Three-dimensional effects in polarization signatures as observed from precipitating clouds by low frequency ground-based microwave radiometers

摘要

Consistent negative polarization differences (i.e. differences betweenthe vertical and the horizontal brightness temperature)are observed when lookingat precipitating systems by ground-based radiometers at slant angles.These signatures can be partially explained by one-dimensional radiativetransfer computations that include oriented non-spherical raindrops. Howeversome cases are characterized by polarization values that exceeddifferences expected from one-dimensional radiative transfer.

A three-dimensional fully polarized Monte Carlo model has been used to evaluatethe impact of the horizontal finiteness of rain shafts with differentrain rates at 10, 19, and 30 GHz. The results show that because of thereduced slant optical thickness in finite clouds, the polarization signalcan strongly differ from its one-dimensional counterpart. At the higherfrequencies and when the radiometer is positioned underneath the cloud,significantly higher negative values for the polarization are found whichare also consistent with some observations. When the observation pointis located outside of the precipitating cloud, typical polarizationpatterns (with troughs and peaks) as a function of the observationangle are predicted.An approximate 1-D slant path radiative transfer model is consideredas well and results are compared with the full 3-D simulations toinvestigate whether or notthree-dimensional effects can be explained by geometry effects alone.The study has strong relevance for low-frequency passive microwavepolarimetric studies.