Conclusions

This paper describes a new dual-source model of heat transfer at heterogeneous land surfaces. This model avoids assumptions on the vertical and horizontal structure of the surface layer by dealing separately with heat transfer in the canopy air space and in the surface layer above the canopy. The model requires observations of surface brightness temperature at two view angles: nadir and a large off-nadir angle. These observations are currently provided at low spatial resolution by the ATSR instruments on-board ERS-1, ERS-2 and ENVISAT in the near future. The Land Surface Processes and Interactions Mission (LSPIM) under consideration at ESA would provide observations at much higher spatial resolution and at additional view angles. The model does also require fractional vegetation cover and a characteristic linear dimension of plant leaves. Fractional vegetation cover can be estimated with a variety of algorithms and observations of the spectro- directional reflectance in the visible and near infrared spectral region. Estimation of leaf size is obviously more difficult, although it may be retrieved by inverting radiative transfer modeling of spectro-directional radiometric measurements. Regional representative leaf size can also be estimated for each canopy given a correct vegetation classification. We note that the alternative single-source model requires the determination of the roughness length for heat transport. Several studies have demonstrated the difficulties involved in the determination of generally applicable values of this land surface property.

Values of sensible heat flux obtained with our model and ATSR data were compared with field measurements collected during two field experiments in China (HEIFE and IMGRASS). Agreement was good in both cases. Detailed field directional measurements of brightness temperature were collected during IMGRASS. This made feasible the comparison with measurements of H throughout the entire campaign. Agreement was good, taking into account the moderate accuracy of eddy correlation systems.

The analysis of field measurements indicates that the dual-source model proposed in this paper describes correctly heat transfer in the canopy air space and to the surface layer above the canopy. The scope of the validation of estimates based on ATSR data was limited since only one ATSR data set was analyzed in each experiment. Future work will address this aspect taking advantage of the algorithms developed for this study and of easier access to ATSR data.

0 0

Post a comment