Cloud processing of aerosol using a hybrid LES/parcel model with solute-following microphysics
Date
1995-08
Authors
Richardson, Wendy A., author
Kreidenweis, Sonia M., author
Cotton, William R., author
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Journal ISSN
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Abstract
The importance of climate forcings due to direct and indirect aerosol influences have been theorized, observed, and modeled, and are accepted on a qualitative level. Though these effects have been identified, the magnitude of their effect locally and globally has been difficult to estimate. Enhancement of the existing Regional Atmospheric Modeling System (RAMS) developed at CSU provides a way to investigate these impacts. The RAMS explicit microphysics has been expanded to include both an aerosol distribution stored in 14 size categories and separate solute mass and concentration information for each water drop size category. The initial aerosol distribution is activated according to chemical and physical principles; that is, it is not some parameterized function. The aerosol is subsequently followed as solute in each water bin, where each droplet size category independently stores a solute concentration. Because the aerosol mass is followed as solute in the drops, information on the changes in aerosol size and number due to cloud processing of an evaporating cloud is available. Schemes for bin representation, aerosol activation, solute transfer during droplet growth, and aerosol regeneration from evaporating droplets were tested in a standalone box model. To determine the effectiveness of using the expanded microphysics in a full scale LES model, a series of tests using a RAMS simulation of a two dimensional hill cap cloud were performed. A hybrid LES/parcel model was also developed. Several passive tracer trajectory environmental profiles through the hill cap cloud were determined during the full RAMS simulation. These profiles were used to drive a parcel model having the same explicit microphysics as the full RAMS simulation. Additionally, the hybrid LES/parcel model was used to drive parcels derived from a RAMS simulation of the same cloud with bulk microphysics. Direct comparisons of aerosol and droplet distributions in time and space were made for the full LES model and for the hybrid LES/parcel simulations. The hybrid LES/parcel model shows promise in its ability to combine very dynamically complex cloud types with complex cloud microphysics and chemistry. Cloud microphysical features, such as cloud base supersaturation maximum, were not well represented in the RAMS simulation, but were in the parcel model simulations. Aerosol which have deliquesced and/or activated to cloud droplets serve as sites for aqueous chemical reactions which can enhance the aerosol mass. An aqueous chemistry module (Kreidenweis, 1992), appropriate for use as a stand-alone model, has been extended. The expanded version can be used to represent both externally and internally mixed aerosol. A seasalt aerosol option is included which can be used to represent marine aerosol distributions. Seasalt constituents that directly affect the chemistry are represented explicitly, along with a parameterization for seasalt alkalinity. Options for multiple droplet sizes and concurrent droplet growth have been created, for incorporation into a dynamical/microphysical cloud model.
Description
August 1995.
Rights Access
Subject
Cloud physics
Aerosols -- Environmental aspects