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Distribution and Diurnal Variation of Warm-Season Short-Duration Heavy Rainfall in Relation to the MCSs in China

í ý), ZHENG Yongguang (Ü ½), ZHANG Xiaoling ( and ZHU Peijun (ý )

CHEN Jiong1 (

1∗

1

),

2

1 National Meteorological Center, China Meteorological Administration, Beijing 100081 2 Zhejiang University, Hangzhou 310027 (Received June 13, 2013; in final form November 19, 2013)

ABSTRACT Short-duration heavy rainfall (SDHR) is a type of severe convective weather that often leads to substantial losses of property and life. We derive the spatiotemporal distribution and diurnal variation of SDHR over China during the warm season (April–September) from quality-controlled hourly raingauge data taken at 876 stations for 19 yr (1991–2009), in comparison with the diurnal features of the mesoscale convective systems (MCSs) derived from satellite data. The results are as follows. 1) Spatial distributions of the frequency of SDHR events with hourly rainfall greater than 10–40 mm are very similar to the distribution of heavy rainfall (daily rainfall  50 mm) over mainland China. 2) SDHR occurs most frequently in South China such as southern Yunnan, Guizhou, and Jiangxi provinces, the Sichuan basin, and the lower reaches of the Yangtze River, among others. Some SDHR events with hourly rainfall  50 mm also occur in northern China, e.g., the western Xinjiang and central-eastern Inner Mongolia. The heaviest hourly rainfall is observed over the Hainan Island with the amount reaching over 180 mm. 3) The frequency of the SDHR events is the highest in July, followed by August. Analysis of pentad variations in SDHR reveals that SDHR events are intermittent, with the fourth pentad of July the most active. The frequency of SDHR over mainland China increases slowly with the advent of the East Asian summer monsoon, but decreases rapidly with its withdrawal. 4) The diurnal peak of the SDHR activity occurs in the later afternoon (1600–1700 Beijing Time (BT)), and the secondary peak occurs after midnight (0100–0200 BT) and in the early morning (0700–0800 BT); whereas the diurnal minimum occurs around late morning till noon (1000–1300 BT). 5) The diurnal variation of SDHR exhibits generally consistent features with that of the MCSs in China, but the active periods and propagation of SDHR and MCSs differ in different regions. The number and duration of local maxima in the diurnal cycles of SDHR and MCSs also vary by region, with single, double, and even multiple peaks in some cases. These variations may be associated with the differences in large-scale atmospheric circulation, surface conditions, and land-sea distribution. Key words: short-duration heavy rainfall, climatology, spatiotemporal distributions, diurnal variation, propagation, mesoscale convective systems (MCSs) Citation: Chen Jiong, Zheng Yongguang, Zhang Xiaoling, et al., 2013: Distribution and diurnal variation of warm-season short-duration heavy rainfall in relation to the MCSs in China. Acta Meteor. Sinica, 27(6), 868–888, doi: 10.1007/s13351-013-0605-x.

1. Introduction Short-duration heavy rainfall (SDHR) is one type of severe convective weather that occurs in China. SDHR events can lead to urban waterlogging and ge-

ological disasters. For example, debris flow resulting from an SDHR event in Zhouqu, Gansu Province on 8 August 2010 caused approximately 2000 deaths. SDHR has therefore become an important focus of weather forecasting in China. The Central Meteorolo-

Supported by the China Meteorological Administration Special Public Welfare Research Fund (GYHY201206004, GYHY201206003, and GYHY200906003) and National (Key) Basic Research and Development (973) Program of China (2013CB430106). ∗ Corresponding author: [email protected]. ©The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2013

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gical Office of China defines SDHR as hourly rainfall in excess of 20 mm, while heavy rainfall in China is defined as daily rainfall in excess of 50 mm. In China, both SDHR and heavy rainfall are mainly produced by mesoscale convective systems (MCSs) and generally accompanied by thunder and lightning. The definition of SDHR emphasizes rain produced by strong convective systems with a short duration. Heavy rain usually lasts a longer time and includes both convective and stratiform precipitation. The climatological distribution of SDHR over China has not yet been studied. Previous studies have focused mainly on the distributions of heavy rainfall ( 50 mm day−1 ), extreme precipitation, thunderstorms, and hail over China (Zhang and Lin, 1985; China Meteorological Administration, 2007; Shen et al., 2010; Huang et al., 2011). Zheng et al. (2007, 2008) used geostationary satellite observations of infrared equivalent black body temperature (TBB) to study the climatological distribution of deep convection over Beijing and China, respectively. Wang and Cui (2012) studied the organizational structure of MCSs in the middle and lower reaches of Yangtze River during the Meiyu period. Yu et al. (2007a, b) showed the relationship between the duration and diurnal variations of rainfall over China during summer using hourly rainfall data. Li et al. (2008a) used hourly raingauge data to study diurnal variations in summer precipitation over Beijing, while Li et al. (2008b) used similar data to derive seasonal variations in the diurnal cycle of rainfall in southern contiguous China. Zhou et al. (2008) studied rainfall during the East Asian summer monsoon period using satellite products and raingauge records. They characterized the spatial patterns of June–August mean precipitation amount, frequency, and intensity, as well as the diurnal and semidiurnal cycles of rainfall. However, their study neglected the diurnal variations of rainfall in spring and autumn and did not analyze the distribution of SDHR over China. Chen et al. (2010) investigated the influences of largescale forcing on the diurnal variation of summer rainfall along the Yangtze River using hourly observational records and reanalysis data at 6-h intervals. Yao et

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al. (2009) analyzed the spatial and temporal distributions of hourly rainfall greater than 8 mm and less than 8 mm based on hourly records from 485 stations in China between 1991 and 2005. These studies have made important contributions to current understanding of diurnal variation, persistence, and propagation of precipitation, and causes of precipitation climate change. However, they have not properly characterized the distribution of SDHR, due in part to the relatively low occurrence frequency of SDHR caused by severe convection. Recently, Zhang and Zhai (2011) analyzed temporal and spatial distributions of the frequency of precipitation events with rain rates exceeding 20 and 50 mm h−1 using hourly precipitation data from 1961 to 2000. They also studied diurnal variations and trends in extreme precipitation during the warm season (May–September). Although summer is the main season for heavy rain in China, convective weather also occurs frequently during spring and autumn in southern China (Huang et al., 1986). This paper uses hourly precipitation data taken over the period 1991–2009 to analyze the climatological characteristics of SDHR during the warm season (April–September).These characteristics are then compared with the climatological characteristics of MCSs as indicated by geostationary satellite observations of low TBBs ( –52℃). The purpose of this study is to improve understanding of the climatology of severe convective weather and to provide a climatological foundation for forecasting these types of weather events. 2. Data and methods The hourly rainfall data are provided by the National Meteorological Information Center of China. This dataset is basically the same as that used by Yu et al. (2007a) and Yao et al. (2009). The data used in this paper cover the period April–September of 1991–2009, and were collected at 876 observation stations over mainland China, including almost all of the first- and second-class national stations in China except those in Taiwan. Hourly rainfall refers to 1-h cumulative rainfall during the previous hour. From

