A new method for first-principles calibration of water vapor Raman lidar

Valentin Simeonov École polytechnique fédérale de Lausanne

Switzerland

Overview

• Raman lidar as meteorological tool• Lidar and Raman lidar principle• Calibration problem• New method for instrumental calibration• Conclusion and perspectives

Time resolution - 10 min, Vertical resolution - 30 m up to 4 km

‘Rain stop’Clouds/Fog < 500 m

Clouds, Rain

Noc

turn

al B

L

Conv

ectiv

e M

ixed

Lay

er

Resid

ual l

ayer

g/kg

Raman Lidar for Meteorological observations (RALMO) EPFL-MeteoSwiss

How does a lidar work?

P0

I(R)

R

R

A

Laser

Telesccope

Spectral unit

2

1R

𝐼 (𝑅 )=

𝑃0 𝐴k𝑅2 𝛽 (𝑅 )𝛤 2 (𝑅 )

I - Signal magnitude P0 -Laser powerA- Telescope areak- Lidar efficiencyR- Distanceβ- Backscatter coefficientΓ - Atmospheric extinctionα- Extinction coefficientFOV- Telescope field of view

FOV

Γ (𝑅 )=−∫0

𝑅

𝛼 (𝑟 )𝑑𝑟

Water vapor Raman lidar

𝜷=𝝈𝑵Quantitative determination

h -Planck constantν – light frequencyc – speed of lightλ – light wavelength

𝝀=𝒄𝝂

High selectivity

σ -Raman cross sectionN – molecular number density

𝐼 𝑋 (𝑅 )=𝑃0𝐴𝑅2 k 𝑋𝜎 𝑋𝑁 𝑋 (𝑅)Γ 𝐿(𝑅)Γ𝑅𝑋 (𝜆𝑋𝑅)

H2O

Wavelength-λ [nm]

Scatt

erin

g in

tens

ity

=

(R)

𝑪=𝝁 𝒌𝑵 𝟐𝝈𝑵 𝟐

𝒌𝑯𝟐𝑶𝝈𝑯𝟐𝑶

q(R) – Water vapor/air mixing ratio C – Calibration constantΔΓ – Differential atmospheric transmissionµ – Constant, converts H2O/N2 to H2O/air mixing ratio

Calibration against a reference instrument(radiosonde)

Disadvantages• Different air volumes sampled• Different spatial and temporal resolution• Auxiliary information (T or T & P profiles ) needed • Additional systematic errors from the conversions -Relative humidity to mixing ratio -Dew point temperature to mixing ratio• ΔΓ included in C• Calibration not traceable to primary standards• Calibration accuracy limited by the reference

instrument accuracy

𝑪=𝒒𝒓𝒆𝒇 (𝑹)𝑰𝑵𝟐(𝑹)𝑰𝑯 𝟐𝑶(𝑹)

𝟏𝜟𝜞 𝝀𝑵 𝟐−𝝀𝑯 𝟐𝑶

Advantages• Simple• Easy comparison with the existing techniques

qref – Reference mixing ratio

407 407.1 407.2 407.3 407.4 407.5 407.6 407.7 407.8 407.90.00E+00

5.00E-32

1.00E-31

1.50E-31

2.00E-31

2.50E-31

3.00E-31

3.50E-31

4.00E-31

4.50E-31

0

0.2

0.4

0.6

0.8

1

στ

Wavelength [nm]

Ram

an c

ross

secti

on σ

[cm

2] τ

Instrumental calibration

𝑪=𝝁 𝒌𝑵 𝟐𝝈𝑵 𝟐

𝒌𝑯𝟐𝑶𝝈𝑯 𝟐𝑶

𝑪(𝑹)=𝝁𝜼𝑵 𝟐𝜺𝑵 𝟐∫𝝉𝑵 𝟐 ( 𝝀 )𝝈𝑵𝟐 (𝝀 ,𝑹)𝒅 𝝀

𝜼𝑯𝟐𝑶𝜺𝑯𝟐𝑶∫𝝉𝑯 𝟐𝑶 (𝝀 )𝝈𝑯𝟐𝑶 (𝝀 ,𝑹)𝒅 𝝀

G. Vaughan et al. (1988)

Sherlock et al. (1999)

Is the lidar calibration constant constant?

