Download Raman Lidar for Meteorological observations (RALMO)

A new method for first-principles calibration
of water vapor Raman lidar
Valentin Simeonov
École polytechnique fédérale de Lausanne
Switzerland
Overview
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Raman lidar as meteorological tool
Lidar and Raman lidar principle
Calibration problem
New method for instrumental calibration
Conclusion and perspectives
Raman Lidar for Meteorological observations (RALMO)
EPFL-MeteoSwiss
Time resolution - 10 min,
Vertical resolution - 30 m up to 4 km
Nocturnal BL
Convective Mixed Layer
Residual layer
g/kg
‘Rain stop’
Clouds/Fog < 500 m
Clouds,
Rain
How does a lidar work?
FOV
1
 2
R
𝐼 𝑅 =
R
𝑃0 𝐴k
2
𝛽
𝑅
𝛤
𝑅
2
𝑅
+𝑏
𝑅
Γ 𝑅 =−
R
𝛼 𝑟 𝑑𝑟
0
I(R)
P0
Spectral unit
Laser
A
Telesccope
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
Water vapor Raman lidar
𝜷 = 𝝈𝑵
σ -Raman cross section
N – molecular number density
𝒉𝝂𝑹𝒂𝒎=𝒉𝝂𝑳𝒂𝒔 ∓ 𝒉𝝂𝑴𝒐𝒍
𝝀=𝒄 𝝂
High selectivity
𝑞 𝑅 =𝐶
𝐼𝐻2𝑂 (𝑅)
𝛥𝛤𝑁2−𝐻2𝑂 (R)
𝐼𝑁2 (𝑅)
𝑪=𝝁
𝒌𝑵𝟐 𝝈𝑵𝟐
𝒌𝑯𝟐𝑶 𝝈𝑯𝟐𝑶
Scattering intensity
Quantitative determination
𝐴
𝐼𝑋 𝑅 = 𝑃0 2 k𝑋 𝜎𝑋 𝑁𝑋 (𝑅) Γ𝐿 (𝑅)Γ𝑅𝑋 (𝜆𝑋 𝑅)
𝑅
h -Planck constant
ν – light frequency
c – speed of light
λ – light wavelength
H2 O
Wavelength-λ [nm]
q(R) – Water vapor/air mixing ratio
C
– Calibration constant
ΔΓ – Differential atmospheric transmission
µ
– Constant, converts H2O/N2 to H2O/air mixing ratio
Calibration against a reference instrument
(radiosonde)
Advantages
• Simple
• Easy comparison with the existing techniques
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𝑰𝑵𝟐 (𝑹)
𝟏
𝑪 = 𝒒𝒓𝒆𝒇 (𝑹)
𝑰𝑯𝟐𝑶 (𝑹) 𝜟𝜞𝝀𝑵𝟐−𝝀𝑯𝟐𝑶
qref – Reference mixing ratio
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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
Instrumental calibration
𝒌𝑵𝟐 𝝈𝑵𝟐
𝒌𝑯𝟐𝑶 𝝈𝑯𝟐𝑶
G. Vaughan et al. (1988)
𝜼𝑵𝟐 𝜺𝑵𝟐 𝝉𝑵𝟐 𝝀 𝝈𝑵𝟐 𝝀, 𝑹 𝒅𝝀
Sherlock et al. (1999)
𝑪=𝝁
𝑪(𝑹) = 𝝁
𝜼𝑯𝟐𝑶 𝜺𝑯𝟐𝑶 𝝉𝑯𝟐𝑶 𝝀 𝝈𝑯𝟐𝑶 𝝀, 𝑹 𝒅𝝀
4.50E-31
1
τ
Raman cross section σ [cm2]
4.00E-31
3.50E-31
0.8
3.00E-31
0.6
2.50E-31
σ
2.00E-31
0.4
1.50E-31
1.00E-31
η – Photodetector efficiency
ε - Optics efficiency
τ - Spectral unit instrumental
function
τ
0.2
5.00E-32
0.00E+00
0
407
407.2
𝑪(𝑹) = 𝝁
407.4
407.6
Wavelength [nm]
𝜼𝑵𝟐 𝑹 𝜺𝑵𝟐 𝑹
407.8
408
𝝉𝑵𝟐 𝝀, 𝑹 𝝈𝑵𝟐 𝝀, 𝑹 𝒅𝝀
𝜼𝑯𝟐𝑶 (𝑹)𝜺𝑯𝟐𝑶 (𝑹) 𝝉𝑯𝟐𝑶 𝝀, 𝑹 𝝈𝑯𝟐𝑶 𝝀, 𝑹 𝒅𝝀
Is the lidar calibration constant constant?
