A 2-SOURCE TIME-INTEGRATED MODEL FOR ESTIMATING SURFACE FLUXES USING THERMAL INFRARED REMOTE-SENSING

We present an operational two-source (soil+vegetation) model for evalu ating the surface energy balance given measurements of the time rate o f change in radiometric surface temperature (T-RAD) during the morning hours. This model consists of a two-source surface component describi ng the relation between T-RAD and sensible heat flux, coupled with a t ime-integrated component connecting surface sensible heating with plan etary boundary layer development. By tying together the time-dependent behavior of surface temperature and the temperature in the boundary l ayer with the flux of sensible heat from the surface to the atmosphere , the need for ancillary measurements of near-surface air temperature is eliminated. This is a significant benefit when T-RAD is acquired re motely. Air temperature can be strongly coupled to local biophysical s urface conditions and, if the surface air and brightness temperature m easurements used by a model are not collocated, energy flux estimates can be significantly corrupted. Furthermore, because this model uses o nly temporal changes in radiometric temperatures rather than absolute temperatures, time-independent biases in T-RAD, resulting from atmosph eric effects or other sources, do not affect the estimated fluxes; onl y the time-varying component of corrections need be computed. The algo rithm also decomposes the surface radiometric temperature into its soi l and vegetation contributions; thus the angular dependence of T-RAD c an be predicted from an observation of T-RAD at a single view angle. T his capability is critical to an accurate interpretation of off-nadir measurements from polar orbiting and geosynchronous satellites. The pe rformance of this model has been evaluated in comparison with data col lected during two large-scale field experiments: the first Internation al Satellite Land Surface Climatology Project field experiment, conduc ted in and around the Konza Prairie in Kansas, and the Monsoon '90 exp eriment, conducted in the semiarid rangelands of the Walnut Gulch Wate rshed in southern Arizona. Both comparisons yielded uncertainties comp arable to those achieved by models that do require air temperature as an input and to measurement errors typical of standard micrometeorolog ical methods for flux estimation. A strategy for applying the two-sour ce time-integrated model on a regional or continental scale is briefly outlined. (C) Elsevier Science Inc., 1997.