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About DMI’s Weather Radar Data
DMI operates a network of five weather radars, located at Bornholm, Rømø, Sindal, Stevns and Virring, to provide optimal radar coverage of Denmark and its surroundings as shown in the figure below. The radars are designed to observe precipitation in the atmosphere at distances of up to 240 km from each radar and at heights of up to several kilometers. It should be noted that precipitation falling on the ground is measured by rain gauges as point measurements whereas the radar provides a measure of precipitation in three-dimensional volume cells in the atmosphere.
The radar data (radardata) API service contains radar data from DMI's radar stations located in Denmark for the past 180 days.
Weather radars make use of the radar principles to observe precipitation. Short electromagnetic pulses are transmitted in a structured set of directions and elevations. When a radar pulse impacts precipitation a small fraction of its energy is reflected and detected by the radar. This is converted to the basic radar observation of so-called radar reflectivity. Other weather radar parameters are also collected, for example the Doppler radial velocity, differential reflectivity and cross correlation which can be useful in advanced radar applications.
In the current operational setup the atmosphere is scanned by rotating the radar’s antenna 360 degrees at fixed elevation angles starting from 0.5° to 15°. A complete scan typically takes about 3 minutes to complete and is repeated at a regular interval of 5 minutes.
Available Radar Data
Three types of radar data are available:
The composite provides the best overview of radar reflectivity by combining radar images from all of DMI’s weather radars into one composite image of the radar reflectivity. In areas of overlap the maximum value is chosen. The time interval of data is 5 minutes and the pixel size is 500 m. On minute numbers 0, 10 ..., 50, a composite based on conventional full range scans is generated while the minute numbers 5, 15, ..., 55 are reserved for a composite based dedicated Doppler scans.
The full range scan composite covers a larger area than the Doppler scan composite due to differences in the range of the full range scan and the Doppler scan (240 km and 120 km, respectively).
Single Radar Images - pseudo CAPPI
Single radar images shows the reflectivity close to the earth’s surface, at a minimum height of 500 m, and are obtained by combining information from the individual elevations of the scan. The time interval of data is 10 minutes and the pixel size is also 500 m.
Volume files are the most basic and raw product and contains the complete dataset as collected by each weather radar. The data is in polar coordinates and is primarily useful for advanced applications. In the current operational setup, a new volume file is available every 5 minutes. On minute numbers 0, 10 ..., 50, a conventional full range scan is collected while the minute numbers 5, 15, ..., 55 are reserved for a dedicated Doppler scan which provides the best Doppler radial velocity observations. In the full range scan and the Doppler scan, the raw data are measured to a distance of 240 km and 120 km, respectively.
File Format and Data Model
Radar data from DMI is available in the HDF5 file format and follows the OPERA Data Information Model (ODIM) specification defined by EUMETNET’s radar programme OPERA.
Information about OPERA can be found here: https://www.eumetnet.eu/activities/observations-programme/current-activities/opera/ and the data model specification can be found here: https://www.eumetnet.eu/wp-content/uploads/2019/05/OPERA-ODIM_H5-v2.01.pdf for volume and composite and https://www.eumetnet.eu/wp-content/uploads/2019/05/OPERA-ODIM_H5-v2.2.pdf for pseudo CAPPI.
Radar Reflectivity and Precipitation Intensity
The radar reflectivity is related to precipitation intensity in an empirical way and converting from reflectivity to intensity can be very uncertain. As a rule of thumb, reflectivity around 20 dBZ is light precipitation (<1 mm/hour), 35 dBZ is moderate precipitation (approx. 5 mm/hour), and above 45 dBZ is heavy precipitation (above 20 mm/hour) or hail.
To compute precipitation intensity from radar reflectivity a standard relation such as the Marshall-Palmer formula can be used.
The radar’s electromagnetic pulses are frequently reflected by other objects than precipitation. For example, the land and sea surface, buildings, wind turbines and migrating birds reflect the radar’s pulses and are recorded by the radar. These non-precipitating echoes are called radar clutter. External electromagnetic signals from wireless computer networks and the sun also give rise to radar clutter.
Radar clutter is present in the volume files as the radar records it, however for the single radar and composite images the data have gone through a range of image processing algorithms to attempt to remove as much radar clutter as possible. However, a perfect removal of clutter is not possible and thus clutter will be present in the radar images and at the same time echoes from precipitation can in some cases have been incorrectly removed from the radar images.
More information about weather radar can be found here: https://en.wikipedia.org/wiki/Weather_radar
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