LIDAR stands for LIght Detection And Ranging and is an active remote sensing instrument which gains informations. about the atmosphere by using the effect of scattering of electro-magnetic waves on molecules, aerosols or clouds. A LIDAR consists of an transmitting and a receiving unit. A scheme of the LIDAR priciple is given in fig.1.
A pulsed laser beam is send out by the transmitting unit. The backscattered light is collected by the receiver and trough the runtime of the laser pulse and the speed of light the distance between the laser and the scattering volume can be calculated by
The received power for elastic backscattering at a given wavelangth is given by the LIDAR equation
Were the overlap function O(z)describes the overlap between the laser beam
and the receivers field of view.
The height independent constant Cs(λ) icludes information about the LIDAR system itself as telescope size and detector efficiency.
The backscatter coefficient β(z,λ) describes the part of the laser light scattered in backward direction.
The exponential term describes the extiction of light by scattering and absorption and is called transmission term.
The Leosphere ALS300 is an elastic backscatter LIDAR with an additional channel for detection of perpendicular light. The emitted wavelength of 355nm is achieved by a frequency-tripled Nd:YAG laser. The emitted light pulses have an energy of 16 mJ and a repetition rate of 20 Hz with a divergence of the laser beam of 0.25 mrad. The telescope collecting the backscattered light has a diameter of 150 mm. Therefore a complete overlap is achieved between 150 m and 200 m. For the acquisition of the incoming signal both analogue-to-digital-converter and photomultiplier can be used separately or together. A scanning device is also part of the setup for azimuthal and zenithal alignment of the ALS300. The scanning device can not only be used just to angle the lidar but also for vertical, horizontal and volume scanning.
Clouds are crucial components of the Earth's climate system (IPCC, 2007). Depending on the cloud properties (height, effective diameter, thermodynamic phase and optical thickness) they have the potential either to warm or cool the atmosphere beneath the cloud or the surface. In spite of a small global coverage of about 3% (ISCCP webpage) convective clouds play an important role in the climate system (water cycle or extreme weather events). Due to their complex dynamics the aerosol-cloud-interactions of convective clouds are much more complicated compared to stratiform clouds (Lee et al., 2008, Lee et al., 2009). In fact of the complexity vertical profile informations about the cloud microphysics and radiative properties are essential. Therefor lidar measurements will be combined with radiance measurements of spectral solar radiation reflected by cloudsides.
Letzte Aktualisierung am 20. 7. 2010 von Jörg Walter