.3 1 Transmission at Central Wavelength (%)

55 60 40

Transmission at 355nm and 532nm

10-12 10-12

Transmission at 200nm~1200nm

10-6 10-6 10-5

Transmission at 375nm, 387nm, 580nm and 607nm

10-8

Diameter (mm) 25.4 25.4 25.4 Thickness (mm) 10 9 5

Characteristics of three interference filters

400 405 410 415 4200

10

20

30

40

50

60

70

80

90

100

Raman water vapor filter

Tran

smis

sion

(%)

Wavelength (nm)375 380 385 390 395

0

10

20

30

40

50

60

70

80

90

100

Raman nitrogen filter

Tran

smiss

ion

(%)

Wavelength (nm)

Transmission function of two Raman interference filters

( ) ( ) ( ) ( ) ( )z,z,qz,z,qznzk

zS HHHH

H 0002 λλπσλλ =

Raman Lidar Equation

Raman Water Vapor:

Raman Nitrogen:

( ) ( ) ( ) ( ) ( )z,z,qz,z,qznzk

zS NNNN

N 0002 λλπσλλ =

Water Vapor Mixing Ratio:

( ) ( )( )

( )( )

( )( )znzn

MM

znzn

MM

znznzw

dry

N

dry

H

N

H

dry

H

dry

H ==

where Cw is the system calibration constant ( )

( ) dry

N

dry

H

H

N

H

Nw n

nMM

kkC

πσπσ

=

( )z,z0wqΔ is the transmission correction function

( ) ( )( )z,z,q

z,z,qz,z0H

0N0

wq λ

λΔ =

( ) ( ) ( )( )zSzSz,zCzw

N

Hwqw 0Δ=

Water Vapor Mixing Ratio:

( )( )

( ) ( )( ) ( )

( )( )

( )( )zz

zSzS

zzSzzS

zSzS

H

N

N

H

NN

HH

N

H

γγ

γγ

⋅==''

/'/'

( )( )zz

H

N

γ

γ

can be obtained through simultaneously measuring the return signals at 386.7nm for Raman water vapor and nitrogen channels.

Retrieval for signals within the overlap region ( )( )z'Sz'S

N

H

0.0 0.5 1.0 1.5 2.0 2.5 3.01.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

May.19,1999 Jun.02,1999

Altitude(km)

γγΝΝ // γγ

ΗΗ

0.80 0.85 0.90 0.95 1.000

1

2

3

4

5

6

532nm τ=0.0 τ=0.25 τ=0.5 τ=1.0

Altit

ude(

km)

Transmission RatioVertical profiles of the transmission correction function for different aerosol optical depth

Transmission Correction Function ( )z,z0wqΔ

30

35

40

45

50

55

60 The mean calibration constant

C--w=41.59

The standard deviation of calibration constant δδCw=2.56

May.10 15 20 25 30 Jun.05 10 15 20

Cw

Date

Calibration Constant Cw

0 1 2 3 4 5 6 7 80

1

2

3

4

5 Lidar radiosonde 20% Relative humidity

May 6,1999

Alti

tude

(km

)

Water vapor mixing ratio (g/kg)0 1 2 3 4 5 6 7 8 9 10

0

1

2

3

4

5

Lidar radiosonde 20% Relative humidity

May 11,1999

Alti

tude

(km

)

Water vapor mixing ratio (g/kg)

0 2 4 6 8 10 120

1

2

3

4

5

Lidar radiosonde 20% Relative humidity

May 14,1999

Alti

tude

(km

)

Water vapor mixing ratio (g/kg)

Vertical Profiles of the water vapor mixing ratio

0 1 2 3 4 5 6 7 80

1

2

3

4

5 May 26, 1999 20:24-20:33 20:33-20:41 20:41-20:50 20:50-21:00 21:00-21:09 21:09-21:18

Alti

tude

(km

)

water vapor mixing ratio (g/kg)

Spatial and temporal variation of water vapor

0 2 4 6 8 10 12 14 16 18 20 220

1

2

3

4

5

6 Lidar radiosonde 100% relative humidity 355nm

Jun.02,1999

Alti

tude

(km

)

Water vapor mixing ratio (g/kg)

Water vapor mixing ratio in the cloud

Error Analysis

2222

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎠

⎞⎜⎝

⎛+⎟⎠

⎞⎜⎝

⎛=⎟⎠

⎞⎜⎝

⎛

w

w

CC

TT

SS

ww δδδδ

0 10 20 30 40 501.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Jun.02,1999

Altitude(km)

δδS/S(%)0 2 4 6 8 10

0

1

2

3

4

5

6

Altit

ude(

km)

δT/T (%)

0 10 20 30 40 501.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Jun.02,1999

Alti

tude

(km

)

δδw/w (%)

Rotational Raman lidar for temperature measurement

Examples of return signals

Examples of results