Modelling Aerosol-Cloud-Meteorology Interaction in an On-line Air Quality Model

The impact of aerosol-cloud interaction on the climate system has been an active area of research for some time now, and most of the current climate models include some representation of this aerosol indirect effect. On much shorter time scales, e.g., synoptic scales, a recent multi-model intercomparison study was conducted under the second phase of the Air-Quality Model Evaluation International Initiative (AQMEII) comparing model simulations with and without the representation of aerosol feedbacks over North America and Europe. This study showed that the inclusion of aerosol feedbacks has a significant impact on model predicted meteorology and chemistry and the aerosol indirect effect was found to often dominate the feedbacks compared to the aerosol direct effect.

In participating in the phase 2 of AQMEII model intercomparison study, aerosol feedbacks to radiative transfer and cloud microphysics were introduced to an on-line air quality model GEM-MACH. To represent the aerosol feedback to cloud microphysics, a physically-base aerosol activation parameterization (Abdul-Razzak and Ghan, 2002; for sectional representation of aerosol size spectrum), coupled with on-line size- and chemically-resolved aerosols, was used to replace the cloud droplet nucleation calculation in the existing double-moment microphysics scheme in the model. For that implementation the standard deviation of updrafts is parameterized as a function of liquid water content (Hoose et al., 2010), and the aerosol wet size at critical supersaturation is used as a first guess for evaluating droplet size dependent parameters. The model with revised aerosol activation (or droplet nucleation) resulted in much enhanced cloud droplet number concentration which subsequently affected precipitation production and cloud amount.

In this follow-up study, we carry out further investigation into model sensitivity to the details of aerosol activation parameterization and its implementation in the coupled model. A turbulent energy-base parameterization for updraft velocity was tested with the Abdul-Razzak-Ghan activation scheme. As well, a different approach for aerosol activation based on a single parameter representation of hygroscopic growth and cloud condensation nucleus activity (Petters and Kreidenweis, 2008) was implemented. Model simulations with the various implementations of aerosol activation were carried out for an aircraft study case during the ICARTT field campaign in summer 2004 when detailed in-situ microphysics and chemistry measurements were conducted in and below stratocumulus clouds over southern Michigan. The impact of the parameterization and implementation of aerosol activation in the coupled model on predicted meteorology and chemistry will be discussed.