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Multifractal modelling of liquid water clouds : cloud spatial structure and its effect on the solar radiation field


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Fienberg, KS 2003 , 'Multifractal modelling of liquid water clouds : cloud spatial structure and its effect on the solar radiation field', PhD thesis, University of Tasmania.

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At any point in time a significant fraction of the globe is covered by liquid
water clouds. Understanding the relationship between cloud and solar radiation
is therefore of great importance for both climate modelling and remote sensing.
Previous studies have found that cloud spatial structure has a significant effect on
cloud albedo, with spatial inhomogeneities leading to less reflection of solar radiation
than was predicted by the traditional homogeneous cloud model. This thesis
investigates further the consequences of cloud spatial variability on atmospheric
radiation, with the aim of developing methods to improve radiation modelling and
remote sensing of cloud properties.
The basis of this study is the quantification of the scaling and intermittency
of liquid water fields using a multifractal model. The fractionally integrated flux
(FIF) model is used to both describe and numerically simulate cloud fields, with
model parameters being determined from aircraft measurements made during 98
flights over northern Tasmania, Australia. The aircraft data set is divided into
three broad cloud types: stratocumulus, altostratus and low level cumulus. The
horizontal fluctuations in all three cloud types are shown not only to be scale
invariant and non-stationary, but also to have very similar statistics with only one
out of three model parameters varying significantly between cloud types.
Clouds with horizontal structure described by the FIF model but constant vertical
profiles are then used in Monte Carlo radiative transfer calculations. The
differences between the multifractal and homogeneous cloud results are larger than
those previously reported for marine stratocumulus, due to the larger degree of
inhomogeneity in the cloud types considered. The results of the Monte Carlo simulations
are used to derive the "effective optical properties" of the multifractal cloud
fields, defined as the optical properties of a homogeneous cloud producing the same
radiative transfer results as the multifractal cloud. This allows the well-known and
efficient radiative transfer techniques for homogeneous cloud to be applied to multifractal
cloud. The effective optical properties were found to vary with the spatial
scale under consideration, and an empirical parameterisation for the effective optical
properties is presented that is a function of spatial scale, mean cloud optical
depth and single-scattering albedo.
The range of conditions under which the effective optical depth approximation
can be used is then examined, with vertical fluctuations in cloud liquid water,
radiance changes with viewing angle, and differing single-scattering properties all
considered. The approximation is found to be reasonable for most low-level cloud conditions, with the greatest discrepancies occurring for absorbing clouds with vertical
fluctuations and significant vertical extent. Finally, the effective optical depth
parameterisation is tested in the satellite remote sensing of cloud liquid water path,
with the results being compared to simultaneous aircraft measurements. The agreement
between the data sets is significantly improved when the results are corrected
for cloud inhomogeneity using the effective optical depth approximation, with the
remaining errors being shown to be of the level expected due to the discrepancies
in measurement scales. These (root-mean-square) errors due to the mismatch of
measurement scales when comparing satellite-based and in situ measurements are
estimated, using the spatial statistics of liquid water, to be approximately 27%.

Item Type: Thesis - PhD
Authors/Creators:Fienberg, KS
Keywords: Cloud physics, Clouds, Solar radiation
Copyright Holders: The Author
Copyright Information:

Copyright 2003 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

No access until 28 August 2005. Thesis (Ph.D.)--University of Tasmania, 2003. Includes bibliographical references

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