8.5.2 Extreme Precipitation

Sun et al. (2006) investigated the intensity of daily precipitation simulated by 18 AOGCMs, including several used in this report. They found that most of the models produce light precipitation (<10 mm day^–1) more often than observed, too few heavy precipitation events and too little precipitation in heavy events (>10 mm day^–1). The errors tend to cancel, so that the seasonal mean precipitation is fairly realistic (see Section 8.3).

Since the TAR, many simulations have been made with high-resolution GCMs. Iorio et al. (2004) examined the impact of model resolution on the simulation of precipitation in the USA using the Community Climate Model version 3 (CCM3). They found that the high-resolution simulation produces more realistic daily precipitation statistics. The coarse-resolution model had too many days with weak precipitation and not enough with intense precipitation. This tendency was partially eliminated in the high-resolution simulation, but, in the simulation at the highest resolution (T239), the high-percentile daily precipitation was still too low. This problem was eliminated when a cloud-resolving model was embedded in every grid point of the GCM.

Kimoto et al. (2005) compared the daily precipitation over Japan in an AOGCM with two different resolutions (high res. and med res. of MIROC 3.2) and found more realistic precipitation distributions with the higher resolution. Emori et al. (2005) showed that a high-resolution AGCM (the atmospheric part of high res. MIROC 3.2) can simulate the extreme daily precipitation realistically if there is provision in the model to suppress convection when the ambient relative humidity is below 80%, suggesting that modelled extreme precipitation can be strongly parametrization dependent. Kiktev et al. (2003) compared station observations of rainfall with the simulations of the atmosphere-only GCM HadAM3 forced by prescribed oceanic forcing and anthropogenic radiative forcing. They found that this model shows little skill in simulating changing precipitation extremes. May (2004) examined the variability and extremes of daily rainfall in the simulation of present day climate by the ECHAM4 GCM. He found that this model simulates the variability and extremes of rainfall quite well over most of India when compared to satellite-derived rainfall, but has a tendency to overestimate heavy rainfall events in central India. Durman et al. (2001) compared the extreme daily European precipitation simulated by the HadCM2 GCM with station observations. They found that the GCM’s ability to simulate daily precipitation events exceeding 15 mm per day was good but its ability to simulate events exceeding 30 mm per day was poor. Kiktev et al. (2003) showed that HadAM3 was able to simulate the natural variability of the precipitation intensity index (annual mean precipitation divided by number of days with precipitation less than 1 mm) but was not able to simulate accurately the variability in the number of wet days (the number of days in a year with precipitation greater than 10 mm).

Using the Palmer Drought Severity Index (PDSI), Dai at al. (2004) concluded that globally very dry or wet areas (PDSI above +3 or below –3) have increased from 20% to 38% since 1972. In addition to simulating the short-duration events like heat waves, frost days and cold air outbreaks, models have also shown success in simulating long time-scale anomalies. For example, Burke et al. (2006) showed that the HadCM3 model, on a global basis and at decadal time scales, ‘reproduces the observed drying trend’ as defined by the PDSI if the anthropogenic forcing is included, although the model does not always simulate correctly the regional distributions of wet and dry areas.