By Tad Stoner
Cayman’s Doppler radar system for weather prediction is essentially the same, with slight variations, as that used in police speed guns, healthcare and radiology, satellites and surface-to-air missiles.
In its basics, it operates like any radar/sonar system, shooting out an electromagnetic signal, then interpreting the results when it bounces back, determining the size and velocity – a measure of both speed and direction – of the target.
Doppler systems can be light enough for mobile ground surveillance, used by infantry, surface ships and the police, detecting motion from vehicles and personnel, enabling night and all-weather combat operation.
In one configuration, it is a high-resolution velocity system, efficiently locating and tracking the movement of weather, “detect[ing] radial wind speeds within storms to better determine [their] strength,” says John Tibbetts, director general of the National Weather Service.
“The Cayman radar is a dual pole radar,” he says, “meaning that the wave it sends out senses both on the horizontal and vertical axis,” yielding detail unavailable in older radars.
Radial wind speeds are measured along a straight line in regard to the radar tower and the electromagnetic waves it produces. As the wind crosses that line at an angle, it reduces estimates of the wind speed, but nonetheless helps determine the direction of the wind and the overall strength of the storm.
Tibbetts said the Doppler system – named for Austrian physicist Christian Doppler in 1842 – helped detailed prediction of weather through detection of motion and direction: “It helps in ‘nowcasting,’ short-term weather forecasting/warnings,” he said.
Construction of the single, €4.1 million, 60foot tower in East End – on a fenced site with minimal bush at High Rock Road, where the old prison farm used to be – started in 2012. The system opened in 2013.
Engineers mounted a dish and a dome on the tower, which, Tibbetts said, “provides advance near ‘in situ’ measuring of meteorological variables within approaching weather phenomena for the Cayman Islands, NHC [the National Hurricane Committee] and others, as well as to fill in a gap in radar coverage over the northwest Caribbean.”
The “variables” the system sought to measure included “radial wind, vertical structure, shear/turbulence and rainfall … to name a few,” he said.
The purely radar function detects rainfall. It works by bouncing a beam off objects – such as rain drops – then reading the returning signal. Its Doppler feature uses the rainfall data to determine upper-level wind speeds, “analyzing how the frequency of the returned signal has been altered by the object’s motion,” according to one summary.
A wide perception that Doppler radar is used in air traffic control is only partially true, Tibbetts says.
“It’s not used directly by air traffic,” he says, “but advance warnings affect ground operations.”