Decrease of light rain events in summer associated with a warming environment in China during

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L11705, doi: /2007gl029631, 2007 Decrease of light rain events in summer associated with a warming environment in China during Weihong Qian, 1 Jiaolan Fu, 1 and Zhongwei Yan 2 Received 23 February 2007; revised 30 April 2007; accepted 8 May 2007; published 13 June [1] Based on daily rainfall and temperature data for the summer monsoon season in mainland China during , this paper demonstrated an overall decreasing trend in the frequency of light rain events in association with regional warming, and different local trends in the frequency of moderate rain events in association with atmospheric circulation anomalies. The frequencies of moderate and extreme rain events increase in eastern China centered over the middle-lower reaches of the Yangtze River, but decrease in North China and Southwest China. This pattern of rainfall trends in China is different from that in the Indian monsoon area, where an increase of extreme rain events in the monsoon season during was found, indicating different responses of regional monsoons to a warming environment. Citation: Qian, W., J. Fu, and Z. Yan (2007), Decrease of light rain events in summer associated with a warming environment in China during , Geophys. Res. Lett., 34, L11705, doi: /2007gl Introduction [2] Many studies have shown the recent trends in annual and summer total precipitation, and the large regional precipitation variability in China [Chen et al., 1998; Shi et al., 2003]. Scientists have paid increasing attention to extreme weather and climate events, such as heavy rainfall and flooding in summer, as these have caused serious natural disasters in China [Liu et al., 2005]. Based on the daily precipitation data, a decreasing trend in annual mean precipitation and extreme rain events was found in a zone extending from the southern part of Northeast China southwestward to the upper Yangtze River valley, whereas increasing trends were found in Xinjiang and Southeast China [Zhai et al., 2005; Qian and Lin, 2005]. Some studies have shown the interdecadal variations of rainfall in association with changes in the large-scale temperature structure [Gong and Wang, 2000; Hu et al., 2003] and circulation patterns in the East Asian summer monsoon system [Hu, 1997; Yang and Lau, 2004]. Analyses of trend for different rainfall intensities may help us to obtain additional insight into these processes.this paper will document the trend patterns of different graded rain events and their association with the global warming and circulation anomalies in China during the past 45 years. 1 Monsoon and Environment Research Group, School of Physics, Peking University, Beijing, China. 2 Key Laboratory of Regional Climate-Environment Research for Temperate East Asia (RCE-TEA), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China. Copyright 2007 by the American Geophysical Union /07/2007GL Data and Method [3] We used daily rainfall and temperature data based on 565 stations from China National Meteorology Centre [Qian and Lin, 2005; Qian and Qin, 2006] for the subtropical summer monsoon season (JJA) during The daily rain rates at the stations were classified into six grades of intensity from trace (no amount), small (0.1R<10 mm/ day), medium (10R<25 mm/day), large (25R<50 mm/ day), heavy (50R<100 mm/day), to very heavy (R100 mm/day) rains. In our analysis, we found that climatic and trend distributions of light rain (recorded but R1 mm/day) were different from those of other small (1<R<2, 2R<3 mm/day, etc.) rains. Thus, we re-categorized them into seven grades of rainfall intensity, including trace, light, small (1<R<10 mm/day), medium, large, heavy, and very heavy rains. As some grades showed similar trends, we only present the results of light, moderate (1<R<50 mm/ day) and extreme (R50 mm/day) rains. Also, the daily mean temperature was used to calculate its trend distribution in mainland China. [4] In order to explain the trends of regional rainfall structures, the NCEP/NCAR reanalysis data [Kistler et al., 2001] was used to calculate the regional water vapor transport and the summer monsoon index. The zonal difference of summer sea-level pressure (SLP), as an index, was used to represent the intensity of the summer monsoon in East Asia. It was defined as the sum of the summer SLP difference between 110 E and 160 E under the condition P 110 E 160 E 5hPa from 10 Nto50 Nwith a 10-degree latitude interval [Guo et al., 2003]. Each summer sum of SLP difference is divided by a climatological mean value of 50 summers from 1951 to A high monsoon index indicates a strong monsoon with strong southerly wind in eastern China under a large sea-land pressure difference. It is thought that changes in atmospheric circulation may result in the variations of water vapor transport and rain events. Regional budgets of summer water vapor over Southwest China and the Xinjiang region were also calculated based on the reanalysis data. 3. Results [5] In general, China can be divided into two parts, the subtropical monsoon region to the southeast and the nonmonsoon region to the northwest [Qian et al., 2007]. Climatologically, there were more than 15 trace rain events (days) in summer in the non-monsoon region and events in the monsoon region. There were light rain events in most of the country, except two regions: parts of western China and the North China Plain, where there were fewer than 10 days. For the small (medium) rain L of5

