RadarScope displays data from 51 radars operated by the Australian Bureau of Meteorology. The product suite differs from one radar to the next. Some have reflectivity, velocity, and spectrum width. Others offer reflectivity and velocity, and a few have reflectivity only.
Doppler radars work by bouncing radio waves off particles in the air. Those particles could be raindrops, hail, snow, or even dust and insects. The amount of energy that bounces off of those particles and returns to the radar is called “reflectivity” and is represented by the variable “Z”. Reflectivity covers a wide range of signal strength, from very weak to very strong, so it is measured on a decibel (logarithmic) scale in units of dBZ, or decibels of Z. The higher the dBZ value, the larger the number and/or size of the particles the radar beam is seeing.
The dBZ values increase as the strength of the signal returned to the radar increases. The scale of dBZ values is related to the intensity of rainfall. It is important to remember, however, that the radar shows only areas of returned energy and not necessarily precipitation. So the presence of a return, especially a very weak return below 20 dBZ, doesn’t always mean that it’s raining.
The colors along the bottom of the map correspond to precipitation types and intensities. When you move your cursor across the squares, RadarScope will display a value for each color. NEXRAD radars can’t distinguish between different types of precipitation with absolute certainty. However, reflectivity values can be somewhat roughly associated with different precipitation types:
• 10 dBZ (blue) – Very light rain or light snow
• 20 dBZ (green) – Light rain or moderate to heavy snow
• 30 dBZ (yellow) – Moderate rain or sleet showers
• 40 dBZ (orange) – Moderate to heavy rain or sleet showers
• 50 dBZ (red) – Heavy thunderstorms
• 60 dBZ (pink) – Intense to severe thunderstorms with hail
The Australian super-res reflectivity product has a resolution of 0.25 kilometers by 0.5 degrees, it provides the highest-resolution reflectivity available from Australian radars to a distance of 230 kilometers from the radar. Super-res reflectivity is available at four different tilts or beam angles, with tilt 1 being the lowest to ground level.
Doppler velocity products indicate storm motion toward or away from the radar. The velocity products in RadarScope use the Doppler effect to determine how fast the particles in the air are moving relative to the radar itself. Negative values (green in RadarScope) indicate motion toward the radar, while positive values (red in RadarScope) indicate motion away from the radar. They can be difficult to interpret without training and experience, but Doppler velocity products can be used to detect the overall movement of a storm as well as relative motion within the storm itself, such as rotation. Note that the radar can only detect the component of the velocity vector along the radar beam, so this isn’t a full picture of the wind field. But it gives you a fairly good idea which way a storm is heading. The Australian super-res velocity product has a resolution of 0.25 kilometers and a range of 230 kilometers.
Super-Res Spectrum Width
Within any volume that is sampled by a radar, there can be a wide range of motions being observed. Spectrum width is a measure of that variation. Higher values of spectrum width correlate to a wider range of velocities being observed (turbulent flow; e.g. mesovortices); lower values indicate a narrower range (smooth flow; e.g. straight line winds). With proper interpretation, spectrum width can provide an indication of turbulence, which can be helpful in identifying conditions associated with severe thunderstorm activity.