Radiative forcing (RF) of aerosol and its effects on climate have been identified as the largest source of uncertainty in understanding climate change in the Intergovernmental Panel on Climate Change 4th Assessment Report (IPCC AR4). For example, the RF due to Twomey effect (also referred to as first indirect effect) is estimated to be -0.7 with an error range of -1.1 to +0.4 W m-2, indicating the lowest level of scientific understanding. Different aerosol activation (or droplet nucleation) parameterizations may influence cloud condensation nuclei (CCN) predictions. The default mechanism option for simulating the effects of aerosols on CCN formation in the Global Weather Research and Forecasting Model with Chemistry (G-WRF/Chem) is the coupled Carbon-Bond Mechanism version Z (CBM-Z) and the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) with the Abdul-Razzak and Ghan (A-R & G) 's sectional aerosol activation parameterization. To better understand aerosol-climate interactions in terms of aerosol indirect effects through acting as CCN, we are coupling an alternative aerosol activation parameterization of Fountoukis and Nenes (FN) with CBM-Z/MOSAIC in G-WRF/Chem, which is considered to be more accurate in terms of aerosol growth and mass transfer treatments than the A-R & G parameterization. Simulations with A-R & G and FN coupled with CBM-Z/MOSAIC are being intercompared to assess the influence of aerosols on CCN predictions and cloud droplet numbers through nucleation scavenging. CCN predictions are also affected by aerosol size representations, aerosol microphysics treatments, and coupled gas-phase mechanisms through aerosol formation. In addition to MOSAIC that is based on a sectional size representation, G-WRF/Chem includes the Modal Aerosol Dynamics Model for Europe/the Secondary Organic Aerosol Model (MADE-SORGAM) that is based on a modal size representation. A new gas-phase mechanism, the Carbon-Bond Mechanism version 2005 (CB05), has been previously incorporated into G-WRF/Chem by this group for global-through-urban applications and is being coupled with MOSAIC and MADE-SORGAM. In this work, the FN module will also be coupled with CB05/MADE-SORGAM. Simulations with A-R & G coupled with CB05/MOSAIC and CBM-Z/MOSAIC will provide the sensitivity of CCN predictions to different gas-phase mechanisms through secondary aerosol formation. Simulations with FN coupled with CB05-MOSIAC and CB05-MADE/SORGAM will provide the sensitivity of CCN predictions to different aerosol modules through aerosol activation/droplet nucleation scavenging.

To assess the relative contributions of the aerosol direct and indirect effects on climate, three separate simulations will be conducted with the CB05-MADE/SORGAM-FN combination to simulate aerosols without any aerosol direct and indirect effect, with the direct effect, and with both direct and indirect effects. We will use measurements from satellite instruments, such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and the International Satellite Cloud Climatology Project (ISCCP), to evaluate the model results in terms of CCN number concentration, cloud droplet number, cloud fraction, cloud effective radius, precipitable water, total precipitation, and long/short wave radiation budget at the top of atmosphere. This work will provide insights into CCN predictions and associated uncertainties, the relative importance of aerosol's direct and indirect effects on climate, as well as an assessment of the capability of current on-line coupled climate-chemistry models in simulating such effects.