Opportunistic Sensing of Precipitation and Evaporation Using Microwave Links From Cellular Communication Networks

Precipitation and evaporation are the two fluxes coupling the atmospheric and terrestrial compartments of the hydrologic cycle. Accurate and robust observations of the spatial and temporal variability of these two fluxes over the Earth’s continents is crucial to help understand the intricacies of land surface – atmosphere interactions. Improving our understanding and our ability to quantify these interactions is not only important for scientific purposes (such as developing better earth system models) but also for societally relevant applications (such as flood and drought forecasting). Here, we demonstrate the potential and address the limitations of microwave links from cellular communication networks for estimating both precipitation and evaporation.

Previous research has shown that attenuation of microwave signals propagating through rainfall from the transmitting to the receiving antennas of microwave links can be related to the average rainfall intensity along the path between transmitter and receiver. Over the past two decades, this notion has been successfully applied to retrieve rainfall fields from existing microwave links which are part of cellular communication networks. Rain-induced signal loss due to absorption and scattering of microwave signals by raindrops, a source of “noise” for mobile network operators, has turned out to be a “signal” for hydrometeorological science and applications. The approach of using existing cellular communication infrastructure for environmental monitoring (in this case rainfall measurement) has been dubbed “opportunistic sensing”.

However, atmospheric constituents between the transmitters and receivers of microwave links do not only affect signal propagation when it rains. When it is dry, refractive index fluctuations induced by temperature and water vapor variations resulting from rising turbulent eddies in the atmospheric boundary layer between transmitters and receivers cause received signals to “scintillate”. The variance of these scintillations has been shown to be related to the structure parameter of the refractive index, which in turn can be related to sensible and latent heat fluxes across the microwave link path using Monin-Obukhov Similarity Theory (and the aid of auxiliary information). This principle is used by microwave scintillometers, commercially available instruments for observing turbulent fluxes in the atmospheric boundary layer.

Recent research results show that microwave links from cellular communication networks can, under certain conditions, also be employed as boundary layer scintillometers. Combining this notion with the previous finding that such microwave links can also be used as path-average rain gauges suggests that there is potential to use each of the roughly five million backhaul links from cellular communication networks worldwide as combined precipitation-evaporation sensors. Hence, gaining access to received signal level data from this enormous number of microwave links would allow large-scale rainfall and evaporation mapping, also for regions across the globe which are currently poorly served in terms of dedicated meteorological stations.

We present both the physical basis of this approach and empirical results from previous and ongoing measurement campaigns to discuss the potential and challenges of opportunistic sensing of two hydrologic fluxes with one single instrument: precipitation and evaporation.