Precipitation radar of the DWD in Emden, the radar is located in the spherical radome on the top of the tower
A precipitation radar (including Doppler radar is called), the most commonly used form of a weather radar . With the help of the precipitation radar, the water content of a cloud can be measured in a limited area , which in turn allows conclusions to be drawn about possible precipitation ( rain , hail or snow ). By evaluating Doppler frequencies , wind speeds can also be determined. In meteorology, precipitation radars are used to collect current data for a weather warning or weather forecast. In aviation, the precipitation radar is mainly used to avoid flights through thunderstorm areas and to be able to assess the expected turbulence .
Precipitation radar is based on the primary radar principle . Precipitation radar emits microwaves and receives the portion of these waves that is reflected on its way through the atmosphere . Operational, ground-based precipitation radars in Europe mostly work in the C-band , i. H. with frequencies around 5.6 GHz (approx. 5.4 cm wavelength ).
The more drops of water , snow crystals or grains of ice the atmosphere contains per volume, the more microwave radiation it reflects back. From the time difference between the transmission of the radiation and the reception of the reflected radiation, conclusions can be drawn about the distance between the precipitation particles and the radar system. From the intensity of the received echo signals, conclusions can be drawn about the size and physical state to a limited extent. This gives you a picture of the distance and precipitation of the cloud.
During the measurement process itself, there are some physical problems:
Decrease in energy density over distance: The electromagnetic waves propagate divergently, and according to the law of distance they lose energy density with increasing distance and the backscattered signal becomes weaker. This attenuation is called free space attenuation . In the case of precipitation radar, according to the radar equation for volume targets , this decrease is proportional to the square of the distance. The remedy is to readjust the reception sensitivity depending on the distance, i. H. After sending the transmission pulse, the reception sensitivity is initially set to be low and it increases with increasing time after the pulse. This time-dependent gain control is called sensitivity time control .
The strength of the echo signal depends on the size and number of reflecting objects as well as their state of aggregation (moist snow reflects better than raindrops or even hail and sleet). Which of these influences predominates and determines the strength of the echo signal cannot be determined so easily by the radar.
Radar shadow: Due to a large accumulation of water in clouds, so much incoming transmission energy is reflected that the remaining energy still penetrating the cloud is no longer sufficient to generate an echo on the radar screen - a so-called radar shadow is created. Remedy: Operation of several precipitation radars distributed throughout the country, whose detection areas overlap.
English Ground clutter or floor clutter are reflections on elevations and mainly a problem in air surveillance radar. At fixed ground stations, ground clutters can be wiped out by manipulating the sensitivity. The size of these disturbances areregistered andstatistically in an electronic clutter map
Interference from 5 GHz WLAN in the rain radar
subtracted from the precipitation echoes. In aviation, attempts are made to hide elevations in the ground using a Doppler ( Moving Target Indication ) method .
Interpretation problem when used in aviation: The measured backscattering of the clouds does not necessarily allow conclusions to be drawn about the strength of the associated turbulence or the type of precipitation. This can also be remedied by the Doppler method and the use of different transmission frequencies.
The 5 GHz WLAN stations also work as secondary users in the same frequency band as the precipitation radar. To avoid interference, they must use an automatic channel selection and transmission power control. However, this is not consistently implemented everywhere.
Precipitation radar stations
Prötzel
Dresden-Klotzsche
Emden knock
Food shuir
Feldberg
Diemelsee-Flechtdorf
Dreieich-Offenthal
Memmingen
Sniffing
Boostedt
Hanover
Neuhaus am Rennweg
Neuheilenbach
Rostock
iceberg
Geislingen-Türkheim
Ummendorf (Börde)
La Dole
Albis
Plaine Morte
White flue
Monte Lema
Schwechat
Feldkirchen
Zirbitzkogel
Patscherkofel
Valluga
Precipitation radar in Germany, Austria and Switzerland
Germany
In Germany, the German Weather Service operates a radar network with 17 precipitation radar stations.
Austro Control is responsible for the operation of the radar systems in Austria . The first operational operation of a precipitation radar started in 1965 in Rauchwarth near Vienna-Schwechat Airport . Polarimetric radar devices have been in use since 2011 . The precipitation radar network in Austria comprises 5 locations (Rauchwarth, Feldkirchen , Zirbitzkogel , Patscherkofel , Valluga ) and thus enables complete coverage of the national territory. Since 2001 the spatial resolution has been 1x1x1 kilometers, the temporal resolution 5 minutes.
↑ Canli E., Loigge B., Glade T. (2017): Spatially distributed rainfall information and its potential for regional landslide early warning systems . In: Natural Hazards. doi : 10.1007 / s11069-017-2953-9
