AUSTRALIA WEATHER RADAR TODAY
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The dBZ Scale
The colors on the legend are the different echo intensities (reflectivity) measured in dBZ. "Reflectivity" is the amount of transmitted power returned to the radar receiver. Reflectivity covers a wide range of signals (from very weak to very strong). So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used.
The dBZ values increase as the strength of the signal returned to the radar increases. Each reflectivity image you see includes one of two color scales. One scale represents dBZ values when the radar is in clear air mode (dBZ values from -28 to +28). The other scale represents dBZ values when the radar is in precipitation mode (dBZ values from 5 to 75).
The scale of dBZ values is also related to the intensity of rainfall. Typically, light rain is occurring when the dBZ value reaches 20. The higher the dBZ, the stronger the rainrate. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rain rates which are associated to the dBZ values. These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Hail is a good reflector of energy and will return very high dBZ values. Since hail can cause the rainfall estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.
Ground Clutter, Anomalous Propagation and Other False Echoes
Echoes from objects like buildings and hills appear in almost all radar reflectivity images. This "ground clutter" generally appears within a radius of 25 miles of the radar as a roughly circular region with a random pattern. An mathematical algorithm can be applied to the radar data to remove echoes where the echo intensity changes rapidly in an unrealistic fashion. These "No Clutter" images are available on the web site. Use these images with caution; ground clutter removal techniques can remove some real echoes, too.
Under highly stable atmospheric conditions (typically on calm, clear nights), the radar beam can be refracted almost directly into the ground at some distance from the radar, resulting in an area of intense-looking echoes. This "anomalous propagation " phenomenon (commonly known as AP) is much less common than ground clutter. Certain sites situated at low elevations on coastlines regularly detect "sea return", a phenomenon similar to ground clutter except that the echoes come from ocean waves.
Radar returns from birds, insects, and aircraft are also rather common. Echoes from migrating birds regularly appear during nighttime hours between late February and late May, and again from August through early November. Return from insects is sometimes apparent during July and August. The apparent intensity and areal coverage of these features is partly dependent on radio propagation conditions, but they usually appear within 30 miles of the radar and produce reflectivities of <30 dBZ.
However, during the peaks of the bird migration seasons, in April and early September, extensive areas of the south-central U.S. may be covered by such echoes. Finally, aircraft often appear as "point targets" far from the radar.
Base Reflectivity
This is a display of echo intensity (reflectivity) measured in dBZ. The base reflectivity images in Precipitation Mode are available at four radar "tilt" angles, 0.5°, 1.45°, 2.40° and 3.35° (these tilt angles are slightly higher when the radar is operated in Clear Air Mode). A tilt angle of 0.5° means that the radar's antenna is tilted 0.5° above the horizon. Viewing multiple tilt angles can help one detect precipitation, evaluate storm structure, locate atmospheric boundaries, and determine hail potential.
The maximum range of the "short range" base reflectivity product is 124 nautical miles (about 143 miles) from the radar location. This view will not display echoes that are more distant than 124 nm, even though precipitation may be occurring at greater distances.
Composite Reflectivity
This display is of maximum echo intensity (reflectivity) measured in dBZ from all four radar "tilt" angles, 0.5°, 1.45°, 2.40° and 3.35°. This product is used to reveal the highest reflectivity in all echoes. When compared with Base Reflectivity, the Composite Reflectivity can reveal important storm structure features and intensity trends of storms.
The maximum range of the "short range" composite reflectivity product is 124 nm (about 143 miles) from the radar location. This view will not display echoes that are more distant than 124 nm, even though precipitation may be occurring at greater distances.
Base Radial Velocity
This is the velocity of the precipitation either toward or away from the radar (in a radial direction). No information about the strength of the precipitation is given. This product is available for just two radar "tilt" angles, 0.5° and 1.45°. Precipitation moving toward the radar has negative velocity (blues and greens). Precipitation moving away from the radar has positive velocity (yellows and oranges). Precipitation moving perpendicular to the radar beam (in a circle around the radar) will have a radial velocity of zero, and will be colored grey. The velocity is given in knots (10 knots = 11.5 mph).
Where the display is colored pink (coded as "RF" on the color legend on the left side), the radar detected an echo but was unable to determine the wind velocity, due to inherent limitations in the Doppler radar technology. RF stands for "Range Folding".
Storm Relative Mean Radial Velocity
This is the same as the Base Radial Velocity, but with the mean motion of the storm subtracted out. This product is available for four radar "tilt" angles, 0.5°, 1.45°, 2.40° and 3.35°.
Determining True Wind Direction
The true wind direction can be determined on a radial velocity plot only where the radial velocity is zero (grey colors). Where you see a grey area, draw an arrow from negative velocities (greens and blues) to positive velocities (yellows and oranges) so that the arrow is perpendicular to the radar beam. The radar beam can be envisioned as a line connecting the grey point with the center of the radar. To think of it another way, draw the wind direction line so that the wind will be blowing in a circle around the radar (no radial velocity, only tangential velocity).
In order to determine the wind direction everywhere on the plot, a second Doppler radar positioned in a different location would be required. Research programs frequently use such "dual Doppler" techniques to generate a full 3-D picture of the winds over a large area.
sources : https://radblast.wunderground.com