![]() ![]() Because the radar has an unobstructed view of Port Phillip Bay out to about 25km, sea-clutter may be seen over the bay during strong winds. These usually become more noticeable on cold, clear, winter nights or early winter mornings. These echoes usually occur along ridges and peaks as isolated, stationary patches, being most common near the Yarra Ranges to the ENE and Mount Macedon to the north. Occasionally, some permanent echoes will not be completely removed from the display. Non-meteorological aspects In most cases the processing of the radar signal removes permanent echoes caused by obstructions such as hills, buildings and other solid objects rather than rainfall. The location on the floor of a wide basin is ideal for Doppler observations which provide wind speed information. Similarly, cold fronts and associated rain and thunderstorms approaching from the northwest, through west and south are well detected. Summer thunderstorms that develop on the surrounding hills and mountains may be observed in detail. The high ground from the east, through north and to the south west tends to obscure shallow rain falling further away. Meteorological Aspects The radar is well sited to provide very good coverage for the Greater Melbourne Metropolitan Area. The most significant obstructions, starting from the east, are: Mt Dandenong 644m high, 53km to the east Yarra ranges 1219m, 83km to ENE Mt Macedon 1013m, 56km to the north Pentland Hills 763m and the Brisbane ranges 421m, 45km from the NNW through to the west and the You Yangs 363m, 30km to the SW. The Great Dividing Range dominates the topography from the east, through the north to the west. ![]() ![]() ![]() The radar is on a tower 24m above ground level. 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.Geographical Situation The radar is situated on the western plains of the Melbourne basin some 19km west-south-west of the Central Business District, about six kilometres from the western shores of Port Phillip bay and on a low rise about 20m above mean sea level. Hail is a good reflector of energy and will return very high dBZ values. These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rainrates which are associated to the dBZ values. The higher the dBZ, the stronger the rainrate. Typically, light rain is occurring when the dBZ value reaches 20. The scale of dBZ values is also related to the intensity of rainfall. The value of the dBZ depends upon the mode the radar is in at the time the image was created. Notice the color on each scale remains the same in both operational modes, only the values change. The other scale (near left) represents dBZ values when the radar is in precipitation mode (dBZ values from 5 to 75). One scale (far left) represents dBZ values when the radar is in clear air mode (dBZ values from -28 to +28). Each reflectivity image you see includes one of two color scales. The dBZ values increase as the strength of the signal returned to the radar increases. So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used. Reflectivity (designated by the letter Z) covers a wide range of signals (from very weak to very strong). "Reflectivity" is the amount of transmitted power returned to the radar receiver. The colors are the different echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan. ![]()
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