ndacc and water vapor raman lidars
DESCRIPTION
NDACC and Water Vapor Raman Lidars. Thierry Leblanc JPL - Table Mountain Facility, CA [email protected]. NDACC and Water Vapor Raman Lidar 1. Measuring goals. Water vapor plays significant role in radiative balance of the UTLS - PowerPoint PPT PresentationTRANSCRIPT
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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NDACC and Water Vapor Raman Lidars
Thierry Leblanc
JPL - Table Mountain Facility, CA
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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NDACC and Water Vapor Raman Lidar 1. Measuring goals
• Water vapor plays significant role in radiative balance of the UTLS
• Water Vapor variability is very high in both space and time
• High resolution water vapor measurements in the UTLS has remained sparse until today
2002: It was proposed to add water vapor Raman lidars to the set of NDACC instruments with the specific following
requirements:
• Capable of measuring water vapor near and above the tropopause
• Capable of measuring down to a few ppm
• Capable to sustain relatively high vertical and temporal resolution
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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NDACC and Water Vapor Raman Lidar 2. Existing wv lidars
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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NDACC and Water Vapor Raman Lidar: 3. Technique/retrieval
• Vibrational Raman backscatter technique at two wavelengths
• UV:
– Laser emission at 355 nm
– Reception at 387 nm (Raman N2) and 407.5 nm (H20)
• Visible:
– Laser emission at 532 nm
– Reception at 607 nm (Raman N2) and 660 nm (H20)
• Lidar signals corrected for background noise, saturation/pile-up effects, signal induced noise, range, Rayleigh extinction.
• Small additional correction due to temperature dependence of the H2O cross-section is needed occasionally if using very narrow filter
• After correction, the ratio of the lidar signals at the two wavelengths is proportional to water vapor mixing ratio.
Needs calibration !
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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NDACC and Water Vapor Raman Lidar: 4. Calibration
• Calibration techniques:
– Internal/theoretical: Very challenging because requires accurate knowledge of transmission ratios of ALL the lidar optical and electro-photonic components
virtually impossible to achieve at required accuracy
– Semi-empirical: Use of a Calibration lamp to illuminate lidar receiver in conditions that mimic real measurements: Difficult, but possible
Accuracy depends mostly of lamp calibration accuracy
– External: Use of independent measurements, e.g., radiosonde, microwave
Easy to implement but accuracy limited by that of independent measurement
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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NDACC and Water Vapor Raman Lidar 5. CONCLUSION
Important requirements for NDACC long-term measurements and associated challenges to lidar community:
1. High capability lidar
Capable of detecting a few ppm at tropopause
2. Stable calibration constant
Simultaneous multiple calibration techniques must be available to cross-validate calibration constants from each technique, and insure long-term stability
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The Raman Lidar at Table Mountain Facility: Overview 1
FIBER FROM0.91 m TELESCOPE
L1
B2 B3
P
P
P
P
F1
F3
F2
P
B4B4
387M Ch6Licel 5
387H Ch4Licel 3 407H Ch3
Licel 2
407M Ch5Licel 4
355H Ch1Licel 0
P
P
P
F2
F3
NEAR CHANNELSF1
407L Ch7Licel 6
387L Ch8Licel 7
355M Ch2Licel 1
SKY
Laser energy:
700 mJ/pulse
8 channels:
3 w.v. ranges
3 telescopes:
One 90 cm
Three 6 cm
Hamamatsu PMTs
Licel
photocounting
8000 7.5-m bins
Vertical resolution
75-m
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The Raman Lidar at Table Mountain Facility: Overview 2
• Calibration used so far:– Radiosondes: Vaisala Humicap RS-92 sensors
• Future plans:– UV lamp, GPS, Microwave– Simultaneous, multiple calibration techniques
• Important requirement for NDACC long-term measurements:– Stable calibration constant
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The Raman Lidar at Table Mountain Facility: Overview 3
TMF– 50 miles NE of Los Angeles– Lat: 34.4ºN– Long: 117.7ºW– Alt: 2285 m (7500 ft)
> 340 clear nights/year
Dataset
• TMF water vapor measurement program started in late 2004
• November 2004 – Present:– Radiosonde P,T, (2.3-20 km), RH (2.3-15 km)
• April 2005 – Present:– Raman Lidar (4-19 km)
• Lidar vertical resolution and accuracy:– 75 m instrumental, 2-h routine integration (5-minutes minimum)– WV total error estimated to ~5-8 ppm at tropopause
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The Raman Lidar at Table Mountain Facility: Overview 4
20
15
10
5
0
Alti
tude
(km
)
0.001 0.01 0.1 1 10Water Vapor Mixing Ratio (g/kg)
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The Raman Lidar at Table Mountain Facility: Overview 5
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The Raman Lidar at Table Mountain Facility: Overview 6
Water Vapor Mixing Ratio (g/kg)
Alti
tude
(km
)
0.01 0.1 1 10
11
10
9
8
7
6
5
4
3
GSFC data and this figure gracefully provided
by T. McGee, L. Twigg, G. Sumnicht, and D. Silbert
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The Raman Lidar at Table Mountain Facility: Overview 7
Overall mean Differences
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The Raman Lidar at Table Mountain Facility: Overview 8
Watch time variability!
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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Aura/MLS - Lidar and
Aura/MLS - sonde
The Raman Lidar at Table Mountain Facility: Overview 9
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The Raman Lidar at Table Mountain Facility: Overview 10
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The Raman Lidar at Table Mountain Facility: Summary
- So Far:- TMF Water Vapor Raman Lidar is doing well- Capable to reach 15-18 km for a 2-hour integration- As of today, used only radiosonde for calibration
- Next:- Introduce new calibration techniques:
- Lamp- GPS?- Microwave?
- Improve lidar power/aperture capability to reach final objectives of detection level of 2 ppm at 15 km
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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% Difference-100 -50 0 50 100
Alt
itu
de
(km
)
3
4
5
6
7
8
9
10
116/13/2005
The Raman Lidar at Table Mountain Facility: Overview 1
6/13/2005
Red = JPL-AT
Green = Sonde-AT
Orange = STROZ-Lite – AT
From:
Tom McGee and Larry Twigg, NASA-GSFC
6 July 2006 First NDACC Water Vapor Working Group Workshop, Bern, Switzerland.
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The GSFC Raman Lidars: Overview 2
From: Tom McGee and Larry Twigg, NASA-GSFC