Combination Methods for Turbulent Fluxes - Transport in the Atmosphere-Vegetation-Soil Continuum - page 275

upon condensation), whereas the second term will be positive (drier air will suppress
the formation of dew through evaporation of part of the formed dew). Figure 7.9
shows an example of the skill of Eq. ( 7.21 ) in predicting the dewfall on grassland.
The upper limit for dew formation will be reached when the aerodynamic term
in Eq. ( 7.21 ) (which counteracts dewfall) is zero. The so-called potential dewfall
then is:

( )
+

sQ * G

LD

pot =

(7.22)

v

s γ

For example, for a night with a mean air temperature of 10 °C, mean Q*- G of
- 40 W m -2 , the potential dewfall rate will be 0.032 mm h -1 , or 0.26 mm during a night
of 8 hours.
There are two ways for the aerodynamic term in Eq. ( 7.21 ) to become zero:

• The air is saturated, resulting in

()−=0 . But if the air is so saturated that fog
occurs, the radiative cooling will be suppressed, leading to a low value for the potential
dewfall.
The aerodynamic resistance tends to ininity. This occurs when the wind speed is zero or

eT e
sat

a a

•

so low that stability effectively suppresses turbulence completely. In that case all turbulent
transport is absent: the supply of water vapour to the surface will vanish and the produc-
tion of dew will cease quickly once all water vapour close to the surface has condensed.

Thus, favourable conditions for dewfall occur when the air is close to saturation and
the wind speed is low, but not so low that turbulence is suppressed.
It should be noted that for situations other than low canopies or wet soils, the mod-
elling of dewfall (and dewrise) will be more complicated. For higher canopies (higher
than about 35 cm) the energy balance needs to be treated in a number of separate lay-
ers (e.g., Jacobs et al., 1990 , 2005 ). Consideration of the energy balance of a control
volume rather than a surface is needed (see Chapter 1 ). For dry soils the assumption
of a wet surface will no longer be valid and the model needs to be adjusted accord-
ingly. Furthermore, in the case of sloping terrain the effect of differences in insolation
(and subsequent longwave cooling at night) between slopes needs to be taken into
account, as well as the possibility of katabatic drainage lows leading to local cooling
in depressions (see Jacobs, 2000 ).

Question 7.14: Estimation of dewfall using the Penman equation.
a) Which conditions are favourable for dewfall to occur (consider time of day, cloudi-
ness, water vapour content in the air and wind speed; explain for each variable why
this is a favourable condition).
b) For dewfall to occur, the air does not need to be saturated. Use the Penman - Monteith
equation to derive an expression for the maximum vapour pressure deicit (VPD) at
which dewfall can still occur.