Geoscience Reference
In-Depth Information
0.1
model (Penman)
mean of lysimeters
0.08
0.06
0.04
0.02
0.0
18
20
22
0
2
4
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8
Time (hours GMT)
Figure 7.9 Comparison of observed (dots) and modelled (black line) cumulative
dewfall for a single night over grass. (Data from Jacobs et al., 2006 )
c) Given the following observations: Q* = - 60 W m -2 , G = - 10 W m -2 , T a = 15 °C, e a =
15 hPa, p = 1013 hPa and r a = 100 s m -1 . Assume L v = 2.47·10 6 J kg -1 , ρ = 1.22 kg
m -3 , c p = 1013 J kg -1 K -1 . What is the dewfall rate in mm h -1 ?
7.5 Summary
With the surface-energy balance and the formulations for the turbulent luxes a num-
ber of combination methods can be developed. The Bowen ratio method uses obser-
vations of temperature and humidity at two heights, in combination with observations
of the available energy to derive the sensible and latent heat lux. The method can be
extended to other scalars, if their vertical concentration difference is measured in con-
junction with either temperature or humidity differences.
The Penman method combines the available energy with expressions for the tur-
bulent luxes based on observations at two levels: one atmospheric level and one
at the surface. The surface is supposed to be wet. The lux expression contains an
aerodynamic resistance that expresses the ease with which heat and water vapour are
transported between the surface and the atmosphere. Because the air at the surface
is saturated, the vertical humidity difference can be determined from the humidity at
the atmospheric level and the temperature at the surface. To arrive at a closed-form
equation, the surface temperature is eliminated through a linearization. The Penman
equation consists of two terms: the radiation term and the aerodynamic term.
The Penman - Monteith method extends the Penman method to vegetated surfaces
by including an extra resistance, the canopy resistance, in the pathway of humidity
transport. For practical applications one needs to model the canopy resistance (dis-
cussed in Chapter 9 ), or assume a value for it. The relative magnitudes of canopy
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