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these observations, we selected 15 yr of observations taken at 549 stations (Fig. 1). The average distance between two neighboring stations is approximately 100 km. In general, the densest observations occur in eastern China. Although the Central Meteorological Office of China has defined SDHR as hourly rainfall  20 mm, there is currently no strictly uniform definition of SDHR in China. Flash floods in the United States are generally associated with hourly rainfall  20 mm (Davis, 2001). Zhang and Zhai (2011) found that the annual occurrence frequency of days with rainfall intensity  50 mm day−1 is approximately equal to the annual occurrence frequency of hourly rain rates in excess of 20 mm h−1 . This result suggests that 20 mm h−1 is a suitable threshold for short-duration “rainstorms” in eastern China. Scale analysis and the relationship between precipitation and vertical velocity suggest that precipitation rates  10 mm h−1 are generally associated with meso- and micro-scale weather systems, while precipitation rates  50 mm h−1 are mainly produced by microscale weather sys-

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tems. The sensitivity of the climatology of SDHR to the threshold intensity is explored by calculating climatological distributions of hourly rainfall  10, 20, 30, 40, and 50 mm over China. The hourly rainfall rates used in this paper are derived on an hourly basis. As a result, an hour of heavy rainfall that occurs across two 1-h intervals may be classified into two hourly data points with rain rates less than the intensity during the heavy rainfall period. Accordingly, the frequencies of SDHR reported in this paper are likely underestimates of the actual frequencies. The following methodology is adopted in this study. Given a specified analysis period and threshold rainfall intensity (for example, 20 mm), the SDHR frequency at any station in China is calculated by dividing the number of hourly data points exceeding the specified threshold intensity by the total number of hourly data points. This frequency is referred to as the frequency of SDHR occurrence. The spatial distribution and average diurnal variations of maximum hourly rainfall are also presented.

Fig. 1. Orography and geography of China (shading), with selected meteorological observation stations (red dots) and analysis regions (white solid lines labeled I, II, III, and IV denote the positions of different cross-sections; rectangles labeled A, B, C, D, E, F, G, and H indicate regions used to calculate diurnal cycles of SDHR and MCSs (TBB  –52℃); see text for details).

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Geostationary satellite observations of infrared TBB are used to analyze the diurnal variations of MCSs. These diurnal variations are then compared with the diurnal variations of SDHR. The TBB dataset is effectively the same as that used by Zheng et al. (2008), but with temporal coverage from June to August 1996–2007 (excluding 2004). The spatial resolution of the data is 0.1◦ ×0.1◦ (latitude by longitude). The methodology mirrors that applied by Zheng et al. (2008), in which the frequency of MCSs is calculated for each grid cell as the frequency of TBB  –52℃. The spatial resolution of the TBB data is significantly finer than the spatial resolution of the hourly rainfall data (approximately 100 km). 3. Spatial distribution Figure 2 shows the spatial distributions of SDHR events exceeding 20 and 50 mm h−1 and the spatial distribution of maximum hourly rainfall over China. The spatial distributions of SDHR events exceeding 10, 30, and 40 mm h−1 (figure omitted) are similar to that of SDHR events exceeding 20 mm h−1 (Fig. 2a). These distributions are also consistent with that reported by Zhang and Zhai (2011), who based their analysis on hourly rainfall observations over eastern China during the warm seasons of 1961–2000. The spatial distribution of SDHR events exceeding 50 mm h−1 (Fig. 2b) is substantially different from the distributions of SDHR events exceeding the lower intensity thresholds. This difference may be because extreme rainfall events with intensity exceeding 50 mm h−1 are mainly associated with rare microscale weather systems that have a very low occurrence frequency. This paper focuses on the spatial and temporal distributions of SDHR events exceeding 20 mm h−1 for consistency with the definition of SDHR given by the Central Meteorological Observatory of China, the threshold for extreme hourly rainfall over eastern China identified by Zhang and Zhai (2011), and the threshold of hourly rainfall intensity typically associated with flash floods in the U.S. (Davis, 2001). This simplification is also supported by the consistency among the spatial distributions of SDHR event frequencies in China when

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SDHR is defined using intensity thresholds between 10 and 40 mm h−1 . The spatial distribution of SDHR events exceeding 20 mm h−1 is very similar to the distribution of the annual mean number of heavy-rain days in China (Zhang and Lin, 1985; China Meteorological Administration, 2007). SDHR events generally occur more frequently in southern China than in northern China. They also occur more frequently in eastern China than in western China, and are more common over plains and valleys than over the adjacent plateaus and mountains. Seasonal rainfall over most of China is closely associated with the movement of the East Asian summer monsoon (Tao, 1980). SDHR events therefore occur more frequently in regions strongly affected by the summer monsoon than in regions where the influence of the summer monsoon is small. The distribution of SDHR frequency over China is also quite similar to the distribution of lightning density according to satellite observations (Ma et al., 2005; China Meteorological Administration, 2007). This result indicates that the climatologies of heavy precipitation and lightning activity are closely related. The distributions of SDHR frequency, thunderstorm activity (China Meteorological Administration, 2007), and MCS occurrence (Zheng et al., 2008) are generally consistent, although there are some important differences. Thunderstorms and MCSs generally occur more frequently over mountain and plateau regions, while SDHR events occur more frequently over plains and valleys. SDHR activity is largest in South China, with a maximum frequency of 0.62% (Fig. 2a). Other regions with high SDHR activity include the southwestern Sichuan basin, the southeastern part of Southwest China, the eastern Huang-Huai River basin, the JiangHuai River basin, Jiangxi Province, the coastal areas of Zhejiang Province, and most of Fujian Province. The spatial distribution of the average annual number of days with SDHR (days on which the rain rate exceeds 20 mm h−1 at least once; figure omitted) is consistent with the spatial distribution of SDHR frequency. The maximum annual mean number of SDHR days in South China is 30 days. The spatial distribution of SDHR and the locations of centers of high

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Fig. 2. Frequencies (%) of SDHR events exceeding (a) 20 and (b) 50 mm h−1 during the warm season (April–September), and (c) maximum hourly rainfall (mm) observed during the warm season.

