About Radar-Derived Rainfall Accumulations

Radar-Derived Rainfall Accumulations

Radar-derived rainfall accumulation products are images which represent the estimated total rainfall over a particular duration (in 5 mins, in 1 hour, in 24 hours, and since 9 am).

Figure 1. Example of a frame from a loop of accumulation of rain over the last 24 hours.

The radar-derived rainfall accumulations are available on the radar viewer, along with the radar reflectivity products. They are available for the following radars in the Bureau’s network:

• Adelaide (Buckland Park)
• Adelaide (Sellicks Hill)
• Brisbane (Mt Stapylton)
• Canberra (Captains Flat)
• Gympie (My Kanigan)
• Hobart (Mt Koonya)
• Kalgoorlie
• Melbourne
• Mount Isa
• Newcastle
• Newdegate
• Perth (Serpentine)
• South Doodlakine
• Sydney (Terrey Hills)
• Watheroo
• N. W. Tasmania (West Takone)
• Wollongong (Appin)

What do the images show?

• For the 5 minute product, there is a loop of 7 images, updated every 5 minutes. Each image is of 5 minutes accumulated rain.
• For the 1 hour product there is a loop of 7 images, updated every 5 minutes. Each image is of 1 hour accumulated rain
• For the since 9am product, there is a loop of 7 images, updated every 15 minutes. Each image is an accumulation of rain since 9am local time.
• For the 24-hour product, there is a loop of 7 images, updated every 24 hours at 9 am local time. Each image is a 24 hour accumulation of rain.

Colour palette

The colour palette for the radar-derived rainfall accumulations is different from the colour palette for the current radar images, to distinguish clearly between radar observations and radar-derived products. The palette is based on the hydrological rain gauge palette, as seen on the Bureau's Rainfall and River Information webpage.

Figure 2. Colour palette for radar-derived rainfall accumulations.

Figure 3. Hydrological rainfall and river observation colour palette.

How the rainfall accumulations are made

Rain gauges accurately measure rainfall at the points they are situated, but don't capture variation over an area. In contrast, radars have a good geographic coverage, but measure precipitation in the atmosphere rather than the amount of rain reaching the ground.

The rainfall accumulation images are made by blending measurements from rain gauges (accurate point data) and radar (excellent spatial coverage). Radar data is calibrated with rain gauge data and is used to fill in the "gaps" between rain gauges. The use of radar information in combination with rain gauge measurements helps to improve rainfall estimates over those based on either form of measurement alone.

The rain gauges record rain accumulation every 15 minutes. The rainfall accumulation images are computed as a 5 minute accumulation so that the radar rainfall estimates can be adjusted in real time using the 15 minute rain gauge data. This provides more accurate rainfall accumulation for radars with large numbers of rain gauges (e.g. capital cities).

Learn about how the radar-derived rainfall accumulations are calculated

Accuracy of the rainfall accumulations

The products are dependent on accurate radar reflectivity, accurate rain gauge measurement, and error and bias correction. With these limitations, the reliability of the images is currently assessed in the number of radar volume scans available to create the image. This is expressed as a fraction. For example, a fraction of 0.3 is the fraction of images (30%) that are missing during an accumulation period. This can occur for different reasons, such as radar outages or communications link outages. This statistic is shown in the line of text at the bottom of each image. A large number of missing scans means that the accuracy of the image is likely to be reduced.

Figure 4. Example of a frame from the 24 hour rainfall accumulation in Brisbane, showing the fraction of images missing.