Conductances for Heat and Mass Transfer
- 1
0
stability
FIGURE 7.1. Diabatic influence and profile correction factors for heat and
momentum as a function of stability (c).
the same. Rearranging Eq. (7.25) and substituting Eq. (7.24) for u* gives:
H =
0.4~fic,u(z>[T(d + ZH) - T(z)l
[h($
+ . MI [h($ + .HI '
The conductance between the "canopy surface" (at height d + ZH) and a
height z above the canopy is therefore
Clearly the conductance depends on the height of the top of the air layer
being considered.
Many of the computations performed simply ignore the profile diabatic
correction factors (they are zero for neutral stability). When they are
important, they are a bit of a challenge to compute, since the fluxes depend
on the correction factors, but the correction factors depend on the heat
flux density. One has to use an iterative approach and usually a computer
to obtain a result. To give some idea of the importance of the stability
correction, Fig. 7.2 shows atmospheric conductance for a range of wind
speeds and canopy;iir temperature differences. It can be seen that for
stable conditions (canopy cooler than air) and low wind speed, the stability
effect can be very large. At high wind speeds there is a smaller effect,
- 1
0
stability
FIGURE 7.1. Diabatic influence and profile correction factors for heat and
momentum as a function of stability (c).
the same. Rearranging Eq. (7.25) and substituting Eq. (7.24) for u* gives:
H =
0.4~fic,u(z>[T(d + ZH) - T(z)l
[h($
+ . MI [h($ + .HI '
The conductance between the "canopy surface" (at height d + ZH) and a
height z above the canopy is therefore
Clearly the conductance depends on the height of the top of the air layer
being considered.
Many of the computations performed simply ignore the profile diabatic
correction factors (they are zero for neutral stability). When they are
important, they are a bit of a challenge to compute, since the fluxes depend
on the correction factors, but the correction factors depend on the heat
flux density. One has to use an iterative approach and usually a computer
to obtain a result. To give some idea of the importance of the stability
correction, Fig. 7.2 shows atmospheric conductance for a range of wind
speeds and canopy;iir temperature differences. It can be seen that for
stable conditions (canopy cooler than air) and low wind speed, the stability
effect can be very large. At high wind speeds there is a smaller effect,