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

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 scatterers by optical properties (e.g. wavelength and scattering cross section)

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/...

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

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

Light Interactions with the Atmosphere (II)

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

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

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)

Lidar System: Schematic Drawing laser detection telescope

3. Lidar Application: Aerosols 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

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

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

3.1 Aerosol Determination by “Slope Method” (II)

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

NLC photos photo: P. Parviainen photo: M Alpers

3.2 Aerosol Determination by “Klett Method” (I)

3.2 Aerosol Determination by “Klett Method” (II)

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

Polar Stratospheric Clouds photo: M. Rex

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

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

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

Methods for Aerosol Determination: A Comparison   Slope Method requires only one channel suitable for weak, noisy signals no extinction correction  only suitable for thin clouds Klett Method extinction considered needs assumption on extinction to backscatter ratio (lidar ratio) Ansmann Method requires no assumption on lidar ratio requires additional Raman channel (small cross section)  range limit

4. Lidar Application: Temperature 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

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

4.1 Temperature Profile from Resonance Lidar IAP Potassium 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 Figure courtesy of J. Höffner

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

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

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

Temperature profile from air density profile

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

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]

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)

Comparison of temperature sounding principles range complexity limits Rayleigh-Integration Strato- and Mesosphere  aerosol inhibits sounding hydrostatic equilibrium assumed Raman-Integration (Troposphere) Stratosphere aerosol disturbs sounding Resonance-Doppler Mesopause region (80-105 km)  limited to atomic metal layer Brilloiun-Doppler lower troposphere very weak signal Rotation-Raman Tropo- and Stratosphere  weak signal

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

Detector of the IAP T lidars