| Consistent negative polarization differences (i.e. differences between
the vertical and the horizontal brightness temperature)
are observed when looking
at precipitating systems by ground-based radiometers at slant angles.
These signatures can be partially explained by one-dimensional radiative
transfer computations that include oriented non-spherical raindrops. However
some cases are characterized by polarization values that exceed
differences expected from one-dimensional radiative transfer.
A three-dimensional fully polarized Monte Carlo model has been used to evaluate
the impact of the horizontal finiteness of rain shafts with different
rain rates at 10, 19, and 30 GHz. The results show that because of the
reduced slant optical thickness in finite clouds, the polarization signal
can strongly differ from its one-dimensional counterpart. At the higher
frequencies and when the radiometer is positioned underneath the cloud,
significantly higher negative values for the polarization are found which
are also consistent with some observations. When the observation point
is located outside of the precipitating cloud, typical polarization
patterns (with troughs and peaks) as a function of the observation
angle are predicted.
An approximate 1-D slant path radiative transfer model is considered
as well and results are compared with the full 3-D simulations to
investigate whether or not
three-dimensional effects can be explained by geometry effects alone.
The study has strong relevance for low-frequency passive microwave
polarimetric studies. |