Duration is a key feature of rainfall events that is closely related to rainfall mechanisms and influences. This study analyzes the decadal change in the duration-related characteristics of late-summer (July–August) precipitation over eastern China during 1966–2005. Accompanying the southern-flooding and northern-drought (SFND) pattern of rainfall amount over the eastern China in recent decades, the duration-related rainfall structure also experienced significant changes. In North China, the frequency of short duration rainfall events decreased and their intensity increased. The decadal decreases of rainfall amount over North China are largely contributed by long duration rainfall events, especially those occurring between midnight and morning. In the mid-to-lower reaches of the Yangtze River valley, both the frequency and amount of long duration precipitation have significantly increased. The mean and maximum duration time of late-summer precipitation has increased 0.85 and 7.61 h, respectively. Considerable increases of rainfall amount of two kinds of precipitation, the short and medium duration rainfall events in the late afternoon and the long duration rainfall events in the early morning, contribute to the “southern-flooding.” Despite the differences between the northern and southern region, there is a common feature of their decadal precipitation changes that the intensity of short duration rainfall in the late afternoon has shown an increasing trend.
Duration time is an important feature of rainfall events. Compared with daily rainfall amount, frequency, and intensity, hourly based rainfall duration is more closely related to the nature of precipitation events. Yu et al. (2007a) discussed the relationship between rainfall duration and diurnal cycle of the warm season precipitation over central eastern China. They proposed that the duration time is a key factor to separate out different kinds of rainfall events. In central eastern China, the short duration rainfall events, which last for less than 3 h, tend to be triggered by strong solar heating and reach the diurnal maximum rainfall around late afternoon. In contrast, the long duration events (longer than 6 h) mostly have their peak rainfall in early morning and are often caused by organized rainfall systems rather than isolated convection. Yuan et al. (2010) has reported that long duration rainfall is closely related to the large-scale monsoon circulation and experiences evident subseasonal movement coinciding with East Asian monsoon rain belt. Hence quantification of the changes in rainfall duration can help to improve the understanding of corresponding mechanisms. Rainfall duration is also an important factor that has significant influences on the local hydrological cycle. The changes in rainfall duration can exert a strong control on the variation of groundwater and overland flow. However, duration-related characteristics of rainfall have been subject to far fewer analyses than rainfall amount and frequency.
In the second half of the twentieth century, the late summer [July and August (JA)] precipitation over central eastern China has experienced considerable interdecadal change. A prominent feature of the change is the southern flooding and northern drought (SFND) pattern, which refers to the increased rainfall over the mid-to-lower reaches of the Yangtze River valley and the decreased rainfall over North China (Hu 1997; Xu 2001; Menon et al. 2002; Hu et al. 2003; Yu et al. 2004; Chen et al. 2004). Yu et al. (2010) showed the spatial pattern of the 20-yr mean changes (1986–2005 mean minus 1966–85 mean) in JA rainfall amount in their Fig. 2a. They pointed out that the area-averaged JA rainfall amount has significantly increased (decreased) by 44.8 mm (22.4 mm) over the mid-to-lower reaches of the Yangtze River valley (North China). The decadal change reaches 17.2% (9.8%) of the climatological mean JA rainfall amount over the “south” (“north”) region.
