OPERA Radar Guide
Understanding pan-European weather radar data enables precise, real-time monitoring of precipitation systems as they develop and traverse the continent. The OPERA Radar Explorer uses the data made available via the EUMETNET OPERA program, combining observations from dozens of national meteorological services into a seamless, high-resolution visual display.
Understanding the Composite Products
The OPERA network provides three distinct observational layers, each serving a specific meteorological purpose.
Maximum Reflectivity (DBZH)
The Maximum Reflectivity product, developed under the OPERA CIRRUS project and generated by Météo-France, offers a high-frequency (5-minute), high-resolution (1 km) view of severe weather potential.
Rather than showing the precipitation strictly at the ground level, this composite displays the highest radar reflectivity value detected anywhere within the vertical column above each 1 km grid square. This makes it an essential tool for nowcasting. Deep convective clouds, which produce heavy downpours and hail, exhibit intense reflectivity aloft. By mapping the maximum vertical value, the DBZH product clearly highlights the core structures of thunderstorms and frontal systems. Values are measured in decibels (dBZ).
Instantaneous Surface Rain Rate (RATE)
When you need to know how intensely it is raining right now, the Instantaneous Surface Rain Rate product is the primary metric. Generated under the OPERA NIMBUS project by GeoSphere Austria, this 2 km resolution composite updates every 15 minutes.
Unlike the vertical maximums shown in the DBZH layer, the rain rate is calculated using only the lowest available elevation scan from the contributing radars, aiming to measure the precipitation closest to the ground. The raw radar reflectivity factor (Z) is mathematically converted into a rainfall intensity (R, in mm/h) using the standard Marshall-Palmer equation (\(Z = 200 \times R^{1.6}\)).
1-Hour Rainfall Accumulation (ACRR)
For assessing flood risks and hydrological impacts, instantaneous rates must be translated into total volumes. The 1-Hour Rainfall Accumulation composite, also updated every 15 minutes at a 2 km resolution, calculates the total precipitation (in mm) that has fallen over the preceding 60 minutes.
It is derived by summing the previous four 15-minute rain rate composites. This product is critical for identifying areas that have experienced persistent, heavy rainfall, making it a key indicator for potential flash flooding or overwhelmed drainage systems.
Practical Considerations and Limitations
When interpreting the OPERA radar composites, keep several technical and physical constraints in mind:
Quality Control and Low-Level Noise
The raw data supplied by national networks undergoes central preprocessing using the BALTRAD toolbox. This involves applying filters to remove non-meteorological echoes (such as anomalous propagation, ground clutter, and satellite interference). However, these filters are not applied universally to all incoming data. As a result, users may occasionally see low-reflectivity artifacts that resemble light rain but are actually noise. For visual clarity, applying a minimal display threshold (e.g., ignoring values below 0.1 dBZ or 0.01 mm/h) is generally recommended.
Network Latency and Gaps
A pan-European composite is only as complete as its constituent parts. The final product relies on the timely delivery of data from individual national weather services. Occasionally, network disruptions, radar maintenance, or transmission delays can result in missing data for specific countries or regions. If a country’s radar data is delayed beyond the processing window, that area will appear blank in the composite, which should not be confused with clear weather.
The Spanish Extrapolation
Because the Spanish national radar network provides data at a 10-minute frequency (rather than the 5-minute standard expected by the CIRRUS DBZH product), the processing system employs a specialized forward-extrapolation technique (Lucas-Kanade) for Spanish data. This mathematical estimation ensures a fluid 5-minute composite over the Iberian Peninsula, but it represents an algorithmic prediction rather than a direct measurement for those intermediate time steps.
Data Sources
The OPERA radar composites are coordinated by the EUMETNET OPERA program. The Maximum Reflectivity (CIRRUS) product is produced by Météo-France, while the Surface Rain Rate and Accumulation (NIMBUS) products are produced by GeoSphere Austria, with developmental support from KNMI and SMHI. Data access and licensing are governed by EUMETNET policies.