7.2.3.2 Land Feedback to Precipitation

Findell and Eltahir (2003) examine the correlation between early morning near-surface humidity over the USA and an index of the likelihood of precipitation occurrence. They identify different geographical regions with positive, negative or little correlation. Koster et al. (2003) and Koster and Suarez (2004) show during summer over the USA, and all land 30°N to 60°N, respectively, a significant correlation of monthly precipitation with that of prior months. They further show that their model only reproduces this correlation if soil moisture feedback is allowed to affect precipitation. Additional observational evidence for such feedback is noted by D’Odorico and Porporato (2004) in support of a simplified model of precipitation soil moisture coupling (see, e.g., Salvucci et al., 2002, for support of the null hypothesis of no coupling). Liebmann and Marengo (2001) point out that the interannual variation of precipitation over the Amazon is largely controlled by the timing of the onset and end of the rainy season. Li and Fu (2004) provide evidence that onset time of the rainy season is strongly dependent on transpiration by vegetation during the dry season. Previous modelling and observational studies have also suggested that Amazon deforestation should lead to a longer dry season. Fu and Li (2004) further argue from observations that removal of tropical forest reduces surface moisture fluxes, and that such land use changes should contribute to a lengthening of the Amazon dry season. Durieux et al. (2003) find more rainfall in the deforested area in the wet season and a reduction of the dry season precipitation over deforested regions compared with forested areas. Negri et al. (2004) obtain an opposite result (although their result is consistent with Durieux during the wet season).

7.2.3.3 Properties Affecting Radiation

Albedo (the fraction of reflected solar radiation) and emissivity (the ratio of thermal radiation to that of a black body) are important variables for the radiative balance. Surfaces that have more or taller vegetation are commonly darker than are those with sparse or shorter vegetation. With sparse vegetation, the net surface albedo also depends on the albedo of the underlying surfaces, especially if snow or a light-coloured soil. A large-scale transformation of tundra to shrubs, possibly connected to warmer temperatures over the last few decades, has been observed (e.g., Chapin et al., 2005). Sturm et al. (2005) report on winter and melt season observations of how varying extents of such shrubs can modify surface albedo. New satellite data show the importance of radiation heterogeneities at the plot scale for the determination of albedo and the solar radiation used for photosynthesis, and appropriate modelling concepts to incorporate the new data are being advanced (e.g., Yang and Friedl, 2003; Niu and Yang, 2004; Wang, 2005; Pinty et al., 2006).