Instruments:
LIDAR (light detection and ranging)
The LIDAR is an active remote sensor using a laser as an emitter. The LIDAR emits short pulses of light into the atmosphere. The emitted radiation encounters diffusion by atmospheric particles and molecules along the line of sight. A part of the radiation is scattered backward and collected by an opto-electronic device into the LIDAR reception system. The optical signal is then translated into a voltage over time and distance by multiplication of the speed of light. An accurate range dependent profile of backscattered light is obtained by calibrating the optical signal.
LIDAR Fundamentals
Ceilometer CL31
Ceilometers are single-channel lidars, an active remote sensing instruments which emits a finite pulse of light and measures the photons scattered back along the path by atmospheric targets. These instruments are principally employed to automatically identify the height of the cloud base above the instrument. The backscattered power is a function of the area of the target illuminated by the ceilometer beam; therefore backscatter from larger targets like clouds will be of a greater magnitude than backscatter from aerosol targets. In fact cloud returns tend to be at least one order of magnitude greater than backscatter from aerosol, allowing clouds to be easily identified by gradient or limit based algorithms.
Ceilometer Fundamentals
Doppler Wind Lidar
This LIDAR enables users to measure the wind speed and direction, using the Doppler effect on small particles, as a radar could follow large droplets or planes, depending on its frequency. Hence using several line of sights at different angles, windspeed and direction are retrieved by projection with a very high spatio-temporal resolution. DWL are especially reliable for the characterisation of the boundary layer (mixing-level height, source of turbulence,...) .
DWL Fundamentals
Wind Profiler
Radar wind profilers are special Doppler radars designed for measuring the vertical profile of the wind vector in the lowest 5 - 20 km of the atmosphere (depending on the operating frequency), on timescales ranging from seconds to years. RWP are also able to provide additional information about the atmospheric state through the profiles of backscattered signal intensity and frequency spread (spectral width) of the echo signal.
The unique characteristic of RWP is their use of longer wavelengths, in comparison with classical weather radars. The typical wavelength range is from about 20 cm (L-Band) to about 6 m (VHF). Electromagnetic waves in this range are scattered at fluctuations of the refractive index of particle-free 'clear air' which are almost omnipresent due to the turbulent state of the atmosphere. This effect is called clear-air scattering, it can be fully understood only on the basis of the theory of radio-wave propagation through the turbulent atmosphere.
Wind Profilers Fundamentals
Raman Lidar
The Raman lidar uses the scattering properties of molecules and aerosols to derive profiles of water vapor and other species, aerosols and temperature with a high vertical and temporal resolution. Raman lidars can cover an altitude range from a few hundreds of meters to the lower stratosphere. The most important application is the profiling of aerosols and water vapor and it is one of the few instruments, that can resolve the high spatial and temporal variability of water vapor. The Raman LIDAR provides data with a high level of resolution (vertical: 3.75 metres, temporal: around 60 seconds).
Raman Lidar Fundamentals
COST - European Cooperation in Science and Technology:
Final Report