New instrumental calibration method
𝑪=𝝁
𝜼𝑵𝟐 𝜺𝑵𝟐 𝝉𝑵𝟐 𝝀 𝝈𝑵𝟐 𝝀 𝒅𝝀
𝜼𝑯𝟐𝑶 𝜺𝑯𝟐𝑶 𝝉𝑯𝟐𝑶 𝝀 𝝈𝑯𝟐𝑶 𝝀 𝒅𝝀
𝒒𝒓𝒆𝒇 =
𝒎𝑯𝟐 𝑶
𝒎𝒅𝒓𝒚 𝒂𝒊𝒓
=
=𝒒𝒓𝒆𝒇
𝑰𝑵𝟐
𝑰𝑯𝟐𝑶
𝒎𝑯𝟐 𝑶
𝒑𝑴𝒂
𝑽
𝒁𝑹𝑻
mX – mass of X
p – air pressure
Ma – molecular mass of air
V – cell volume
T – air temperature
z – compressibility factor
Detection
266 nm beam
Laser
P, T RH
D
Gas inlet
Evaporator
Gas exit
Ventilator
Cell
FOV
“Telescope”
Optical fiber
W
Spectral unit
Laser
beam
Experimental setup
P sensor
output
T, RH
Beam output
Laser beam
Cell
T, RH
Gas inlet
Ventilator
Optical fiber
Laser beam
XYZ adjustable
fiber holder
Evaporator
Calibration function
Reference sample mixing ratio [g/kg]
16
14
y = 6.8924x - 1.5725
R² = 0.9985
12
10
8
Linear (fit)
6
4
2
0
0
0.5
1
1.5
Ratio of H2O/N2 Raman signals
2
2.5
Experimental uncertainties
Parameter
Value
Uncertainty
Cell length [m]
1.8
±0.001
Cell width [m]
0.284
±0.0005
Cell height [m]
0.300
±0.0005
P [Pa]
97560
±300
Ma [kg/mol]
0.0289654
±5.0.10-8
R [J/molK]
8.31447
±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
2.387±0.058
6.182±0.0651
8.345±0.0712
10.714±0.0789
13.414±0.0890
Uncertainty %
2.42
1.05
0.85
0.73
0.66
Calculated RH
Measured RH%
%
10.66
11.92
29.11
30.4
38.76
39.4
48.13
50
60.01
60.4
High resolution Raman lidar
Water vapor mixing ratio
Spatial resolution 1.2 m
Temporal resolution 1 s
Operational distance 50-500m
Whole hemisphere scanning ability
Lake internal boundary layer
Lidar
Horizontal wind speed
Wind direction
v
Vertical wind
28/08/08 18:45
28/08/08 19:00
Sodar
0
2
[m/s]
4
0
180
[°]
-1
0
[m/s]
1
23
24
[°C]
25
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
HRSRL spectral unit
3
2
1
Raman lidar for meteorological observations RALMO
Lidar Windows
Laser Beam
Aerosol / Temperature
spectral unit
Water Vapor
spectral unit
Telescope array
Beam Expander
Laser Power Supply
Laser
Water vapor
Temperature
Aerosol
Time resol- 30 min
Spatial resolution
30-300m
Distance range
Day 5 km
Night 12 km
Transciever RALMO
Transmitter
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
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
New calibration cell- design
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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