Decreasing trends in surface-observed total cloud amount over China: another effect of anthropogenic aerosol loading?
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Tuesday, 19 January 2010: 2:00 PM
Various studies of surface (human)-observed cloud amount datasets have shown evidence of significant, decreasing total cloud amount trends over China, particularly north of about 30°N latitude, over roughly the last half of the 20th century. These decreasing trends are generally thought to be “real” in the sense that it is likely that individual observers over much of China have indeed seen smaller amounts of cloud covering their local skies over recent decades. In this talk we will focus on examining potential changes and challenges in the ability to observe all clouds present. This question arises in light of two main factors: (1) recent increases in anthropogenic aerosol burden over much of China; and (2) generally decreasing sunshine duration and decreasing solar irradiance (“dimming”) over China for most of the last 50 years or so. The first factor has come to be generally regarded in the literature as the primary cause of the second factor, since the observed decrease in cloud amount would, taken alone, serve to increase sunshine duration and solar irradiance.
Our main science question we hope to answer: Is atmospheric aerosol loading complicating the ability of human observers to visibly detect the presence of clouds and thus imparting an anthropogenic effect on surface-observed cloud amount? To attempt to answer this question definitively, we will examine trends in related variables such as visibility and the frequency of occurrence of reported present weather and sky cover conditions (e.g., clear, ranges of partly cloudy, overcast, and obscured). We will also conduct comparisons of satellite-observed cloud amount with surface-observed data for common periods of record.