Special Issue “Rain Sensors”

In situ weather sensors aiming at the measurement of liquid atmospheric precipitation (rainfall) experienced limited conceptual innovation in recent decades, except for the data recording and transmission components. The most relevant advancement in modern rainfall measurement devices remains the transition from catching to non-catching instruments. The former are used to measure the integral properties of rainfall (the total amount and intensity of the rainwater collected through a funnel), whereas recent sensors (most of them contactless) can detect the microphysical features of the rainfall process (e.g., the drop size distribution and fall velocity) or the kinetic energy of each drop. These sensors are based on modern measurement techniques and involve optical, acoustic and radar measuring principles, rather than the more traditional gravimetric (see, e.g., Cauteruccio et al. [1]) and seldom adopted thermodynamic ones (see, e.g., Cauteruccio et al. [2]). However, the lack of any standardized calibration procedure for non-catching instruments (Lanza et al. [3]) and the differences still observed between measurements from co-located modern and traditional (reference) instruments (see, e.g., Lanza and Vuerich [4]) still make the reliability and accuracy of the new sensors largely questionable. The role of external influencing variables, such as the environmental conditions at the measurement site, is also rarely accounted for, and no adjustments are usually applied to the raw data to correct, e.g., for the wind-induced bias (Chinchella [5]). Therefore, their use in applications out of the research framework should not be encouraged before sufficient knowledge of both the instrumental and environmental sources of bias is achieved and suitable adjustments are implemented. The development of opportunistic sensors is the second main innovation recently experienced, suggesting a high potential for large-scale application due to the low cost of their installation and operation. These sensors exploit already-existing, but usually unrelated, microwave (MW) or millimeter wave (mmW) links to infer the rainfall amount or intensity by interpreting the extra attenuation induced by the precipitation on the received signal level. Communication technologies that can be opportunistically used for rain sensing include commercial MW links (CMLs) of cellular phone networks, satellite MW links (SMLs), including broadcast satellite links (BSLs), but also wireless sensor networks (WSNs), e.g., for Internet of things (IoT) applications, moving vehicles, surveillance cameras, etc. (see, e.g., Uijlenhoet et al. [6], Giannetti et al. [7], Haberlandt and Sester [8] and Allamano et al. [9]). These sensors turn out appropriate for large-scale installation (e.g., within citizen scientist initiatives) and coverage of wide areas, where in situ rainfall measurements are rarely sufficient or even possible. However, the availability of comprehensive and convincing validation exercises is still scarce (see, e.g., Colli et al. [10]) and assessing their accuracy is difficult. Indeed, opportunistic sensors in most cases provide cumulative information about rainfall along some geometric path (from linear to unpredictable), which does not match the coverage of any traditional instrument that could be used as a reference. Weather radars and remote sensors on board meteorological satellite platforms have been largely developed and used to measure rainfall over wide areas, with increasingly enhanced spatial and temporal resolution. These are based on various remote imaging systems operating in different bands of the light spectrum (from visible to MW wavelengths). Their main advantage is the spatial coverage of wide geographical areas, with the capability to provide areal averaged estimates of the rainfall field over hydrological units (the catchment area) where many applications operate (e.g., flood forecasting, water resources management, etc.). However, the associated rainfall products largely rely on in situ rainfall measurements for calibration and validation purposes. Following Michaelides et al. [11] “measurements at the ground have been proved indispensable, despite advances in several areas of remotely sensing of precipitation”. To provide specific guidance about instrument calibration and their achievable accuracy, perform laboratory and field tests, and develop research/technical activities about the measurement of precipitation intensity and the related data analysis and interpretation, the first Measurement Lead Centre on Precipitation Intensity was designated by the World Meteorological Organization (WMO) in Italy in 2010 [12]. The Lead Centre is dedicated to the memory of, and named after, Benedetto Castelli and his historical work on precipitation measurements. A reference system for rainfall intensity observations is available at the field test site of the Lead Centre in Vigna di Valle (Rome, Italy) for testing the performance of various existing and new rain sensors. Installed in a pit, it is composed of a set of working reference instruments employing a selection of measuring principles tested during the previous WMO Field Intercomparisons of Rainfall Intensity Gauges (see Lanza and Vuerich [4]).