November 19, 2023

Lidar or Radar

Although lidars and radars are similar in theory, their detection capabilities are quite different, as their wavelengths are very far apart on either side of the distribution range for atmospheric particles. The laser source emits visible radiation at 532 nm and near-infrared radiation at 1,064 nm, whereas radars operate at 94 GHz, or 3.2 mm.

Lidars can detect cloud tops and bases through which they can travel very precisely; beyond an optical thickness of about 3, the signal is too weak to be measured. The base of a cloud might therefore no longer be detected by lidars, which could also miss potential layers underneath the cloud. Measurements taken in clear skies are used to detect aerosol layers, the atmospheric boundary layer and the surface, whose echo is used to calibrate the instrument.

At 94 GHz, microwaves penetrate ice clouds practically without attenuation. Radar signals are sensitive to the size of particles multiplied to the power of six: consequently, it does not see aerosols and is more sensitive to ice clouds than liquid water clouds. It can also detect precipitation. Radars can therefore detect both the top and the base of clouds, even when they are thick, as long as there is no precipitation.

As a result, lidars are more effective when studying thin clouds and aerosols, whereas radars work better with low clouds. The complementarity of these two techniques explains why Calipso and CloudSat flew in formation—CloudSat trailing Calipso at an interval of less than 15 seconds.

Lidar profiles

Left: lidar measurements taken at night at 532 nm over the Tropics by the LITE lidar on the Space Shuttle, in September 1994. Right: simulation based on LITE measurements of what Calipso would observe for daytime measurements.

On the left: lidar measurements taken at night at 532 nm over the Tropics by the Lite lidar on the Space Shuttle, in September 1994. On the right: simulation based on Lite measurements of what Calipso would observe for daytime measurements.

Profils lidar
 

Schema de fonctionnement du lidar simplifie

Green ray

Unlike sunlight, the "green ray" emitted by the CALIOP lidar on Calipso is completely polarized in one direction.

Atmospheric scattering modifies this incident polarization and so the measurement of this depolarization provides a wealth of information about the nature of particles, especially their geometry.

Since the analysis of ground-based measurements has shown that depolarization of ice clouds depends to a large extent on the shape and orientation of the crystals of which they are comprised, it is thus possible to classify particles into four types according to their shape: spheres, platelets, hexagonal columns or polycrystals.