Millimetre-wave radar measurement of rain and volcanic ash
Abstract
This thesis presents the development of various methods for measuring rainfall
rates using horizontally-pointing millimetre-wave radars. This work builds from
the combination of a T-matrix scattering model that allows the scattering from
almost arbitrarily pro led rotationally symmetric particles to be calculated, and
drop shape models that allow the effects of temperature and pressure on the shape
to be taken into account.
Many hours of rain data have been collected with 38 and 94 GHz FMCW radars,
as well as with a disdrometer and weather station. These have been used to develop
single- and dual-frequency techniques for measuring rainfall rate.
A temperature, polarisation and attenuation corrected application of simple
power-law relationships between reflectivity and rainfall rate has been successfully
demonstrated at 38 GHz. However, at 94 GHz it has been found that more detailed
functions relating reflectivity, attenuation and rainfall rate are beneficial. A
reflectivity-based determination of attenuation has been adapted from the literature
and successfully applied to the 94 GHz data, improving the estimate of rainfall rate
at longer ranges.
The same method for estimating attenuation has also been used in a dualfrequency
technique based on the ratio of the extinction coefficients at 38 and 94
GHz, but with less success. However, a dual-frequency reflectivity ratio based approach
has been successfully developed and applied, producing good estimates of
rainfall rate, as well as reasonable estimates of two drop-size distribution parameters.
Simulations of radar measurements of airborne volcanic ash have also been carried
out, demonstrating that for most reasonable measurement configurations the
optimal frequencies would typically be 35 GHz or 94 GHz, not the more commonly
used 3-10 GHz. It has also been shown that various existing millimetre-wave radars
could be used to detect ash. Finally, there is a discussion of the optimal frequencies
for dual-frequency measurement of volcanic ash.
Type
Thesis, PhD Doctor of Philosophy
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