Contribution of Aerosol–Cloud Interaction to the Surface Warming at Barrow Site

The aerosol in the Arctic, with large contribution from long­range transport from mid­latitudes, plays an important role to the Arctic climate by modifying cloud properties. Using 4 years (2000­2003) ground­based observations of aerosol, clouds, radiation and other meteorological variables, we find Arctic clouds are dominated by thin clouds and haze events mainly occur in winter and spring at Barrow site. For winter and spring time with heavy haze events, it shows that for similar ranges of liquid water path, the longwave emissivity of clouds under polluted condition is about 0.05-0.08 larger than that under clean condition, corresponding to a strong warming effect of 3­5 W/m2 or 1­2 K. By separating clouds into clean and polluted conditions for each month based on aerosol light scattering, it further shows that there are even more significant positive warming effect due to aerosol-cloud interaction in winter and spring, which is about 8 W/m2, while it plays a negative cooling effect in summer. While the annual averages of aerosol-cloud interaction effect is close to neutral, the seasonal distribution along with complicated feedbacks could help the melting of ice and the warming of Arctic.