Polarimetric radar
A polarimetric radar is a special type of precipitation radar that works with polarized electromagnetic waves , which means that it evaluates, among other things, changes in polarization. In practice, polarimetric radars are mostly used, which emit horizontally and vertically polarized electromagnetic waves and receive the reflected waves in these two polarizations simultaneously.
background
Conventional precipitation radars are able to measure the intensity of the backscattered signal (and thus of the precipitation) using the reflectivity Z and the radial velocity of the precipitation using the Doppler effect . Through comparative measurements, this radar receives reliable information about the amount of precipitation and (as a statistical statement) about the size of the individual raindrops.
By using different polarizations, one receives additional information to make a statement about the shape of the precipitation particles ( hydrometeors ). Polarimetric radars are thus able to classify precipitation particles according to their type.
Working method
The horizontally polarized wave is used to measure the horizontal expansion of the reflecting objects. The measurement result of the reception channel for horizontally polarized is considered Z H , respectively. Similarly, with the vertically polarized wave, the vertical extent of Z V . The ratio Z H / Z V is called the differential reflectivity Z DR .
Falling raindrops have an aspherical, flattened shape due to air resistance. A falling raindrop is always slightly wider than it is tall. The larger the raindrop, the more pronounced the disproportion. Therefore, when raindrops are falling, the horizontally polarized component measures a greater reflectivity than the vertical component. The dielectric constant of compact ice is only about 20% that of water, so the particle shape has a much smaller effect on hail than on rain. Hailstones are also irregular in shape and tumble, sometimes even on edge. The polarimetric radar will therefore measure a larger vertical component Z V at a very low Z DR value and thus be able to distinguish a hail area from a normal rain area without a doubt.
Polarimetric radar quantities
In addition to the classic radar parameters that a precipitation radar can measure, such as direction, distance and reflectivity, the following can also be determined by comparing the polarization planes received:
- the differential reflectivity Z DR , describes the orientation of the hydrometeor and the radar's viewing angle to it;
- the differential phase φ DP and the specific differential phase K DP (phase difference between the echo signals parallel and orthogonal), if no phase difference can be measured, it is an isotropic scatterer, i.e. very small spherical particles;
- the linear depolarization ratio LDR (or, in the case of circularly polarized radar devices, the circular depolarization ratio CDR ) describes a tumbling of the hydrometeor: it can then only consist of ice;
- Correlation coefficient ρ HV , is used to distinguish exactly spherical scatterers (rain) from other hydrometeors.
The radar only needs to transmit in both polarization directions to measure the first two parameters. The other polarimetric radar variables are determined by the fact that the radar only transmits in one polarization direction (mostly horizontally polarized because of the greater range), but receives in both polarization directions.
Block diagram
The transmission energy from the powerful generator is divided in the power divider. One part is led to a horn antenna with horizontal polarization, the other part to a horn antenna with vertical polarization.
The duplexers (here: ferrite circulators ) switch the antenna to the transmitter when it is transmitted and to the receiver when it is received. At the moment of transmission, they must protect the sensitive receiver from the high transmission power.
The antenna converts the high-frequency energy from the transmitter into electromagnetic waves and distributes them in certain directions. This process is reversible for receiving the echo signals. The energy received has approximately the same polarization as the energy sent.
The energy received by the antenna is processed in two separate reception channels and processed into a weather image in the radar signal processor.
The changeover switch between the power dividers enables the correct phase merging of the two power sections and thus the work on only one (shown here: the horizontal) polarization plane. The reception path is not affected by this. Because of possible depolarization , the receiver will still be able to receive echo signals on the polarization plane that is not transmitted.
literature
- Richard J. Doviak, Dusan S. Zrnic: Doppler Radar & Weather Observations. Academic Press, 1993, ISBN 0122214226 .