atmospheric corrections determined using raman/backscatter lidar measurements valentin mitev

HEAPnet meeting, 19-20 February 2007, Amsterdam

Atmospheric corrections determined using Raman/backscatter lidar measurements 1

LIDAR

Atmospheric corrections determined using Raman/backscatter lidar

measurements

Valentin Mitev

Observatory of NeuchâtelRue de l’Observatoire 58, CH2000 NeuchâtelSwitzerlandTel.: +41–32–889 8813E-mail: [email protected]



LIDAR

Content:

• Measurement requirements• Concept for the Lidar set-up• Extinction derivation, vibrational Raman• Numerical performance simulations

for Extinction derivation, Raman lidar• Extinction derivation, elastic backscatter • Temperature derivation, pure Rotational Raman• Conclusion

• Annex: Compact backscatter lidar in field measurements



LIDAR

~7kmTotal transmission

Range-resolved transmission (extinction coefficient)

Zenith angle0°-60°

Measurement requirements

Direction of probing

Temperature profile



LIDAR

Raman-elastic backscatter lidar – Concept:

• One laser with two/optional three separate receivers for increased dynamic range and decrease of the « blind » range

• Transmitted wavelength: 355nm, 532nm, 3rd/2nd harmonics of Nd:Yag laser

• Receiverd wavelengths: 355nm (elastic); 387nm (Raman N2), 532nm elastic + polarisation/depolarisation; Rotational Raman at (533nm, 531nm)+ (529nm, 535nm)

• Lidar on pointing platform for collocation of the direction of probing with te line-of-sight of the Cerenkov camera;

• Optical&Laser part in environmental housing



LIDAR

Raman backscatter lidar: Basics

• One laser line transmitted (UV/ vis)

• Received Raman vibrational: N2, O2, H2O/Rorational

• Determined: extinction, water vapours, temperature

• Development and use: since early 1980s / in atmospheirc probing for aerosol extinction and microphysics, humidity, temperature, …



LIDAR



LIDAR

1. Laser;

2a, 2b, 2c. Telescope long/med/short range

3a, 3b, 3c. Spectral selection

4a, 4b, 4c. Detectors

5. Pointing platform/environnmental housing

6. Synchronisation: Acqusition and Laser pulse& Main Experiment

7.Signal acquisition electronics

Synch out

1

5

2a3a4a

2b3b4b

6

7

Data out 2c3c4c



LIDAR

532nm, 355nm

532nm

387nm

532nm-s

532nm -p

355nmRR1…RR4

532nm (e)

355nm (e)

356/8nm (2*RR-S)352/4nm (2*RR-aS)

aS1/ aS2/ 355nm/ S1/ S2

Laser

Receiver

123

3

4

51-Coupling optics2-Dichroic beamsplitter3-Interference filter4-Depolarisation beamsplitter5-Grating spectrometer

2



LIDAR

r

0RR2RLR 'dr)'r()'r(exp)r(

2

c

r

A)r(OKE)r(E

Extinction derivation from vibrational Raman backscatter

d

)v(d)r(N)r( Ram

MR

)r()r()r(S/)r(Nlndr

d2N2N2N

… two times the averaged value of the extinction coefficient in the spectral range 355nm – 387nm



LIDAR

Inputs for the performance simulations:

Lidar subsystems specifications• Pulse energy at 355nm: 300mJ/PRR : 20Hz• Telescope diameter of the « long-range » receiver: 80cm • Efficiency transmitter/receiver (without filter): 07./07• Transmission, filter: 0.6• Detector, Quantum efficiency: 0.2

Lidar measurement parameters• Integration time: 600sec• Zenith angle (from zenith): 60°• Range resolution: 120m at 60• Ambient optical background:

full moon – 7*10-4 Wm-2m-1



LIDAR

Atmosphere:

• Molecular model: hydrostatic

• Aerosol model: PBL/dust, 0 - 2 kmtropospheric layer, 3 - 5kmcirrus cloud, 9 - 10,4km

PBL/Dust layer, 0-2km

Tropopsphere/Desert Dust, 3-5km

Cirrus cloud, 9 – 10.4km



LIDAR

Vibrational -Raman signal – simulated, at slant path 60 deg



LIDAR

Extinction from the vibrational Raman signal



LIDAR

Error of the extinction coefficient obtained from the vibrational Raman signal



LIDAR

Error of the extinction coefficient obtained from the vibrational Raman signal- ZOOM

Range x104m , @60° zenith angle



LIDAR

Total atmospheric transmission of the marked layers, derived from the simulated Raman signal« TRmod » = model value; « TRmeas » = derived value

TRmodel = 0.5836TRmeasured = 0.5830

PBL/Dust layer

Tropopsphere/Desert Dust

Cirrus cloudTRmodcloud = 0.9498TRmeas cloud = 0.9508



LIDAR

Concept for derivation of the extinction coefficient inside aerosol layer using elastic backscatter

Assumptions:- The layer contains the same type of aerosol (e.g.,subvisible cirrus cloud)- Aerisol-free atmosphere above the cloud- Total layer (cloud) transmision is determined from the Raman signal



