6 - DISCUSSION
Comparisons between the Cupid model and field measurements provide reasonable evidence
that the model is capturing essential characteristics of directional infrared signatures in soil-
vegetation systems. The Cupid model can be used to predict several of the different "canopy
temperatures" discussed in the Definitions section of this paper. Figure 4 contains air temper
atures along with predictions of the nadir brightness temperature, the soil and mean canopy
kinetic temperatures, and the aerodynamic temperature considering the excess resistance for
heat over momentum (thermal aerodynamic temperature). From this prediction, one might be
tempted to conclude that nadir brightness temperature and aerodynamic temperature are simi
lar, but on the previous day they were 5 C different. The difference between aerodynamic
temperature, based on an excess resistance for heat and aerodynamic temperature assuming
equivalence between heat and momentum transport (momentum aerodynamic temperature),
is shown in Figure 5. One might be tempted to conclude that brightness temperature at 50°
zenith view angle is similar to momentum aerodynamic temperature, as Huband and Monteith
(1986) did, but on the previous day these two temperatures were 6 C apart.
Clearly aerodynamic and radiative temperatures are not the same, but the model Cupid
has been used to derive a simple relation between these two temperatures (Norman et al.,
1994). Essentially the difference between aerodynamic and radiative temperatures arises from
two sources: 1) The emissivity of the soil-vegetation system is less than unity, and 2) the
weighting of temperature differences in the soil-vegetation system is different for radiative
than aerodynamic temperatures. Radiative temperatures are weighted by radiometer field-of-
view and aerodynamic temperatures weighted by aerodynamic resistances of various
components of the soil-vegetation system.
The combination of a mechanistic model of soil-plant-atmosphere processes, which
incorporates thermal radiative exchanges, with field measurements provides a valuable tool
for interpreting remote measurements of infrared temperature and radiation.
Figure 4. Predictions of various temperatures from the model for FIFE site 11 on Day
156. Tcpy is the mean temperature of all the leaves in the canopy, Nadir IR is the
brightness temperature from the nadir view, and Taero-h the thermal aerodynamic
temperature.
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