Abstract
A simulation study to understand the influence of topography on the surfaceemissivity observed by a satellite microwave radiometer is carried out. We analyze theeffects due to changes in observation angle, including the rotation of the polarization plane.A mountainous area in the Alps (Northern Italy) is considered and the information on therelief extracted from a digital elevation model is exploited. The numerical simulation refersto a radiometric image, acquired by a conically-scanning radiometer similar to AMSR-E,i.e., flying at 705 km of altitude with an observation angle of 55°. To single out the impacton surface emissivity, scattering of the radiation due to the atmosphere or neighboringelevated surfaces is not considered. C and X bands, for which atmospheric effects arenegligible, and Ka band are analyzed. The results indicate that the changes in the localobservation angle tend to lower the apparent emissivity of a radiometric pixel with respectto the corresponding flat surface characteristics. The effect of the rotation of thepolarization plane enlarges (vertical polarization), or attenuates (horizontal polarization)this decrease. By doing some simplifying assumptions for the radiometer antenna, theconclusion is that the microwave emissivity at vertical polarization is underestimated,whilst the opposite occurs for horizontal polarization, except for Ka band, for which bothunder- and overprediction may occur. A quantification of the differences with respect to aflat soil and an approximate evaluation of their impact on soil moisture retrieval areyielded.
Highlights
Spaceborne microwave radiometric observations of land are mainly determined by surface emissivity and temperature, especially at frequencies where the atmosphere is more transparent
The results indicate that the changes in the local observation angle tend to lower the apparent emissivity of a radiometric pixel with respect to the corresponding flat surface characteristics
We have focused our analysis on a mountainous area in the Alps (Northern Italy) and we have derived the topography from a digital elevation model (DEM) with a spatial resolution of 250×250 m
Summary
Spaceborne microwave radiometric observations of land are mainly determined by surface emissivity and temperature, especially at frequencies where the atmosphere is more transparent. Large-scale relief effects on the upwelling brightness temperature (TB) measured by a spaceborne radiometer are [2,3]: i) modification of the atmospheric contributions due to their dependence on the altitude of the emitting surface; ii) shadowing of the downwelling atmospheric radiation; iii) local modification of the observation angle with respect to a flat terrain, which may cause shadowing of the upwelling surface radiation; iv) rotation of the linear polarization plane. These effects imply an overall change of the apparent emissivity with respect to the surface characteristics that can affect the retrieval of bio-geophysical parameters, such as soil moisture
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