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Atmospheric corrections determined using Raman/backscatter lidar measurements Valentin Mitev

Atmospheric corrections determined using Raman/backscatter lidar measurements Valentin Mitev Observatory of Neuchâtel Rue de l’Observatoire 58, CH2000 Neuchâtel Switzerland Tel.: +41–32–889 8813 E-mail: valentin.mitev@ne.ch. Content: • Measurement requirements

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Atmospheric corrections determined using Raman/backscatter lidar measurements Valentin Mitev

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  1. Atmospheric corrections determined using Raman/backscatter lidar measurements Valentin Mitev Observatory of Neuchâtel Rue de l’Observatoire 58, CH2000 Neuchâtel Switzerland Tel.: +41–32–889 8813 E-mail: valentin.mitev@ne.ch

  2. 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

  3. Measurement requirements Zenith angle 0°-60° Range-resolved transmission (extinction coefficient) Temperature profile Direction of probing ~7km Total transmission

  4. 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

  5. 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, …

  6. 5 1 6 4a 4c 3a 3c 2a 2c 7 4b 3b 2b Data out Synch out • 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

  7. Laser 532nm, 355nm Receiver 3 532nm (e) 355nm (e) 356/8nm (2*RR-S) 352/4nm (2*RR-aS) 4 2 532nm 532nm-s 1 2 3 532nm -p 387nm 355nm RR1…RR4 5 1-Coupling optics 2-Dichroic beamsplitter 3-Interference filter 4-Depolarisation beamsplitter 5-Grating spectrometer aS1/ aS2/ 355nm/ S1/ S2

  8. Extinction derivation from vibrational Raman backscatter … two times the averaged value of the extinction coefficient in the spectral range 355nm – 387nm

  9. 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-2mm-1

  10. Atmosphere: • Molecular model: hydrostatic • Aerosol model: PBL/dust, 0 - 2 km tropospheric layer, 3 - 5km cirrus cloud, 9 - 10,4km Cirrus cloud, 9 – 10.4km Tropopsphere/Desert Dust, 3-5km PBL/Dust layer, 0-2km

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

  12. Extinction from the vibrational Raman signal

  13. Error of the extinction coefficient obtained from the vibrational Raman signal

  14. Error of the extinction coefficient obtained from the vibrational Raman signal - ZOOM Range x104m , @60° zenith angle

  15. TRmodcloud = 0.9498 TRmeas cloud = 0.9508 Cirrus cloud TRmodel = 0.5836 TRmeasured = 0.5830 Tropopsphere/Desert Dust PBL/Dust layer Total atmospheric transmission of the marked layers, derived from the simulated Raman signal « TRmod » = model value; « TRmeas » = derived value

  16. 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

  17. Extinction from Elastic backscatter signal - simultion Aerosol layer (Cirrus cloud) reference

  18. The elastic-backscatter lidar equation

  19. The Fernald's inversion method for derivation of the backscatter coefficient; l is omitted 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 • “b(rf)” is known ( typically, the molecular backscatter)

  20. Assuming: “b(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

  21. Derivation of the atmospheric temperature profile using pure rotational Raman backscatter Rotational Raman Spectra of N2 and O2, Excitation at 532nm

  22. Spectral intervals in pure RR where the scattering cross-sections derivative has opposite sign Temperature derivative of the Rotational Raman lines of N2 (red) and O2 (black) R(T)=exp(a – b/T) Typically dR/dT ~0.05% « - » « - » A calibration of the lidar is critical. « + » « + »

  23. Uncertainty - ZOOM 60° zenith angle Integration time: 30min Range resolution: 120m

  24. 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

  25. 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

  26. ANNEX: Possibility for atmospheric characterisation at potential CTA sites with a compact elastic backscatter lidars

  27. MAL 1 MAL 2 32 cm MAL-1 MAL-2 Micro-pulse lidarson stratospheric aircraft (M55)

  28. Micro-pulse lidarson stratospheric aircraft (M55) SCOUT O3/ Brunei - Darwin, 12 November 2005 Backscatter Ratio= (ba+ bm)/ bm

  29. • Ground-based LIDAR, transportable development, observations, data analysis 600mmx600mmx700mm 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

  30. • 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 …)

  31. 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)

  32. Thank you! Valentin Mitev (valentin.mitev@ne.ch) Observatory of Neuchâtel Rue de l’Observatoire 58, CH2000 Neuchâtel Switzerland

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