OPERA Radar Products Explained: DBZH, RATE and ACRR
Europe’s weather-radar mosaic is not one measurement with three labels. DBZH, RATE, and ACRR use related radar inputs, but each product answers a different question. DBZH highlights the strongest radar echo in a vertical column; RATE estimates how intensely rain is falling near the surface at one time; ACRR estimates how much rain accumulated during the preceding hour.
That distinction matters whenever a colourful radar pixel is used as evidence. A strong DBZH signal should not automatically be read as a measured surface rainfall rate, and an ACRR value is not a rain-gauge observation. This explainer follows the official EUMETNET OPERA production chain from national radar inputs to the three European composites, then shows how Climate Explorer makes selected catalogue frames easier to inspect and export without claiming to produce the official data.
OPERA turns national radar scans into European composites
Weather radars transmit microwave pulses and record energy scattered back from precipitation and other targets, as explained in the NOAA radar overview. National meteorological services operate the contributing C- and S-band radars, and OPERA’s CUMULUS/STRATUS line receives scans and volume data for downstream production. CIRRUS, operated at Météo-France, creates maximum reflectivity. NIMBUS, operated at GeoSphere Austria, creates surface rain rate and one-hour accumulation.
A composite is therefore more than a collage. The inputs have different scan timing and strategies, selected quality-control procedures are applied, the observations are mapped to a common European grid, and a documented rule determines which value each output pixel receives.
DBZH shows the strongest echo in the column
The CIRRUS maximum-reflectivity product is a 1 km, 5-minute pan-European grid. At a given output pixel, the process selects the maximum reflectivity among the contributing single-scan pixels. Reflectivity is expressed in dBZ, a logarithmic representation of the radar reflectivity factor.
The practical reading is: DBZH is useful for locating intense radar echoes and storm structure, including strong echoes aloft. It is not the same as a surface rain rate. Hail, melting precipitation, distance from a radar, attenuation, beam geometry, and non-meteorological echoes can all influence how a reflectivity field should be interpreted.
Use DBZH when the question is “Where are the strongest echoes?” Move to RATE when the question is “What surface rainfall intensity does the radar processing estimate?”
RATE converts low-level reflectivity into a rain-rate estimate
The NIMBUS RATE product is a 2 km, 15-minute product expressed in millimetres per hour. The documented method starts from each radar’s lowest elevation scan. For a given composite pixel, it selects the contribution from the radar providing data closest to sea level; the reflectivities are not averaged. It then converts reflectivity to rain rate with the Marshall–Palmer relationship, Z = 200R1.6.
This conversion makes RATE easier to relate to rainfall intensity, but it remains an estimate. As NOAA’s reflectivity explanation illustrates, the same reflectivity can correspond to different rainfall rates when drop sizes differ. The radar beam may also sample precipitation above the surface. RATE should therefore be read as a spatially continuous radar estimate, not as a replacement for a gauge at an exact point.
Use RATE when the question is “How intense is the estimated rainfall in this frame?” A value in mm/h is an intensity rate at the product time, not the amount that necessarily accumulated during the full preceding hour.
ACRR adds four RATE frames to estimate the previous hour
NIMBUS ACRR uses the same 2 km grid and a 15-minute product interval. It sums the previous four 15-minute RATE composites to obtain a rolling one-hour rainfall accumulation in millimetres.
This makes ACRR the most direct of the three layers for asking where rainfall has persisted or accumulated during the preceding hour. But it also inherits uncertainty from each RATE input. A missing or biased contributing frame can affect the one-hour result, so the product time, interval, data gaps, and quality context still matter.
Use ACRR when the question is “How much rainfall did the radar products estimate over the previous hour?” For safety-critical flood or warning decisions, consult the responsible national meteorological or hydrological authority rather than relying on this independent interface.
Quality information helps, but it does not remove every artefact
Both official product documents stress dependence on the availability and timeliness of national radar inputs. The NIMBUS documentation also notes that low-reflectivity noise may remain and that quality filters are selected by data providers rather than applied identically to every input. A blank area can represent unavailable data or a radar limitation; a faint echo can represent light precipitation, noise, or another non-meteorological target.
Quality indices are useful context, not an error-free truth flag. The CIRRUS and NIMBUS documents describe indices from 0 to 1, while warning that their calculation may be revised or should be used cautiously. Interpretation should combine the variable, map pattern, time sequence, quality information, nearby observations, and official warnings.
Climate Explorer makes selected OPERA frames easier to access
Climate Explorer provides an independent interface for the three OPERA composites. It does not create the official CIRRUS or NIMBUS products and is not affiliated with EUMETNET. The explorer lets a reader select DBZH, RATE, or ACRR, move through catalogued UTC timestamps, inspect the mapped field, and download the selected frame as a georeferenced GeoTIFF.