SDHR activity are closely related to the characteristics of the terrain. This result is consistent with the conclusions of several previous studies that the distribution of heavy rainfall in China is strongly affected by the topography (Tao, 1980; Huang et al., 1986). The frequency of extreme SDHR (hourly rainfall  50 mm) is very low throughout China (Fig. 2b). The maximum frequency is only 0.08%, which corresponds to only 8 h of extreme SDHR in every 10000 h (approximately 417 days). The maximum number of hours in April–September with extreme SDHR  50 mm h−1 during 1991–2009 is 64 h at Yangjiang station. Extreme SDHR events occur most frequently over the coastal areas of Fujian and Zhejiang, central Henan, southern Hebei, and southwestern Liaoning provinces. The spatial distribution of extreme SDHR  50 mm h−1 is very similar to the spatial distribution of heavy rain  100 mm day−1 reported by Zhang and Lin

(1985). Extreme SDHR activity (rain rates  50 mm h−1 ) is much more scattered and heterogeneous than SDHR activity (rain rates  20 mm h−1 ). This may be because extreme SDHR is mainly produced by extreme microscale weather systems, or it may be attributable to special terrain characteristics. Extreme SDHR is more common along the coastal areas of southeastern China than in inland areas because the coastal areas are more commonly affected by extreme weather systems associated with typhoons or tropical easterly waves. The spatial distribution of maximum hourly rainfall (Fig. 2c) reveals another aspect of severe convective weather systems. The distribution of maximum hourly rainfall is different from the distribution of SDHR frequency over China. The maximum hourly rainfall observed at most stations outside the typical domain of the summer monsoon was less than 50 mm,

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likely due to lower concentrations of water vapor in the atmosphere. In eastern China (where the annual cycle of rainfall is strongly affected by the summer monsoon), most of the stations that observe a maximum hourly rainfall larger than 120 mm are located in southern China; however, the maximum hourly rainfall exceeds 80 mm at several stations in northern China, and even exceeds 120 mm at a few stations. This result suggests that the intensities of the most severe convective rainfall events are similar throughout eastern China. The hourly rainfall data used for this analysis only include observations from most of the first- and second-class national meteorological stations in China. The data at each station only represent the average hourly rainfall in a very specific area. Furthermore, the data record only spans 19 yr. The maximum hourly rainfall observed at each station during this time period may not be truly representative of extreme precipitation. For example, there are reports of hourly rainfall of 185.6 mm at Gangyaoling, Liaoning Province on 11 July 1978 (Hydrological Bureau of the Yangtze River Water Resources Commission of MWR of China, 1995; Hydrology Bureau of MWR of China, 2006), 198.3 mm at Linzhuang, Henan Province on 5 August 1975 (Ding and Zhang, 2009), 220.2 mm at Maodong (Yangjiang), Guangdong Province on 12 May 1979, and 245.1 mm at Dongxikou (Chenghai), Guangdong Province on 11 June 1979 (Huang et al., 1986). 4. Seasonal and pentad variations 4.1 Seasonal variations Heavy rain and convective weather in China occur most frequently during summer. Summer (June– August) is also the most active season for SDHR (Fig. 3), which is consistent with common knowledge of precipitation patterns in China. The second highest SDHR frequency during the warm season is in late spring (April–May), while the frequency of SDHR drops substantially in early autumn (September). The spatial distribution of SDHR events during summer (Fig. 3b) is very similar to the distribution during the

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entire warm season (Fig. 2a). This similarity suggests that the distribution of SDHR during the warm season is primarily determined by the distribution during summer, and the summer monsoon plays a key role in determining the distribution of SDHR. Springtime SDHR events occur mainly in South China, particularly in the regions south of the Yangtze River and southern Yunnan (Fig. 3a). The largest frequencies are located in the eastern part of the Guangxi Region and western Guangdong Province. These high frequencies are closely associated with precipitation during the first rainy season in South China. Although the warmest air masses during spring are located over southern China, SDHR events occasionally occur in the southern part of North China and western Shandong Province. For example, an SDHR event with a rain rate greater than 20 mm h−1 occurred at Jiyang, Shandong Province on 17 April 2003. Another SDHR event with a rain rate greater than 20 mm h−1 occurred over a wide area comprising southern Hebei, Shandong, and northern and western Henan provinces during 9–10 May 2009. An event of similar magnitude occurred over a broad area comprising central and southern Hebei, northwestern Shandong, and central and southern Liaoning provinces during 24–25 April 2012. As mentioned above, the spatial distribution of SDHR during summer largely determines the mean distribution for the entire warm season; however, the centers of high SDHR frequency are more prominent during summer than when the warm season is considered as a whole (Fig. 3b). SDHR events also occur relatively frequently in the boundary regions of the summer monsoon (e.g., southern Gansu, Shaanxi, Shanxi provinces, and central and eastern Inner Mongolia) during this season. An SDHR event with a peak hourly rainfall of 77.3 mm resulted in a large and damaging flow of debris at Zhouqu, Gansu Province on 8 August 2010. The frequency of SDHR events weakens considerably in early autumn (September), toward the end of the warm season (Fig. 3c). The frequency of early autumn SDHR events is highest in the coastal areas of southeastern China such as Fujian, Zhejiang, Hainan,

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Fig. 3. Frequencies (%) of SDHR events with rain rates greater than 20 mm h−1 during (a) late spring (April–May), (b) summer (June–August), and (c) early autumn (September).

and so on. SDHR events also occur relatively frequently in southern Yunnan Province and in the Sichuan basin. SDHR events are not limited to these regions, however; for example, an SDHR event with a peak hourly rainfall of more than 60 mm occurred over a broad area south of the Yangtze River on 9–10 November 2009. The distribution of rainfall in China during summer is determined primarily by the advance and retreat of the summer monsoon (Tao, 1980). SDHR events in spring are often associated with the summer monsoon or cold air incursions. Most precipitation in South China during April is related to frontal activity. After the summer monsoon becomes established in mid-May, convective activity and the occurrence frequency of heavy precipitation increase significantly in South China (Huang et al., 1986; Zhou et

al., 2003). In summer (especially in July and August), there may be two or three active bands of SDHR over China on one day. One of these bands may be related to cold air incursions at middle and high latitudes, while the other(s) may be related to the activity of the subtropical high and summer monsoon. SDHR events are much more common under the latter situation than under the former. The western Pacific subtropical high at 850 hPa weakens considerably in September, so weather conditions in eastern China are dominated by the deformation of the anticyclonic circulation in the Arctic high pressure zone south of 40◦ N (Central Meteorological Bureau, 1975). The frequency of SDHR events therefore drops rapidly at this time of year over most of China; however, the ITCZ (Intertropical Convergence Zone) also reaches its northernmost location during September. This seasonal