LIDAR

Extinction from Elastic backscatter signal - simultion

reference

Aerosol layer (Cirrus cloud)



LIDAR

The elastic-backscatter lidar equation

r

02L 'dr)'r(2exp)r(

2

c

r

A)r(OKE)r(E

2r)r(E)r(S

r2

dr

rd

r

1

dr

)r(dS



LIDAR

The Fernald's inversion method for derivation of the backscatter coefficient; is omitted

r

fr

r

fr

mol

f

f

r

rf

mol

)r(rd)0lrlr(2exp)r(Srdlr2r

)r(S

)r(rd)0lrlr(2exp)r(S

r

Additional conditions: • “lr” is constant (extinction to backscatter ratio, initial approximation taken from model values, here the depolarization ratio may help to classify the cloud particles), • “rf” is a reference range• “(rf)” is known ( typically, the molecular backscatter)

)r()r()r( aermol



LIDAR

Assuming: “(r)” is derived from elastic lidar Total double trip transmission “DT” is derived from Raman lidar, Molecular backscatter is known/type of particles may be “guessed”

Then we may determine “lr” from

And the profile of the aerosol extinction in the cloud

2r

1r

molaer 'dr)'r()'r(.lr2exp)2r,1r(DT

)r(.lr)r( aeraer



LIDAR

Derivation of the atmospheric temperature profile using pure rotational Raman backscatter

Rotational Raman Spectra of N2 and O2,Excitation at 532nm



LIDAR

Temperature derivative in Rotational Raman spectraof N2 (red) and O2 (black)

-1

-0,5

0

0,5

1

1,5

2

2,5

3

3,5

525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540

wavelength, nm

deriv

ativ

e, re

lativ

e un

its

Spectral intervals in pure RR where the scattering cross-sections derivative has opposite sign

)T/baexp(

)T(I)T(I)T(I)T(I

)T(I)T(I)T(I)T(IK

)T(R

ast2Ost2Oast2Nst2N

ast2Ost2Oast2Nst2N

A calibration of the lidar is critical.

« + »

« - »« - »

Temperature derivative of the Rotational Raman lines of N2 (red) and O2 (black)

« + »

R(T)=exp( – /T)

Typically dR/dT ~0.05%



LIDAR



LIDAR

Uncertainty - ZOOM

60° zenith angleIntegration time: 30minRange resolution: 120m



LIDAR

Summary:A Raman-backscatter Lidar for CTA-site is a technically feasible solution for the requirements in CTA:

• Advantages: « Real time » and « Real direction » coinciding with the pointing direction the Cherenkov Telescope(s) • The necessary lidar methods and algorithms are developed, adaptation to the tasks will be possible ;• Realistic subsystem specifications, compatible with the commercially available hardware;• Additional /Optional lidar tasks: laser backscatter for calibration of the Cherenkov telescope;

Remark: This presentation is not with system optimisation. The final specifications may be different from the specifications used for numerical simulations



LIDAR

Next step for the Raman lidar - a design study with the following objectives:

• Detailed numerical simulations of the various detection modes with respect to the finalised detection requirements

• Concept design and optimisation;

• Algorithm developments;

• Optional 1: Participation in atmospheric characterisation at the potential CTA sites;• Optional 2: Raman lidar bread-board/ lower aperture and power



LIDAR

ANNEX: Possibility for atmospheric characterisation at

potential CTA sites with a compact elastic backscatter lidars



LIDAR

Micro-pulse lidars on stratospheric aircraft (M55)

MAL 1 MAL 2

MAL-1 MAL-2

32 cm



LIDAR

Micro-pulse lidars on stratospheric aircraft (M55)SCOUT O3/ Brunei - Darwin,

12 November 2005 Backscatter Ratio=

(a+ m)/ m



LIDAR

• Ground-based LIDAR, transportable development, observations, data analysis

The lidar on the balcony of the 5th floor of the University of Basel; Project BUBBLE (2001-2002) . The lidar was remotely operated from ON

Example for 24h- measurement of the aerosol load above Basel in project BUBBLE

600mmx600mmx700mm



LIDAR

• Ground-based three-wavelength elastic Raman LIDAR, in Observatory of Neuchatel

Operational, Presently under refurbishment

Concerning the CTA-activity:

• Not transportable

• May be a base for the Raman lidar bread-board/test bench wrt the CTA requirements

• Possibility to be deployed on site (with limitations for steering, schedule …)



LIDAR

Summary for the “compact lidar” capabilities:

- Possibility for qualitative characterisation of the aerosol vertical/slant path profile: Backscatter coefficient profile (~30% uncertainty, systematic), altitude of layers,

-Convenient transportation and implementation on the field

- Limitations: The qualitative evaluation is not adequate to the requirements in CTI, i.e., NOT a replacement for the Raman lidar)



LIDAR

Thank you!

Valentin Mitev([email protected])

Observatory of NeuchâtelRue de l’Observatoire 58, CH2000 NeuchâtelSwitzerland

atmospheric corrections determined using raman/backscatter lidar measurements valentin mitev

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