The SFND is closely associated with the shift in large-scale circulation patterns. During the last several decades, strong tropospheric cooling occurs over East Asia (Yu et al. 2004). Yu and Zhou (2007) stated that the cooling weakens the normal northward progression of the southerly monsoon in July and August, which reduces the water supply in North China and keeps the monsoon trough over the mid-to-lower reaches of the Yangtze River valley. Under this anomalous circulation pattern, significant SFND appears in July and August in recent decades. Despite the changes of large-scale circulation, there are also studies linking the rainfall changes to anthropogenically induced changes in extreme precipitation by analyzing the simulation from coupled models (Hegerl et al. 2004; Kimoto et al. 2005). Therefore, the specific mechanism responsible for SFND remains an open question. To identify the root causes of SFND, it is necessary to grasp its essential features. Based on daily rainfall products, previous studies have investigated the interdecadal changes of precipitation characteristics. Wang and Zhou (2005) analyzed long-term trends in mean and extreme precipitation over China and showed that the change of mean precipitation varies with seasons and regions. Patterns of the trends in extreme daily precipitation events are similar to those in the annual and seasonal mean precipitation except in northwest China where most areas show increasing trends in extreme events only in summer. Zhai et al. (2005) indicated that the number of rain days has significantly decreased throughout most parts of China with northwest China being an exception and precipitation intensity has significantly increased. The long-term trends in mean rainfall, rainfall intensity, and extreme precipitation in Japan are also studied (Fujibe et al. 2005, 2006a,b). However, rainfall is often confined to a short time and it is of interest to analyze hourly precipitation for interdecadal changes. Dai (1999) analyzed hourly precipitation data and found that the hourly precipitation frequency and diurnal amplitude have changed from 1963 to 1993 over the contiguous United States. Using hourly rainfall data, Yu et al. (2010) found that the hourly rainfall intensity has decreased (increased) in the mid-to-lower reaches of the Yangtze River valley (North China) during 1966–2005. And the SFND pattern is mostly attributed to changes in precipitation with moderate and low intensity.
In this study, quality controlled hourly rainfall records were used to analyze the duration-related characteristics of the SFND pattern. The findings could provide some detailed features of the SFND, which are closely related to the precipitation process.
2. Data and methods description
The dataset of hourly rain gauge records at 575 stations covering mainland China for the period from 1954 to 2007 has been used in this study. This dataset was obtained from the national climatic reference network and national weather surface network of China and was collected and quality controlled by the National Meteorological Information Center (NMIC) of the China Meteorological Administration (CMA). To avoid biases introduced by missing data, the analysis was restricted to the period from 1966 to 2005. Further quality control has been applied as follows: daily rainfall amount Rh is calculated by the hourly data and compared with the record of daily rainfall amount Rd; if |Rh − Rd| > Rd/10, the records of hourly data in this day are regarded as missing values. The records from 371 stations (marked in Fig. 1) have been analyzed to quantify the changes in late-summer precipitation. All of these stations are located in eastern China and have more than 900 days without missing or suspicious values in JA during both the former (1966–85) and the latter (1986–2005) 20 years. Two key regions are selected following previous work by Yu et al. (2010). In Fig. 1, North China (37°–42°N, 107.5°–125°E) is enclosed by a solid-line box and is defined as region north in this paper. The mid-to-lower reaches of the Yangtze River valley (27°–33°N, 107.5°–122.5°E) is marked by a dashed-line box and is referred to as region south.
At each station a rainfall event starts when measurable precipitation (≥0.1 mm h−1) occurs after two or more dry hours. Following Yu et al. (2007a), the duration of a rainfall event is defined as the number of hours from the beginning to the end of the event, during which time there is no intermittence or at most a 1-h intermittence. Precipitation frequency referred to in this paper was defined as the number of hours having measurable precipitation during each late-summer. For one JA period, if there are two rainfall events that last for 10 and 20 h respectively, then the frequency is 30 h, and the number of rainfall events is 2. The rainfall intensity was calculated as the mean rainfall rate averaged over all rainy hours in JA.