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shift of the ITCZ increases the influence of tropical weather systems (such as typhoons and tropical easterly waves) and related SDHR events along the coast line of southern and southeastern China. 4.2 Monthly and pentad variations Figure 4a shows the mean variations of SDHR frequency during the warm season at monthly and pentad time resolutions averaged over all of China. The monthly variation of SDHR frequency during the warm season has a single peak. The frequency increases gradually from April to July, peaks in July, and then begins to decrease in August. This decrease becomes even more pronounced in September, so the overall evolution is characterized by a low increase followed by a rapid decrease. This evolution is reminiscent of the seasonal evolution of the East Asian summer monsoon, which becomes established around midMay, peaks near the end of July, and rapidly retreats southward in September (Chen et al., 1991). SDHR events during the warm season in China are most common in July and least common in April. SDHR events occur most frequently on the fourth pentad of July, followed by the first and second pentads of August. Among warm season pentads, SDHR events occur least frequently on the first pentad of April. The evolution of the SDHR frequency appears to develop in intermittent phases, with multiple maxima (active phases). This evolution is closely related to the seasonal development of precipitation associated with the East Asian summer monsoon, which tends to advance and withdraw in stages (Ding and Zhang, 2009). The initial increase of SDHR activity that occurs in April and May corresponds to the beginning of the heavy-rain season in South China. A second increase in the SDHR frequency follows in mid-May, with a peak in early June. This change is associated with the onset of the South China Sea Summer Monsoon (SCSSM) and the heaviest rainfall of the first rainy season in South China (Huanget al., 1986). The peak of SDHR activity in the sixth pentad of June is associated with the late June–early July Meiyu period over the Yangtze-Huai River basin. The rainy season in North China stretches from late July to early

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August. The peaks in SDHR frequency in the fourth pentad of July and the first and second pentads of August are associated with the second Meiyu precipitation and strong convective activity along the rim of the subtropical high over the Yangtze-Huai River basin. The last two pentads in August correspond to the second rainy season in South China, which is strongly influenced by tropical weather systems such as typhoons and tropical easterly waves. The minor peak in SDHR activity in the fourth pentad of September is associated with the northward displacement of the ITCZ and associated rainfall in the coastal areas of southern and southeastern China. Figure 4b shows the pentad-latitude cross-section of SDHR frequency along 116◦ E. A similar analysis has been done to examine variations in SDHR activity along 105◦ E (figure omitted). The locations of these two meridians are shown in Fig. 1. The frequencies of SDHR events along both meridians show similar evidence of intermittent development. The number of peaks in SDHR activity during the warm season varies with latitude. More SDHR events occur at lower latitudes than at higher latitudes. The evolution of SDHR activity during the warm season is characterized by a longer active period containing multiple peaks at low latitudes but a shorter active period with a single peak at high latitudes. The SDHR frequency along meridian 116◦ E (Fig. 4b) increases slowly and then decreases rapidly as the warm season progresses. This slow northward advance and rapid southward retreat of SDHR activity are closely associated with the evolution of the East Asian summer monsoon (Chen et al., 1991). SDHR events along 116◦ E are mainly restricted to South China and the area south of the Yangtze River during late spring (April–May), but SDHR events also occur infrequently in the Yangtze-Huai River basin and the southern part of North China. SDHR activity is still mainly located in South China and the area south of the Yangtze River through mid June, although it gradually advances northward. In late June, SDHR activity rapidly advances northward into the central part of North China (near 40◦ N). The northernmost SDHR events typically occur in mid July, fol-

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Fig. 4. (a) Monthly (dashed line) and pentad (solid line) variations in the frequency (%) of SDHR events with rain rates greater than 20 mm h−1 . (b) Pentad-latitude cross-section of variations in SDHR frequency (%) along 116◦ E.

lowed by the southward retreat of SDHR activity into the southern part of North China by late August. This activity then quickly retreats further southward into South China by late September. This evolution of SDHR activity in China is entirely consistent with the northward advance and southward retreat of the East Asian summer monsoon, which is related to northward and southward displacements of the western North Pacific subtropical high (Ding and Zhang, 2009). Pentad variations in SDHR activity over the coastal area of South China (near 23◦ N) are small, especially during summer (June–August) (Fig. 4b). This result is consistent with pentad variations in convective activity implied by geostationary satellite observations of infrared TBB in summer (Zheng and Chen, 2011), but differs from pentad variations in total rainfall averaged over all of South China (Ding and Zhang, 2009). The frequencies of SDHR events over 24◦ –25◦ N of South China, the area south of the Yangtze River, the Yangtze-Huai River basin, and southern North China along meridian 116◦ E peak at different times during the warm season. SDHR activity in these areas is influenced by different types of weather systems at different stages of the summer monsoon. The frequency of SDHR events in northern North China (north of 40◦ N) generally peaks only once during the warm season. This single peak is related to the maximum northward advance of the East Asian summer monsoon, which occurs in July. The main periods of SDHR activity differ substantially over dif-

ferent regions, but the common feature is that these active periods are associated with the northward advance or peak activity of the East Asian summer monsoon. The secondary peaks typically occur after the summer monsoon has advanced further northward or when the summer monsoon retreats back toward the south. 5. Diurnal variations Diurnal variations in precipitation and convective activity (such as thunderstorms, hail, and MCSs) are evident. A number of previous studies have analyzed these diurnal variations (Zhang and Lin, 1985; Yu et al., 2007a; China Meteorological Administration, 2007; Zheng et al., 2007, 2008; Li et al., 2008a). Zhou et al. (2008) analyzed the characteristics of precipitation as observed by both raingauge and satellite. Zhang and Zhai (2011) reported diurnal variations of more than 20 mm h−1 in hourly precipitation over eastern China during the warm season (May– September). No previous studies have characterized the relationships between diurnal variations in SDHR and convection. This section presents the characteristics and propagation patterns of diurnal variations in SDHR over different parts of China, and explores their relationships with diurnal variations in MCSs. MCSs are defined as regions with TBB  –52℃ at a horizontal resolution of 0.1◦ ×0.1◦ . This definition can identify both meso-α-scale convective systems

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(MαCS) and meso-β-scale convective systems (MβCS) with horizontal scales exceeding 20 km (Zheng et al., 2007, 2008). These systems comprise a variety of atmospheric convective systems, including weak convective systems (such as isolated thunderstorms and lightning storms) and severe convective systems (such as hailstorms and SDHR with damaging winds). The spatial resolution of the hourly rainfall data is much coarser than the spatial resolution of the geostationary satellite TBB data. 5.1 Mean diurnal variations over China Figure 5 shows the mean diurnal variations of SDHR frequency and maximum hourly rainfall averaged over all stations in China (excluding Taiwan) during the warm season (April–September). Mean diurnal variations over the active precipitation region (Anhui, Fujian, Jiangsu, Shandong, Shanghai, Zhejiang, Jiangxi, Heilongjiang, Jilin, Liaoning, Beijing, Hebei, Inner Mongolia, Shanxi, Tianjin, Guangdong, Guangxi, Hainan, Henan, Hubei, Hunan, Yunnan, Guizhou, Sichuan, and Chongqing) are also shown, as are the mean diurnal variations of MCSs during summer (June–August). The mean SDHR frequency and maximum hourly rainfall are calculated based on data at stations with at least 15 years of observations. SDHR frequencies and maximum hourly rainfall are identified for every hour of the day at each station. The results are then averaged across stations to illustrate mean diurnal variations of SDHR frequencies and maximum hourly rainfall in China. The most prominent feature of the diurnal variations in SDHR is the presence of three peaks. The main peak occurs in the afternoon (between 1600 and 1700 BT), with secondary peaks just after midnight (0100–0200 BT) and in the early morning (0700–0800 BT). SDHR is least common during the morning. The mean diurnal variations in maximum hourly rainfall are largely consistent with those in mean SDHR frequency. The diurnal variations of both variables are fully consistent from midnight to early morning; however, variations in mean maximum hourly rainfall are slightly behind those in mean SDHR frequency from

