Daily Solar Radiation (Global Horizontal Exposure)

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 It may contain outdated information.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 daily solar radiation exposure across Australia derived by the Bureau of Meteorology using geostationary satellite remote sensing observations.

Key specifications

Background

What is Solar Radiation Exposure?

Global solar radiation exposure is the total amount of solar energy falling on a horizontal surface. The daily global solar exposure is the total solar energy for a day. Typical values for daily global exposure range from 1 to 35 MJ m^-2 (megajoules per square metre). For mid-latitudes, the values are usually highest in clear sun conditions during the summer, and lowest during winter or very cloudy days.

Solar Radiation Data

Satellite-derived global solar exposure estimates are based on images from the Geostationary Meteorological Satellite GMS-4, GMS-5, MTSAT-1R, MTSAT-2 (from 2010) and Geostationary Operational Environmental Satellite (GOES-9) satellites which are provided with permission of the Japan Meteorological Agency (JMA) and the United States National Oceanic & Atmospheric Administration (NOAA).

At each location for each satellite acquired image, the brightnesses are averaged over each grid cell and used to estimate solar irradiance at the ground. Essentially, the irradiance at the ground can be calculated from the irradiance at the top of the earth's atmosphere, the amount absorbed in the atmosphere (dependant on the amount of water vapour present), the amount reflected from the surface (surface albedo) and the amount reflected from clouds (cloud albedo). These instantaneous irradiance values are integrated over the day to give daily insolation (daily radiant exposure) in megajoules per square metre. The daily exposure gridded datasets cover Australia with a resolution of 0.05° in latitude and longitude.

Algorithm

The primary data input is visible-band imagery from the geostationary meteorological satellites that have been stationed over the western Pacific Ocean, namely GMS-4, GMS-5 and GOES-9 and MTSAT-1R in the past, and MTSAT-2 currently. The satellite images have 1.25-km or 1-km resolution at the subsatellite point and are typically produced hourly. The images are acquired in real-time by the Bureau.

The images are geolocated, calibrated and regridded to a 0.01° grid, then spatially averaged to the final 0.05° grid. Instantaneous surface solar radiation (irradiance) at each grid point is calculated for the time of each satellite image with a physical model that parameterises the important aspects of the radiative transfer in clear and cloudy atmospheres in two spectral bands, covering visible and near-infrared wavelengths respectively (Weymouth and Le Marshall, 2001). The physical parameterisations are adapted to the spectral response characteristics of each satellite. Total column water vapour amount, an ancillary input to the model, is taken from a numerical weather prediction (NWP) model field: the ERA and NCAR/NCEP reanalyses for archival processing, and the forecast field from the Bureau’s LAPS and ACCESS regional models for near real-time processing. Total column ozone is taken as a fixed function of latitude. There is no explicit treatment of aerosol.

The hourly surface irradiance estimates for each day are integrated to give the daily solar exposure at each grid point. The bias in the satellite daily exposure estimates is subtracted, where the bias is estimated as a linear function of exposure by a regression against surface measured values for all sites and days in each month.

Quality

Quality and limitations

The source of uncertainties associated with calculations of solar radiation includes:

• Anisotropy of cloud-top reflectance.

• Water vapour in the atmosphere.

• Satellite calibration.

• The availability of hourly images.

Data completeness

The exposure values are derived only for land-filled grid cells. The values are spatially extrapolated by nearest neighbour into inland water bodies and by several cells out from the coast. This ensures that values are present for all coastal locations, close-in islands and lakes, but for such locations the value may be derived from a grid cell up to 70 km away.

The exposure grids are missing for some days, with years in the early 1990s having the most missing days. This is because each day is the integration (a sum with interpolation in time) of values produced from hourly satellite images, and the accuracy of the integrated daily value is unacceptably poor if the data from more than two consecutive hours is missing. Missing data are due to:

• Interruption to satellite operations, including technical problems on the satellite

• Problems with ground receiving equipment or data processing systems

• Rejection of images that are noisy due to solar radio interference at the receiving station at certain times of the year

• Resolving problems as data processing systems are adapted to each new satellite.

Validation

The accuracy of the satellite estimates of daily solar exposure has been assessed with observations from the Bureau of Meteorology’s surface radiation monitoring network. The station-mean absolute difference (mean absolute error) between the bias corrected satellite estimate and the surface-based measurement generally cycles annually from approximately 0.8 MJ m⁻² in winter to 1.5 MJ m⁻² in summer, compared with an exposure range of approximately 5 to 30 MJ m⁻².

Product variants

Time series of averages of daily solar exposure over periods of 1 week, 1 month and longer are available from the Bureau of Meteorology's solar radiation maps webpage.

Climatological monthly averages and the annual average of daily solar exposure are available from the Bureau of Meteorology's solar radiation average maps webpage.

Other specifications

References