water vapour raman lidars in the utls: where are we now? or “the jpl-table mountain experience”

GEWEX/GlobVapour Workshop, Mar 8-10, 2011, ESRIN, Frascati, Italy

WATER VAPOUR RAMAN LIDARS IN THE UTLS:Where Are We Now?

or“The JPL-Table Mountain Experience”

Thierry Leblanc

NASA Jet Propulsion LaboratoryCalifornia Institute of Technology

Wrightwood, CAUSA


Lidar Technique: Brief Overview

Raman-backscatter Lidar Equationfor any atmospheric molecule M:

SM(z) = PL t(L,R,z) M M(R) OM(z)AL z

(z - zL)2

M(R) NM(z)





(z - zL)2

M(R) NM(z)

Signal collectedin Raman channelfor altitude z…





(z - zL)2

…is proportional to the numberof photons emitted by laser…

M(R) NM(z)






(z - zL)2

…is proportional to the numberof photons emitted by laser…

…and after a fewtypical lidar corrections…

M(R) NM(z)






(z - zL)2

Signal collectedin Raman channelfor altitude z… …to the number of

scattering moleculesat altitude z…is proportional to the number

of photons emitted by laser…

…and after a fewtypical lidar corrections…

M(R) NM(z)


Lidar Technique: Bref Overview

This equation applies similarly to water vapour (our target)and Nitrogen (a well-mixed “reference” gas)



(z - zL)2

M(R) NM(z)

R(z) =SH2O(z)

SN2(z)= k q(z)

H2O H2O H2O OH2O

N2 N2 N2 ON2with k = t

The determination of k is referred to as the lidar calibration

NH2O(z)

NN2(z)= k

…so that the ratio of the signal collected in the H2O channel to thatcollected in the N2 Raman channel is proportional to H2O mixing ratio q:


Calibration: Two Approaches

1. Independent, also known as “absolute” or “experimental”Calculate each individual term in lidar ratio equationNeed calibrated lamp, manufacturer specs of all optics and electronics, etc…Difficult to achieve, especially on a routine basisEstimated uncertainty: 7-20%

2. External, also known as “absolute” or “a priori”Estimate all the altitude-dependent terms in lidar ratio equation,then normalize to a well-known external measurement,for example, radiosonde in the lower troposphere, or GPS total columEasier to achieve than method 1, but not independent from other instrumentsEstimated uncertainty (radiosonde): 3-15%Estimated uncertainty (GPS): 7-15%

Hybrid Method: A combination of the two methods above1. Perform external calibration during (yearly) campaigns2. Monitor lidar receiver stability between campaigns using lampNot more accurate than external calibration, but saves money and timeby avoiding systematic routine radiosonde launches

Recommended (but not mandatory) practice by NDACC


Radiosonde in blue

Calibration:One example (radiosonde)


Calibration:One example (radiosonde)

Lidar in red

Best fit to radiosonde Calibrated


1. Vibrational Raman scattering is 1000 times weaker than Rayleigh scattering

Why are H2O Lidar MeasurementsChallenging in the UTLS?

2. Water Vapor mixing ratio drops down to less than 10 ppmv in the UTLS

As a result: H2O signal is 109 times weaker than Rayleigh signal in UTLSand therefore very challenging to detect…


Resulting in a Long and Winding Road…

Oct 2006:Validation campaign MOHAVE Fluorescence Detected!

Oct 2007:Validation campaign MOHAVE-II

Fall 2008:H2O Raman lidar becomes an official NDACC instrument

June 2005 - Present:Routine measurements 2h/night, 3-4 nights per week

April 2005:TMF WV Raman Lidar First Light

Summer 2009:Six H2O Raman lidars provisionally accepted (need validation)

Fall 2009:TMF H2O Raman lidar Validated

2002:NDACC considers Raman WV lidar as new NDACC instrument





June 2005 – Present:Routine measurements 2h/night, 3-4 nights per week




Resulting in a Long and Winding Road…


Early Results: 2005-2006

RS92 (corrected) vs. LidarComparison shows: 2005-2007 Climatology (average of 202 profiles)

Lidar Very Wet or RS92 Very Dryor Both!


A Long and Winding Road…









MOHAVE Campaign (Oct 2006):Fluorescence Detected

Strong Rayleigh signal (355 nm) induces fluorescence in lidar receiverComparison below shows lidar vs. CFH mean profileswhen 355 nm blocked at the receiver’s entrance (right), andwhen 355 nm is not blocked (left)

New anti-fluorescent optics were ordered, custom-madeand installed on the TMF lidar during summer 2007


Since July 2007: Fluorescence is gone

RS92 (corrected) vs. LidarComparison shows: 2007-2009 by Season (154 profiles)

The corrected RS92 is still too dry: Not a lidar problem anymore


Prior to 2009:No Water Vapor Lidars in NDACC

(as of 2008)


(as of 2009)

Since 2009:Six Water Vapor Lidars provisionally in NDACC

“Provisionally” because (more) validation is required









Fall 2009: MOHAVE-2009 CampaignTMF H2O Raman Lidar Validated


October 2009:TMF Lidar Profiles Fully Validated

Latest (optimized) receiver configuration since July 2009Comparison below shows lidar vs. CFH mean profiles during MOHAVE-2009Below 14 km: Average of 12 one-hour-integrated profiles coincident with balloonAbove 14 km: Average of 8 full nights with coincident CFH flights


Conclusions and Perspectives

TMF Water Vapor Lidar Program:Started in 2005

Re-Analysis Back to 2007:Expected to be Available Late 2011

Period 2005-2007:Plagued with fluorescence

(no satisfactory results above 12 km)

Early Analysis (2007-2009):Pronounced Annual Cycle (summer max)


TMF = Significant NDACC past and presentcontribution to Ozone and Temperature Trends…

THANK YOU !THANK YOU !from Steinbrecht et al., 2008from Steinbrecht et al., 2008

OZONEOZONE TEMPERATURETEMPERATURE

15

-25

km

wate

r vap

or

an

om

aly

(%

)

?Table Mountain

2010 2020 2030

WATER VAPORWATER VAPOR

Please visit JPL-TMF lidar website: http://tmf-lidar.jpl.nasa.gov/

Back to NDACC (long-term) Context

We hope for a similar contributionfor Water Vapor… in 2030!


Backup Slides


Tropopause

All 2005-2009 RS92 Temperature Profiles:WMO TP in BlueCold-Point TP in red

Text

Text

Typically Two Regimes:Well-defined Tropical-Type Unique Cold-Point TP in Summer

Separate WMO and Cold-Point TP Typical of mid-lat. in Winter


What is the Hybrid Calibration Method?

1. A calibrated laboratory lamp is permanently mounted above the lidar’s primary telescope mirror

2. Routine 5-min lamp data acquisition is performed just before and after the normal data acquisition

3. Once a year, absolute calibration campaigns are organized (lamp runs still included)

4. The “transfer function”, which is the quotient of the lamp ratio by the absolute calibration constant, is monitored with time, and should remain constant if the lidar receiver remains stable


Title Texttext

Text

Text

water vapour raman lidars in the utls: where are we now? or “the jpl-table mountain experience”

Documents