Geography Reference
In-Depth Information
slightly in the grassland, with an increment of only 0.01 C/year (Fig
4.16
). The
result is largely consistent with the result of the research of Lim et al. (
2005
), in
which the climate in the northern hemisphere was simulated with the ''CRU-NNR''
model at the 5 9 5 resolution (the OMR trend value of the urban and built-up land,
crop land, broadleaf forest, and bare land was 0.034/year, 0.02 C/year, 0.002 C/
year, and 0.02 C/year, respectively). However, the simulated result is still some-
what higher, which may be because the ERA40 reanalysis indirectly included the
ground-based observation data and consequently made the OMR trend values
smaller than the results obtained with the numerical simulation.
The vegetation plays an important role in influencing the near-surface tem-
perature. For example, one of the main reasons for the near-surface temperature in
different land-cover and land-use types is the amount and the density of the
vegetation. On the whole, the better the vegetation cover is, the lesser will be the
temperature rise. It may be caused by little evaporation in the barren land, and the
land surface heat mainly gets into the atmosphere in the form of sensible heat. By
contrast, there is higher soil humidity in the densely vegetated land, which makes
the land surface heat mainly get into the atmosphere in the form of latent heat and
consequently reduces part of the temperature rise of the land surface. In addition,
the heat island effect in the urban region also leads to the rise of the near-surface
temperature. By contrast, the temperature increment is less in the water bodies,
mainly because the specific heat capacity of water bodies is very large, which
makes the temperature increase very slow, and consequently makes the near-
surface temperature lower (Su et. al 2005). Furthermore, there is great difference
between the irrigation intensities of the dryland cropland and irrigated cropland,
which leads to great difference in their physical characteristics, and consequently
makes the temperature increments in them differ greatly.
The change of the average near-surface temperature corresponding to each kind
of land use/cover change was summarized in this study. The following figure
shows the temperature change in the eight major kinds of land use/cover change
that involve a large area of land (Fig
4.17
). The result showed that the conversion
from dryland crop to forest and built-up land made the near-surface temperature
increase by 0.13 C/year, while the conversion from dryland crop to grassland
made the near-surface temperature decrease by 0.1 C/year. By contrast, the other
conversion
types
only
made
the
near-surface
temperature
increase
by
0.01-0.04 C/year.
The conversion of croplands to built-up lands can lead to the changes in the
roughness and albedo of the land surface, which cause the change in the radiation
flux of the land surface and consequently make the regional near-surface tem-
perature increase. Besides, changes of the underlying surface due to the urbani-
zation can alter physical processes, such as the energy balance of the land surface
and lead to the ''five island effects'' (i.e., dark islands, heat islands, dry islands, wet
islands, and rain islands), decreasing the wind velocity and lead to the variable city
climate, and consequently influencing the structure and development of the
boundary layer to change the climate in a large area. Moreover, the conversion
from grasslands to dryland croplands can decrease the albedo of the land surface,
