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Precipitation Radar

Figure 1: Precipitation radar of the Deutscher Wetterdienst (German Weather Service) in Essen-Bredeney

Figure 1: Precipitation radar of the Deutscher Wetterdienst (German Weather Service) in Essen-Bredeney

How does the precipitation radar work?

Precipitation Radar

A precipitation radar is a weather radar operating in the S-Band or C-Band (seldom in the X-Band) with a rotating and swiveling symmetrical parabolic antenna. Contrary to amateurish ideas, clouds are not measured with a precipitation radar. The precipitation radar measures only a reflectivity, which is used as a measure for the amount of water in clouds. Additionally (but only if any reflectivity can be measured at all), a Doppler frequency and a difference in polarity can be measured. The radar is used to locate areas of precipitation and to quantitatively measure the local precipitation. By evaluating Doppler frequencies, the distribution of wind speeds can be determined at the same time.

Precipitation radars are always 3D-radars. They change the elevation angle of the antenna (the antennas tilt) after each rotation and thus made each elevation scan around the antenna individually. This results in a so-called volume scan, which (measured on the data renewal rate of air traffic control radar), however, can only be updated quite slowly within 5 to 15 minutes.

Receiver
Parabolic
reflector
Orthomode
transducer
Horn feed
Transmitter
Radome
Lightning
conductor
Turn table
Waveguide

Figure 2: Schematic representation of a weather radar under a radome

Transmitter
Lightning
conductor
Turn table

Figure 2: Schematic representation of a weather radar under a radome

Technical specifications

Precipitation radar usually uses only a narrow-band short transmission pulse, which is generated by a high-power vacuum tube (magnetron or klystron). Solid State Power Amplifiers (SSPA) are also possible but have only a lower pulse power due to the limited dielectric strength of semiconductors. The resulting necessity to use more broadband intra-pulse modulated transmission pulses also worsens the accuracy due to their time-sidelobes. Also, the receiver sensitivity is limited because a broadband receiver captures more noise.

Figure 3: Due to the divergence of the electromagnetic waves, the resolution cell is larger at longer distance, therefore the radar with the smaller distance is more accurate.

Figure 3: Due to the divergence of the electromagnetic waves, the resolution cell is larger at longer distance, therefore the radar with the smaller distance is more accurate.

The maximum range of precipitation radar results from the necessary number of such radars per area. The accuracy of a precipitation radar also decreases with the range (see the the resolution cell in Figure 3). In the often only poor developed tropical and subtropical regions, mostly radars in the S-Band are used. They can achieve ranges of up to 800 km. In Europe, which is very well developed in terms of weather radar (temperate climate), radar sets in the C-Band are mostly used. This frequency range allows higher accuracy. The range of these radars is limited to about 250 km, of which mostly only 150 to 180 km is used because of the higher accuracy. Also, the curvature of the Earth has a negative influence in far away distances, so that deeper-lying meteorological phenomena disappear behind the horizon.

To achieve the required linear receiver dynamics of up to 105 dB, several receivers with different receiver sensitivities operate in parallel. The software then selects the signal with the best signal-to-noise ratio, which is not yet overdriven.

Modern precipitation radars have a very dynamic rotational speed and pulse repetition frequency. Since the weather takes place in the tropopause, the absolute altitude range is limited to about 18 km. Because of this, quite steep elevation angles do not have to be driven with the maximum possible range. Here a much higher pulse repetition frequency can be used and the antenna can rotate faster than in the lower elevation scans.

Range [km]
Height [km]
Precipitation Scan

Figure 4: Scan strategy for a precipitation radar
(The scale of the height scale is increased 1:5)

Range [km]
Height [km]
Precipitation Scan

Figure 4: Scan strategy for a precipitation radar
(The scale of the height scale is increased 1:5)

Scanning strategy

A scanning strategy is shown in Figure 4. A cycle starts with a precipitation scan, whose antenna tilt depends on the coverage angle (horizon) of the radar site. During this rotation, the radar works with a pulse repetition frequency of 600 Hz. The antenna rotates at a speed of 12° per second. Then the volume scan with the antenna pattern starts at 5.5° and turns down one degree after each further rotation. These scans use a pulse repetition frequency of 600 or 800 Hz. The antenna rotation speed increases slightly to 18° per second, only the lowest diagram uses the 12° per second again (suppression of fixed clutter interference). In the eighth rotation, the scan is performed with an elevation angle of 8° and a pulse repetition frequency of 800 or 1 200 Hz. The elevation angles of 12°, 17°, and 25° use 2 410 Hz at a rotation speed of 30° per second. Finally, a scan is performed vertically upwards, which is used for internal calibration. The antenna aligns itself to the north and waits for the start of the next cycle. All precipitation radars of the DWD start simultaneously with the rotation for the next cycle. Therefore the rotation of the antennas is roughly synchronized and thus prevents the antennas from radiating each other. At the same time, it supports the formation of composite radar products (calculation of radar images from data of several radars), because the data from the precipitation scan, for example, are all from the same minute.

Figure 5: Horizontal radar coverage of a network of weather radars in Germany (Source: Deutscher Wetterdienst)

Figure 5: Horizontal radar coverage of a network of weather radars in Germany (Source: Deutscher Wetterdienst)

Radar Distribution

The Deutsche Wetterdienst (German Weather Service) maintains a network of 17 precipitation radars in Germany, which are distributed in such a way that they cover the entire territory of the Federal Republic of Germany. They partly overlap each other, so that if one radar fails, the data of neighboring devices can be used. One of these radars is transportable, so that in case of larger planned downtimes a relocation of the radar to this site is possible.