A Polar Perspective on Aerosol-Cloud Interactions

The high Arctic has long been recognized as a region for extensive and climatologically significant aerosol-cloud interaction. For most of the year, low-level stratiform cloud cover is a predominant meteorological condition. For half the year (winter and spring), Arctic atmospheric circulation traps aerosol of anthropogenic and biomass-burning origin in extensive optically thick layers known as the “Arctic haze.” Within the past decade, aerosol-cloud interactions have been observed in the Arctic from both surface and space-based instruments. These observations have revealed some unique manifestations. In the Arctic, the longwave radiative manifestation of the first indirect effect is generally more significant than its shortwave counterpart. The surface radiative impact of the first indirect effect is comparable to that of doubling atmospheric carbon dioxide abundance. In addition, increasing cloud emissivity from aerosol entrainment can enhance the positive feedback loop between cloud-top radiative cooling and new droplet condensation. Space-based observations have revealed Arctic contrasts in aerosol-cloud interaction strength between plumes of anthropogenic versus biomass-burning origin. During summer over the Arctic Ocean, the generally low aerosol burden (after Arctic haze removal by late spring) makes clouds and their radiative properties especially sensitive to localized changes in aerosol abundance. During the Arctic winter, sulfate interaction with cloud ice water can lead to sedimentation of ice particles and a negative aerosol-cloud-radiative feedback. This presentation will review recent US, Canadian, and European research, both empirically and in the context of ongoing efforts to improve mixed-phase cloud treatment in climate models.