L-band Microwave Emission of the Biosphere (L-MEB) Model: Description and calibration against experimental data sets over crop fields

Abstract

In the near future, the SMOS (Soil Moisture and Ocean Salinity) mission will provide global maps of surface soil moisture (SM). The SMOS baseline payload is an L-band (1.4 GHz) two dimensional interferometric microwave radiometer which will provide multi-angular and dual-polarization observations. In the framework of the ground segment activities for the SMOS mission an operational SMOS Level 2 Soil Moisture algorithm was developed. The principle of the algorithm is to exploit multi-angular data in order to retrieve simultaneously several surface parameters including soil moisture and vegetation characteristics. The algorithm uses an iterative approach, minimizing a cost function computed from the differences between measured and modelled brightness temperature (T_B) data, for all available incidence angles.

In the algorithm, the selected forward model is the so-called L-MEB (L-band Microwave Emission of the Biosphere) model which was the result of an extensive review of the current knowledge of the microwave emission of various land covers. This model is a key element in the SMOS L2 algorithm and could be used in future assimilation studies. There is thus a strong need for a reference study, describing the model and its implementation. In order to address these needs a detailed description of soil and vegetation modelling in L-MEB is given in this study. In a second step, the use of L-MEB in soil moisture retrievals is evaluated for several experimental data sets over agricultural crops. Calibrations of the soil and vegetation L-MEB parameters are investigated for corn, soybean and wheat. Over the different experiments, very consistent results are obtained for each vegetation type in terms of calibration and soil moisture retrievals.

Introduction

L-band (1.1–1.7 GHz) microwave radiometry is one of the most relevant remote sensing techniques to monitor soil moisture over land surfaces at the global scale (Jackson et al., 1999, Kerr, in press, Njoku et al., 2003, Schmugge, 1998). Two proposed space missions, SMOS (Soil Moisture and Ocean Salinity, Kerr et al., 2001), and Hydros (Hydrosphere State, Entekhabi et al., 2004) are based on that technique in order to obtain global maps of the surface soil moisture in the near future. The SMOS mission was proposed to the European Space Agency in the framework of the Earth Explorer Opportunity Missions in 1998; it is planned for a launch in 2007. The baseline SMOS payload is an L-band (1.4 GHz) two dimensional (2-D) interferometric radiometer that is Y shaped with three 4.5 m arms. SMOS aims at providing global maps of soil moisture, with an accuracy better than 0.04 m³/m³ every 3 days, with a space resolution better than 50 km (Kerr et al., 2001).

As the satellite moves over the Earth, a given point within the Field Of View (FOV) is observed from different view angles by the 2-D interferometer. The series of dual-polarized multi-angular measurements allow simultaneous retrievals of several surface parameters including soil moisture and vegetation optical depth (Wigneron et al., 2001). As part of the SMOS mission, geophysical products such as soil moisture (SM) and vegetation opacity (τ) will be produced by an operational algorithm. The principle of the algorithm is based on an iterative approach, minimizing a cost function computed from the sum of squared weighted differences between measured and modelled microwave brightness temperature (T_B) data, for a variety of incidence angles (Kerr et al., 2006). In the algorithm, for each incidence angle, the different cover types (bare soil and vegetated area, open water, urban area, etc.) present within the SMOS footprint are estimated from high resolution land use maps. For low vegetation and forest categories, these maps used a large number of sub-categories corresponding, for instance, to grasslands, crops, scrubs, tropical and boreal forests, which were distinguished for a variety of climatic and geographic conditions. Currently, the ECOCLIMAP data base (Masson et al., 2003) that distinguishes 218 ecosystems at 1 km resolution was selected as the reference landcover map. Within each pixel, the brightness temperatures from each cover type are simulated with a forward model and then aggregated, accounting for the SMOS field of view and antenna pattern. Parameters driving the forward model are selected and tabulated based on the selected vegetation classes and on maps of soil properties (for soil texture, roughness and bulk density). This manuscript will only describe the forward model used over each homogeneous vegetation type and the description of the whole algorithm and of the aggregation process over heterogeneous pixels is described in Kerr et al. (2006). The forward model selected is the so-called L-MEB (L-band Microwave Emission of the Biosphere; Wigneron et al., 2003) model which was used in the first ESA studies aiming at evaluating SMOS capabilities from synthetic data sets (Pellarin et al., 2003a, Pellarin et al., 2003b, Pellarin et al., 2003c). The L-MEB model was the result of an extensive review of the current knowledge of the microwave emission of various land cover types (herbaceous and woody vegetation, frozen and unfrozen bare soil, etc.), with the objective of being accurate while remaining simple enough for operational use at global scale and while allowing developments to be incorporated as they occur.

Since the first version of L-MEB, a large number of experimental campaigns have been carried out for a variety of vegetation/soil characteristics and climatic conditions (De Rosnay et al., 2006a, Fenollar et al., 2006, Grant et al., 2007, Hornbuckle et al., 2003, Schwank et al., 2005). Combined experimental and modelling activities have contributed to improving very significantly our knowledge of the key processes that drive the emission of the soil and vegetation canopy such as rainfall interception within the canopy, mulch and litter in prairies and forests, surface roughness, effective soil temperature, dependence of vegetation attenuation on configuration parameters (incidence angle, polarization), etc. These results were integrated in L-MEB. As L-MEB is the forward model used in the processing of the SMOS level-2 soil moisture products, it may also be used in assimilation studies of microwave brightness temperature observations developed by assimilation centres (Dirmeyer and Gao, 2004, Seuffert et al., 2003). There is thus a strong need for a study describing L-MEB in detail and providing key information for model calibration over a variety of vegetation types.

The objective of this study is to describe the new version of L-MEB and analyse its calibration and validation over cropped fields. First, a reference description of L-MEB will be given, including key results which have contributed to recent improvements of the model. The focus of the present paper is on vegetation canopies with low levels of biomass and soil; the case of forests will be the subject of another paper. Second, the calibration and validation of L-MEB will be investigated from SM retrievals using experimental data sets over cropped fields. L-MEB calibration over grassland was investigated separately in another paper (Saleh et al., submitted for publication).