Projections of Extreme Rainfall in Hong Kong in the 21st Century

The possible changes in the frequency of extreme rainfall events in Hong Kong in the 21st century were investigated by statistically downscaling 30 sets of the daily global climate model projections (involving a combination of 12 models and 3 greenhouse gas emission scenarios, namely, A2, A1B, and B1) of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. To cater for the intermittent and skewed character of the daily rainfall, multiple stepwise logistic regression and multiple stepwise linear regression were employed to develop the downscaling models for predicting rainfall occurrence and rainfall amount, respectively. Verification of the simulation of the 1971-2000 climate reveals that the models in general have an acceptable skill in reproducing past statistics of extreme rainfall events in Hong Kong. The projection results suggest that, in the 21st century, the annual number of rain days in Hong Kong is expected to decrease while the daily rainfall intensity will increase, concurrent with the expected increase in annual rainfall. Based on the multi-model scenario ensemble mean, the annual number of rain day is expected to drop from 104 days in 1980-1999 to about 77 days in 2090-2099. For extreme rainfall events, about 90% of the model-scenario combinations indicate an increase in the annual number of days with daily rainfall >= 100 mm (R100) towards the end of the 21st century. The mean number of R100 is expected to increase from 3.5 days in 1980-1999 to about 5.3 days in 2090-2099. The projected changes in other extreme rainfall indices also suggest that the rainfall in Hong Kong in the 21st century may also become more extreme with more uneven distributions of wet and dry periods. While most of the model-emission scenarios in general project consistent trends in the change of rainfall extremes in the 21st century, there is a large divergence in the projections among different model/emission scenarios. This reflects that there are still large uncertainties in model simulations of future extreme rainfall events.