a. Changes of rainfall events with different duration time
At each station rainfall events are divided into two groups according to their duration. Precipitation that lasts for 1–6 h are defined as short duration events, and those that persist longer than 6 h are labeled as long duration events. The number of these two kinds of precipitation events is an important feature of regional climate. In late summer, the 1966–2005 mean number of short (long) duration rainfall events is 20.3 (4.9) in region north and 21.8 (4.7) in region south. Figure 2 shows the changes in the number of JA rainfall events between the 1986–2005 mean and the 1966–85 mean. For short duration rainfall (Fig. 2a), the number of events significantly decreases in region north. Compared to the former 20 years, there are around 3.0 fewer events in the 1986–2005 mean JA period. However, changes in short duration events are small and not significant in region south. Figure 2b presents the changes in the number of long duration events. The zero line locates around 35°N in eastern China, with negative (positive) values north (south) of it. In region north the number of long duration rainfall events significantly decreases around 0.81 during each JA. In contrast, there are 0.87 more long duration events in region south in each JA in the latter 20 years. Accompanying the “southern flooding and northern drought,” the duration-related rainfall structure also changes in central eastern China, and the changes in region north and south exhibit different features. The rainfall frequency and its changes as a function of duration are shown in Fig. 3. The JA rainfall frequency area averaged over region north and south are presented in Figs. 3a and 3b, respectively. For the rainfall events persisting longer than 2 h, the JA rainfall hours decrease as the duration increases in both regions. In region north the rainfall frequency decreases in the last several decades. And around 73.5% of the decreases in rainfall frequency is contributed by the rainfall events shorter than 13 h. The changes in short duration events (1–6 h) account for approximately 40.8% of the total decreases. In contrast, the rainfall events lasting longer than 12 h do not show considerable changes in frequency. In region south, the line for the latter 20 years is above that of the former 20 years except the 1-h duration events. The frequency changes are almost evenly distributed among different duration events. The 1–6- (1–12-) h duration events account for 11.4% (31.0%) of the total increases.
Figure 4 shows the changes in rainfall amount of short (a) and long (b) duration events. For precipitation within 6 h, the rainfall amount changes exhibit strong local features. The positive and negative centers are alternatively distributed and there is no unified pattern in either region north or south. Nevertheless, the rainfall amount changes of precipitation lasting longer than 6 h present negative (positive) values in region north (south). In the last several decades, the long duration rainfall decreases 19.4 mm in region north and increases 32.8 mm in region south in each JA. Comparing with the 1966–2005 mean JA rainfall amount of long duration events (139.3 mm in region north and 153.6 mm in region south), the decrease (increase) in region north (south) accounts for 13.9% (21.4%) of the climatic mean. The “southern flooding and northern drought” is mostly contributed by the changes of long duration rainfall events. Figure 5 shows the area-averaged rainfall amount as a function of rainfall duration. In both regions, the accumulated rainfall amount decreases as duration increases from 2 h, which is similar to the frequency pattern in Fig. 3. In region north (Fig. 5a), the rainfall amount of short duration events does not show decreasing trend, which is in sharp contrast to the changes in frequency. Comparison between the rainfall amount and frequency changes of short duration events indicates that the rainfall intensity has changed. During the period from 1966 to 2005, the intensity of short duration rainfall in region north increases 0.35 mm h−1 and the linear trend is significant at 1% confidence level. Most of the rainfall reduction (96.5%) over region north comes from the long duration events. The strongest decrease occurs in the 11-h duration events and it accounts for 18.3% of the total decline. In region south (Fig. 5b), the rainfall changes are evenly distributed among different duration events and the corresponding variation is analogous to that of the frequency (Fig. 3b). The short duration precipitation events account for 15.6% of the increase in rainfall amount over region south and around 84.4% is contributed by long duration events.
b. Changes in rainfall duration
Figure 6 shows the time series of the mean (Fig. 6a) and maximum (Fig. 6b) duration time averaged over region north and south. During the 40 years from 1966 to 2005, the mean rainfall duration time in region south increases 0.85 h, which is statistically significant at the 1% level. And the maximum duration time in region south also exhibits remarkable upward trend, which reaches 7.61 h (40 yr)−1 and is statistically significant at the 1% level. There are no significant linear trends in both the mean and the maximum duration time averaged over region north. During 1966 to 1985, the mean (maximum) duration time of region north is 0.28 (0.08) hours longer than that of region south. In contrast, during the latter 20 years, the mean (maximum) duration of region south is 0.20 h (5.29 h) longer than that of region north. There is a noticeable relationship between the mean duration of these two regions at interannual time scale. After removing the linear trend, the two series shown in Fig. 6a have a negative correlation of −0.33, which is significant at the 5% level.
