Measurements Of Atmospheric Water Vapor using the Raman Lidar Technique: Summary and status within NDACC

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1 Measurements Of Atmospheric Water Vapor using the Raman Lidar Technique: Summary and status within NDACC Thierry Leblanc Jet Propulsion Laboratory, California Institute of Technology, Wrightwood, CA. USA

2 Vibrational Raman backscatter technique (brief summary) 1/ Laser beam out 2/ Raman-shifted backscattered light by N 2 (or O 2 ) molecules and H 2 O molecules is collected by telescope 3a/ Signals corrected for background noise, saturation/pile-up up effects, signal induced noise, range, Rayleigh extinction. 3b/ Small additional correction due to temperature dependence of the H 2 O cross-section section is sometimes needed if using very narrow filter 4/ After correction, the ratio of the H 2 O and N 2 (or O 2 ) lidar signals is proportional to water vapor mixing ratio. Needs calibration!

3 NDACC and Water Vapor Raman Lidars Water vapor plays significant role in radiative balance of the UTLSU Water Vapor variability is very high in both space and time High resolution water vapor measurements in the UTLS has remained ed sparse until today 2002: It was proposed to add water vapor Raman lidars to the set of NDACC instruments given the following conditions: 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

4 NDACC water vapor Raman lidar sites

5 FROM MOHAVE (October 2006)

6 Short timescale variations (10 October 2006) Color contours = log-scale >200% variations in 1.5 hour

7 JPL lidar Vaisala RS92 comparison (from Leblanc et al., JTECH-A A 2008)

8 (from Leblanc et al., JTECH-A A 2008) Fluorescing fiber 2007: : 355H signal and detector sent out before the fiber

9 FROM MOHAVE II (October 2007)

10 JPL lidar and Vaisala RS92 total WV seasonal cycle (from Leblanc et al., JTECH-A A 2008)

11 Other WV Raman lidars: NOAA/GMD/ESRL lidar at Mauna Loa, Hawaii (John Barnes) -55 deg C

12 Calibration Several 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: Absolute calibration remains difficult, partial calibration OK 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 and non-simultaneity and non-co-location of lidar and external measurements New 2008! : Hybrid method using routine lamp calibration and occasional (campaign-basis) external Most limitations of each method taken separately now suppressed

13 New hybrid calibration principle Consider 2-hour routine measurements 4 nights per week all-yearround Each measurement is preceded and followed by a few minuteslong lamp run that consists of measuring signals acquired in the two lidar channels from which wv ratio is needed The ratio obtained during the lamp run provides a partial calibration, and in normal conditions, varies from one experiment to another by less than 1-2% Once a year, an intercomparison campaign is setup, with intensive co-located and simultaneous external and lidar measurements The lamp routine partial calibration value is then scaled to the mean absolute calibration value obtained externally during the campaign Any small calibration changes associated with instrumental changes can be tracked on a daily basis

14 Top 4 figures: Variability of calibration constant using radiosonde Bottom figure: Variability of partial calibration constant using lamp

15 Top: Mean calibration from radiosonde Middle: Mean (partial) calibration from lamp Bottom: Mean absolute calibration from hybrid method

16 Current status of instrumental issues Issues in / Lidar power-aperture and rarity of water vapor limit sensitivity at the tropopause 2/ Instrument calibration need proper address 3/ Detection limit cause suspicious undesired fluorescence signal Progress status as of / New/mature technology driving lidar power-aperture up slowly but steadily at no additional cost Some progress 2/ New hybrid calibration technique Some progress 3/ Undesired fluorescence signal identified and supressed Significant advance BUT altitude ceiling of measurement has dropped by 2 km

17 CONCLUSION Slow but noticeable progress made in the past 2 years Technique has reached a near-mature stage and is ready for routine measurement Accuracy at the tropopause is expected to reach NDACC target within 5 years Presently 8 operating and potential NDACC-class water vapor Raman lidars: -Mauna Loa, Hawaii (Barnes, NOAA) -Haute Provence, France (Keckhut, CNRS) -Table Mountain, California (Leblanc, JPL) -Tor-Vergata (non-ndacc), Italy (Congedutti, CNR) -La Reunion Is. (Keckhut, CNRS) -MARL NDACC-Germany, (Schrems, AWI) -AT (mobile-ndacc) US (McGee, GSFC) -ALVICE (mobile non-ndacc) US (Whiteman, GSFC) Presently no synergy between instrument PIs on routine and longterm water vapor measurement strategy Need improvement