Mean monthly radiation surfaces for Australia at 1 arc-second resolution

Abstract

Solar radiation data are useful for many purposes including ecological and hydrological modelling and for assessing available solar energy. Using the new national 1 arc-second DEM, the SRAD model, and national climate, albedo, and vegetation cover data, we have created mean monthly radiation surfaces for Australia: total shortwave on a sloping surface (SWS), total shortwave on a horizontal surface (SWH), the ratio of sloping to horizontal shortwave (SWR), incoming atmospheric longwave (LIN), outgoing surface longwave (LOUT), net longwave (LNET), and net radiation (RNET). The output datasets will be available through the TERN Data Discovery Portal (http://portal.tern.org.au) and the CSIRO Data Access Portal (http://data.csiro.au/dap). Modelling solar radiation using DEMs can be undertaken with varying degrees of complexity and parameterisations. Calculating the position of the sun and length of the day are relatively straightforward but the effects of clouds, scattering and absorption in the atmosphere, topographic shadowing, and reflection from sloping surfaces make precise calculation of radiation at the surface exceedingly difficult. SRAD models the solar radiation incident upon a sloping surface accounting for the solar geometry on a given day, the surface orientation, shadowing by surrounding terrain and the transmittance of the atmosphere and clouds, and calculates incoming and outgoing longwave radiation based on air and surface temperatures and emissivities. The model uses elevation, slope and aspect, and 12 mean monthly surfaces for albedo, fractional vegetation cover, 9 am and 3 pm cloud cover, minimum and maximum air temperature, and 9 am and 3 pm vapour pressure. The relationship between cloud fraction and sunshine fraction was modelled by linear regression with observed cloud and sunshine from the BoM, while observed cloud fraction, sunshine fraction and incoming shortwave radiation data were used to determine the clear sky and cloud transmittance parameters. Figure 1 shows examples of the SRAD SWS outputs for January and July in south-western Tasmania. A preliminary comparison of the SWH outputs for January and July with observed radiation data for 6 stations shows differences of less than 2% for 7 of the 12 comparisons and differences of 5% to 15% in the other 5. The causes of these differences are not yet known but differences in the period of record for parameterisation and validation is one possibility. Expanding the validation to include 12 months and 37 BoM stations should help to make this clearer.