Typically, meteorologists determine the amount of rain with the help of automatic rain gauges or by rain radar. Since 2010 scientists of the Institute of Meteorology and Climate Research of Karlsruhe Institute of Technology (KIT) in Garmisch-Partenkirchen (KIT Campus Alpin) have been studying a new measurement technique. It takes advantage of the physical effect of rain drops attenuating electromagnetic radiation and uses the existing infrastructure of commercial mobile radio networks. A specifically developed software analyses radiation fluctuations within the links between the radio towers and deduces information about rainfall intensity and duration. The dense network of mobile radio towers ensures a high regional coverage. Another benefit of the new technique is the minimum time delay until the rainfall information can be provided.

Radio links are operated at the microwave frequency domain between 15 and 40 Gigahertz. Raindrops approximately have the same size as the wavelength of the utilized frequency band. For this reason liquid precipitation strongly attenuates the microwave radiation of the radio links. The heavier the rainfall, the more pronounced the decrease of power between the two antennas. Mobile phone users hardly even notice this impairment. Only in case of extremely strong rain the radiation may be attenuated to such an extent that the communication between the radio towers fails and the telephone connection breaks off. For the KIT research group of Professor Harald Kunstmann the power fluctuations are sufficient to determine the attenuation rates and hence the rain fall intensity and duration.

The measurement technique has the same sensitivity detection limit of one millimeter rain per hour as the classical methods of rain gauges. However, due to the structural characteristics of snow, the technique can’t yet be used for this type of precipitation.

Christian Chwala and Felix Keis, scientists at Kunstmann’s research group, are currently testing the technique with attenuation rates from 450 radio links in Southern Bavaria. The cooperation with Ericsson Deutschland and the specifically developed software allow an immediate data transfer from the computing center of the mobile network provider to KIT Campus Alpin. There the data are processed and searched for conspicuous power fluctuations. The researchers only need the transmitted and received power level of the radio links. Sensitive information such as details of the communication is not collected.

In Germany there are roughly 100.000 radio links that theoretically could be included in the measurement technique and that could supplement the approximately 1.000 conventional measurement points for precipitation. Use of the new technique might e.g. allow for a quicker and more reliable flood warning in mountain regions. But above all, the method is of high potential for countries, where there are few or no weather stations or weather radars but a dense mobile radio network. In regions like West Africa or the Middle East the method might help to improve the weather and climate modeling, which is urgently required for the local water resources management. For this reason the KIT scientists are in contact with colleagues and mobile network providers in, amongst others, Burkina Faso, Ghana and Palestine and try to establish the innovative approach there. For example in March 2015 the RainCell Africa Workshop in Ougadougou, Burkina Faso has been organized in close cooperation with international partners. During this workshop scientists, engineers, met service representatives and students from 18 different African, American and European countries have been introduced to the scientific and infrastructural basics of the new measurement technique.

Publications

 Real-time data acquisition of commercial microwave link networks for hydrometeorological applications. Atmos. Meas. Tech., 9, 991-999, 2016. doi:10.5194/amt-9-991-2016
 Improving Rainfall Measurement in Gauge Poor Regions Thanks to Mobile Telecommunication Networks. BAMS, Volume 97 No. 3, 2016.

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