Optimal Length of Weather Records

Climate data sets like our Reference Meteorological Years (RMYs) are specially compiled to be representative of “average” conditions from the historical weather record. In a changing climate, the “average” is moving. This presents a few challenges: in particular, we seek to balance the desire between considering a longer period to provides a stable reference baseline and a shorter, more recent period as an implicit predictor of the most likely conditions in the near future.

In 2015, the global average temperature exceeded the 1961–1990 average by approximately 0.76 ± 0.09°C (World Meteorological Organisation, 2016). Moreover, the years spanning from 2015 to 2022 have marked the eight warmest years on record, and it is expected that this trend of increased warming will persist due to rising anthropogenic greenhouse gas emissions. Within the context of a warming climate, conventional reference periods for which weather data are derived—such as CSIRO’s reference period of 1990-2015—may no longer accurately represent the current or projected future climate at a given location. Hence, the adoption of more current reference periods that better characterise the climate dynamics of today are necessary to improve the quality and suitability of climate data for building simulations, especially with regards to energy modelling. 

We conducted simulations through EnergyPlus using historical weather data (from 1990-2022) for Sydney to obtain heating and cooling energy data for our archetype supermarket, 3-storey and 10-storey office buildings. The results showed a general decrease in heating demand (energy consumption), and an increase in cooling demand and total energy usage over time. Similar shifts were seen across all three building archetypes. Notably, the cooling demand from 2015 to 2019 was relatively high, meeting our expectations from the knowledge that these were extremely hot years.

We conducted a temporal analysis by calculating the monthly averages of selected meteorological elements over (i) a 15-year reference period from 2008-2022, (ii) a 33-year reference period from 1990-2022, and (iii) the CSIRO reference period of 1990-2015. Given the ongoing effects of global warming, we propose the 15-year reference period as a more current perspective.

Comparing the monthly averages of 1990-2015 with 2008-2022 showed an increase in wind speed of 6.64%, an increase in dry bulb temperature and dew point temperature of 0.24°C and 0.38°C, respectively; and a decrease in Global Horizontal Irradiation (GHI) and Direct Normal Irradiation (DNI) of 2.40% and 7.36%, respectively. Likewise, comparing the monthly averages of 2008-2022 with 1990-2022 showed an increase in wind speed of 5.77%, an increase in dry bulb temperature and dew point temperature of 0.18°C and 0.24°C, respectively; and a decrease in GHI and DNI of 1.92% and 5.78%, respectively. The monthly averages for the periods 1990-2022 and 1990-2015 were comparable, although the GHI and DNI tended to be higher each month in the 1990-2015 period. 

These findings collectively suggest recent substantial shifts in the local climate which affect heating and cooling demand, highlighting the need for updated climate data sets for building energy simulation. More frequent updates and shorter reference periods can yield greater predictive accuracy for meteorological conditions, and by extension, building simulations. Meanwhile, outdated climate data will produce inaccurate energy performance estimates and poorly optimised systems. 

We hope to elaborate on this work in an upcoming presentation to the Asia Pacific Solar Research Conference. Stay tuned for further details.

Exemplary Energy can provide full data sets for the 1990-2022 period for all Australian capital cities to address the need for dynamically evolving and more relevant reference periods. Our program aims to complete the data for all NatHERS Climate Zones by the end of September. In parallel with that work, we will be producing the latest-15-years Reference Meteorological Years (RMYs) and eXtreme Meteorological Years (XMYs) to represent Australia’s current climate.

References

World Meteorological Organisation, 2016, ‘WMO Statement on the status of the global climate in 2015’. Available at: https://public.wmo.int/en/resources/library/wmo-statement-status-of-global-climate-2015. Accessed 5 September 2023.