PROCEEDINGS: Index of Abstracts
AN ENERGY BALANCE MODEL FOR FOREST CANOPIES: A CASE
1-University of Maine, Orono, ME 04473. 2-Laboratory
for Terrestrial Physics-NASA/GSFC, Greenbelt, MD 20771.
The use of thermal scanning devices to map underlying terrain surface
temperatures has been recognized as a potential tool for estimating
evapotranspiration and latent heat flux densities in forest canopies.
Ecologically, knowledge of the temperature distribution of phyto-elements
and the underlying soil surface is directly applicable to understanding
photosynthetic, heat and mass transfer and soil decomposition rates.
Such knowledge of the current state and surface characterization
of terrestrial ecosystems is a critical starting point for understanding
Global Change processes and modeling the underlying energy exchanges.
A steady-state thermal radiance model to compute thermal exitance
and energy balance components within forest canopies is presented.
The model treats fully leafed canopies as discrete ensembles of
leaves partitioned into slope-angle and height classes. Short-wave
energy flux absorbed within the canopy is estimated by solving simplified
radiosity equations. Sensible heat exchange is estimated using a
logarithmic wind profile above the canopy and a modified exponential
profile within the canopy. The latent heat boundary layer resistance
is estimated from site-specific measurements summarizing the effects
of solar irradiance, air temperature and vapor pressure deficit
on stomatal conductance. Comparisons are presented of the basic
model with field measurements conducted at a dense spruce-fir forest
study site at Howland, Maine. Field data included eddy correlation
values of latent and sensible heats as well as IR measurements of
foliage temperatures. For clear days the resulting root mean square
error in modeled versus measured canopy temperatures was 1.2°
C. Corresponding errors in latent and sensible heat flux energy
budget terms were 30 and 32 W m-2, respectively. For
partly cloudy days the root mean square error in predicted temperature
was 1.0° C and corresponding errors in latent and sensible heat
were 40 and 110 W m-2.