Rainfall Map Information

Rainfall

The rainfall analyses and associated maps use data contained in the Bureau of Meteorology climate database, the Australian Data Archive for Meteorology (ADAM). The analyses are initially produced automatically from real-time data with limited quality control. They are intended to provide a general overview of rainfall across Australia as quickly as possible after the observations are received.

What is rainfall?

According to the American Meteorological Society's Glossary of Meteorology, rainfall is the amount of precipitation of any type (including the liquid equivalent of frozen hydrometeors such as hail and snow); usually taken as that amount measured by means of a rain gauge in millimeters of liquid water depth over a specified period of time. For Australia, rainfall is typically measured for a 24-hour period.

A more accurate term would be precipitation or precipitation amount. However, the broad use of 'rainfall' is firmly established in meteorology.

Daily rainfall maps

Daily rainfall for the current day is the 24-hour total rainfall from local time 9am the day before to 9am the current day. At about 1:30pm EST, these rainfall values from about 3000 sites across the country are analysed onto 0.25x0.25 and 0.05x0.05 degree grids with limited quality control.

The national map shown on the web is based on the 0.05x0.05 degree grid, sub-sampled at every fifth point to give an effective resolution of 0.25x0.25 degrees. The regional maps are based directly on the 0.05x0.05 degree grids, so there may be some differences in the fine detail between the national map and the regional maps.

All analyses and maps are progressively updated over the following six months, as new data become available and as the data in the climate database are improved through quality control. The schedule of updates is available here, although additional unscheduled updates may occur from time to time. Subsequent versions will tend to be more accurate, as they will be based on larger quality-controlled input datasets and will contain more data from non-real-time reporting sites. A date stamp at the bottom right-hand corner of each map indicates when the analysis was produced.

An additional set of daily rainfall analyses and maps is also available, the 'recalibrated' daily rainfall products. The ordinary daily and monthly rainfall analyses (see below) use different spatial scales, and in most cases sums of daily rainfall grids are only approximately equal to the corresponding monthly analyses. The 'recalibrated' daily rainfall analyses are derived from grids which have been scaled so that they sum to equal the corresponding monthly rainfall analyses. Accordingly, the 'recalibrated' daily rainfall maps are only generated once the complete monthly rainfall analysis becomes available at the end of the month.

The daily rainfall gauge network has varying density across the country, and is in some places not dense enough to support a daily rainfall analysis. In some of the daily rainfall maps, these data 'voids' are shown in grey shading. As the daily rainfall gauge network varies over time, the shaded data void regions likewise change over time.

Monthly and multiple-monthly rainfall maps

The latest rainfall maps for periods of one calendar month and longer are usually produced within the first few days of the new month and updated several times at later dates. The schedule of updates is available here. Updates will typically be more accurate, as they will be based on larger and more accurate input datasets. A date stamp at the bottom right-hand corner of each map indicates when the analysis was produced.

Analyses over 3, 6 and 12 months or longer are based on the summation of the one-month grids which comprise the period in question.

Other rainfall variables

For information on the various rainfall analysis variables mapped, use the following links - amounts, deciles, drought, percentages and anomalies. The rainfall deciles and drought maps based on a reference period comprising all gridded monthly analyses from 1900 to the present available at the time the deciles are calculated. The rainfall percentages and anomaly maps are calculated with respect to the 30-year reference period 1961-1990.

Analysis technique

The analyses are computer generated using a sophisticated analysis technique described in Jones et al. (2009). This method uses an optimised Barnes successive correction technique that applies a weighted averaging process to the station data. Topographical information is included by the use of rainfall ratio (actual rainfall divided by monthly average) in the analysis process. On the maps each gridpoint represents an approximately square area with sides of about 5 kilometres (0.05 degrees). The size of the grids is limited by the data density across Australia.

This gridpoint analysis technique provides an objective average for each grid square and enables useful estimates in data-sparse areas such as central Australia. However, in data-rich areas such as southeast Australia or in regions with strong gradients, 'data smoothing' will occur resulting in gridpoint values that may differ slightly from the exact rainfall amount measured at the contributing stations.

Map formats

Most of these rainfall maps are produced as both colour and black/white GIF images, with low and high resolution versions available in each case. The low resolution colour GIF images are the ones usually displayed, with links to the other three types placed under the main image. Place names are generally to be found on the high resolution versions. Portable Document Format (PDF) versions of the images are also generated for high-quality printing. Please note however that PDF versions is not archived for reasons of space. PDF versions of older maps may be obtained from the National Climate Centre via feedback form, but charges may be imposed for their provision.

Map Projections

The map projections used are either Cylindrical Equidistant (CE) or Lambert Conformal (LC). The Lambert Conformal projection takes three parameters; the central longitude (in degrees east of the Greenwich Meridian) and two standard parallels of latitude (in degrees south of the equator).

Region Aus. Qld NSW Vic. Tas. SA WA NT
Map projection LC 134° 10°, 40° CE CE LC 140.8° 10°, 40° LC 146.5° 10°, 44° CE CE CE

The Victoria and Tasmania maps are based on a finer resolution analysis than the remaining maps. Consequently there may be slight inconsistencies in the detail represented on the Vic./Tas. maps as compared against the Aus./SA/NSW maps.

Grid format

Daily, weekly and monthly rainfall totals grids may be downloaded from the Bureau's website. These grids are in an ASCII format suitable for ingesting into geographic information systems (GISs), compressed using the UNIX compress utility. The ASCII grids have appended to them their original AIFS ASCII grid header (a Bureau of Meteorology grid format), to provide additional grid metadata. Note that some GISs may require the user to change the grid file extension from '.grid' to '.txt', prior to ingestion into the GIS.

Quality control

The analyses use data collected through electronic and paper communication channels. These data have been screened for errors, using an automated technique, and make use of quality control which has been undertaken on the climate database. Full quality control is completed some weeks to months after the end of the most recent month when (a) extreme values are confirmed by written reports, and (b) data more generally are compared with those of nearby stations so that values and dates of occurrences are similar.

Occasionally in the data-sparse areas, errors may enter the analyses because they cannot be detected by comparison with other reports. In these instances, the erroneous maps will be amended as soon as is practicable.

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