Ice Scattering Experiments (ISE)
Cirrus clouds, despite being thin and wispy in appearance, have a
profound influence on the Earth's climate. By redirecting solar and
thermal radiation they can have an overall warming effect - unlike other
types of cloud. Moreover, cirrus may be causing a positive climate
feedback: higher temperature can result in more cirrus, potentially
leading to further warming and so on. This would have the effect
of amplifying man-made climate change. For these reasons, it is very
important that properties of cirrus are incorporated into climate
models. While such models have become very comprehensive and accurate,
cirrus clouds are the source of some of the greatest remaining
uncertainties. This is because the properties of cirrus, most notably
the manner in which small ice crystals contained in the clouds scatter
light, are not yet accurately known.
We have been developing new methods of studying cirrus, making it
possible to measure light scattering in the laboratory, instead of
having to take complex equipment into high-altitude clouds. The methods
are based on combining techniques like electrodynamic
levitation with materials resembling ice - ice analogues.
Levitation allows us to measure scattering properties like the phase
function (for scattering angles from 0.5˚ to 177˚),
the degree of linear polarization and the asymmetry parameter
for single ice-analogue crystals. We have developed two ice analogues:
fine glass fibres with hexagonal cross-section and crystals with shape,
size and optical properties closely resembling those of atmospheric ice.
Unlike ice, these materials can exist at room temperature and do not change
their shape and size under normal conditions, which makes them much
easier to study than ice. They can also be transported between different
laboratories. Therefore, in addition to light scattering studies, the analogues
are very useful for the testing and calibration of in situ cloud probes such as
the SPEC Cloud Particle Imager
and the University of Hertfordshire Small Ice Detector
series of instruments. We have produced a variety of crystals,
including simple shapes such as hexagonal columns or platelets and more
complex ones such as rosettes and aggregates - see pictures on this
page. Further examples can be viewed in the Ice-Analogue
Image Gallery, which also features the
Ice-Analogue Flower Garden.
Scattering theory
Observations of scattering effects produced by the analogues have led to the development of a novel, efficient computational model describing the scattering of light from faceted objects such as crystals. It approximates scattering by combining ray tracing with a new formulation of diffraction. Tests done using the analogues have shown the model to be more accurate than the most widely used method for calculating scattering from cirrus crystals - the Geometric Optics approach.
Halos
A beautiful way in which cirrus
clouds manifest themselves is through halos - circles, arcs and spots of
white or coloured light seen when sunlight or moonlight strikes thin
ice-clouds. Cirrus is a virtually unique source of these displays
because they are produced by the refraction of light-rays passing
through ice crystals. A tangible demonstration that the new ice analogues do
indeed resemble real ice was provided in September 2002. We used the
crystals to produce realistic halo displays - apparently the first time
this was achieved in a laboratory - see the picture here and the Analogue Halos page.
Polarimetry
The non-sphericity of cirrus ice crystals affects the way they scatter light, and consequently the radiative balance of the atmosphere. Conversely, light scattering can be used to determine the shape of cirrus ice crystals and thus obtain information necessary for the modelling of the radiative effects. A property of scattered light especially sensitive to particle shape is polarization. We are developing an imaging polarimeter for the remote sensing of cirrus - see our Cloud Polarimetry page.
This project has been funded by the NERC Clouds Climate and Water Vapour thematic programme.