Doppler Radar Weather Forecasting
Meteorologists Use Radar to Predict Storm Precipitation Intensity
Since the 1940s, meteorologists have used Doppler radar to detect storm intensity and movement, predict precipitation amounts and help give weather warnings and alerts.
Using Radar to Detect Weather
Radar, or RAdio Detection And Ranging, is the most effective tool used to detect and predict precipitation and storm movement by meteorologists. Radar has been used since the 1940s, available for public use after World War II. During the 1980s and 1990s the National Weather Service set up a weather surveillance system known as WSR-88D, or Weather Surveillance Radar - 1988 Doppler. There are 155 antennas used for radar detection located across the United States. A number of television stations have their own antennas for detecting and forecasting weather.
How Radar Detects Weather
Doppler radar, first discovered by Christian Doppler in 1842 by studying sound waves, uses electromagnetic radio waves to detect weather, particularly precipitation. As these radio waves are sent out from antennas, raindrops and other precipitation bounce the waves back at the antenna. These reflected waves are evaluated and plotted by a computer on a map. Radar can detect if an object is moving towards or away from the antenna, as the waves either become more frequent or decrease in frequency. Radar can detect precipitation but it requires evaluation of atmospheric conditions by meteorologists to determine if this precipitation is rain, snow, sleet or hail.
Types of Radar Images
The National Weather Service has identified six different types of radar images available from the WSR-88D.
- Reflectivity Images
- Base Reflectivity are considered the basic scan of the radar range, these images display the amount of power that is returned to the radar receiver. These radar images are used to detect precipitation, assess a storm’s structure and dimension.
- Composite Reflectivity images show the intensity of precipitation, using scans from all elevations and features of storm structure
- Velocity Images
- Base Velocity images are used to detect motion, either away from or towards the radar receiver, these images display the overall wind field, speed of fronts, and areas of strong wind. There is a maximum range that can be scanned from the radar location of 140 miles.
- Storm Relative Motion images have the same range as Base Velocity images. They show radial velocity of wind to a storm’s motion, which is important when evaluating rotating storms like supercell thunderstorms.
- Precipitation Images
- One-hour Precipitation images display basically what its name states – estimated one-hour accumulation amounts of precipitation. These images are important when evaluating flood warnings, rainfall intensities or other weather warnings.
- Storm Total Precipitation images show the estimated accumulation of precipitation since the last one-hour break. These are used to estimate run-off and potential flooding. These images are constantly being updated.
Limits of Radar
There are limits to using radar to predict and detect weather and precipitation. Radar cannot detect the height of precipitation. Precipitation that occurs but doesn’t reach the ground, called virga, is detected and recorded by radar. Mountains, trees or buildings can block the radar waves. Sea and ground clutter occur when radar waves reflect off the ocean or birds, planes and insects. Doppler radar also looses its ability to detect precipitation with increased distance. The curvature of the Earth plays a part in limiting weather detection by radar.
Australia’s Weather Watch and Windfinding
Australia’s Bureau of Meteorology uses weather radar coupled with computer analysis to present 3-D images of the sections within a storm. By analyzing each cross-section of a storm, meteorologists can more accurately pinpoint a storm’s intensity, movement and issue improved weather warnings.