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afternoon to midnight. The temporal rates of change in the two diurnal cycles indicate that SDHR frequency and maximum hourly rainfall change rapidly from afternoon to midnight. Both variables change less rapidly after midnight. Mean diurnal variations in SDHR frequency and maximum hourly rainfall over the active precipitation region are consistent with those over China as a whole; however, mean SDHR frequency over the active precipitation region is higher than mean SDHR frequency over China as a whole. The lower frequency of SDHR over the inactive precipitation region depresses mean SDHR frequencies averaged over all of China. Mean diurnal variations in MCSs have only a single peak, regardless of whether the average is calculated over China as a whole or over the active precipitation region. There are some differences between variations over China as a whole and variations over the active precipitation region. First, diurnal variations in MCSs over China as a whole lag about 1 h behind diurnal variations in MCSs over the active precipitation region. This difference is caused by diurnal variations in MCSs over the Qinghai-Tibetan Plateau, which is among the most active regions for MCSs in China. MCS activity over the Qinghai-Tibetan Plateau lags that over the active precipitation region due to differences in longitude. Second, mean MCS frequencies between midnight and early morning are significantly higher over the active precipitation region than over China as a whole. Several areas within the active precipitation region (e.g., Guangdong and Guangxi, the Sichuan basin, the northeastern Yunnan-Guizhou Plateau, and the Yangtze-Huai River basin) have significant nocturnal MCS activity (Zheng et al., 2008, 2010; Zheng and Chen, 2011). Mean diurnal variations in MCSs and SDHR over China as a whole and over the active precipitation region have several common features, as well as significant differences. Common features include: 1) the most active period occurs in the afternoon (because convection is most active in the afternoon); and 2) all four diurnal variations change rapidly from afternoon to midnight, and then change less rapidly (and relatively smoothly) after midnight. Differences include:

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Fig. 5. Diurnal variations in mean frequency and maximum hourly rainfall for SDHR events with rainfall  20 mm h−1 and brightness temperature (TBB) less than –52℃. Diurnal variations in the mean frequencies are shown for SDHR averaged over all of China (thin black solid line), SDHR averaged over the active precipitation region (thick black dashed line), TBB  –52℃ averaged over all of China (blue solid line), and TBB  –52℃ averaged over the active precipitation region (blue dashed line). The thick black solid line shows the maximum hourly rainfall averaged over all of China. The blue y-axis indicates the frequency (%) of TBB  –52℃, the black y-axis on the left indicates hourly rainfall (mm), and the black y-axis on the right indicates SDHR frequency (%).

1) the frequencies of SDHR and MCSs are significantly different (i.e., MCSs occur much more frequently than SDHR events, with the peak frequency of MCSs approximately 40 times that of SDHR); 2) the pattern of diurnal variations in MCSs is relatively continuous and smooth when compared with that of variations in SDHR; and 3) SDHR frequency has three peaks (two of which occur at night), while the frequency of MCSs has a single peak and no nocturnal maximum. Although convective activity peaks in the afternoon in China during the warm season, only a very small fraction of this activity reaches SDHR intensity. Moreover, the ratio of convective events that reach SDHR intensity is substantially higher after midnight than in the afternoon, even though overall convective activity is weaker. This result indicates that nocturnal convective activity in China during the warm season often produces relatively large amounts of precipitation. The occurrence frequency of other types of severe convective activity is substantially reduced during nighttime, although previous work has shown that

hailstorms may occur at night in some parts of China (Zhang et al., 2008). Both the SDHR frequency and the maximum hourly rainfall associated with thermal convection in the afternoon are higher than those associated with convection that occurs after midnight. The main reason for the increase in relative SDHR activity after midnight is the close relationship between SDHR and MαCSs, which can last for more than 12 h. This result is consistent with the conclusions of Yu et al. (2007b), who showed that precipitation often peaks after midnight during long-duration rainfall events. Nocturnal SDHR events are associated with different periods of active convection over different regions (Zheng et al., 2008, 2010; Zheng and Chen, 2011). The following section provides further analysis of regional differences in the diurnal variations of SDHR. Figure 6 shows distributions of SDHR frequency at different times of a day. As in Fig. 5, SDHR events occur most often in the afternoon (1400–2000 BT) and least often in the morning (0800–1400 BT), but peri-

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ods of high SDHR activity in China differ by region. Moreover, diurnal variations in SDHR activity appear to propagate in characteristic ways. SDHR frequencies are high from afternoon to evening (1400–2000 BT; Fig. 6c) over wide spread areas. These high frequencies are associated not only with large-scale weather systems that provide favorable environments for SDHR (such as low pressure troughs and the Meiyu front), but also with topography and the distribution of thermal convection in the afternoon (Zheng et al., 2007, 2008). SDHR frequencies are substantially weaker between the evening and early morning (2000–0200 BT; Fig. 6d) over most of China, but nocturnal SDHR events are relatively common over the southwestern Sichuan basin, southern Guizhou, northwestern Guangxi, and the coastal areas of Guangdong and Guangxi. Areas of high SDHR frequency continue to shrink from midnight to early morning (0200–0800 BT; Fig. 6a). SDHR activity weakens substantially over the southwestern Sichuan basin and southern Guizhou, but SDHR activity over Guangxi and the coastal areas of Guangdong strengthens. SDHR events are less common over all of China during the morning hours (0800–1400 BT; Fig. 6b), but SDHR events are still relatively frequent over the coastal areas of southeastern China, the eastern Sichuan basin, and the lower and middle reaches of Yangtze River. This relatively high SDHR activity may indicate that these areas are strongly influenced by local atmospheric circulations caused by the land-sea transition (for coastal areas), topography, or large-scale weather systems (such as the Meiyu front or easterly waves). SDHR frequencies over the coastal areas of South China are higher during the 0800–1400 BT period than between midnight and early morning. This diurnal cycle in SDHR activity may be associated with the strengthening of the land-sea breeze during the morning hours (Huang et al., 1986). Zhang and Zhai (2011) pointed out that, over the Sichuan basin and Guizhou, many of the events with hourly precipitation exceeding 20 mm h−1 happen at night. Nighttime SDHR activity is centered primarily in three regions of China. The first region includes Guizhou, Guangxi, and

