Daily Rain Gauge Precipitation

Disclaimer: This page is a copy of the record for the BOM AGCD dataset once held by TERN AusCover. It may contain outdated information. Users should verify any key information with the BOM.

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Overview

These datasets are gridded analyses of daily rainfall measurements from the Bureau of Meteorology's rain gauge network across Australia.

Monthly datasets represent the accumulation of the daily values over the observed month.

Key specifications

Background

What is rainfall?

According to the American Meteorological Society's https://glossary.ametsoc.org/wiki/Rainfall , 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.

Rainfall Data

The gridded rainfall data is derived from daily rainfall values from several thousand stations across the country. These data are contained in the Bureau of Meteorology climate database, the Australian Data Archive for Meteorology (ADAM). The daily rainfall data represents the amount of precipitation of any type observed by means of rain gauges stationed across Australia measuring millimeters of liquid water depth over a 24-hour period. The 24-hour period runs from local time 9am the day before to 9am the current day.

At about 1:30pm AEST each day these rainfall values from sites across the country are analysed onto a 0.05° × 0.05° grid with limited quality control. The Bureau of Meteorology rainfall products are progressively updated over the following six months, as new data become available and as the data in the Bureau of Meteorology climate database are improved through quality control. Hence, the highest quality products are a least six months old because 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. TERN provides the data after the last of these updates, to ensure stable values and best quality, and so the most recent data is 6 months before the present.

Monthly observations represents accumulation of the 24-hour values over the observed month.

Algorithm

The analyses (grids) are computer generated using a sophisticated analysis technique described in Jones et al. (2009). It incorporates an optimised Barnes successive correction technique that applies a weighted averaging process to the station data. The meteorological variable being analysed is decomposed into a long-term average (climatological) component and an anomaly component. Topographical information is included by the use of rainfall ratio (actual rainfall divided by monthly average) in the analysis process.

Quality

Quality and limitations

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.

The observational (station) data on which the analyses were based have an associated positional accuracy of the order of 0.01 degrees (approximately 1km) or better.

The grid-point analysis technique provides an objective average for each grid square and enables useful estimates in data-sparse areas such as central Australia. The size of the grid is limited by the data density across Australia. However, in data-rich areas such as southeast Australia or in regions with strong gradients, "data smoothing" will occur resulting in grid-point values that may differ slightly from the exact rainfall amount measured at the contributing stations.

The highest quality products are a least six months old because the rainfall products are progressively updated over the following six months after the observation date. Rainfall site stations can take time to report, hence, as new data become available the product quality control improves.

Data completeness

Temporal coverage is complete.

The grids are derived only from land-based observations. Although the entire grid domain is populated with values, over ocean the values are valid only up to one or two grid cells beyond the coast.

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. Thus some of the daily rainfall grids include these data 'voids', indicated as regions of missing data values. As the daily rainfall gauge network varies over time, the void regions of missing data likewise change over time. The monthly dataset does not have such voids, because monthly rainfall is representative of a larger spatial region than is daily rainfall and so the monthly analyses are based on a larger spatial scale.

Validation

The accuracy of the spatial analyses has been determined through a cross-validation procedure which repeatedly deleted 5% of the stations at a time and calculated the errors in an analysis of the remaining stations. The daily rainfall values have a root mean square error of 3.1 mm and a mean absolute error of 0.9 mm. The monthly rainfall values have a root mean square error of 21.2 mm and a mean absolute error of 11.5 mm. Details are in Jones et al. (2009).

Product variants

Gridded rainfall data are available back to 1900 from the Bureau of Meteorology's Rainfall maps web page.

Rainfall Deciles

Deciles are used to give an element a ranking. For example, a decile rainfall map will show whether the rainfall is above average, average or below average for the time period and area you have already chosen.

Rainfall Percentages

One useful way of representing rainfall across a wide area like Australia is in terms of percentages of a mean rainfall over some base period. Percentages between 0% and 100% indicate lower than average rainfalls, while percentages greater than 100% indicate higher than average rainfalls. In some northern parts of the country, dry season rainfalls are often very low, and weeks or even months may go by without any rainfall being recorded. In these places and times of the year, the monthly rainfalls may be below average (that is, percentages less than 100%) in rather more than 50% of years.

Rainfall Anomalies

Anomalies denote the departure of an element from its long-period average value for the location concerned.

Drought

Drought in general means acute water shortage. The drought maps highlight areas considered to be suffering from a serious or severe rainfall deficiency. These classes are assigned by first examining rainfall periods of three months or more for selected places throughout Australia to see whether they lie below the 10th percentile (lowest 10% of records). The terms serious and severe are defined by:-

• Serious rainfall deficiency:- rainfall lies above the lowest five per cent of recorded rainfall but below the lowest ten per cent (decile 1 value) for the period in question,

• Severe rainfall deficiency:- rainfall is among the lowest five per cent for the period in question.

Areas where the rainfall is lowest on record for the given time period are also shown.

1-year Difference

Interannual rainfall difference from current year to previous year.

2-year Difference

Interannual rainfall difference from current year to 2 years ago.

3-year Difference

Interannual rainfall difference from current year to 3 years ago.

Other specifications