Chemistry, Aerosols & Dynamics Interactions Research

Goal: To develop an understanding of the interactions between atmospheric dynamics, aerosols and chemistry at the meso- and cloud- scales, particularly with respect to the coupling between transport, cloud physics, and chemistry.

Atmospheric chemistry can be greatly influenced by the dynamics governing air motion and meteorology at meso- and cloud-scales. For example, deep convection can rapidly transport species and aerosols, such as anthropogenically produced nitrogen oxides, into the upper troposphere where they have a longer lifetime and are more effective at modifying ozone concentration, which plays an important role in oxidizing trace gases in the troposphere. Similarly, mixing across the top of the planetary boundary layer can redistribute constituents into the free troposphere. In addition, liquid and solid particles in clouds provide locations for chemical reactions to occur. Gas-phase chemistry (in particular sulfur chemistry) influences the quantity and size of aerosols, which can become cloud condensation nuclei, providing surfaces for cloud drop formation. In the clouds, chemical reactions and microphysical processes alter aerosols and their properties as nuclei for cloud and ice formation. Thus, interactions of chemistry, aerosols, and the dynamics of clouds are important to several aspects of atmospheric research.

MMM continues to take a lead role in research on the coupling of dynamics, chemistry and aerosols at small scales. We are developing several state-of-the-art cloud-chemistry models for cloud-topped convective boundary layers and for deep convective storms.

To understand dynamical interactions with chemical reactions, chemistry has been incorporated into MMM's LES model and into the COMMAS convective-cloud model. The LES focuses on eddy transport and entrainment at the top of the convective PBL. Ongoing development of this coupled LES includes incorporating cloud and aerosol physics and chemistry, so that it can be applied to the cloud-topped marine boundary layer. Transport, turbulent mixing and chemistry within deep convection are being investigated using COMMAS coupled with chemistry. This model is currently being applied to thunderstorms observed during the STERAO-Deep Convection Experiment to determine the contributions of transport from the boundary layer and from lightning to the nitrogen oxides. This model is also being used to assess the relative importance of chemical species transport versus chemical reactions for the high-plains thunderstorms observed during STERAO. On a larger scale, ACD's Regional Chemistry and Transport Model (HANK) and MM5 simulations of STERAO cases are being used to examine the regional/synoptic scale transport and chemistry for the STERAO convective events. Information learned from the convective cloud model coupled with chemistry simulations can be incorporated into HANK to improve descriptions of convective clouds and chemistry.

Sulfur chemistry directly affects the number and mass of sulfate aerosols, which are the predominant cloud condensation nuclei. This in turn affects the development of precipitation through the condensation-coalescence or the ice process when aerosols act as ice nuclei. These changes can have large effects on radiative balance, precipitation rates, and even dynamics. MMM scientists are playing significant roles in the Indian Ocean Experiment (INDOEX) and subsequent analysis, examining how aerosols affected cloud microphysical properties. Studies that parameterize these effects for numerical models and that investigate interactions between radiation, microphysics, and aerosols are also underway. Future studies will further examine effects of aerosols and entrainment on drizzle suppression and cloud radiative properties, possibly through participation in ACE-Asia. MMM is collaborating with the development of an NCAR-wide box model that describes size segregated aerosol physics and chemistry in detail. This model will be used to guide the model development of prediction of mass, number concentration, and composition of aerosols and cloud hydrometeors in the 3-D cloud and meso-scale models mentioned above.

To provide better representations of chemical transport and the interactions between chemical species, aerosols, and dynamics, chemistry is being incorporated in the Weather and Research Forecast (WRF) model. This coupled chemistry-meteorological model will be used for the cloud scale and the regional scale, will replace the COMMAS coupled with chemistry convective model, and will be merged with ACD's HANK model. Work on this project began with a workshop that assessed the approaches and methodologies of chemistry modeling in cloud and mesoscale models. Next a version of WRF coupled with chemistry will be created so that subgrid parameterizations and depiction of chemistry processing can be developed, working towards a complete description of chemical processing on the mesoscale. The completed model can then be used for many studies including chemical redistribution by convection, regional-scale transport of chemical species, and aerosol-cloud interactions.

Coupling of the modeling and theoretical studies with field programs is crucial; many specific questions being addressed do not have sufficient observational datasets to guide the theory and modeling. Efforts in organizing and leading the STERAO experiment will be continued with similar participation in upcoming field programs emphasizing deep convection, chemistry, and aerosols in the mid-latitudes and the tropics. The Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II), which was conducted during the summertime 2001 off the California coast and used several trace species as tracers of entrainment and mixing within and across the top of marine stratocumulus. The ongoing work represents cooperative and interdisciplinary investigations coupling small-scale dynamics with chemistry, aerosols, and cloud physics. Ultimately, issues to be addressed are either small-scale in nature, such as air quality (local, regional), or larger scale in nature, such as climate/chemistry issues related to ozone production-loss and the role of sulfur species on cloud microphysics and dynamics. The investigations into clouds and chemistry will also lead to improvements in, or development of, parameterizations for large-scale models. Thus, the work directly supports the goals of the GTCP (Global Tropospheric Chemistry Program) and climate research.