1. A new method for first-principles calibration
of water vapor Raman lidar
Valentin Simeonov
École polytechnique fédérale de Lausanne
Switzerland

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

3. Raman Lidar for Meteorological observations (RALMO)
EPFL-MeteoSwiss
Nocturnal BL
Convective Mixed Layer
Residual layer
g/kg
Time resolution - 10 min, Vertical resolution - 30 m up to 4 km
‘Rain stop’ Clouds/Fog < 500 m
Clouds, Rain

4. How does a lidar work? 𝐼 𝑅 = 𝑃 0 𝐴k 𝑅 2 𝛽 𝑅 𝛤 2 𝑅 Spectral unit Laser
FOV
𝐼 𝑅 = 𝑃 0 𝐴k 𝑅 2 𝛽 𝑅 𝛤 2 𝑅 R +𝑏
Γ 𝑅 =− 0 𝑅 𝛼 𝑟 𝑑𝑟 R
I - Signal magnitude
P0 -Laser power
A- Telescope area
k- Lidar efficiency
R- Distance
β- Backscatter coefficient
Γ - Atmospheric extinction
α- Extinction coefficient
FOV- Telescope field of view
I(R) P0
Spectral unit
Laser A
Telesccope

5. Water vapor Raman lidar
h -Planck constant
ν – light frequency
c – speed of light
λ – light wavelength
𝜷=𝝈𝑵
σ -Raman cross section
N – molecular number density
𝒉 𝝂 𝑹𝒂𝒎 =𝒉 𝝂 𝑳𝒂𝒔 ∓𝒉 𝝂 𝑴𝒐𝒍
𝝀= 𝒄 𝝂
Quantitative determination
High selectivity
H2O
Wavelength-λ [nm]
Scattering intensity
𝐼 𝑋 𝑅 = 𝑃 0 𝐴 𝑅 2 k 𝑋 𝜎 𝑋 𝑁 𝑋 (𝑅) Γ 𝐿 (𝑅) Γ 𝑅𝑋 ( 𝜆 𝑋 𝑅)
𝑞 𝑅 =𝐶 𝐼 𝐻2𝑂 (𝑅) 𝐼 𝑁2 (𝑅) 𝛥 𝛤 𝑁2−𝐻2𝑂 (R)
q(R) – Water vapor/air mixing ratio
C – Calibration constant
ΔΓ – Differential atmospheric transmission
µ – Constant, converts H2O/N2 to H2O/air mixing ratio
𝑪=𝝁 𝒌 𝑵𝟐 𝝈 𝑵𝟐 𝒌 𝑯𝟐𝑶 𝝈 𝑯𝟐𝑶

6. Calibration against a reference instrument
(radiosonde)
Advantages
Simple
Easy comparison with the existing techniques
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= 𝒒 𝒓𝒆𝒇 (𝑹) 𝑰 𝑵𝟐 (𝑹) 𝑰 𝑯𝟐𝑶 (𝑹) 𝟏 𝜟 𝜞 𝝀𝑵𝟐−𝝀𝑯𝟐𝑶
qref – Reference mixing ratio

7. Instrumental calibration
𝑪=𝝁 𝒌 𝑵𝟐 𝝈 𝑵𝟐 𝒌 𝑯𝟐𝑶 𝝈 𝑯𝟐𝑶
G. Vaughan et al. (1988)
𝑪(𝑹)=𝝁 𝜼 𝑵𝟐 𝜺 𝑵𝟐 𝝉 𝑵𝟐 𝝀 𝝈 𝑵𝟐 𝝀,𝑹 𝒅𝝀 𝜼 𝑯𝟐𝑶 𝜺 𝑯𝟐𝑶 𝝉 𝑯𝟐𝑶 𝝀 𝝈 𝑯𝟐𝑶 𝝀,𝑹 𝒅𝝀
Sherlock et al. (1999)
η – Photodetector efficiency
ε - Optics efficiency
τ - Spectral unit instrumental
function
𝑪(𝑹)=𝝁 𝜼 𝑵𝟐 𝑹 𝜺 𝑵𝟐 𝑹 𝝉 𝑵𝟐 𝝀,𝑹 𝝈 𝑵𝟐 𝝀,𝑹 𝒅𝝀 𝜼 𝑯𝟐𝑶 (𝑹) 𝜺 𝑯𝟐𝑶 (𝑹) 𝝉 𝑯𝟐𝑶 𝝀,𝑹 𝝈 𝑯𝟐𝑶 𝝀,𝑹 𝒅𝝀
Is the lidar calibration constant constant?

