A Novel Method for Assessing Rainfall Extremes from GCMs and RCMs using Areal Reduction Factors.

Design of hydraulic structures requires information on the maximum amount of rainfall that could occur for a particular catchment area over a specific duration. However, design rainfall estimates only provide the rainfall information at a point scale. To transform the point design rainfall to an equivalent design rainfall over the catchment area, areal reduction factors (ARFs) are commonly used. Although there has been some research into the impacts of anthropogenic climate change on design rainfalls, there has been much less work examining the likelihood of spatial changes to design rainfalls which ARFs represent.
This study investigates the impact of climate change on ARFs over the Sydney region by using a high- resolution Weather Research and Forecasting (WRF) model. The WRF model is driven by a General Circulation Model (GCM), which is CSIRO Mk3.5, under current (1990-2009) and future (2040-2059) climate scenarios. The domain of this WRF model extends to the Hunter Valley in the north, Jervis Bay in the south and Orange in the west. The oceanic region on the WRF domain was excluded in this study since the ARFs over ocean are not of much practical use. The study area was then divided into two climatically similar regions: inland and coastal. ARFs were estimated from the model simulations using the modified Bell’s method for Annual Exceedance Probabilities (AEPs) from 1 in 2 to 1 in 20. To assess the ability of WRF model to correctly represent the areal properties of extreme rainfall, the modeled ARFs were compared to existing observed ARFs for Australia. It was found that ARFs are simulated well in both global reanalysis and GCM driven WRF simulations with less than 5% error for durations longer than 3 hours.
The assessment of the future change of ARFs was then conducted, and results show that ARFs tend to decrease in the future for durations longer than 2 hours for AEP of 1 in 2 over the inland region. The reductions in ARFs are larger for larger contributing areas. It is conjectured that this could indicate that there are likely to be more small-scale convective storms in the future over the inland region. One of the important implications of this finding is that if the future climate conditions are incorporated into the hydrological structural design and flood risk management, the extent of the reduction of the model predicted precipitation to a catchment scale can be more than that suggested by the ARFs derived for the current climate. Changes in the ARFs for less frequent events were not found to be significant over the inland region. Over the coastal region, the magnitude of the change of ARFs is rather small (i.e. <1%) and insignificant for AEP of 1 in 2, but for AEP of 1 in 20 a clear increasing trend (i.e. ≤ 10%) of ARFs was found. Although these increases were found to be significant for over half of the areas and durations, the spatial variability of the changes is quite high.