Opportunities for opportunistic sensing of rainfall with Commercial Microwave Links in Nigeria

Near-surface rainfall estimates from Commercial Microwave Links (CMLs) are a viable source of rainfall information in data scarce regions, notably Low and Middle-Income countries in the tropics. CMLs are point-to-point radio links commonly used in cellular telecommunication networks. When it rains, the radio signal between two cell phone towers is (partially) attenuated, and this rain-induced attenuation can be used to infer the average rainfall intensity along the path. Typically, every 15 minutes the minimum and maximum received signal levels are stored in network management systems by mobile network operators for quality monitoring purposes. Based on these signal levels it is possible to estimate path-averaged rainfall intensities, which can be interpolated to produce high-resolution rainfall maps.

In this study we investigate the use of several thousands of CMLs, predominantly located in heavily urbanized areas, during one rainy season in Nigeria. We use 32 hourly rain gauges (12 from Nigeria’s Meteorological Agency, and 20 from the Trans-African Hydro-Meteorological Observatory) as a reference to compare with the path-averaged rainfall intensities from CMLs within 5 km of a gauge. To quantify the uncertainties in CML rainfall estimates we compare the performance of these links with different frequency and polarization across the same path. We make a similar comparison by comparing interpolated rainfall maps from CMLs to available gridded (satellite) rainfall products on a seasonal basis. As such, this study aims to highlight the added value of using CMLs as an opportunistic source of rainfall estimation in a region where reference rainfall information from dedicated ground-based sensors is very limited. It offers a balanced outlook for the use of these near-surface rainfall estimates with their associated uncertainties as input for hydrometeorological applications at the kilometer scale, ranging  from numerical weather prediction to calibration of satellite precipitation products, and hydrological modelling.