LIDAR: Introduction to selected topics

1. LIDAR: Introduction to selected topics Leibniz-Institut für Atmosphärenphysik Schlossstraße 6, 18225 Kühlungsborn E-mail: gerding@iap-kborn.de Michael Gerding

2. LIDAR: What’s it all about? • LIDAR = light detection and ranging (similar to radar, sodar) • light: pulsed laser (nanosecond-range) • scatterer: air molecules and aerosols • detector: telescope and photon detectors • ranging: time-resolved detection • distinction between scatterersby optical properties (e.g. wavelengthand scattering cross section)

3. LIDAR: What’s the use of it? • vertical distribution of scatterers • aerosol particles from troposphere to mesosphere • existence, phase, optical thickness (extinction) • particle size, particle number ??? • trace gases in troposphere/stratosphere/mesosphere/ thermosphere • concentration of pollutants: NO2, SO2, ..., H2O, O3, OH, metal atoms (Fe, Na, K, Ca, ...), Ca+ • temperature of the troposphere/stratosphere/mesosphere/...

4. 1. Introduction and Overview 2. Lidar Basics 3. Lidar Application: Aerosols 4. Lidar Application: Temperature

5. Light Interaction with the Atmosphere • Rayleigh scattering • elastic; atoms or molecules • Mie (particle) scattering • elastic; aerosol particles • Raman scattering • inelastic, molecules • Resonance fluorescence • elastic at atomic transition; large cross section • Fluorescence • inelastic, broadband emission; atoms or molecules • Absorption • attenuation in bands; molecules or particles

6. Light Interactions with the Atmosphere (II)

7. Light Interactions with the Atmosphere (III) Rayleigh-Raman spectrum Q branch total intensity of the rotational Raman bands Rayleigh/Mie line pure rotation Raman bands vibrational Raman scatter relative intensity wavelength [nm] from: A. Behrendt, PhD thesis, University Hamburg, 2000

8. Elastic Lidar Backscatter Signal M. Gerding, PhD thesis, IAP Kühlungsborn, 2000

9. Basic Lidar Equation background bin width detector sensitivity total backscatter coefficient geometric overlap between laser and telescope FOV transmission between ground and scattering altitude zi solid angle of visible telescope aperture emitted intensity at the wavelength  intensity at the emitted wavelength received from altitude zi (z=c·t/2)

10. Lidar System: Schematic Drawing laser detection telescope

11. 1. Introduction and Overview 2. Lidar Basics 3. Lidar Application: Aerosols 3.1 Aerosol Determination by “Slope Method” 3.2 Aerosol Determination by “Klett Method” 3.3 Aerosol Determination by “Ansmann Method” 4. Lidar Application: Temperature

12. Aerosol in the Atmosphere noctilucent clouds aerosol free polar stratospheric clouds Junge layer and volcanic aerosol clouds boundary layer and tropospheric aerosol

13. 3.1 Aerosol Determination by “Slope Method” (I) n: molecule number density

14. 3.1 Aerosol Determination by “Slope Method” (II)

15. Application of the “Slope Method” Figure courtesy of M. Alpers

16. NLC photos photo: P. Parviainen photo: M Alpers

17. 3.2 Aerosol Determination by “Klett Method” (I)

18. 3.2 Aerosol Determination by “Klett Method” (II)

19. Application of the “Klett Method”: Polar Stratospheric Clouds Ny-Ålesund, January 20, 2001, 0:21-2:10 UT Slope (smoothed) Klett (unsmoothed)

20. Polar Stratospheric Clouds photo: M. Rex

21. 3.3 Aerosol Determination by “Ansmann Method” (I) R. Schumacher, PhD thesis, Alfred Wegener Institute, 2001

22. 3.3 Aerosol Determination by “Ansmann Method” (II) aAer(lR) k Ångström-coefficient No need of “L”!

23. Application of the “Ansmann Method”: Tropospheric Aerosol LR=25: sea salt Koldewey Aerosol Raman Lidar KARL March 12, 2002, 21:00-23:30 UT

24. Methods for Aerosol Determination: A Comparison

25. 1. Introduction and Overview 2. Lidar Basics 3. Lidar Application: Aerosols 4. Lidar Application: Temperature 4.1 Temperature Profile from Resonance Lidar 4.2 Temperature Profile from Rayleigh Lidar 4.3 Temperature Profile from Raman Lidar

26. IAP Mobile Potassium Lidar Potassium Temperature Lidar of Leibniz-Institute of Atmospheric Physics on the Plateauberget near Longyearbyen (78°N, 16°E) photo: J. Höffner

27. 4.1 Temperature Profile from Resonance Lidar • atomic K exists in the mesopause region (like Si, Mg, Fe, Na, Ca ...) • investigation by resonance lidar • alkali metals show hyperfine structure of electronic transitions • temperature dependent Doppler broadening of resonant backscatter can be detected by narrowband laser IAP Potassium Lidar Figure courtesy of J. Höffner

28. above 80 km: K resonance lidar Hyperfinestructure and Doppler broadening of a K resonance line von Zahn and Höffner, 1996

29. 80-105 km: K resonance lidar Hyperfinestructure and Doppler broadening of a K resonance line Measured and fitted shape of the resonance line

30. 22-90 km: Rayleigh lidar hydrostatic equation ideal gas law relative density profile required - derived from (aerosol free) lidar backscatter signal

31. Temperature profile from air density profile temperature air density

32. 1-30 km: Rotation-Raman lidar Rotation-Raman spectrum of air for excitation at 532.05 nm Alpers et al., 2004

33. 4.3 Temperature Profile from Raman Lidar • Rotation-Raman spectrum depends on temperature • Intensity of transitions to high J-numbers increase with temperature, intensity of transitions to low J-numbers decrease • Intensity ratio between two different wavelengths depends on temperature • For lidar choose narrow fractions of the spectrum high-J filter low-J filter A. Behrendt, Uni Hamburg, 2000 wavelength [nm]

34. 5.3 Temperature Profile from Raman Lidar (II) • Backscatter signal at the different wavelengths depend on temperature, but also on the filter characteristic, the transmission of the detection system, atmospheric extinction • temperature dependence of the signal can (hardly) be calculated or • lidar can be calibrated with respect to temperature response (comparison with other methods like radiosondes)

35. Comparison of temperature sounding principles

36. 1. Introduction and Overview 2. Lidar Basics 3. Lidar Application: Aerosols 4. Lidar Application: Temperature 5. … add on’s

40. Detector of the IAP T lidars