LIDAR- Light Detection and Ranging

1. LIDAR- Light Detection and Ranging • Lidar = “laser-radar” • RADAR-wavelengths: mm, cm • LIDAR-wavelengths: 250 nm-10 μm • Principle: short energetic pulses of electromagnetic radiation are emitted into the atmosphere, hiting objects and scattered in all directions. Parts of the radiation will be backscattered to a receiver, which is usually close to the emitter.

2. LIDAR continued • Since the radiation propagates at the speed of light, you can determine the distance to the scattering object by measuring the time from puls emission to signal reception. • LIDARs are well suited for observations of fairly small objects, while RADARs cannot observe objects smaller than raindrops. • The LIDAR-pulses are scattered by molecules and aerosols which are always present in the atmosphere. Consequently, the pulses are significantly reduced with height (this causes problem in fogy or cloudy conditions).

3. The LIDAR equation 1 Backscattered flux from one single particle:

4. The LIDAR equation 2 We can rewrite the equation in the following way:

5. The LIDAR equation 3 The incoming flux can be written in terms of the transmitted power:

6. The LIDAR equation 4 The received backscattered power can be written in the following way:

7. The LIDAR equation 5 We can define the mean scattering cross section: So the backscattered power for an average particle is given by →

8. LIDAR equation 6 The pulse length transmitted by the lidar system is Δh. At any given time the receiver receives scattered energy from ½ pulse length. The total number of particles within an effective scattering volume will therefore be: N·At·Δh/2

9. Effective scattering volume:

10. LIDARequation 7 We can rewrite the equation again: But we must also take extinction into account:

11. LIDAR equation 8 We end up with the final LIDAR equation: Ar,Pt and Δh are all well known parameters, but….

12. 2 unknown in the lidar-equation,βe & βπ • βe og βπare depending on the consentration and the optical properties of the scattering particles/molecules • For Rayleigh-scattering there is a simple relationship between the two parameters: βπ/βe=1.5 • For Mie-scattering: The phasefunction is strongly dependent on the sizeparameter α=2πa/λ. Strong forward scattering is characteristic. • Common approximation for warm (liquid phase) clouds: βπ/βe=0.63

13. βe & βπ The use of a backscattered LIDAR-signal to calculate a vertical profile for the extinction koefficient is central within LIDAR theory.

14. Vertical profiles of βe • You start by defining the variable S(r) • If there is an established relationship between βe and βπ, βe(r) can be calculated from S(r). From βe,the size and number concentration of the particles can be calculated.

15. Differential Absorption Lidar Technique (DIAL) • Emission of laser pulses at two different wavelengths: One central in one of the absorption lines of the gas, the other corresponding to low absorption. • For clear sky the total extinction coefficient is given by:

16. DIAL technique The ALOMAR ozon lidar uses the DIAL technique

17. DIAL technique cont. • It is important to choose two wavelengths with practically equal optical properties with respect to aerosols (and equal Rayleigh scattering). • By taking the log of the lidar equation for both wavelengths (λ1 and λ2) and defining…

18. DIAL technique continued For the right choice of λ1 and λ2,scattering/ backscattering will be practically equal for the two wavelengths. We end up with:

19. DIAL technique cont. • In this equation, ρa(r) is the only unknown parameter → we can obtain a vertical density profile.☺ • With the newest laser technology, lasers can be tuned exactly to the absorptionlines for different gases like H2O, NO2,SO2 and O3.

20. Studies of clouds and aerosols using depolarization-principles • The emitted power (Pt) from the lidar can have either a vertical or horizontal polarization. • The reciever can be constructed for reception of both polarization-components. • This will provide information about the properties of the scattering particles – this technique is called the depolarization-technique.

21. Depolarization technique continued • Symmetric, spherical particles: Backscattered radiation will keep its original polarization • Non-spherical particles:Backscattered radiation with opposite polarization compared to incident radiation is produced through multiple internal reflection. • Consider the case where the radiation incident on a particle is vertically polarized (Pver). We can define the backscatterratio δ=Phor/Pver • By measuring δ we can determine the phase of clouds. (Ice phase, δ=0.5-0.7; liquid phase, δ=0).

22. Principles of depolarization

23. Mekanical chopper • A mekanical chopper is a disk rotating at high frequency. • It is used for blocking intense backscattered radiation from the lower levels of the atmosphere. This radiation would otherwise “overload” the receiver system. • The received signal is not reliable within the time interval where the chopper starts to open until it is completely open. This timeinterval should therefore be as short as possible .

24. BACKGROUND • I reality there will always be a background signal and noise in addition to the signal described by the lidar-equation. • Ncount= N(r) + N backgr. (N = counted photones) • The background consists of scattered sunlight, moonlight and starlight etc. • We can reduce the background by reducing the LIDAR “field of view (FOV)” since Nbackgr is proportional to the FOV. But, the FOV must always be larger than the divergence of the emitted beam.

25. BACKGROUND continued • The background can also be reduced by reducing the receiver bandwidth. • This can be achieved by using filters. • However, the receiver bandwidth cannot be more narrow than the laser bandwidth. END OF LIDAR LECTURE

26. PSC – Polar Stratospheric Clouds Also known as mother-of-pearl clouds Developes when stratospheric cooling reaches a critical level (193 K, -80oC) For these temperature water will condense, despite the very low water vapour concentration in the stratosphere (~ 1 ppmv) This occurs over arctic and antarctic areas in winter PSCs can be divided into two types : - Type 1: Condensed water and nitric acid, sometimes combined with sulfuric acid - Type 2: Consist of practically pure ice crystals (H2O)

27. Beautiful but dangerous….

28. PSCs and ozone depletion • Chemical reactions take place on the surface of the cloud droplets in the PSCs: • Then, when sunlight returns in spring ☼ OZONE DEPLETION!!!

29. Polar Vortex • Large-scale cyclonic circulation in the UTLS (“Upper Troposphere, Lower Stratosphere”) in polar regions. • The air is isolated from stratospheric air from lower latitudes for long periods each winter. This produces cold and inert air in the center of the cyclones. • The isolation is due to strong zonal winds circulating around the poles and preventing in- and outflow of air

30. Cont. Polar Vortex • The isolation is more efficient over antarctic than over arctic areas, probably because the topography around the north pole tends to break up the circulation pattern.

31. Polar vortex right now: • A potential vorticity > 70·10-6 Km2/kgs is characteristic within the vortex:

32. Polar vortex right now: • A temperature lower than 193 K is characteristic within the vortex: