Study of mechanisms of aerosol indirect effects on glaciated clouds

Description

The Tropical Warm Pool-International Cloud Experiment (TWP-ICE) campaign, partly funded by the U.S. Department of Energy, involved observations of aerosol size distributions and aerosol composition with probes flown on aircraft near Darwin in Australia, as well as microphysical properties of glaciated clouds. In this presentation, microphysical improvements to an aerosol-cloud model using our newly developed scheme of 2-moment bulk microphysics, including an empirical scheme to treat heterogeneous ice nucleation, and prognostic treatment of six aerosol species are described. These improvements include treatment of size-dependent morphology (bulk density, shape) of ice particles and use of emulated spectral (bin) microphysics to treat collision and sticking efficiencies of coagulation.

A month-long simulation of the TWP-ICE case by the aerosol-cloud model is compared against observations of ice and droplet concentrations for cases with coincident observations of aerosol and thermodynamic conditions as its input. Detailed treatments of droplet evaporation near -36°C during homogeneous droplet freezing and of the evolving mean size of sulphate aerosol are shown to enable correct prediction of ice concentrations by the aerosol-cloud model.

For cold clouds generally, the problem of whether it is possible to predict correctly the observed ice concentration using as input the observed aerosol conditions is discussed.