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Guangdong. It is interesting to note the propagation of SDHR across this region. SDHR activity over Guangxi and Guangdong develops and strengthens between 1400 and 2000 BT. A sharp weakening follows after 2000 BT, although SDHR activity near the coast remains strong. This coastal center of SDHR activity then begins to strengthen and propagate inland after 0200 BT, followed by continued strengthening and propagation into mountainous areas during 0800– 1400 BT. By contrast, SDHR activity over southern Guizhou and northern Guangxi develops and strengthens during 2000–0200 BT. This active property then propagates southeastward after 0200 BT, as SDHR activity over northwestern Guangxi increases. SDHR activity then propagates into the central region of Guangxi (and even to Guangdong) during 0800–1400 BT. These characteristic propagation patterns are consistent with the movement of MCSs as revealed by geostationary satellite TBB data (Zheng et al., 2008, 2010; Zheng and Chen, 2011). The patterns are closely related to topography, which may cause local valley wind or land-sea breeze circulations. The mechanism by which local circulations and the associated synoptic systems lead to these characteristic propagation patterns requires further study. The second region is the Sichuan basin. SDHR events in the southwestern Sichuan basin are most frequent during 2000–0200 BT.This center of SDHR activity then weakens and propagates toward the northeast during 0200–0800 BT, before largely disappearing around 1400–2000 BT. SDHR activity in this region is weakest in the afternoon and strongest from evening to midnight. Nocturnal rain events are accordingly relatively common. These features are consistent with the diurnal variation and propagation of MCSs in this region (Zheng et al., 2008, 2010), which may also have close relationships with local valley circulations caused by topography. The third region covers the middle and lower reaches of the Yangtze River and the Yangtze-Huai River basins. This is the main Meiyu precipitation region (Tao, 1980), and a region of frequent MCS activity (Zheng et al., 2008). SDHR in this region is remarkably strong during the afternoon (1400–

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Fig. 6. Diurnal variations of the frequency (%) of SDHR with hourly rainfall  20 mm h−1 for (a) 0200–0800 BT, (b) 0800–1400 BT, (c) 1400–2000 BT, and (d) 2000–0200 BT.

2000 BT). This center of SDHR activity is weak during the evening and late night (2000–0200 BT), but strengthens again by the early morning (0200–0800 BT). 5.2 Diurnal variations over different regions Differences in diurnal variations of SDHR and MCSs over different regions are examined using timelatitude cross-sections along the 105◦ and 116◦ E meridians, a time-longitude cross-section along the 31◦ N parallel, and a cross-section along a straight line from southwestern Sichuan to southeastern Guangxi (Fig. 7). The positions of all cross-sections are indicated by white lines in Fig. 1. Diurnal variations in SDHR and MCSs are very similar, but diurnal variations in MCSs are smoother and more continuous, and the MCSs propagate more clearly. These differences result from differences in the observations. Ob-

servations of TBB (which are used to identify MCS anvil clouds) are instantaneous rather than hourly, and TBB is a more continuous quantity than precipitation. Diurnal variations in SDHR and MCSs along the ◦ 105 E meridian (Figs. 7a and 7b) indicate two different regimes of nocturnal convection. SDHR and MCS activities over the Yunnan-Guizhou Plateau (24◦ – 28◦ N) are strong from afternoon until midnight. This center of convective activity remains strong but propagates from south to north after midnight. Convection in this region is active in the afternoon and at night, with minimum activity in the morning. This pattern of diurnal variations is consistent with diurnal variations in lightning activity (Wang et al., 2009). SDHR and MCS activities over the Sichuan basin (28◦ –32◦ N) strengthen after 2200 BT, and then weaken substantially after sunrise. This center of convective activity also propagates from south to north, but it is weakest

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Fig. 7. Temporal cross-sections of diurnal variations in the frequencies (%) of SDHR (hourly rainfall  20 mm; left column) and MCSs (TBB  –52℃; right column) along (a, b) the 105◦ E meridian, (c, d) the 116◦ E meridian, (e, f) the 31◦ N parallel, and (g, h) the straight line connecting points (28.9◦ N, 101.7◦ E) and (21.5◦ N, 109.9◦ E).

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in the afternoon. Convection in the region is active from midnight to morning, with a minimum in the afternoon. Diurnal variations in SDHR and MCSs along the ◦ 116 E meridian (Figs. 7c and 7d) show evidence of diurnal cycles with a single peak, multiple peaks, and longer duration. Convection along this transect is most active in the afternoon. North-south propagation of SDHR and MCSs is largely absent, particularly when compared with their movement along 105◦ E (Figs. 7a and 7b). SDHR and MCSs in the coastal area near 23◦ N, 116◦ E have longer durations (particularly SDHR). The area from 23.5◦ to 28◦ N covers Guangdong and southern Jiangxi, where diurnal variations of MCSs typically have a single peak. Diurnal variations of SDHR show two types: single-peak and multiple-peak. The differences between them may be related to the distribution of rainfall forced by mesoscale topographic variations. SDHR and MCSs within 23◦ –26◦ N along this meridian appear to propagate from south to north. This result is consistent with that obtained by Zheng and Chen (2011). The diurnal variations of SDHR over the area from 28◦ to 32◦ N (Jiangxi and southern Anhui) typically have multiple peaks, with substantial nocturnal activity. Diurnal variations in MCS activity in general do not have multiple peaks. Diurnal variations in both SDHR and MCSs over the area from 34◦ to 46◦ N (Henan, Shandong, and North China) commonly have double peaks, with a primary peak in the afternoon and a secondary peak near midnight. Figures 7e and 7f show diurnal variations in SDHR and MCSs along the 31◦ N parallel. Diurnal variations along this transect indicate multiple peaks in convective activity. SDHR and MCSs over 102◦ –110◦ E (Sichuan) and 111◦ –118◦ E (Hubei and Anhui) propagate towards the east, and are more active during nighttime and morning. SDHR and MCSs along 31◦ N in the Sichuan basin are largely nocturnal, particularly over 103◦ –108◦ E. These events typically propagate eastward after midnight. By contrast, MCSs over the plateau (to the west of the Sichuan basin near 102◦ E) are more active in the afternoon. SDHR rarely occurs over this plateau at this time of

