Observed changes in extreme weather indices in Hong Kong



Past trends of the occurrences of extreme temperature and rainfall events in Hong Kong from 1885 to 2008 were examined using a suite of 27 extreme indices adopted from the core indices developed by the Expert Team on Climate Change Detection, Monitoring and Indices (ETCCDMI) under the auspices of the World Meteorological Organization (WMO), with appropriate modification to suit the sub-tropical climate of Hong Kong. Results showed that the extreme daily maximum and minimum temperatures, annual numbers of very hot days (daily maximum temperature ≥ 33.0 °C) and hot nights (daily minimum temperature ≥ 28.0 °C) as well as the warm spell duration index (WSDI) in Hong Kong exhibited statistically significant long-term rising trends, whereas the annual number of cold days (daily minimum temperatures ≤ 12.0 °C) and cold spell duration (CSDI) index had a statistically significant decreasing trend. Regarding rainfall, the frequency of occurrence of extreme hourly, 2- and 3-hourly rainfall amounts increased significantly. The fraction of annual total precipitation due to events exceeding the daily 95th percentile of the climatological normal (1971–2000) increased by 22 mm per decade in the same period, indicating that the contribution of heavy rain to the annual rainfall amount was increasing with time. However, the maximum length of dry spell in the summer months from April to September increased at a rate of 0.3 days per decade from 1885 to 2008. Using the time-dependent generalized extreme value (GEV) distribution technique, the return period for the daily minimum temperature of 4 °C or lower had significantly lengthened from 6 years in 1900 to 163 years in 2000. The return period for an hourly rainfall of 100 mm or more had shortened from 37 years in 1900 to 18 years in 2000. Copyright © 2010 Royal Meteorological Society