What is TMYs? How does TMY data produced? How do we ensure the data quality? What is the time convention for our climate data? What data file format do we provide?
Typical Meteorological Years, when used for regulatory purposes in Australia, these are referred to as Reference Meteorological Years (RMYs) to avoid confusion with the internationally recognised TMY weather data format (now most common as TMY2).
We develop a Typical Meteorological Year (TMY) by concatenating the twelve Typical Meteorological Months (TMM) that each most closely fits the longer term average - currently data from 1990 to 2013. Note that this is usually a different calendar year for each month for each location. Typicality is assessed by a cumulative difference function with weightings for each of the weather elements selected to suit the end use application. See "Megnus", Vol 35, No 8 (ACADS-BSG, September 2014).
Prior quality assurance work comparing BOM satellite estimated solar irradiation data with BOM ground-based measurements in Darwin, Alice Springs and Tullamarine (Melbourne) suggest significant overestimation at low solar altitudes. Accordingly, we routinely apply a capping algorithm to reduce this impact as described below. We can include the solar geometry and theoretical indicative (but not maximum) clear sky values in the data sets for client direct comparison.
All times are at Standard Time for the location concerned. In places and years where Daylight Saving applies over summer, software and/or users must make the necessary adjustments for calculation and simulation purposes.
Solar and Cloud Cover
1. Solar data is supplied by BOM as instantaneous values at 60 minute intervals stamped for Universal Time (UTC or GMT) but the time of observation is off the clock hour and specific to each location (pixel) as the satellite scans the earth - ~270,000 values for mainland Australia.
2. Hourly data in ACDB format provided by Exemplary Energy is stamped for the local standard time (0 to 23 hours).
3. In keeping with the convention of the Australian Climate Data Bank for solar data, each hourly value is calculated based on the minute by minute interpolated value from 30 minutes before the hour to 30 minutes after the hour.
4. Hourly values are calculated precisely for the capping algorithm and for the temporal interpolation and the result is recorded as the nearest integer value to avoid spurious implications of accuracy of any individual value.
5. Alternatively, in keeping with the convention of the TMY2 and EPW formats for solar data, each hourly value is calculated based on the minute by minute interpolated value for 60 minutes before the hour stated (1 to 24 hours).
6. Cloud cover data is inferred from solar data with the same time convention by relating it to coincident Clear Sky values.
Temperature, Atmospheric Pressure and Humidity
All values are instantaneous on the hour as measured by BOM.
All values are averages for the 10 minutes preceding the stated hour as measured by BOM.