Measuring urban rainfall with a dense Commercial Microwave Link network in Lagos, Nigeria

Measuring urban precipitation adds extra difficulty to the already challenging task of quantitative precipitation estimation. Buildings form obstructions that can block ground-based precipitation radar signals, and the complex urban microclimate makes gauge measurements representative for only a very small area. Performing precipitation measurements in an urban setting thus benefits from using many different data sources, to capture the largest possible range of scales. As such, opportunistic sensing techniques are especially valuable for urban hydrometeorological research: the use of unconventional data sources to extract valuable data that can allow us to estimate urban precipitation. One of the more prominent data sources is the use of Commercial Microwave Links –CMLs – to measure rainfall, by making use of the signal attenuation between cell phone towers. This method of estimating rainfall has been mostly tested and applied in developed countries that already have reasonable coverage of conventional precipitation measurements. However, the most benefits are to be made in developing regions lacking such measurement networks. Only few studies address this, generally using relatively small datasets.

This research focuses on tropical CML rainfall estimation in Lagos, Nigeria. This African megacity has a dense network of CMLs and few official measurement stations, making it an interesting area to study the effectiveness of urban CML precipitation measurements in such a region. We employ the open-source R package RAINLINK to obtain 15-min rainfall maps based on data from a few thousand CMLs during the rainy season. We optimise the most important RAINLINK parameters by comparing to rain gauge data, considering local network and environmental conditions. In addition, disdrometer data from Nigeria or similar climates are used to compute the values of the physically-based coefficients relating specific attenuation to rainfall rate.