Periodic characteristics and variation trend of summer cloud amount over China

In order to explore the periodic characteristics and long-term trends of cloud variability, Mainland China region is divided into nine subregions based on results of precipitation cluster analysis and geoclimatic characteristics of China. Precipitation and cloud amount data collected at 2400 stations in China from 1961 to 2010 are used. The ensemble empirical mode decomposition (EEMD) and wavelet analysis methods are used to extract multi-scale signals of the variation series of daily mean cloud amount in summer in each region. The combined results of EEMD and wavelet analysis demonstrate that the periodic components of 2—4 a, 5—7 a, 8—12 a and 25—32 a are primarily present in the summertime climatic changes of total and low cloud amounts in China. The 2—4 a is a global time domain significance cycle and the 5—7 a is a local time domain significance cycle for cloud amounts in most areas of China. It indicates that the 2—4 a and 5—7 a are real periodic oscillations in the cloud amount variation series. The dominating cycle of the cloud amount series is the quasi-3 a, which is most prevalent, significant, and stable. Over 70% of the variance contribution is accounted for by the inter-annual scale signals, i.e., 2—4 a and 5—7 a. Although there are certain similarities on the multiscale period signals and distribution circumstances for total and low cloud amount series, there are also some differences in cycle length, variance contribution, and trend shift, while the last one of which is particularly important. From the trend items of total and low cloud amount series, it can be found that half of the trend items are significantly different from the linear trend, which illustrates the intricacy of trend variations and reasons. The summer total cloud amount in Northeast China showed a general decreasing trend during the 50-year period, but it turned to an increasing trend after 2000. The low cloud amount remained a stable low state until 1980, and then it showed an increasing trend and an overall increasing trend. The low cloud amount in the eastern arid zone was decreasing from the partial state, and there was a significant decreasing trend. The total summer cloud amount in the western arid zone had an increasing trend from the low state. The trend term of low cloud amount was also increasing from the low state, and its contribution was as high as 61.2%. The total summer cloud amount in North China basically remained unchanged before 1980 and turned to an obvious increasing trend after 1980. The low cloud amount was decreasing from a partial state. In southern Central China, the low cloud amount in summer increased with time from the low state to the high state around 1995, and the low cloud amount changed little after the 21st century. There was little variation in the total cloud amount in summer in Southern China until 1980, and there was a declining trend later. Low cloud amount had gradually increased over time. The trend of low cloud amount in summer in Southwest China decreased first and then increased, with 1980 as the turning point. The total cloud amount in summer in the Qingzang plateau region showed a trend of decreasing and then increased from a high state, with 1995 as the turning point. The trend term of low cloud amount also showed a decreasing trend followed by an increasing trend, with 1980 as the turning point.