2 Figure 1. Trends (days/decade for rain events, C/decade for temperature) of (a) light rain events, (b) daily mean temperature, (c) moderate rain events, and (d) extreme rain events in mainland China for summer (JJA) from 1961 to Solid dots denote increasing trends and circles denote decreasing trends (squares and diamonds indicate significant trends at the 0.05 confidence level). The boxes in Figure 1c indicate four typical regions: Xinjiang (Northwest China), Southwest China, North China, and the middle-lower reaches of the Yangtze River. events, there were fewer than 10 (1) days in the inland dry region, more than 30 (10) days in the upper reaches of the Yangtze River and (5 10) days in Central Eastern China. Fewer than 5 days of large rain events were observed in the non-monsoon region, more than 20 days in Southwest China and along the coast of South China, and 5 10 days elsewhere. Annually, 4 5 days of extreme rain events can be found along the Yangtze River and in South China, but only a few events during the last 45 years can be found in Northwest China. [6] A significant decreasing trend of trace rain events was found in mainland China except several sites in Southwest China, confirming the conclusion of Yan and Yang [2000] based on the data from a sparse network of 61 stations. There was also a decreasing trend of light rain events in whole China except Northwest China and several sites in the Yangtze River valley (Figure 1a). [7] In China as a whole, consistent warming trends of daily maximum and minimum temperature have been well documented [Zhai and Ren, 1997; Zhai and Pan, 2003; Qian and Lin, 2004]. Figure 1b shows the trend distribution of daily mean temperature for summer. Significant increasing trends of temperature occurred in whole China except the central part of China and several sites in Xinjiang. This distribution is basically consistent with previous results for the period [Chen et al., 1998]. The relationship between the temperature and light precipitation trends will be discussed in the last section. [8] The trends of moderate rain events are shown in Figure 1c. Significant increasing trends occurred in Xinjiang and the middle-lower reaches of the Yangtze River, while decreasing trends occurred in North China and Southwest China. For the extreme rain events, a significant increasing trend was still found in the middle-lower reaches of the Yangtze River and a decreasing trend was found in North China (Figure 1d). These trend distributions were similar with the previous results of summer precipitation in China [Hu, 1997; Qian and Lin, 2005]. [9] The time series of total precipitation amount (R0.1mm/day) averaged over all sites in China showed an increasing trend of 4 mm/decade, but it was not significant during (Figure 2a). The increasing rainfall was mainly concentrated in the late 1990s. The time series of total rain events ( R0.1mm/day) in China, however, showed a significant decreasing trend during (Figure 2b). A decreasing trend of 0.75 days/decade for the light rain events was calculated over mainland China (Figure 2c). There was no significant trend of moderate rain events in China due to the regional feature. Extreme rain events were mainly located in eastern China over the monsoon region. In China, extreme rain events showed an increasing trend of 0.04 days/decade, but it was not significant (Figure 2d). An increasing trend of 0.02 days/decade for the very heavy rain events was found (Figure 2e). The increasing trend of extreme rain events and very heavy rain events in eastern China appeared consistent with the increase of very heavy rainfall that occurred in central India [Goswami et al., 2006]. The increasing trend of daily mean temperature (Figure 2f) coincided with the decreasing trend of light rain events in China. [10] There was an overall increasing trend of moderate rain events with a rate of 0.54 days/decade in the middlelower reaches of the Yangtze River, but it was not significant (Figure 3a). In contrast, an overall decreasing trend of moderate rain events occurred in North China (Figure 3b). For comparison, Figure 3c shows the index of the East 2of5

