Geoscience Reference
In-Depth Information
g
H
E
=
(22.24)
l
Δ
Finally, combining Equations (22.19) with Equation (22.24) gives:
Δ
E
R
l
=
(22.25)
n
Δ+
g
This evaporation rate is called the '
Equilibrium evaporation rate
. It is the rate of
evaporation that would occur from natural surfaces in response to incoming energy
if the overlying atmosphere was saturated and remained saturated. As such, it is a
useful concept because it defines a lower limit on natural evaporation rates. However,
in the real world, evaporation rates are higher than this because the atmosphere is
usually not saturated because water is removed aloft by precipitation.
Evaporation into an unsaturated atmosphere
Most often natural evaporation occurs during the day into an unsaturated ABL
which is partly (but not wholly) contained by a stable inversion. The surface
evaporation rate is determined by the surface resistance and aerodynamic
resistance but also by atmospheric variables (which may be measured) in the
surface layer, specifically by net radiation, VPD, wind speed, and temperature.
However, because the ABL is partly contained, the values of these atmospheric
variables are themselves influenced by the surface energy inputs, and the potential
for surface-ABL feedbacks therefore exists.
If daytime containment of the ABL were totally effective, evaporation rate
would presumably be close to the equilibrium evaporation rate. But this is not the
case. Precipitation processes (and the associated loss of water vapor and release of
latent heat aloft) mean that the air in the free atmosphere above the daytime
inversion is on average drier and (in terms of potential temperature) warmer than
that in the ABL. As discussed in Chapter 18, intermittent breakdown of the
inversion layer during the day allows some entrainment of this drier, warmer air
from the free atmosphere into the ABL, and the ABL grows as a result. Again as
discussed in Chapter 18, some of the moisture evaporated from the ground may
remain in the ABL, but most is used to moisten the incoming drier air from the
free atmosphere. As a result the change in absolute moisture content in the ABL
can be small. On the other hand, during the day the air in the ABL is warmed both
by sensible heat from the ground and by the incoming warmer air from above.
Consequently, the temperature of the ABL rises and the VPD remains finite.
Because the VPD in the ABL is finite, natural evaporation rates from moist surfaces
are greater (typically 25% greater) than the equilibrium evaporation rate.
The extent to which dry, warm air from the free atmosphere is entrained
depends on the strength of the stable inversion. The stronger the inversion the
