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Impacts of vegetation on global water and energy budget as represented by
the Community Atmosphere Model (CAM3)
Zhongfeng XU and Congbin FU
RCE-TEA, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China, xuzhf@tea.ac.cn
1. Introduction
Vegetation exerts very important influences on the
climate system through modifying momentum, energy, and
water flux between land and air. Over the last decades many
studies have been focused on the climatic impacts of
vegetation over various regions such as the tropical forest
(e.g., Dickinson and Henderson-Sellers, 1988), temperate
forests and grasslands (e.g., Bounoua et al 2002; Fu, 2003),
boreal forests (e.g., Bonan et al, 1992), and dry lands (e.g.,
Charney 1975; Xue, 1997). The previous studies showed
that the effect of vegetation is different from place to place
and from season to season. In a general sense how much the
Earth’s climate depends on vegetation? This study is aiming
to estimate the contributions of vegetation to global and
regional hydrological cycle and land surface energy budget
by using a community atmosphere model.
2. Model and experimental design
The model used is the NCAR community atmosphere
model (CAM3) with the resolution T85 and 26 layers in the
vertical. Two numerical experiments were performed. The
first one, ctrl run, was integrated by using the standard
vegetation type map supplied with the CLM distribution. In
the second one, noveg run, the standard vegetation type map
was replaced by bare ground. The soil color, percentages of
lake and glacier in each gridcell were not changed. Each
simulation is integrated 15 years with prescribed monthly
varying SST forcing periodically. The first three years of the
simulation were discarded for spin up.
3. Influence on global water and energy budget
The annual mean water and energy budget are shown in
Figure 1 in which the bold fonts indicate the difference
between the ctrl run and noveg run at the confidence level of
95%. In the ctrl run, land surface evapotranspiration and
precipitation increase by 10.6% and 1.5%, respectively,
indicating an enhanced water exchange between land and
atmosphere when the vegetation is included. Meanwhile the
runoff reduces by 13.2% due to the enhanced
evapotranspiration. The atmospheric precipitable water
content increases by 1.5% over land. In addition, the
evaporation significantly reduces by 1.1% over the oceans
(Fig. 1a).
The land-surface energy balance is also widely
influenced by vegetation (Fig. 1b). In the ctrl run, the
reflected solar radiation decreases by 6.7%, while the
incident solar radiation decreases only 0.6%, indicating a
reduced land surface albedo. As a result, there is a 1.3%
increase in the solar radiation absorbed by the continents
(Fig. 1b). The decrease of incident solar radiation suggests
the increase of cloud albedo associated with the increasing
precipitation over the continents (Fig. 1a). The net longwave
radiation at land surface decreases by 3.6%, although the
incident and reflected longwave radiations both show a
slight decrease (0.1% and 0.8%, respectively). The
increasing land-surface evapotranspiration leads to a
considerable increase in the latent heat flux (7.9%).
However, no significant change can be found in the sensible
heat flux between the ctrl run and noveg run. This is likely
related to the following two effects of vegetation: (1) the
inclusion of vegetation leads to an increase in net solar
radiation due to the reduced land-surface albedo and a
decrease in net longwave radiation at land surface, which
tends to increase the land surface temperature. (2) On the
other hand, the presence of vegetation leads to a
considerable increase in evapotranspiration which tends to
decrease the land surface temperature. In this case, the
annual mean global land-surface temperature doesn’t show
significant difference between the ctrl run and the noveg
run because two effects are largely offset with each other.
Thus the influence of vegetation on annual and global
mean sensible heat flux appears to be insensitive.
However, the relative contributions of two effects of
vegetation to surface temperature vary with time and space,
therefore substantial difference of sensible heat flux can
still be found in some latitudes.
703.1
703.6
+0.1%
Ocean
Atmosphere
409.8 456.2
120.3 68.5
409.7 451.4
122.1 76.6
0.0% -1.1% +1.5% +10.6%
39.6
52.2
46.1
-13.2%
(a)
100.0
67.2
100.0 38.9
66.9
0.0% -1.8% -0.4%
Atmosphere
53.5 14.3
90.2 110.2
53.2 13.4
90.1 109.3
-0.6% -6.7% -0.1% -0.8%
192.8
195.7
+1.5%
Land
39.2
20.0
8.2 11.7
39.7
19.3 Land 8.2 12.7
+1.3% -3.6% 0.0% +7.9%
shortwave
radiation
longwave
radiation
sensible
heat flux
latent
heat flux
(b)
Figure 1. Annual means of (a) global water cycle and (b)
the surface energy balance over land for the noveg
(upper), ctrl (middle) run, and ctrl minus noveg in percent
(lower). Annual mean water fluxs in 10 15 kg yr -1 ,
precipitable water in 10 15 kg, and heat/radiative fluxes in
percent incoming radiation (100% = 341.3 W m -2 ). Bold
fonts indicate the difference between the ctrl run and the
noveg run at significance level of 0.05.
4. Hydrological cycle and energy budget in
different latitudes
To examine the effects of vegetation on land surface
energy and water budget in different latitudes, Figure 2
shows the zonal mean differences of some key variables
between the ctrl run and noveg run as a function of