The microphysical properties of tropical convective anvil cirrus: A comparison of models and observations

This paper describes the use of two-dimensional (2-D) cloud-resolving numer ical model simulations of a period of active convection from the Tropical O cean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment to gene rate statistics of the distribution of total ice water content (IWC) and of the relative contribution to IWC of small (sub-200 mum) particles with tem perature. The model data are sampled in a way which excludes contributions from active convective updraught regions, so that the model results may be compared with similar such distributions derived from in situ sampling of a number of tropical anvil ice clouds in the vicinity of Kwajalein in the we st Pacific. The model gives a good description of the mean and mode IWC in each of seve n temperature ranges and, for temperatures warmer than -40 degreesC, correc tly predicts the dominant contribution of large particles to the IWC. At lo wer temperatures, the model retains an excessive fraction of its IWC in lar ge particles. Estimates of the maximum crystal length (MCL) that would be s ampled by a 2-D Optical Array Probe exceed the observed values in this temp erature range but are otherwise in good agreement. Tests of the sensitivity of model results to the cloud ice bulk density, and the absence of a homog eneous freezing source of ice crystals, suggest that the excessive MCL valu es are due in part to excessive rates of autoconversion and aggregation of cloud ice.