𝑪(𝑹)=𝝁𝜼𝑵𝟐 (𝑹)𝜺𝑵 𝟐 ( 𝑹)∫𝝉𝑵 𝟐 (𝝀 ,𝑹 )𝝈𝑵 𝟐 ( 𝝀 ,𝑹)𝒅 𝝀

𝜼𝑯𝟐𝑶 (𝑹)𝜺𝑯𝟐𝑶(𝑹)∫𝝉𝑯𝟐𝑶 (𝝀 ,𝑹 )𝝈𝑯 𝟐𝑶 (𝝀 ,𝑹)𝒅 𝝀

η – Photodetector efficiencyε - Optics efficiencyτ - Spectral unit instrumentalfunction

New instrumental calibration method

𝑪=𝝁𝜼𝑵 𝟐𝜺𝑵𝟐∫𝝉𝑵 𝟐 (𝝀 )𝝈𝑵 𝟐 (𝝀 )𝒅 𝝀

𝜼𝑯𝟐𝑶𝜺𝑯𝟐𝑶∫𝝉𝑯𝟐𝑶 (𝝀 )𝝈𝑯𝟐𝑶 ( 𝝀 )𝒅 𝝀

mX – mass of Xp – air pressureMa – molecular mass of airV – cell volumeT – air temperaturez – compressibility factor

LaserbeamDetection

 

   Cell

  Laser

 

       Ventilator

Gas inlet EvaporatorGas exit

W 

Spectral unit

P, T RH

266 nm beam

D

FOV

“Telescope”

Optical fiber

=

Experimental setup

Cell

Optical fiber

T, RH

Gas inletXYZ adjustablefiber holder

P sensor output

Beam output

Laser beam

Evaporator

Ventilator

T, RH

Laser beam

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.40

2

4

6

8

10

12

14

16

f(x) = 6.89238116446633 x − 1.57246216539675R² = 0.998539174974694

Ratio of H2O/N2 Raman signals

Refe

renc

e sa

mpl

e m

ixin

g ra

tio [g

/kg]

Calibration function

Parameter Value UncertaintyCell length [m] 1.8 ±0.001Cell width [m] 0.284 ±0.0005Cell height [m] 0.300 ±0.0005P [Pa] 97560 ±300Ma [kg/mol] 0.0289654 ±5.0.10-8

R [J/molK] 8.31447 ±1.10-7

T [K| 299.2 ±0.3Z 0.9971 ±0.0032Liquid water mass [g] from 0.40 to 2.32 ±0.01

MR g/kg Uncertainty % Calculated RH %

Measured RH%

2.387±0.058 2.42 10.66 11.926.182±0.0651 1.05 29.11 30.48.345±0.0712 0.85 38.76 39.4

10.714±0.0789 0.73 48.13 5013.414±0.0890 0.66 60.01 60.4

Experimental uncertainties

High resolution Raman lidar

Water vapor mixing ratioSpatial resolution 1.2 mTemporal resolution 1 sOperational distance 50-500mWhole hemisphere scanning ability

0 2 4

Horizontal wind speed

[m/s]0 180

Wind direction

[°]-1 0 1

Vertical wind

[m/s]23 24 25

v

[°C]

28/08/08 18:4528/08/08 19:00

Lidar

Sodar

Lake internal boundary layer

Conclusion

Results:• New method for first-principle calibration of a Raman lidar proposed• High accuracy and precision of the calibration constant possible• Calibration constant potentially traceable to primary standard of mass ?

Potential applications:

Operational water vapor observations for weather nowcasting and climatology

Use as reference instrument for water vapor mixing ratio profiling in:• balloon sonde tests and intercomparison• GPS water vapor calibration

1

2

3

HRSRL spectral unit

Laser

Laser Power Supply

Water Vapor spectral unit

Aerosol / Temperature spectral unit

Lidar Windows

Laser Beam

Telescope array Beam Expander

2005 mm

Raman lidar for meteorological observations RALMO

Water vaporTemperatureAerosolTime resol- 30 minSpatial resolution 30-300mDistance rangeDay 5 kmNight 12 km

Transciever RALMO

TransmitterNd:YAG laser400 mJ & 355 nm30 Hz rep. rateBeam expander 15 X

ReceiverMatrix telescope offour mirrors30 cm in diameter0.2 mrad FOV

Polychromator RALMO

RALMO specifications

•Distance range 150 m-up to 5 km day/ 12km night•Temporal resolution 30 min (optional 10 min)• Spatial resolution - variable 15-300 m•Detection limit water vapor 0.05 g/kg•Temperature resolution 0.5 K•Aerosol extinction and backscatter coefficients at 355 nm•Statistical error < 10 %•Automatic operation and data treatment•Eye safe

•Water vapor channel -Experimental operation since 2007-Fully operational since 2008•Temperature/aerosol channel operational since 2009

• Stainless steel- low wall deposition• Can be evacuate to 10-4 torr • Volume 72 liters• Designed for precise weighing of dry air mass (uncertainty 0.02%)• Total uncertainty of the mixing ratio < 0.05%• Temperature stabilization from -30° C to +40°C (double-wall cell)• Signal duration up to 200 ns

New calibration cell- design