Active Remote Sensing of the Atmosphere - Lidar -

1. Active Remote Sensing of the Atmosphere - Lidar - Remote Sensing I Lecture 9 Summer 2006

2. LIDAR (Light Detection And Ranging) • Idea: • Use of an active system that emits light pulses and measures the intensity of the backscattered light (from air molecules, aerosols, thin clouds) as a function of time (optical Radar) • Instrument: • a strong laser with short pulses • possibly several wavelengths emitted • a large telescope to collect the weak signal • Measurement quantity: • time lag gives altitude information (z = 1/2 t c, with c speed of light) • signal intensity gives information on backscattering at given altitude and extinction along the light path • measurements at different wavelengths provide information on absorbers and aerosol types • polarisation measurements provide information on phase of scatterers • => Very good vertical resolution can be achieved!

3. Review: Scattering in the Atmosphere Summary Mie and Rayleigh Scattering

4. Comparison of Rayleigh and Mie phase functions • The larger the size parameter, • the larger the forward scattering • peak Fig. from Liu, An introduction to atmospheric radiation

5. LIDAR-Types and Target Quantities • Applications: • altimeter • Rayleigh Lidar: temperature • DIAL (Differential Absorption)-Lidar: trace gases • multi wavelength aerosol Lidar: aerosol amount and aerosol properties (size distribution, type) • Raman-Lidar: trace gases • Doppler-Lidar: particle velocities • Fluorescence-Lidar: temperature in the upper atmosphere

6. LIDAR: Instrument • Laser: • short pulses (small dead range above instrument) • high pulse power (high backscattered signal) • typical lasers: • solid state laser (e.g. Nd-YAG) • gas laser (e.g. XeCl) • dye lasers • Detector: • excellent quantum efficiency needed (low signal) • low noise needed (low signal) • typical detectors • Photomultiplier • Photodiodes • CCDs • wavelength selective (use of filters)

7. LIDAR: Example G. Beyerle, PhD thesis, 1994

8. LIDAR: Measurement Example • two wavelengths (353 nm and 532 nm • minimum altitude: 11 km • maximum altitude: 45 km • background signals of calibration • exponential scale • signature of volcanic aerosol • signature of PSCs

9. Lidar equation • The detected intensity Pd(z,λ) is proportinal to • Emitted intensity • Backscatter coefficient • Observed solid angle (with A area of telescope) • Transmission along the light path • Sensitivity of the detector in this channel (including geometric overlap):

10. Lidar equation Taking these factors together will give the so calledLidar-Equation: with

11. DIAL LIDAR • Idea: • two wavelengths are emitted, one at an absorption line, the other one off the absorption but close enough to have small changes in scattering properties and absorption by other absorbers • Application: • ozone profiles • H2O profiles http://www.etl.noaa.gov/et2/

12. Forming the ratio between the received signals Pon and Poff: ... And then the logarithm:: DIAL Lidar equation Start from the Lidar-equation for two wavelength on/off:

13. If the two wavelength are nearby, scattering properties will be Similar, and we finally get: DIAL Lidar equation Differentiating wrt altitude z gives:

14. DIAL LIDAR: Examples Tropospheric O3 Stratospheric O3

15. Aerosol LIDAR • Idea: • Backscattering at different wavelengths is used to derive information on aerosol properties • for each wavelength, the backscattering coefficient βMie(z, λ) is computed from the Lidar equation using the Klett-algorithm: • profiles of temperature and pressure as Input • use of reference height with known backscatter coefficient (Rayleigh only) • Mie scattering ratio determined from model: LMie(z, λ)= αMie(z, λ)/ βMie(z, λ) • Measurement quantity is the backscattering ratio R.

16. Aerosol Lidar: Example PSC

17. Aerosol Lidar: Example Cirrus Clouds • airborne lidar measurements • OLEX instrument (http://www.dlr.de/~flentje/olex.html ) • very good detection limit • high spatial and vertical resolution • detection of cirrus clouds, thin and even “subvisible“ • particle size from colour ratio • particle phase from depolarisation

18. LIDAR: Overview

19. Lidar In-space Technology Experiment (LITE) • Instrument: • flashlamp-pumped Nd:YAG laser • 1064 nm, 532 nm, and 355 nm • 1-meter diameter lightweight telescope • PMT for 355 nm and 532 nmavalanche photodiode (APD) for 1064 nm • Mission Aims: • test and demonstrate lidar measurements from space • collect measurements on • clouds • aerosols (stratospheric & tropospheric) • surface reflectance • Operation: • on Discovery in September 1994 as part of the STS-64 mission • 53 hours operation http://www-lite.larc.nasa.gov/index.html

20. LITE: Example of Aerosol Measurements Clouds (ITCZ) Atlas mountains complex aerosol layer maritime aerosol layer http://www-lite.larc.nasa.gov/index.html

21. More LIDARS in space • ICESat (January 12, 2003) • 532 nanometer lidar • ice sheet mass balance • aerosol and cloud heights • vegetation and land topography • http://icesat.gsfc.nasa.gov/ • CALIPSO (2005?) • 532 nm and 1064 nm) polarization-sensitive lidar • clouds and aerosols • http://www-calipso.larc.nasa.gov/ • WALES (2008?) • water vapour DIAL • high resolution water vapour profiles • http://www.esa.int/export/esaLP/ASE77YNW9SC_wales_0.html