Significant decreasing cloud cover during 1954–2005 due to more clear-sky days and less overcast days in China and its relation to aerosol

An updated analysis of cloud cover during 1954– 2005 in China was performed using homogeneous cloud cover data from 314 stations. Long-term changes in frequencies of different cloud cover categories and their contributions to long-term changes in cloud cover were assessed. Furthermore, aerosol effects on cloud cover trends were discussed based on comparison of cloud cover trends in polluted and mildly polluted regions. Frequencies of clear sky (cloud cover <20 %) and overcast days (cloud cover >80 %) were observed to increase by∼2.2 days and decrease by∼3.3 days per decade, respectively, which accounts for ∼80 % of cloud cover reduction. Larger decreasing trends in cloud cover due to larger increase in clear sky frequency and larger decreases in overcast frequency were observed at stations with lower aerosol optical depth. There is no significant difference in trends regarding cloud cover, clear sky frequency, and overcast frequency between mountain and plain stations. These results are inconsistent with our expectation that larger decreasing trends in cloud cover should have been observed in regions with higher aerosol loading where more aerosols could lead to stronger obscuring effect on ground observation of cloud cover and stronger radiative effect as compared with the mildly polluted regions. Aerosol effect on decreasing cloud cover in China appear not to be supported by this analysis and therefore, further study on this issue is required.