About Radar-Derived Rainfall Accumulations

Introduction

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. This improvement is accomplished by adjusting, or calibrating, radar-rainfall data with data from rain gauges situated within the radar boundary. The rain gauge data allows forecasters to calibrate the radar data in the form of ground truth, and the radar data allows us to fill in the "gaps" between rain gauges.

Rain gauges accurately measure rainfall on the ground at point locations. Generally, rain gauges are distributed evenly across catchment areas, but there may be many kilometres between each gauge. In contrast, radar reflectivity represents precipitation occurring in the atmosphere over a large geographic area, and therefore provides good spatial coverage. However because weather radars do not point at the ground, radar reflectivity does not accurately represent rainfall on the ground. The radar-derived rainfall accumulations combine the benefits of both these systems - the accurate point data from the rain gauges and the excellent spatial coverage of the radar.

Radar-Derived Rainfall Accumulations

Radar-derived rainfall accumulation products are now available for the Bureau's Doppler radars in:

  • Adelaide (Buckland Park)
  • Brisbane (Mt Stapylton)
  • Canberra (Captains Flat)
  • Hobart (Mt Koonya)
  • Melbourne
  • Mount Isa
  • Newcastle
  • Sydney (Terrey Hills)
  • Wollongong (Appin)

These products are images which represent the estimated total rainfall over a particular duration (in 6/10 mins, in 1 hour, in 24 hours, and since 9am).

Number of products 4 for each radar
Types of products

Rainfall accumulations over a given duration:
- in 6 mins (all the above sites bar Adelaide)
- in 10 mins (Adelaide only)
- in 1 hour
- since 9am
- in 24 hours

Units mm
Coverage area 128km radius centred around the radar location

Radar-derived rainfall accumulations are used by meteorologists in weather forecasting, and by hydrologists for flood modelling and flash flood warning. Emergency services also use this product to ascertain if flood mitigation action may be required. The radar-derived rainfall accumulations are geo-referenced, meaning that each rain pixel in the image has a latitude and longitude, so the products can be overlayed onto a map.

Viewing The Radar-Derived Rainfall Accumulations

The radar-derived rainfall accumulations are accessible via an enhanced radar viewer. This viewer allows users to see looped images or the most current image, and offers access to the regular radar reflectivity products as well as the new accumulation products.

  • For the 6 minute product, there is a loop of 6 images, updated every 6 minutes. Each image is of 6 minutes accumulated rain.
  • For the 10 minute product, there is a loop of 4 images, updated every 10 minutes. Each image is of 10 minutes accumulated rain.
  • For the 1 hour product in Brisbane and Melbourne, there is a loop of 6 images, updated every 6 minutes. Each image is of 1 hour accumulated rain.
  • For the 1 hour product in Adelaide, there is a loop of 4 images, updated every 10 minutes. Each image is of 1 hour accumulated rain.
  • For the since 9am product, there is a loop of 7 images, updated every 30 minutes on the hour & the half hour. Each image is an hourly accumulation of rain since 9am.
  • 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.

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

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

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.

Current colour palette

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

Hydrological colour palette

Figure 3. Hydrological Rainfall and River observation colour palette.

Accuracy Of The Radar-Derived Rainfall Accumulations

The science behind radar-derived rainfall accumulations is still relatively new and the products are dependent on accurate radar reflectivity, accurate rain gauge measurement, and error and bias correction. With these limitations, the reliablility of the images is currently assessed in the following terms:

— the number of radar volume scans available to create the image, expressed as a percentage of the expected scans that were missing. This statistic is shown in the line of text at the bottom of each image. A large percentage missing indicates that the accuracy of the image is likely to be reduced because of missing radar information.

— the difference between the rain-gauge totals and the radar-rainfall accumulation at the same points, expressed as a root-mean-square error over the network or rain-gauges around the radar. This statistic is calculated for each image and is available to Registered Users.