A modeling and observational study of the detailed microphysical structure of tropical cirrus anvils

A detailed microphysical analysis of ice particles in a cirrus anvil is presented, using a combination of observational data and model simulations for missions flown during the Central Equatorial Pacific Experiment (CEPEX). The observational data are obtained mainly from the two-dimensional cloud probe, measuring particles as small as 100 mu m in diameter; the model simulations are done with a detailed microphysical model that categorizes cloud particles according to their size, shape, and solute content. Detailed analyses of the simulated particle size spectra are made for the size range of 1 mu m to several millimeters, which are then compared with the observational results. The evolution of the cirrus anvil is divided into four stages: the deep convection, the precipitating anvil, the extended anvil and the detached anvil. Discussions on the properties of the simulated cloud systems are given in detail for each stage. Model results show that the number concentration of ice particles in the anvils can reach over 1000 per liter. Frozen cloud droplets and interstitial aerosol particles are the main sources of these ice particles. In the precipitating anvils, a trimodal size distribution is prevalent at the upper portion, whereas bimodal or unimodal distributions are more common at the lower levels. The size distributions in the extended anvils exhibit a bimodal shape at the upper levels and a unimodal shape at the lower levels. The properties of the extended anvil compare fairly well with the observational results for one case but not as well for the other two cases. Nevertheless, both the observations and simulations show that smaller particles (diameter < 100 mu m) are fractionally more important to the total number concentration and ice water content at higher altitudes in the anvil, whereas larger particles (diameter > 100 mu m) are more abundant in the middle to lower sections. Sedimentation sorting plays a significant role in determining the structures and evolution of the cirrus anvils. A sensitivity test indicates that the amount of condensation nuclei in the convective inflow may have a strong influence on the number concentration of ice particles, as well as the structure and lifetime of tropical cirrus anvils.