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the day. Diurnal variations in SDHR and MCSs over 112◦ –118◦ E indicate multiple peaks in convective activity, with a primary maximum from afternoon to midnight and a secondary maximum after midnight. SDHR and MCSs in this region propagate eastward after midnight. This eastward propagation is more pronounced for SDHR. Previous research on the climatological distribution of deep convection in subtropical China has shown that MCSs over the plateau west of Sichuan propagate southeastward toward the northern Yunnan-Guizhou Plateau after sunset (1900 BT), while MCSs over the northeastern Yunnan-Guizhou Plateau propagate southeastward toward northern Guangxi after sunset (1900 BT) (Zheng et al., 2010). We therefore also analyze diurnal variability and propagation of SDHR and MCSs along the straight line [(28.9◦ N, 101.7◦ E)– (21.5◦ N, 109.9◦ E)], which extends from the southeastern part of the plateau west of Sichuan to Guangxi through the northeastern part of the Yunnan-Guizhou Plateau (southwestern Guizhou). The diurnal variations along this line (Figs. 7g and 7h) show differences in the characteristics of SDHR and MCSs over different underlying surfaces. Several characteristic types of diurnal variations in SDHR and MCSs are identified along the aforementioned line (Figs. 7g and 7h). (1) The first characteristic type is the single-peak diurnal variation in both SDHR and MCSs located over the southeastern part of the western Sichuan plateau [(28.6◦ N, 102◦ E)–(27.2◦ N, 103.6◦ E)]. MCS activity over this region is strongest from the afternoon until after midnight and has a relatively long duration. SDHR events occur mainly at night, and with lower frequencies. (2) The second characteristic type is the thermal convection over western Guizhou [(27.2◦ N, 103.6◦ E)– (26.3◦ N, 104.6◦ E)]. MCS activity over this region occurs mainly in the afternoon, with a relatively short duration. Nighttime MCS activity is lower over this region than over the western Sichuan plateau. SDHR events are very infrequent over this region. (3) The third characteristic type is the single-peak diurnal variation in both SDHR and MCSs over southwestern Guizhou [(26.3◦ N, 104.6◦ E)–(24.8◦ N, 106.2◦ E)].

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Both SDHR and MCSs are active over this region, with longer durations and strong activity from afternoon until just before sunrise. This pattern of diurnal variations is consistent with the pattern of diurnal variations in SDHR and MCSs over 24◦ –28◦ N along the 105◦ E meridian. It also corresponds to the diurnal variation of lightning in this region (Wang et al., 2009). (4) The fourth characteristic type is observed over the western Guangxi basin [(24.8◦ N, 106.2◦ E)– (23.9◦ N, 107.3◦ E)]. SDHR and MCSs in this region occur mainly at night (2000–0800 BT). This diurnal pattern is very similar to that of SDHR and MCSs over the Sichuan basin (28◦ –32◦ N) along the 105◦ E meridian. (5) The fifth characteristic type is observed over the coastal area of southern Guangxi, where SDHR events occur more often and have longer durations, mainly between midnight and sunset (0200–1800 BT). The occurrence frequency of both SDHR and MCSs decreases significantly between sunset and midnight. The typical propagation of SDHR and MCSs along this transect is as follows. First, SDHR and MCSs over the southeastern part of western Sichuan plateau [(28.6◦ N, 102.0◦ E)–(27.2◦ N, 103.6◦ E)] propagate toward the southeast. Second, SDHR and MCSs over western Guizhou (27.2◦ N, 103.6◦ E), which are most active in the afternoon, propagate southeastward. These systems arrive in the border region between Guizhou and Guangxi (24.8◦ N, 106.2◦ E) around 0000 BT. They then continue to propagate southeastward after midnight, reaching the northern part of southern Guangxi (22.9◦ N, 108.3◦ E) at about 0400 BT and the coastal area of Guangdong at about 0600 BT. Third, although SDHR activity over the coastal area of Guangxi (21.5◦ N, 109.9◦ E) is relatively strong from 0200 to 1800 BT, MCS activity strengthens considerably after 0400 BT. This center of strong MCS activity propagates northwestward after 1200 BT, but splits and propagates in two separate directions after 1800–2000 BT. Some of the MCSs continue to propagate northwestward into the boundary region between Guizhou and Guangxi (arriving around 0000 BT). The remainder retreat southeastward and return to the coastal area of Guangxi. Zheng et al. (2008, 2010) and Zheng and Chen

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(2011) showed that diurnal variations in convection over different regions of China are closely related to local circulations caused by thermal differences among water, land, and rough terrain (such as valley winds or land-sea breezes). The results in this paper also show close relationships between local thermodynamic circulations and SDHR. Huang et al. (1986) studied the relationship between the diurnal variations in precipitation and thermodynamic circulations over South China. They found that circulations coupled to valley winds and land-sea breezes amplify the diurnal variations and propagation tendencies of precipitation in this region. Zheng and Chen (2011) showed that this relationship extends to convection as well. However, the diurnal variations and propagation of SDHR and MCSs are affected not only by local thermodynamic circulations but also by large-scale circulation patterns, the movement of large-scale weather systems, and variations in the steering flow. The influence of multi-scale interactions between the local circulation and large-scale or mesoscale weather systems on diurnal variations and propagations of SDHR and MCSs should therefore be addressed in future work. Figure 8 shows the diurnal variations of the mean frequencies of SDHR (hourly rainfall  20 mm) and the MCSs (TBB  –52℃) over active precipitation areas. These results highlight the dependence of these diurnal variations on the underlying surface. The areas (indicated by rectangles in Fig. 1) include the southwestern Sichuan basin (rectangle A; 28◦ – 31◦ N, 102◦ –105◦ E), southwestern Guizhou (rectangle B; 24◦ –27◦ N, 104◦ –107◦ E), central Guangxi (rectangle C; 22◦ –25◦ N, 107◦ –110◦ E), the coastal area of Guangxi (rectangle D; 21◦ –23◦ N, 108◦ –110◦ E), central Guangdong (rectangle E; 22◦ –25◦ N, 112◦ –115◦ E, central and southern Anhui (rectangle F; 30◦ –33◦ N, 116◦ – 119◦ E), southwestern Shandong, northern Jiangsu and northern Anhui (rectangle H; 33◦ –36◦ N, 116◦ –119◦ E), and the eastern Yangtze-Huai River basin (rectangle G; 31◦ –34◦ N, 117◦ –120◦ E). Note that two vertical axes are used in both panels of Fig. 8 to better highlight the diurnal variations of SDHR and MCSs in regions with lower frequencies. Figure 8a shows that SDHR frequencies are high-

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est in the afternoon (1400–2000 BT) over central Guangdong, central and southern Anhui, southwestern Shandong, northern Jiangsu and northern Anhui, and the eastern Yangtze-Huai River basin. This pattern of diurnal variability is reflected in the mean diurnal variability of SDHR frequency over China as a whole, which also indicates peak activity in the afternoon. By contrast, SDHR frequency over the southwestern Sichuan basin is largest at about 0000 BT. Moreover, although SDHR activity is strong in the afternoon over southwestern Guizhou, it is strongest at about 0200 BT. SDHR activity is also strongest from midnight to early morning (0200–0800 BT) over central and coastal areas of Guangxi. The frequencies of SDHR occurrence over central Guangxi, the coastal area of Guangxi, and central Guangdong are significantly higher than those over other regions even during relatively inactive periods; frequencies over these three regions should be analyzed using the right ordinate axis in Fig. 8a. Different regions have several different patterns of diurnal variations in SDHR (e.g., single-peak, double-peak, and multiple-peak), as well as significantly different active periods and durations. Regions with single peaks in the diurnal cycle of SDHR frequency include central Guangdong and the southwestern Sichuan basin. The single peak over central Guangdong occurs in the afternoon and has a long duration, while that over the southwestern Sichuan basin occurs at night and has a relatively short duration. Regions with double peaks in the diurnal cycle of SDHR frequency include central Guangxi, the coastal area of Guangxi, and southwestern Guizhou. The main peak in SDHR activity over central Guangxi occurs around 0200–1000 BT, and the secondary peak occurs around 1600–2000 BT. The difference in the amplitudes of these two peaks is insignificant. SDHR diurnal variations over the coastal area of Guangxi are similar to those over central Guangxi, but with substantially higher SDHR frequencies and a more pronounced main peak between midnight and morning. The main peak in SDHR frequency over southwestern Guizhou occurs around 0000–0200 BT, and the secondary peak occurs from afternoon to early evening

