Aircraft measurements of the microphysics of a tropical convective anvil (attemperatures ~−60 °C) forming above the Hector storm, over theTiwi Islands, Northern Australia, have been conducted with a view todetermining ice crystal aggregation efficiencies from in situ measurements.The observed microphysics have been compared to an explicit bin-microphysicalmodel of the anvil region, which includes crystal growth by vapour diffusionand aggregation and the process of differential sedimentation.It has been found in flights made using straight and level runs perpendicularto the storm that the number of ice crystals initially decreased withdistance from the storm as aggregation took place resulting in largercrystals, 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 anvilCi cloud decreased, but also that the average particle size (weighted bynumber) remained relatively constant along the length of the anvil outflow.Comparisons with the explicit microphysics model showed that the changes inthe shapes of the ice crystal spectra as a function of distance from thestorm could be explained by the model if the aggregation efficiency was setto values of ~0.5 and higher. This result is supported byrecent literature on aggregation efficiencies for complex ice particles andsuggests that either the mechanism of particle interlocking is important tothe aggregation process, or that other effects are occuring, such asenhancement of ice-aggregation by high electric fields that arise as aconsequence of charge separation within the storm.It is noteworthy that this value of the ice crystal aggregation efficiency ismuch larger than values used in cloud resolving models at these temperatures,which typically use ~0.0016. These results are important tounderstanding how cold clouds evolve in time and for the treatment of theevolution of tropical Ci in numerical models.
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