7.2.4.4 Mechanisms for Modification of Precipitation by Spatial Heterogeneity

Clark et al. (2004) show an example of a ‘squall-line’ simulation where soil moisture variation at the scale of the rainfall modifies the rainfall pattern. Pielke (2001), Weaver et al. (2002) and S. Roy et al. (2003) also address various aspects of small-scale precipitation coupling to land surface heterogeneity. If deforestation occurs in patches rather than uniformly, the consequences for precipitation could be different. Avissar et al. (2002) and Silva Dias et al. (2002) suggest that there may be a small increase in precipitation (of the order of 10%) resulting from partial deforestation as a consequence of the mesoscale circulations triggered by the deforestation.

7.2.4.5 Interactive Vegetation Response Variables

Prognostic approaches estimate leaf cover based on physiological processes (e.g., Arora and Boer, 2005). Levis and Bonan (2004) discuss how spring leaf emergence in mid-latitude forests provides a negative feedback to rapid increases in temperature. The parametrization of water uptake by roots contributes to the computed soil water profile (Feddes et al., 2001; Barlage and Zeng, 2004), and efforts are being made to make the roots interactive (e.g., Arora and Boer, 2003). Dynamic vegetation models have advanced and now explicitly simulate competition between plant functional types (e.g., Bonan et al., 2003; Sitch et al., 2003; Arora and Boer, 2006). New coupled climate-carbon models (Betts et al., 2004; Huntingford et al., 2004) demonstrate the possibility of large feedbacks between future climate change and vegetation change, discussed further in Section 7.3.5 (i.e., a die back of Amazon vegetation and reductions in Amazon precipitation). They also indicate that the physiological forcing of stomatal closure by rising atmospheric CO₂ levels could contribute 20% to the rainfall reduction. Levis et al. (2004) demonstrate how African rainfall and dynamic vegetation could change each other.