The export is designed for QGIS, ArcGIS, GDAL, Python, R, and other tools that understand GeoTIFF. It covers the full composite raster rather than only the current map viewport and carries the spatial metadata needed to place the grid. Recent frames are delivered from their source Cloud Optimized GeoTIFF (COG) when the catalogue marks that file ready. For an archived frame, the service reconstructs the selected full-grid time slice from GeoZarr as a one-band floating-point GeoTIFF.
This distinction matters for reproducibility: a recent source COG can contain a different band structure from a generated archive GeoTIFF. The exported measurement should be interpreted using the chosen product and unit, and display opacity or a visual quality threshold should not be assumed to have changed the downloaded values.
Explore or verify the products: Open the OPERA Radar Explorer, choose DBZH, RATE, or ACRR, select a catalogued UTC frame, and compare the map with the product definition above. On a desktop layout, use Download GeoTIFF in the Current frame panel for that selected frame. The OPERA Radar Guide documents the controls, pixel inspection, CSV workflow, and export differences in more detail.
Easy technical access does not replace the upstream terms. EUMETNET documents licensing routes for third-party research, education, and commercial use. Anyone redistributing or using the files beyond personal inspection should check the applicable OPERA, EUMETNET, ECOMET, or national-service terms for the intended use.
Choose the product from the question, not the colour scale
The simplest reliable rule is to begin with the quantity you need:
- choose DBZH for the strongest radar echo and storm structure;
- choose RATE for an estimated surface rainfall intensity at one product time;
- choose ACRR for the radar-estimated rainfall accumulated during the preceding hour.
All three products add valuable continental context, but none removes the physical and operational limits of radar. Treat gaps, faint echoes, extreme values, and point-level interpretations with care, and use official warnings and local observations for consequential decisions.
Editor note: OPERA product specifications were checked against the June 2024 CIRRUS and NIMBUS documents. Climate Explorer access and export behaviour was verified against the application implementation on 3 August 2026.
Frequently Asked Questions
Is DBZH the same as rainfall intensity?
No. DBZH is the maximum radar reflectivity selected within the contributing scans for a composite pixel. It can highlight intense storm structure, but RATE is the OPERA product that converts low-level reflectivity into an estimated surface rain rate.
What is the difference between RATE and ACRR?
RATE is an instantaneous surface rain-rate estimate in mm/h on a 15-minute product grid. ACRR is a rainfall total in mm calculated from the previous four 15-minute RATE composites, representing the preceding hour.
Can I download an OPERA radar frame as GeoTIFF?
Yes. In the Climate Explorer OPERA Radar Explorer, select DBZH, RATE, or ACRR and a catalogued UTC frame, then use Download GeoTIFF in the Current frame panel on the desktop layout. Recent files are delivered from the source COG when available; archived frames are generated from GeoZarr.
Does Climate Explorer produce the OPERA radar composites?
No. CIRRUS and NIMBUS are official EUMETNET OPERA production lines. Climate Explorer provides an independent visualization and selected-frame access layer and is not affiliated with EUMETNET.
Data Annex
| Product | Official production line and centre | Quantity | Unit | Grid and time basis |
|---|---|---|---|---|
| DBZH | CIRRUS, Météo-France | Maximum reflectivity selected from contributing scans | dBZ | 1 km; 5-minute product |
| RATE | NIMBUS, GeoSphere Austria | Radar-derived instantaneous surface rain-rate estimate | mm/h | 2 km; 15-minute product |
| ACRR | NIMBUS, GeoSphere Austria | Sum of the previous four RATE composites | mm | 2 km; rolling preceding hour on a 15-minute product grid |
: Product definitions used in this explainer. Temporal resolution does not guarantee zero delivery latency.
| Access or interpretation question | Verified behaviour or limitation |
|---|---|
| Upstream product owner | EUMETNET OPERA; Climate Explorer is an independent interface. |
| Selected-frame export | Full-composite georeferenced GeoTIFF for the chosen catalogued DBZH, RATE, or ACRR frame. |
| Recent frame | Source COG is delivered when the catalogue marks it ready. |
| Archive frame | The selected GeoZarr measurement slice is generated as a one-band floating-point GeoTIFF. |
| Display controls | Map opacity and visual thresholds are presentation settings and are not export parameters. |
| Reuse | Technical access does not replace the applicable OPERA/EUMETNET/ECOMET or national-service terms. |
: Provenance and access details needed when interpreting or reusing a downloaded frame.
Data Sources
- EUMETNET OPERA data production and access: production-line roles, operational context, data model, and third-party access routes.
- OPERA CIRRUS Maximum Reflectivity Product Sheet, edition 2.0: grid, time step, production method, quality index, and limitations.
- OPERA NIMBUS Product Datasheet, 13 June 2024: RATE and ACRR definitions, processing, quality information, and limitations.
- NOAA radar overview and reflectivity explainer: accessible background on radar returns, dBZ, and the effect of target properties on reflectivity.