Sonia Kreidenweis (PI)
Colorado State University
sonia@chem.atmos.colostate.edu

Modeling Studies of Aerosol-Cold Cloud Interactions

It is proposed to develop two variations of a parcel-type model of aerosol-cloud interactions to be used to study drop formation at cold temperatures, homogeneous freezing, heterogeneous freezing, and the competition for water vapor between condensation, freezing, and crystal growth. Building on our prior modeling work for aerosol-warm cloud interactions, the ambient environment can be represented either as an adiabatic parcel or the ambient conditions can be specified along a trajectory diagnosed from a dynamical simulation. This permits application of the model to a variety of dynamical regimes. The aerosol and its interaction with water vapor are modeled in two ways. In the first, the particle size evolution is treated in a Lagrangian framework, minimizing diffusional errors, easily accommodating a variety of aerosol types and size distributions, and permitting variable resolution in particle size-space. In the second, we employ a more conventional bin-microphysical representation, which permits calculation of particle-particle interactions such as collision-coalescence. There is a need in the community for such models for several reasons. First, the limited numerical studies appearing in the literature have usually assumed a simplified description of the aerosol, such as a single composition or a lognormal size distribution. A few interesting parametric studies have been presented, again using conceptualized aerosol inputs which should be refined. We now have a wealth of in situ aerosol data, due in large part to NASA-sponsored field missions, that can be used to develop more realistic inputs and improved representation of the ice initiation process, and new models are needed to accommodate such data and more fully explore parameter space. Second, only recently have in-situ, airborne measurements of homogeneous freezing nucleation of liquid aerosols and ice formation by heterogeneous ice nuclei been available. Our group has been in the forefront of producing these data on ice nucleation and we are in a unique position to integrate them into improved representations of the ice formation process and its link to aerosol abundance and characteristics.

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