7.2.2.3 Daily and Seasonal Variability

Diurnal and seasonal variability result directly from the temporal variation of the solar radiation driver. Large-scale changes in climate variables are of interest as part of the observational record of climate changes (Chapter 3). Daytime during the warm season produces a thick layer of mixed air with temperature relatively insensitive to perturbations in daytime radiative forcing. Nighttime and high-latitude winter surface temperatures, on the other hand, are coupled by mixing to only a thin layer of atmosphere, and can be more readily altered by changes in atmospheric downward thermal radiation. Thus, land is more sensitive to changes in radiative drivers under cold stable conditions and weak winds than under warm unstable conditions. Winter or nighttime temperatures (hence diurnal temperature range) are strongly correlated with downward longwave radiation (e.g., Betts, 2006; Dickinson et al., 2006); consequently, average surface temperatures may change (e.g., Pielke and Matsui, 2005) with a change in downward longwave radiation.

Modification of downward longwave radiation by changes in clouds can affect land surface temperatures. Qian and Giorgi (2000) discussed regional aerosol effects, and noted a reduction in the diurnal temperature range of –0.26°C per decade over Sichuan China. Huang et al. (2006) model the growth of sulphate aerosols and their interactions with clouds in the context of a RCM, and find over southern China a decrease in the diurnal temperature range comparable with that observed by Zhou et al. (2004) and Qian and Giorgi. They show the nighttime temperature change to be a result of increased nighttime cloudiness and hence downward longwave radiation connected to the increase in aerosols.

In moist warm regions, large changes are possible in the fraction of energy going into water fluxes, for example, by changes in vegetation cover or precipitation, and hence in soil moisture. Bonan (2001) and Oleson et al. (2004) indicate that conversion of mid-latitude forests to agriculture could cause a daytime cooling. This cooling is apparently a result of higher albedo and increased transpiration. Changes in reflected solar radiation due to changing vegetation, hence feedbacks, are most pronounced in areas with vegetation underlain by snow or light-coloured soil. Seasonal and diurnal precipitation cycles can be pronounced. Climate models simulate the diurnal precipitation cycle but apparently not yet very well (e.g., Collier and Bowman, 2004). Betts (2004) reviews how the diurnal cycle of tropical continental precipitation is linked to land surface fluxes and argues that errors in a model can feed back to model dynamics with global impacts.