Observations and modelling of microphysical variability, aggregation and sedimentation in tropical storm cirrus outflow regions
1Centre for Atmospheric Science, University of Manchester, Manchester, UK
2NCAR, P.O. Box 3000, Boulder, CO, 80307-3000, USA
3School of the Environment, Flinders University, GPO Box 2100, Adelaide 5001, Australia
Abstract. Aircraft measurements of the microphysics of a tropical convective anvil (at temperatures ~−60 °C) forming above the HECTOR storm have been performed. The observed microphysics has been compared to a bulk and explicit microphysical model of the anvil region including crystal aggregation and sedimentation.
It has been found that 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 due to sedimentation. At still greater distances from the storm the number of very small crystals increased. This is attributed to the formation of new ice crystals on aerosol particles as the ice super saturation rose following the depletion of the larger ice particles following aggregation and sedimentation. Comparison 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 explicit microphysical model if the aggregation efficiency was set to E~0.02.
It is noteworthy that this aggregation efficiency is much larger than values normally used in cloud resolving models at these temperatures (typically E~0.0016). Furthermore if the bulk model is used then optimum agreement was reached with a collection efficiency for aggregation of E~0.05. These results are important for the treatment of the evolution and lifetime of tropical cirrus clouds.