Geoscience Reference
In-Depth Information
modeling these mechanisms are all assessed as 'medium'. In practice it is likely
that future improvements in understanding and modeling will most effectively
be made by comprehensive studies in which models are challenged using a suite
of sustained observations made over several seasons that includes measure-
ments of the variables associated with all three of these influential mechanisms
simultaneously.
C.
The influence of imposed persistent changes in land cover
As mentioned earlier in this chapter, the land cover and hydrological behavior of
large areas of the globe have now been altered greatly by human intervention and
continued changes seem inevitable as human population continues to grow.
Because land surface exchange helps determine how the overlying atmosphere
behaves, persistent changes in the nature of land surfaces inevitably have an effect
on weather and climate. Small-scale changes have some effect on weather variables
that are measured near the ground (typically at 2 m) because local surface
influences in part determine these. Larger regional-scale changes in land surface
features can have influence higher in the atmosphere and give rise to regional
or even global scale modifications to atmospheric processes and flows. Such
modifications arise in two main ways, either because the area-average surface
exchanges are modified over a large area, or because development introduces
heterogeneity into land surface exchanges at a spatial scale such as to generate
mesoscale circulations in the atmosphere that may alter the probability and
whereabouts of cloud and precipitation. For simplicity, the influence of persistent
changes in land cover are considered separately below, although in practice all
three influences may well act simultaneously.
1.
Effect of imposed land cover change on near surface observations
The values of climate variables measured at (say) 2 m above the ground depend on
vertical profiles that are determined by the land-cover dependent turbulent
transfer of energy, water and momentum fluxes between the surface and the mixed
layer in the Atmospheric Boundary Layer (see Chapter 19 and Fig. 19.6). This fact
is recognized explicitly in the derivation of the Matt-Shuttleworth approach
(Shuttleworth, 2006; Shuttleworth and Wallace, 2009), see the relevant section
in Chapter 23. Observed differences in near-surface climate are easily observed
and have, for example, been reported over small clearings and natural forest in
Amazonia (Figs 25.7a,b,c), and in the form of the 'urban heat island' phenomenon
when regions undergo extensive urbanization (Fig. 25.7d).
Because the changes in near-surface climate that are associated with imposed
land cover change are readily understood in terms of present day turbulent
transport theory and are readily observed, the assessment of likelihood for
this influential mechanism is assessed as 'extremely likely, and the level of
understanding and modeling as 'good' in Table 25.1.
