8.5.1 Extreme Temperature
Kiktev et al. (2003) compared station observations of extreme events with the simulations of an atmosphere-only GCM (Hadley Centre Atmospheric Model version 3; HadAM3) forced by prescribed oceanic forcing and anthropogenic radiative forcing during 1950 to 1995. The indices of extreme events they used were those proposed by Frich et al. (2002). They found that inclusion of anthropogenic radiative forcing was required to reproduce observed changes in temperature extremes, particularly at large spatial scales. The decrease in the number of frost days in Southern Australia simulated by HadAM3 with anthropogenic forcing is in good agreement with the observations. The increase in the number of warm nights over Eurasia is poorly simulated when anthropogenic forcing is not included, but the inclusion of anthropogenic forcing improves the modelled trend patterns over western Russia and reproduces the general increase in the occurrence of warm nights over much of the NH.
Meehl et al. (2004) compared the number of frost days simulated by the PCM model with observations. The 20th-century simulations include the variations in solar, volcano, sulphate aerosol, ozone and greenhouse gas forcing. Both model simulations and observations show that the number of frost days decreased by two days per decade in the western USA during the 20th century. The model simulations do not agree with observations in the southeastern USA, where the model simulates a decrease in the number of frost days in this region in the 20th century, while observations indicate an increase in this region. Meehl et al. (2004) argue that this discrepancy could be due to the model’s inability to simulate the impact of El Niño events on the number of frost days in the southeastern USA. Meehl and Tebaldi (2004) compared the heat waves simulated by the PCM with observations. They defined a heat wave as the three consecutive warmest nights during the year. During the period 1961 to 1990, there is good agreement between the model and observations (NCEP reanalysis).
Kharin et al. (2005) examined the simulations of temperature and precipitation extremes for AMIP-2 models, some of which are atmospheric components of coupled models used in this assessment. They found that models simulate the temperature extremes, especially the warm extremes, reasonably well. Models have serious deficiencies in simulating precipitation extremes, particularly in the tropics. Vavrus et al. (2006) used daily values of 20th-century integrations from seven models. They defined a cold air outbreak as ‘an occurrence of two or more consecutive days during which the local mean daily surface air temperature is at least two standard deviations below the local winter mean temperature’. They found that the climate models reproduce the location and magnitude of cold air outbreaks in the current climate.
Researchers have also established relationships between large-scale circulation features and cold air outbreaks or heat waves. For example, Vavrus et al. (2006) found that ‘the favored regions of cold air outbreaks are located near and downstream from preferred locations of atmosphere blocking’. Likewise, Meehl and Tebaldi (2004) found that heat waves over Europe and North America were associated with changes in the 500 hPa circulation pattern.