Observed Aerosol Influence on Ice Water Content of Arctic Mixed-Phase Clouds

The phase or the phase partitioning of water in a cloud is important to the clouds ability to absorb and emit radiation. Mixed-phase clouds in the Arctic significantly influence the radiation budget of the Arctic climate system. Therefore, it is important to understand the physical processes that contribute to both liquid water and ice formation in these clouds. Aerosols have long been known to impact the liquid drop size distribution of clouds, but how changes to the drop size distribution alter processes such as ice nucleation and riming is poorly understood.

The response of ice water content (IWC) in Arctic mixed-phase stratocumulus to atmospheric aerosols is observed. IWC retrievals from ground based radars operated by the Atmospheric Radiation Measurement (ARM) program in Barrow, Alaska are used to construct composite profiles of cloud IWC from a 9-year radar record starting in January of 2000. The IWC profiles for high (polluted) and low (clean) aerosol loadings are compared. Generally, we find that clean clouds exhibit statistically significant higher levels of IWC than do polluted clouds by a factor of 2-4 at cloud base. For springtime clouds, with a maximum relative humidity with respect to ice (RHI) above 110% in the cloud layer, the IWC at cloud base was a factor of 3.25 times higher in clean clouds than it was in polluted clouds. We infer that the aerosol loading of the cloud environment alters the liquid drop size distribution within the cloud, with larger drops being more frequent in clean clouds. Larger cloud drops promote riming within the cloud layer, which is one potential explanation for the higher IWC levels observed in clean clouds. The drop size distribution may also be a significant control on ice nucleation events within mixed-phase clouds. The physical processes responsible for the differences in IWC in clean and polluted clouds are under investigation at this time.