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h
3.
drying
1.2
entrainment
heating
q
θ
2.
moistening
1.1
direct
heating
H
L
v
E
Figure 7.7
Feedbacks between surface luxes
LE
and
H
and the temperature (
θ
) and
humidity (
q
) in the boundary layer and boundary-layer height (
h
). Solid arrows are
positive feedbacks, open arrows negative feedbacks. Line styles indicate different
feedbacks: heating from surface and through entrainment (long dashes), moistening
from surface (short dashes) and drying through entrainment (solid line). (From van
Heerwaarden et al.,
2009
; used with permission from John Wiley & Sons)
ABL by evapotranspiration limits VPD, and thus the magnitude of the aerodynamic
term (loop 2).
Even though the air does not become saturated, due to dry air entrainment, the
concept of an equilibrium evaporation can still be deined: it is the evaporation that
leads to a stationary VPD (rather than VPD = 0). Its value solely depends on the
speed of entrainment in combination with the contrast in temperature and humid-
ity between the boundary layer and the free atmosphere above. Together these
determine the rate of entrainment heating and entrainment drying, respectively,
both of which affect VPD (see
Figure 7.7
). In equilibrium the surface evapotrans-
piration would exactly cancel the effect of entrainment on VPD. The equilibrium
evaporation does not depend on the surface conditions. But whether or not the
surface evapotranspiration actually reaches this equilibrium value (in the course
of a diurnal cycle) depends mainly on the magnitude of the canopy resistance
and aerodynamic resistance: higher resistances delay the approach to equilibrium,
in many cases beyond the duration of daytime conditions (see van Heerwaarden
et al.,
2009
).
Despite the utility of the concept of equilibrium evapotranspiration in an entraining
boundary layer, we refer - in the remainder of the topic - to the classical expression
in Eq. (
7.17
) as the equilibrium evapotranspiration.
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