A Study of the Impact of Aerosols on Marine Stratocumulus Clouds during the Second Aerosol Characterization Experiment (ACE-2)

Aerosol particles exert an indirect effect on the global climate by acting as cloud condensation nuclei (CCN), and in so doing, modify the cloud microphysical, macrophysical and radiative properties. Substantial evidence, for example, from airborne measurements, ground-based remote sensing and satellite retrievals, supports the first Aerosol Indirect Effect (AIE), that is, the effect of aerosols on cloud droplet size and cloud reflectivity.

However, the second AIE, (i.e., the impact of aerosols on cloud liquid water path, cloud coverage, and lifetime), is more complicated and often entangled with the meteorological conditions and the hydrological cycle. Sometimes the cloud liquid water path (LWP) increases with aerosols, while at other times it does not. Therefore, the second AIE is extremely difficult to quantify and highly uncertain.

The Second Aerosol Characterization Experiment (ACE-2) offers a good opportunity to study the impact of aerosols on marine stratocumulus clouds. ACE-2 took place from 16 June to 24 July, 1997, over the subtropical northeast Atlantic. Previous observations showed that clean marine air alternates with anthropogenic pollution over this area. Two contrasting cases are examined here. One is a clean case on June 26, 1997, and another is a polluted case on July 9, 1997. In this study, a cloud resolving model, Active Tracer High Resolution Atmospheric Model, (ATHAM), is adopted to study the AIE.

In the base test, a 30-hour simulation with the two-dimensional version of ATHAM shows that the model results closely match the available observations (e.g., thermodynamic profiles, cloud morphology, and cloud micro-physical and radiative properties). Sensitivity tests will also be explored to examine the AIE, and meteorological effect (or thermo-dynamic effect) on clouds by swapping the meteorological conditions and aerosol burdens between the clean and polluted cases. The total AIE (i.e., the sum of the first and second AIE), the first AIE and the second AIE will be estimated by subtracting the shortwave radiative flux in the polluted scenario from that in the clean scenario under the condition of same meteorological conditions, under the condition of same cloud macro-physical properties, and under the condition of same cloud micro-physical properties, respectively.