Application of a plane-stratified emission model to predict the effects of vegetation in passive microwave radiometry

Abstract

This paper reports the application to vegetation canopies of a coherent model for the propagation of electromagnetic radiation through a stratified medium. The resulting multi-layer vegetation model is plausibly realistic in that it recognises the dielectric permittivity of the vegetation matter, the mixing of the dielectric permittivities for vegetation and air within the canopy and, in simplified terms, the overall vertical distribution of dielectric permittivity and temperature through the canopy. Any sharp changes in the dielectric profile of the canopy resulted in interference effects manifested as oscillations in the microwave brightness temperature as a function of canopy height or look angle. However, when Gaussian broadening of the top and bottom of the canopy (reflecting the natural variability between plants) was included within the model, these oscillations were eliminated. The model parameters required to specify the dielectric profile within the canopy, particularly the parameters that quantify the dielectric mixing between vegetation and air in the canopy, are not usually available in typical field experiments. Thus, the feasibility of specifying these parameters using an advanced single-criterion, multiple-parameter optimisation technique was investigated by automatically minimizing the difference between the modelled and measured brightness temperatures. The results imply that the mixing parameters can be so determined but only if other parameters that specify vegetation dry matter and water content are measured independently. The new model was then applied to investigate the sensitivity of microwave emission to specific vegetation parameters.

Influence of vegetation on SMOS mission retrievals

“…Avoiding sharp transitions in dielectric at canopy boundaries suppresses internal reflections within the canopy and the associated interference patterns in microwave emission. Lee et al (2002) give a more detailed description of this phenomenon.…”

“…Lee et al (2002) extended the application of the original Wilheit (1978) microwave emission model upwards through the vegetation canopy to create a model of the microwave emission that represents both the soil and the vegetation together. Thus, in this extended Wilheit (1978) model, the vegetation canopy is assumed to be infinite in horizontal extent and to be made up of homogeneous, planar layers of dielectric medium, which lie above similar homogeneous, planar layers of dielectric medium representing the soil.…”

“…In the Lee et al (2002) model, the dielectric of the vegetation matter itself (ε v ) is calculated assuming a linear version of the Dobson et al (1985) mixing model (hitherto used for soils), i.e. from:…”

“…Moreover, in the context of the SMOS mission, understanding the role of vegetation on overall microwave emission is particularly important because the water content of the vegetation is one of the geophysical variables to be retrieved and the large (~50 km) footprint of the sensor means that vegetation cover will rarely be homogeneous across the area sampled. Lee et al (2002) extended an existing coherent radiative transfer model (Wilheit, 1978) to create what is, in effect, a new approach to modelling the effect of vegetation on passive microwave emission and explored the ability of their model to describe field data. This approach results in a plausibly realistic model of intermediate complexity that captures, adequately, most of the important features of the microwave emission of vegetation, such as the influence of canopy type/structure, polarisation and wavelength of the radiation, look-angle of the sensor and temperature and water content of vegetation.…”

“…This approach results in a plausibly realistic model of intermediate complexity that captures, adequately, most of the important features of the microwave emission of vegetation, such as the influence of canopy type/structure, polarisation and wavelength of the radiation, look-angle of the sensor and temperature and water content of vegetation. In the present study, the modelling approach of Lee et al (2002) was adopted and used to simulate synthetic, multi-angle, L-band microwave brightness temperatures of different soil-vegetationatmosphere systems to investigate (a) the retrieval capability of the proposed SMOS retrieval algorithm; and (b) the effect of heterogeneity in vegetation parameters on retrievals made with the proposed SMOS retrieval algorithm.…”