Utility of Radiometric–aerodynamic Temperature Relations for Heat Flux Estimation |
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| Authors: | William P Kustas Martha C Anderson John M Norman Fuqin Li |
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| Institution: | (1) USDA-ARS Hydrology and Remote Sensing Laboratory, Bldg. 007, BARC-West, Beltsville, MD, USA;(2) Department of Soil Science, University of Wisconsin, Madison, WI, USA |
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| Abstract: | In many land-surface models using bulk transfer (one-source) approaches, the application of radiometric surface temperature
observations in energy flux computations has given mixed results. This is due in part to the non-unique relationship between
the so-called aerodynamic temperature, which relates to the efficiency of heat exchange between the land surface and overlying
atmosphere, and a surface temperature measurement from a thermal-infrared radiometer, which largely corresponds to a weighted
soil and canopy temperature as a function of radiometer viewing angle. A number of studies over the past several years using
multi-source canopy models and/or experimental data have developed simplified methods to accommodate radiometric–aerodynamic
temperature differences in one-source approaches. A recent investigation related the variability in the radiometric–aerodynamic
relation to solar radiation using experimental data from a variety of landscapes, while another used a multi-source canopy
model combined with measurements over a wide range in vegetation density to derive a relationship based on leaf area index.
In this study, simulations by a detailed multi-source soil–plant–environment model, Cupid, which considers both radiative
and turbulent exchanges across the soil–canopy–air interface, are used to explore the radiometric–aerodynamic temperature
relations for a semi-arid shrubland ecosystem under a range of leaf area/canopy cover, soil moisture and meteorological conditions.
The simulated radiometric-aerodynamic temperatures indicate that, while solar radiation and leaf area both strongly affect
the magnitude of this temperature difference, the relationships are non-unique, having significant variability depending on
local conditions. These simulations also show that soil–canopy temperature differences are highly correlated with variations
in the radiometric–aerodynamic temperature differences, with the slope being primarily a function of leaf area. This result
suggests that two-source schemes with reliable estimates of component soil and canopy temperatures and associated resistances
may be better able to accommodate variability in the radiometric–aerodynamic relation for a wider range in vegetated canopy
cover conditions than is possible with one-source schemes. However, comparisons of sensible heat flux estimates with Cupid
using a simplified two-source model and a one-source model accommodating variability in the radiometric-aerodynamic relation
based on vegetation density gave similar scatter. On the other hand, with experimental data from the shrubland site, the two-source
model generally outperformed the one-source scheme. Clearly, vegetation density/leaf area has a major effect on the radiometric–aerodynamic
temperature relation and must be considered in either one-source or two-source formulations. Hence these adjusted one-source
models require similar inputs as in two-source approaches, but provide as output only bulk heat fluxes; this is not as useful
for monitoring vegetation conditions. |
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| Keywords: | Aerodynamic temperature Heat flux estimation One-source modelling Radiometric temperature Remote sensing Two-source modelling |
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