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| Titel |
A large scale microwave emission model for forests. Contribution to the SMOS algorithm |
| VerfasserIn |
R. Rahmoune, A. Della Vecchia, P. Ferrazzoli, L. Guerriero, F. Martin-Porqueras |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023624
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| Zusammenfassung |
| 1. INTRODUCTION
It is well known that surface soil moisture plays an important role in the water cycle and the global climate.
SMOS is a L-Band multi-angle dual-polarization microwave radiometer for global monitoring of this variable. In the areas covered by forests, the opacity is relatively high, and the knowledge of moisture remains problematic. A significant percentage of SMOS pixels at global scale is affected by fractional forest. Whereas the effect of the vegetation can be corrected thanks a simple radiative model, in case of dense forests the wave penetration is limited and the sensitivity to variations of soil moisture is poor. However, most of the pixels are mixed, and a reliable estimate of forest emissivity is important to retrieve the soil moisture of the areas less affected by forest cover. Moreover, there are many sparse woodlands, where the sensitivity to variations of soil moisture is still acceptable. At the scale of spaceborne radiometers, it is difficult to have a detailed knowledge of the variables which affect the overall emissivity.
In order to manage effectively these problems, the electromagnetic model developed at Tor Vergata University was combined with information available from forest literature. Using allometric equations and other information, the geometrical and dielectric inputs required by the model were related to global variables available at large scale, such as the Leaf Area Index. This procedure is necessarily approximate. In a first version of the model, forest variables were assumed to be constant in time, and were simply related to the maximum yearly value of Leaf Area Index. Moreover, a unique sparse distribution of trunk diameters was assumed. Finally, the temperature distribution within the crown canopy was assumed to be uniform. The model is being refined, in order to consider seasonal variations of foliage cover, subdivided into arboreous foliage and understory contributions. Different distributions of trunk diameter are being considered. Also the effects of temperature gradients within the crown canopy are being considered.
The model was tested against radiometric measurements carried out by towers and aircrafts. A new test has been done using the brightness temperatures measured over some forests in Finland by the AMIRAS radiometer, which is an airborne demonstrator of the MIRAS imaging radiometer to be launched with SMOS.
The outputs produced by the model are used to fit the parameters of the simple radiative transfer model which will be used in the Level 2 soil moisture retrieval algorithm. It is planned to compare model outputs with L1C data, which will be made available during the commissioning phase. To this end, a number of adequate extended forest sites are being selected: the Amazon rain forest, the Zaire Basins, the Argentinian Chaco forest, and the Finland forest.
2. PARAMETRIC STUDIES
In this paper, results of parametric simulations are shown. The emissivity at vertical and horizontal polarization is simulated as a function of soil moisture content for various conditions of forest cover. Seasonal effects are considered, and the values of Leaf Area Index in winter and summer are taken as basic inputs. The difference between the two values is attributed partially to arboreous foliage and partially to understory, while the woody biomass is assumed to be constant in time. Results indicate that seasonal effects are limited, but not negligible. The simulations are repeated for different distributions of trunk diameters. If the distributions is centered over lower diameter values, the forest is optically thicker, for a given biomass. Also the variations of brightness temperature due to a temperature gradient within the crown canopy have been estimated. The outputs are used to predict the values of a simple first order RT model.
3. COMPARISONS WITH EXPERIMENTAL DATA
Results of previous comparisons between model simulations and experimental data are summarized. Experimental data were collected by tower, in the Julich and Les Landes forest (Bray site) and by aircraft, over some forests in Tuscany. New comparisons have been done between model simulations and brightness temperature data collected by the AMIRAS demonstrator in southern Finland, in the region of the Lohja Lake. The region was dominated by lakes and dense forests, of both broadleaf and coniferous species. The flights took place on June 2006, and the outputs were made available in an earth fixed grid together with the information describing the geometry between the target and the sensor similarly to forthcoming SMOS L1C data. For both forest types, the brightness temperature values predicted by the model are close to the center of the histograms of measured values. The experimental data show a slight difference between vertical and horizontal polarization, which is underestimated by the model. |
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