8. New instrumental calibration method
𝑪=𝝁 𝜼 𝑵𝟐 𝜺 𝑵𝟐 𝝉 𝑵𝟐 𝝀 𝝈 𝑵𝟐 𝝀 𝒅𝝀 𝜼 𝑯𝟐𝑶 𝜺 𝑯𝟐𝑶 𝝉 𝑯𝟐𝑶 𝝀 𝝈 𝑯𝟐𝑶 𝝀 𝒅𝝀
= 𝒒 𝒓𝒆𝒇 𝑰 𝑵𝟐 𝑰 𝑯𝟐𝑶
mX – mass of X
p – air pressure
Ma – molecular mass of air
V – cell volume
T – air temperature
z – compressibility factor
𝒒 𝒓𝒆𝒇 = 𝒎 𝑯 𝟐 𝑶 𝒎 𝒅𝒓𝒚 𝒂𝒊𝒓 = 𝒎 𝑯 𝟐 𝑶 𝒑 𝑴 𝒂 𝒁𝑹𝑻 𝑽
Laser
beam
Detection
Cell
Laser
Ventilator
Gas inlet
Evaporator
Gas exit W
Spectral unit
P, T RH
266 nm beam D
FOV
“Telescope”
Optical fiber

9. Experimental setup P sensor output T, RH Beam output Laser beam Cell
Optical fiber
T, RH
Gas inlet
XYZ adjustable
fiber holder
P sensor
output
Beam output
Laser beam
Evaporator
Ventilator
T, RH
Laser beam

10. Calibration function

11. Experimental uncertainties
Parameter
Value
Uncertainty
Cell length [m]
1.8
±0.001
Cell width [m]
0.284
±0.0005
Cell height [m]
0.300
P [Pa]
97560
±300
Ma [kg/mol] ±
R [J/molK]
±1.10-7
T [K|
299.2
±0.3 Z
0.9971
±0.0032
Liquid 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
11.92
6.182±0.0651
1.05
29.11
30.4
8.345±0.0712
0.85
38.76
39.4
10.714±0.0789
0.73
48.13 50
13.414±0.0890
0.66
60.01
60.4

12. High resolution Raman lidar
I will mention only the key parameters of the lidar.
Water vapor mixing ratio
Spatial resolution 1.2 m
Temporal resolution 1 s
Operational distance m
Whole hemisphere scanning ability 12

13. Lake internal boundary layer
Lidar
Sodar

14. 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

15. HRSRL spectral unit

16. Raman lidar for meteorological observations RALMO
Laser
Laser Power Supply
Water Vapor spectral unit
Aerosol / Temperature spectral unit
Lidar Windows
Laser Beam
Telescope array Beam Expander
2005 mm
Water vapor
Temperature
Aerosol
Time resol- 30 min
Spatial resolution m
Distance range
Day 5 km
Night 12 km

17. Transmitter Receiver Transciever RALMO Nd:YAG laser 400 mJ & 355 nm
30 Hz rep. rate
Beam expander 15 X
Receiver
Matrix telescope of
four mirrors
30 cm in diameter
0.2 mrad FOV

18. Polychromator RALMO

19. RALMO specifications
Distance range 150 m-up to 5 km day/ 12km night
Temporal resolution 30 min (optional 10 min)
Spatial resolution - variable 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

20. New calibration cell- design
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