c. Duration and diurnal variation
As stated in Yu et al. (2007b,a), the summer precipitation over contiguous China has large diurnal variations with considerable regional features, which are closely related to the rainfall duration. The diurnal variations of JA rainfall amount for events with different duration hours over region north (south) are shown in Fig. 7a (Fig. 7b). One common feature of these two regions is that most of the rainfall amount of short duration events is concentrated in the afternoon, with a peak at 1700 LST. And the long duration rainfall events tend to occur in the early morning. Revealing the diurnal cycle of rainfall changes can improve the understanding of shifts in rainfall features. Figure 8 shows the rainfall amount differences between the latter and former 20 years as functions of both duration and diurnal phases. In region north (Fig. 8a), the rainfall amount of short duration events decreases at 1400 LST and increases at 1800 LST. Large and significant decreases of rainfall occur from midnight to morning, especially for rainfall events with fairly long duration (lasting around 11 h). The changes in rainfall accumulated from events occurring between 2300–1000 LST and lasting for 6–12 h account for 43.0% of the total decrease of precipitation amount in region north. Over region south (Fig. 8b), positive values dominate most of the plot and two types of rainfall show strong and significant increasing trend. One is the rainfall between 1500 and 2100 LST with short and medium duration time. The other is the rainfall between 0300 and 0900 LST with long duration time. The corresponding changes in JA rainfall frequency are shown in Fig. 9. Over region north, the strongest frequency decrease occurs during midnight to morning hours (Fig. 9a). Both the short and long duration rainfall hours have significantly reduced during this period. The negative center around 9–11-h duration is coherent with the strong decrease of rainfall amount. Figure 9b presents the decadal rainfall frequency changes of region south. The increase in frequency of long duration early morning rainfall is consistent with the precipitation amount shift shown in Fig. 8b. In contrast to the positive changes of most kinds of rainfall events, the frequency of 1–2-h duration late afternoon precipitation has decreased. Comparing Fig. 8 and Fig. 9, there are positive changes in rainfall amount and negative changes in frequency of short duration late afternoon precipitation in both regions, which indicates the intensity of this kind of rainfall has increased. Figure 10 presents the time series of intensity of late afternoon precipitation with short duration. The intensity averaged over region north has experienced a pronounced increase as high as 0.58 mm h−1 in 40 years. The corresponding trend for region south is also significant, which is around 0.30 mm h−1 (40 yr)−1. The correlation coefficient between these two lines reaches 0.42 (0.31 after detrending). Although the JA rainfall in region north and south has shown out-of-phase features in many aspects, the short duration rainfall in the late afternoon, which is more closely related to local thermal and moisture conditions rather than large-scale circulations, has certain related variations.
4. Summary and discussion
Using hourly station rain gauge data, this paper reveals some duration-related features of the decadal changes in the late summer precipitation over central eastern China. The major findings are summarized here.
In region north, the frequency of short duration rainfall events decreased and the intensity increased. As for the long duration events, both the frequency and amount have decreased.
In region south, the rainfall frequency and rainfall amount of long duration events have significantly increased.
The mean and maximum rainfall duration in region south have considerably expanded during the last several decades.
The “northern drought” can be partly ascribed to the decreases of long duration rainfall in the night and early morning. The “southern flooding” is contributed by both the increases of late afternoon rainfall with short and medium duration and early morning rainfall with long duration.
The intensity of short duration precipitation in the late afternoon has increased in both regions.
The changes in duration-related rainfall features provide new insights for exploring the mechanisms of the decadal rainfall shifts over eastern China. As proposed by Yuan et al. (2010), the rainfall in the East Asia summer monsoon rain belt is dominated by the long duration rainfall with early morning peaks. Thus the SFND is closely related to the changes in monsoon rain belt over eastern China. When using composite analysis to evaluate the influences of circulation field, it is suggested to select the events with significant long duration rainfall. Such composites of circulation fields have more chance to reveal the mechanisms of SFND. Although the linear trends of total precipitation amount over the two regions are opposite, they share one common feature that the late afternoon short duration rainfall has been strengthened. This indicates that multiple forcings contribute to the decadal changes of precipitation. Distinguishing their related rainfall feature is a key step to understand their influences.
This research was supported by the Major National Basic Research Program of China (973 Program) on Global Change under Grant 2010CB951902, National Natural Science Foundation of China under Grants 40625014 and 40921003, and the Basic Research Fund of CAMS (Grant 2010Z003).