There are different types of parameterizations employed in models that describe clouds. 

How does the choice of one of these microphysics parameterizations affect chemistry even if cloud chemistry is not included in the chemical transport model? 

The choice of microphysics parameterization can affect the amount of precipitation on the ground, the cloud lifetime, and ultimately thermodynamic quantities, such as temperature and water vapor. 

1) Explicit microphysics parameterizations describe cloud particles segregated by size into bins.  Generally several (on the order of 10) bins describe cloud condensation nuclei, a half dozen bins depict the cloud water drops, and a few more bins depict rain.  If ice is also included many more bins of frozen hydrometeors would be required.  Therefore, the explicit microphysics parameterization is usually limited to warm rain physics (clouds with only liquid drops). 

2) Bulk water microphysics parameterizations describe a few classes of hydrometeors and prescribe a size distribution to each class. 

2a) Liquid water only bulk microphysics parameterization depicts cloud water and rain hydrometeor classes.  Usually a monodisperse size distribution is used for cloud water and a Marshall-Palmer size distribution is used for the rain.  Physical processes that are described are condensation and evaporation, autoconversion of cloud water to form rain, accretion of cloud water by rain, and sedimentation of rain. 

2b) Two class ice hydrometeor bulk microphysics parameterization depicts cloud water, rain, ice, and snow hydrometeors.  A monodisperse size distribution is assigned to the cloud water and ice, a Marshall-Palmer size distribution is used for rain, and an exponential decay size distribution is used for snow.  Physical processes include the warm rain physics, the freezing/melting and deposition/sublimation processes, accretion of hydrometeors among the different categories, and sedimentation of the rain and snow. 

2c) Three class ice hydrometeor bulk microphysics parameterization depicts cloud water, rain, ice, snow, and graupel or hail.  This parameterization is quite similar to the two-class scheme, but additional processes are included to account for the graupel or hail. 

This talk will illustrate some of these results and the ensuing discussion will help the community choose which type of parameterization is best for studies on chemistry.