Solar exposure is an essential climate variable in the World Meteorological Organisation’s (WMO) Global Climate Observing System (GCOS), as solar energy is the primary source of energy for the earth-atmosphere system. Solar Exposure represents the incident solar energy per unit area of a surface.

Three different types of solar exposure are measured by the Australian Bureau of Meteorology:

• Direct solar exposure – the solar exposure incident on a given plane originating from a small solid angle centred on the sun’s disk
• Global solar exposure – the total solar exposure on a horizontal surface
• Diffuse solar exposure – the global solar exposure, minus the component due to direct solar exposure.

As a member of the WMO, Australia, through the Bureau of Meteorology, has been providing global and diffuse solar exposure data from its monitoring stations to the WMO World Radiation Data Centre since the mid-1960s.

The Bureau is the custodian and distributor of Australia’s climate archive, and has been providing Australian solar data since measurements have been available.

The focus of the Bureau’s solar monitoring network has been to establish a suitable solar climate record. The size of the network has changed over the years, depending on the community’s level of interest in solar energy.

The early years: surface measurements only

The Bureau’s first significant network comprised 28 stations providing measurements of 30 minute global and diffuse solar exposure.

That network was expanded for a short time with external funding from the National Research, Development and Demonstration Council to enable measurements at all Australian capital cities, but by the early 1990s, the network had contracted to six stations measuring only global solar exposure.

Data quality remains an issue with the original network measurements, as trends suspected of being caused by the instruments were modified to reflect a model climatology based on measurements principally from South Africa, rather than by recalibrating the instruments.

Unfortunately, some of the trends removed are now known to have been environmental – not instrumental.

A tiered approach: adding satellites

Developed in 1992, the tiered approach is based on measurements of global solar components from a number of ground sites, supplemented with the satellite-derived estimates that have been derived since 1989 to provide complete coverage of the Australian region with a typical areal resolution of 25 km2.

The new surface network – the Solar and Terrestrial Radiation Network – began in 1993 and collected 1 minute irradiance statistics of direct, diffuse and global solar exposure and terrestrial (long-wave) irradiance. These measurements are combined to provide exposure data in the same 30 minute exposure format as the old network.

From 1993, the Bureau operated up to 17 sites, including one for the New Zealand National Institute of Water and Atmosphere at Lauder. However, in 2005 – after state-based users indicated little interest or priority for high-resolution solar exposure or irradiance data – the network was reduced to roughly one site per Australian state for basic adjustment of the satellite-derived product.

Between 1993 and 1996, the network was supplemented by the Solar Irradiance Monitoring System network, funded by a consortium of the state power authorities in Victoria, New South Wales, Northern Territory, South Australia and Western Australia and the Energy Research Development Council.

Quality and standards today

The Solar and Terrestrial Network sites conform to the measurement and state-of-the-art quality protocols of the GCOS Baseline Surface Radiation Network (BSRN). Three of these sites – at Alice Springs, Cocos Island, and Darwin – provide 1 minute irradiance statistics to the BSRN archive in Germany.

All measurements from the sites are traceable to the International System of Units. As a result, uncertainties are also derived for each set of minute statistics, and enable specific uncertainties to be estimated for other derived quantities.

The satellite-derived product is satellite-specific and, as satellite characteristics change, the derived data are adjusted to the surface station data. Hence quality will vary depending on the available satellite measurements and size and data quality of the network.

Looking to the future

The Bureau currently distributes 30 minute exposure data from all sites as well as the satellite-derived daily and hourly estimates. It is expected that the 1 minute irradiance statistics from the surface sites will be available in the future if the user community gives it sufficient priority.

As in the past, the size and quality of the surface network and associated satellite products will depend on the Australian community’s environmental intelligence priorities.