Impact of ice aggregate parameters on microwave and sub-millimetre
scattering properties
Abstract
Improved observations of ice hydrometeors can lead to better weather
predictions and understanding of the hydrological cycle. Global coverage
is best achieved by satellites, using active and passive microwave
remote sensing due to the inherent penetration capability and
sensitivity to the snow and ice cloud particles which scatter and absorb
the radiation. Snow and cirrus cloud particles are to a large degree
composed of aggregates. While it is known that the shape of these
aggregates influence microwave measurements to various degrees, it is
currently not fully clear to what extent certain particle features are
of importance. For example, of what importance is the shape of
individual crystals in the aggregates? This work is an attempt at
improving our knowledge on the impact of ice aggregate microphysics on
microwave and sub-millimetre scattering properties from a modelling
point of view. A large amount of aggregates (roughly 4000) where
modelled through several semi-physical stochastic simulations. The
aggregates are composed of hexagonal ice crystals of varying axis ratio,
ranging from 1/15 (plates) to 15 (columns), and assumed to be oriented
in the horizontal plane. Single scattering properties of over 1000
aggregates were then assessed for zenith/nadir observations, using the
discrete dipole approximation (DDA) at three typical radar bands (13.4,
35.6, 94.1 GHz) and three passive microwave frequencies (183.3, 325.15
and 664 GHz). An analysis on the sensitivity of these scattering data to
various aggregate parameters is presented. In general, extinction was
found to be less sensitive to shape than back-scattering at investigated
at frequencies. Extinction at 664 GHz in particular was found to be
shape insensitive; promising for the sake of the Ice Cloud Imager (ICI)
on the upcoming Metop-SG satellite. In contrast, evaluation of triple
frequency signatures showed relatively high shape sensitivity; of
relevance to future multi-frequency radars.