3 Asian summer monsoon [Guo et al., 2003]. It is commonly thought that the decreasing moderate rain events in North China and the increasing trend over the Yangtze River are associated with the weakening of summer monsoons in East Asia on the interdecadal scale. [11] Opposite trends of moderate rain events were also observed in Southwest China and Xinjiang (Figures 4a and 4c ). A decreasing trend ( 0.79 days/decade) was located in Southwest China, while an increasing trend (0.66 days/decade) was observed in Xinjiang. Hu [1997] also found that in Southwest China the rainfall anomaly around changes from above normal to below normal, and the temperature changes from below normal to above normal, according to an analysis for the period In Southwest China, the water vapor budget integrated from the surface to 300 hpa showed a decreasing trend (Figure 4c). A correlation analysis suggested that the weakening water vapor transport from South Asia was closely related to the decreasing trend of moderate rain events in Southwest China. In Xinjiang, the relationship is more complex before and after the year The increasing mean rainfall since 1987 in Xinjiang [Qian and Qin, 2007] is consistent with the increases of water vapor budget (Figure 4d) and westerly water vapor transport in the midhigh latitude region [Dai et al., 2006]. 4. Conclusion and Discussion [12] In summary, this paper demonstrated the trend patterns of different graded rain events in China during the past 45 years. Climatologically, there were more trace rain Figure 2. Annual series of (a) total precipitation amount (R0.1mm/day), (b) number (N) of total rain events (R0.1mm/day), (c) light rain events, (d) extreme rain events, (e) very heavy rain events, and (f) daily mean temperature ( C) for summer from 1961 to The dashed lines highlight the linear trends in the series. Asterisks (*, **, ***) indicate the linear trends reaching the 0.05, 0.01, and significance levels, respectively. The t-test method is used to detect whether the trend of a series reaches statistical significance. Figure 3. Same as Figure 2 except for annual series of the day number (N) of moderate rain events in (a) the middlelower reaches of the Yangtze River, (b) North China, and (c) the index of the East Asian summer monsoon from 1961 to of5

4 in decreasing trace and light rain events. Another relevant fact is that both daytime and nighttime total cloud cover exhibited significant decreasing trends of 1 2% sky cover/ decade during over China [Kaiser, 2000]. [14] There were different causes for the trends in moderate rain events. In eastern China, the weakening trend in the East Asian monsoon flows caused the decrease of moderate rain events in North China and the increase along the Yangtze River valley. The increasing trend of extreme rain events and the long-term increase of daily rainfall variation are likely due to the warming trend of tropical sea surface temperature [Goswami et al., 2006] or the global warming [Gong and Wang, 2000]. The observed trends of various graded rain events may be explained by a global warming environment and changes in regional atmospheric circulation, such as the weakening of summer monsoons in East Asia. [15] The decreasing trend of moderate rain events in Southwest China was due to the decrease of large rainfall events in association with the weakening Indian monsoon flows and decreasing water vapor budget. In Xinjiang, the increasing trend of moderate rain events was mainly due to the increase of small rainfall events. Before 1985, the decreasing trend of moderate rain events in Xinjiang was consistent with that of the water vapor budget. Since 1987, however, the strengthening of the water vapor transport by the mid-high latitude westerly flows and the enhancement of the local hydrological cycle may have contributed to the increasing trend of moderate rain events in the Xinjiang region. Figure 4. Same as Figure 2 except for annual series of the day number (N) of moderate rain events in (a) Southwest China and (c) Xinjiang from 1961 to (b and d) Water vapor budgets integrated from the surface to 300hPa for the domains indicated in Figure 1c over Southwest China and Xinjiang. In Figures 4c and 4d, long-term trends for 45 years and short-term trends are indicated. events in the non-monsoon region than in the monsoon region. For light precipitation, there were fewer events in western China and the North China Plain. Fewer trace and light rain events occurred in some places of higher summer temperature in eastern China. The lower air temperature in the western mountain regions results in a larger probability of rainfall, since the air can reach its dew-point temperature more easily than other regions. For small to large rainfall, there were fewer events in the inland dry region and the non-monsoon region due to the large scale circulation. [13] This paper demonstrated the overall decreasing trend of trace and light rain events, accompanied by increasing temperature in China. Previous studies have revealed decreasing trends in surface wind speed, pan evaporation, sunshine duration, and daily temperature range since the 1950s [Chen et al., 1998; Ren and Guo, 2006]. The increasing sulfate aerosols and the weakening wind caused the decreasing sunshine duration and pan evaporation [Ren and Guo, 2006], and the rising nighttime temperature and daily mean temperature. Given a stable level of water vapor content in the lower troposphere, it is harder than usual for the warmer air to reach the dew-point temperature, resulting [16] Acknowledgments. This research was supported by the CAS BRJH project and the NNSF China ( ) and the NBRP China (2006CB40362 and ). The authors thank the two anonymous reviewers for their helpful comments and suggestions. References Chen, L. X., W. Q. Zhu, W. Wang, X. J. Zhou, and W. L. Li (1998), Study on climate change in China in recent 45 years (in Chinese), Acta Meteorol. Sin., 12(1), Dai, X. G., W. J. Li, and Z. G. 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