Frost Protection - UTL Repository

MECHANISMS OF ENERGY TRANSFER
R n supplies energy that heats the air, plants and soil or evaporates water. In this
book, the equation in Figure 3.3 is used for the surface energy balance. Note that
energy storage in the plants, photosynthesis and respiration are generally ignored
in vertical energy fluxes in frost protection. Assuming that all of the energy
fluxes are vertical, energy from R n is partitioned into the components G, H and
LE, so R n is set equal to the sum of G, H and LE (Eq. 3.1).
R LE Wm -2 n = G + H +
Eq. 3.1
Again, the sign of the energy flux component indicates the direction of energy
flow. Radiation adds energy to the surface, so it is positive to the surface. When
G is positive, energy is going into the soil, and when H and LE are positive, the
energy flux is upward to the atmosphere. Therefore, G, H and LE fluxes are
positive away from the surface and negative towards the surface.
Although, most energy transfer on a frost night is vertical, a crop is threedimensional,
and energy can pass horizontally as well as vertically through a
crop. Energy transfer through a crop is often depicted using an energy box
diagram (Figure 3.4), which represents the volume of air to be heated in frost
protection. The energy content of the box in the diagram depends on the sources
and losses of energy (Figure 3.4), where most of the energy fluxes can be in either
direction. The energy balance for the box is given by:
R P R Wm -2 n = G + H + LE + F 1 + F 2 + ∆S
+
Eq. 3.2
where R n is a positive number when more energy from radiation is received than
is emitted and reflected, and it is negative if more radiant energy is lost than
gained. The variables G, H and LE are all positive when the energy is exiting
from the box and are negative if the energy is entering the box. F 1 is horizontal
sensible and latent heat flux into the box (a negative number) and F 2 is horizontal
sensible and latent heat flux out of the box (a positive number). The sum of F 1
and F 2 is the net difference in horizontal flux of sensible and latent heat. The
variable P R is for photosynthesis (a positive number) and respiration (a negative
number). However, P R is small and commonly ignored for energy balance
calculations. The variable ∆S is the change in stored energy (sensible heat) within
the box, which is positive if the energy content increases (e.g. when the
temperature increases) and it is negative when the energy content decreases
(e.g. temperature falls).
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