Simulating L-Band Emission of Coniferous Forests Using a Discrete Model and a Detailed Geometrical Representation

Self Cite

Abstract-In this paper, the L-band Microwave Emission of the Biosphere (L-MEB) model used in the Soil Moisture and Ocean Salinity (SMOS) Level 2 Soil Moisture algorithm is calibrated using L-band (1.4 GHz) microwave measurements over a coniferous (Pine) and a deciduous (mixed/Beech) forest. This resulted in working values of the main canopy parameters optical depth (τ ), single scattering albedo (ω), and structural parameters tt(H) and tt(V), besides the soil roughness parameters H R and N R . Using these calibrated values in the forward model resulted in a root mean-square error in brightness temperatures from 2.8 to 3.8 K, depending on data set and polarization. Furthermore, the relationship between canopy optical depth and leaf area index is investigated for the deciduous site. Finally, a sensitivity study is conducted for the focus parameters, temperature, soil moisture, and precipitation. The results found in this paper will be integrated in the operational SMOS Level 2 Soil Moisture algorithm and used in future inversions of the L-MEB model, for soil moisture retrievals over heterogeneous, partly forested areas.Index Terms-Forest, L-band, microwave radiometry, soil moisture, Soil Moisture and Ocean Salinity (SMOS).

Section: Tor Vergata Modelmentioning

confidence: 99%

“…This paper presents the first calibration of the forward model L-MEB over forests using experimental data. Previous forest studies in the context of SMOS were all based on modeling and/or very short-term experiments (e.g., [16] and [17]). …”

Section: Introductionmentioning

confidence: 99%

Calibration of the L-MEB Model Over a Coniferous and a Deciduous Forest

Grant

Saleh-Contell

Wigneron³

et al. 2008

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“…Several theoretical [7]- [9] and experimental studies [10]- [14] were performed with regard to deciduous and coniferous forests. The Forest Soil Moisture Experiment (FOSMEX) [12] performed at a deciduous forest site at the Research Center Jülich, Germany, showed the semitransparency of the forest canopy at L-band frequencies, allowing for sensing the forest ground radiation.…”

Section: Introductionmentioning

confidence: 99%

Testing a New Model for the L-Band Radiation of Moist Leaf Litter

Schwank

Guglielmetti²,

Mätzler

et al. 2008

Abstract-The crown vegetation of a deciduous forest is known to be semitransparent at low microwave frequencies, and leaf litter covering the forest soil has been recognized to have a significant impact on ground emission. The proposed approach for modeling the L-band radiative transfer through leaf litter consists of an isotropic effective medium approach for the litter permittivities, a coherent radiative transfer model for computing the coherent reflectivities from dielectric depth profiles, and an averaging procedure for computing the reflectivities determining the field-scale brightness temperatures. Evaluations were performed for the case of leaf litter on top of a conducting wire grid (litter-grid formation) and for litter on underlying soil (litter-soil formation). A model sensitivity analysis was performed with respect to parameters characterizing litter thickness variations and boundary roughness. For the litter-soil formation, the model was rather sensitive to local irregularities at the air-to-litter boundary. Modeled microwave signatures reproduced the major features of the measurements performed on a site comprising a litter-grid formation. Under dry conditions, the investigated litter layer was nearly "invisible." When the same litter layer was wetted, it acted as an important radiation source to be taken into account for the quantitative remote soil moisture detection of forested areas. Under certain conditions, the simulations revealed an increasing brightness when the litter is wetted prior to the underlying soil. Further wetting of the litter-soil system then resulted in a decreasing brightness as expected for increased moisture. Such effects are important to know to avoid misleading interpretations of L-band signatures.

“…Discrete radiative transfer approaches used for forward modeling the L-band radiation of forests are presented in [9]- [11]. These sophisticated approaches are using dielectric cylinders with known radiation patterns, suitably arranged for representing the morphology of a forest stand and the corresponding forest radiation.…”

mentioning

confidence: 99%

FOSMEX: Forest Soil Moisture Experiments With Microwave Radiometry

Guglielmetti¹,

Schwank

Mätzler

et al. 2008

Abstract-The microwave Forest Soil Moisture Experiment (FOSMEX) was performed at a deciduous forest site at the Research Centre Jülich (Germany). An L-and an X-band radiometer were mounted 100 m above ground and directed to the canopy. The measurements consist of dual-and single-polarized L-and X-band data and simultaneously recorded ground moisture, temperature, and meteorological data. The canopy L-band transmissivity was estimated from a subset of the FOSMEX data, where the ground was masked with a metalized foil. For the foliage-free canopy, the reflecting foil diminished the L-band brightness by ≈24 K, whereas brightness increased by ≈14 K when the foil was removed from below the foliated canopy. Depending on the assumption made on the scattering albedo of the canopy, the transmissivities were between 0.2 and 0.51. Furthermore, the contribution of the foliage was quantified. Although, the evaluation revealed the semitransparency of the canopy for L-band frequencies, the brightness sensitivity with respect to ground moisture was substantially reduced for all foliation states. The effect of ground surface moisture was explored in an irrigation experiment. The L-band measurements were only affected for a few hours until the water drained through the litter layer. This emphasizes the significance of the presence of litter for soil moisture retrieval from remotely sensed L-band brightness data. The FOSMEX database serves for further testing and improving radiative transfer models used for interpreting microwave data received from future spaceborne L-band radiometers flying over areas comprising a considerable fraction of deciduous forests.