The International Conference on Opportunistic Sensing of Precipitation (OpenSense) took place on 25–26 June 2025 at the Deutscher Wetterdienst (DWD) headquarters in Offenbach, Germany. Organized as the final meeting of the Cooperation in Science and Technology (COST) Action Opportunistic Precipitation Sensing Network (CA20136), the meeting brought together over one hundred participants from across Europe and beyond. The attendees represented a broad spectrum of the meteorological community, including researchers in hydrology, radar meteorology, atmospheric science, computer science, and representatives of several national meteorological and hydrological services (NMHSs). The overarching research topic of the conference was about the advances of opportunistic sensing (OS) for precipitation monitoring. OS uses signals or devices not originally designed for professional, high-quality meteorological purposes—such as commercial microwave links (CMLs), personal weather stations (PWSs), television-satellite microwave links (SMLs), and citizen-science contributions—as rainfall sensors. OS can either complement conventional observations from gauges, radars, and satellites or give observational data in regions with sparse rainfall data. The COST Action OpenSense, launched in 2021, has provided a framework for this community. It aimed to establish a coordinated scientific network, to harmonize processing methods, and prepare the ground for operational uptake of OS methods. Working groups within OpenSense focused on data management, methodological homogenization, data merging, and applications (https://opensenseaction.eu/). The conference was the kickoff of a meeting series on the scientific progress and institutional prospects of OS rainfall estimation. The second edition is already in preparation at the Royal Netherlands Meteorological Institute (KNMI) in De Bilt, the Netherlands, on 23 and 24 June 2026. This first edition of the conference served as the final meeting of OpenSense and also consisted of internal management committee and working group meetings preceding the two conference days. The conference program was structured into thematic sessions described in the following sections and consisted of oral and poster sessions as well as keynote lectures and a panel discussion. This report synthesizes the main themes, cross-cutting insights, and emerging challenges as discussed across the sessions.

Opportunistic sensors are increasingly used for rainfall measurement. However, their raw data are collected by a variety of systems that are often not primarily intended for rainfall monitoring, resulting in a plethora of different data formats and a lack of common standards. This hinders the sharing of opportunistic sensing (OS) data, their automated processing, and, at the end, their practical usage and integration into standard observation systems. This paper summarises the experiences of the more than 100 members of the OpenSense Cost Action involved in the OS of rainfall. We review the current practice of collecting and storing precipitation OS data and corresponding metadata, and propose new common guidelines describing the requirements on data and metadata collection, harmonising naming conventions, and defining human-readable and machine readable file formats for data and metadata storage. We focus on three sensors identified by the OpenSense community as prominent representatives of the OS of precipitation: Commercial microwave links (CML): fixed point-to-point radio links mainly used as backhauling connections in telecommunication networks Satellite microwave links (SML): radio links between geostationary Earth orbit (GEO) satellites and ground user terminals. Personal weather stations (PWS): non-professional meteorological sensors owned by citizens. The conventions presented in this paper are primarily designed for storing, handling, and sharing historical time series and do not consider specific requirements for using OS data in real time for operational purposes. The conventions are already now accepted by the ever growing OpenSense community and represent an important step towards automated processing of OS raw data and community development of joint OS software packages.

Commercial microwave links (CMLs) in telecommunication networks can provide relevant information for remote sensing of precipitation and other environmental variables, such as path-averaged drop size distribution, evaporation, or humidity. The CoMMon field experiment (COmmercial Microwave links for urban rainfall MONitoring) mainly focused on the rainfall observations by monitoring a 38ĝ€¯GHz dual-polarized CML of 1.85ĝ€¯km path length at a high temporal resolution (4ĝ€¯s), as well as a co-located array of five disdrometers and three rain gauges over 1 year. The dataset is complemented with observations from five nearby weather stations. Raw and pre-processed data, which can be explored with a custom static HTML viewer, are available at 10.5281/zenodo.4923125 . The data quality is generally satisfactory for further analysis, and potentially problematic measurements are flagged to help the analyst identify relevant periods for specific study purposes. Finally, we encourage potential applications and discuss open issues regarding future remote sensing with CMLs.