Observations and modelling of microphysical variability, aggregation and sedimentation in tropical anvil cirrus outflow regions 1Centre for Atmospheric Science, SEAES, University of Manchester, UK
26 Jul 2012
2NCAR, P.O. Box 3000, Boulder, CO, 80307-3000, USA
3School of the Environment, Flinders University, GPO Box 2100, Adelaide 5001, Australia
Received: 28 July 2011 – Published in Atmos. Chem. Phys. Discuss.: 22 August 2011 Abstract. Aircraft measurements of the microphysics of a tropical convective anvil (at
temperatures ~−60 °C) forming above the Hector storm, over the
Tiwi Islands, Northern Australia, have been conducted with a view to
determining ice crystal aggregation efficiencies from in situ measurements.
The observed microphysics have been compared to an explicit bin-microphysical
model of the anvil region, which includes crystal growth by vapour diffusion
and aggregation and the process of differential sedimentation.
Revised: 08 June 2012 – Accepted: 18 June 2012 – Published: 26 July 2012
It has been found in flights made using straight and level runs perpendicular
to the storm that the number of ice crystals initially decreased with
distance from the storm as aggregation took place resulting in larger
crystals, followed by their loss from the cloud layer due to sedimentation.
The net result was that the mass (i.e. Ice Water Content) in the anvil
Ci cloud decreased, but also that the average particle size (weighted by
number) remained relatively constant along the length of the anvil outflow.
Comparisons with the explicit microphysics model showed that the changes in
the shapes of the ice crystal spectra as a function of distance from the
storm could be explained by the model if the aggregation efficiency was set
to values of Eagg~0.5 and higher. This result is supported by
recent literature on aggregation efficiencies for complex ice particles and
suggests that either the mechanism of particle interlocking is important to
the aggregation process, or that other effects are occuring, such as
enhancement of ice-aggregation by high electric fields that arise as a
consequence of charge separation within the storm.
It is noteworthy that this value of the ice crystal aggregation efficiency is
much larger than values used in cloud resolving models at these temperatures,
which typically use E~0.0016. These results are important to
understanding how cold clouds evolve in time and for the treatment of the
evolution of tropical Ci in numerical models.
Citation: Gallagher, M. W., Connolly, P. J., Crawford, I., Heymsfield, A., Bower, K. N., Choularton, T. W., Allen, G., Flynn, M. J., Vaughan, G., and Hacker, J.: Observations and modelling of microphysical variability, aggregation and sedimentation in tropical anvil cirrus outflow regions, Atmos. Chem. Phys., 12, 6609-6628, doi:10.5194/acp-12-6609-2012, 2012.