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(1600–2000 BT). SDHR diurnal variations over central and southern Anhui, southwestern Shandong, northern Jiangsu and northern Anhui, and the eastern Yangtze-Huai River basin have multiple peaks. This may be associated with the Meiyu precipitation in the Yangtze-Huai River basin. However, SDHR frequencies in these regions are significantly smaller than those over the southwestern Sichuan basin and southwestern Guizhou. The amplitudes of SDHR diurnal variations over southwestern Shandong, northern Jiangsu, and northern Anhui are significantly larger than those over central and southern Anhui and the eastern YangtzeHuai River basin. The duration of the active SDHR period over the former two regions is also longer than that of the active period over the latter two regions. Figure 8b shows only two patterns of diurnal variations in MCS activity: single-peak and double-peak. Multiple peaks are not apparent in the diurnal variations of MCSs in any of the analyzed regions. Diurnal variations in MCSs over the southwestern Sichuan basin and southwestern Guizhou have a single peak during the nighttime, but the magnitudes and durations of this peak are substantially different. Diurnal variations in MCSs over all other analyzed regions have a main peak in the afternoon and evening, although MCSs over the coastal area of Guangxi are active around 0400 BT (earlier than over the other regions). Post-midnight secondary peaks in the diurnal variations of MCSs are apparent but small over central Guangxi, central and southern Anhui, and the eastern Yangtze-Huai River basin. Comparison of Figs. 8a and 8b indicates that diurnal variations in the frequencies of SDHR and MCSs are largely consistent. The main periods of SDHR and MCS activities are coherent with each other over eight of the nine regions (the southwestern Sichuan basin, southwestern Guizhou, central Guangxi, central Guangdong, central and southern Anhui, southwestern Shandong, northern Jiangsu and northern Anhui, and the eastern Yangtze-Huai River basin). Although the main period of MCS activity over the coastal area of Guangxi (afternoon to evening) does not correspond to the main period of SDHR activity (after midnight to early morning), the frequency of MCSs is also rela-

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Fig. 8. Diurnal variations in frequency (%) of (a) SDHR (hourly rainfall  20 mm) and (b) MCSs (TBB  –52℃). The blue curves (for central Guangxi, the coastal area of Guangxi, and central Guangdong) use the blue (right) ordinate axes; the curves for other regions use the black (left) ordinate axes.

tively high during the main period of SDHR activity. Beyond this qualitative consistency, there are some differences between the diurnal variations in SDHR and MCS frequencies. As mentioned above,

SDHR frequencies are significantly lower than those of MCSs, and the diurnal variations in SDHR are much less continuous and smooth than the diurnal variations in MCSs. With the exception of diurnal varia-

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tions over the coastal area of Guangxi, the differences between diurnal peaks in SDHR frequency are not so large as those between diurnal peaks in MCS frequency. Except for southwestern Sichuan and southwestern Guizhou, peak MCS activity occurs during the afternoon and evening. The secondary peak in MCS frequency after midnight is insignificant relative to the main peak for most of these regions. This indicates that the proportion of nighttime MCSs that produce SDHR is larger than the proportion of afternoon MCSs that produce SDHR. The main reason for these differences between the diurnal cycles of SDHR and MCSs is that the MCSs (as denoted by TBB  –52℃) encompass a variety of atmospheric convective processes. Most MCSs produce precipitation, but some types of MCSs (such as afternoon MCSs related to thermal convective processes) cannot produce precipitation at rates exceeding 20 mm h−1 . This limitation implies differences in the diurnal cycles of the frequencies of SDHR and MCSs. 6. Conclusions and discussion This paper summarizes the spatial distribution, seasonal, monthly, pentad and diurnal variations of SDHR and the relationships between diurnal variations in SDHR and in MCSs over China during the warm season. All results are based on hourly precipitation data between April and September during the period 1991–2009. SDHR is a low probability event with small occurrence frequencies. Despite some differences, the spatiotemporal frequency distributions of MCSs and SDHR exhibit substantial coherence. This coherence holds when SDHR is defined as hourly rainfall intensity  10, 20, 30,or 40 mm h−1 , and confirms the synoptic and climatological significance of the SDHR distributions analyzed in this paper. In general, the spatial distributions of SDHR in China defined using thresholds of 10, 20, 30, or 40 mm h−1 are very similar to the distribution of heavy rainfall with intensity  50 mm day−1 . By contrast, the spatial distribution of SDHR with intensity  50 mm h−1 is more scattered, with a stronger similarity to the spatial distribution of heavy rainfall with intensity 

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100 mm day−1 . The heaviest hourly rainfall observed in China during this period was more than 180 mm. Although SDHR with intensity  50 mm h−1 is rare, it has been observed to occur even in regions with very weak SDHR activity. The monthly mean frequency distribution of SDHR averaged over all of China is unimodal, and is closely related to the variability of the East Asian summer monsoon. The average frequencies of SDHR increase gradually from April to July and then decrease rapidly in September. The pentad mean frequency distribution of SDHR is largest in the fourth pentad of July, followed by the first and second pentads of August. The SDHR frequency develops intermittently and in phases. The pentad-by-pentad development of SDHR activity may be unimodal or multimodal, depending on the region, but generally contains multiple periods of strong SDHR activity. The frequency of SDHR occurrence is lower during spring and fall, but SDHR may still occur within a broad area of northern China during these transitional seasons. The mean diurnal cycles of SDHR frequency and maximum hourly rainfall averaged over China as a whole are trimodal. The primary maximum in this diurnal cycle occurs during the afternoon (1600–1700 BT); this peak is consistent with the primary maximum in the diurnal cycle of MCS activity. There are also two secondary peaks during the midnight and early morning hours (0100–0200 and 0700–0800 BT, respectively), which do not appear in the diurnal cycle of MCS activity. SDHR frequency averaged over all of China is minimum in the morning. Changes in the mean SDHR frequency are more gradual after midnight. The mean diurnal cycles of SDHR averaged within different regions of China have several different characteristic types (including single-peak, doublepeak, and multiple-peak). The main periods and relative durations of SDHR differ substantially over different underlying surfaces. Diurnal variations in SDHR and MCSs have many common features over most regions of China, but often differ significantly after midnight. The diurnal variations of SDHR and MCSs along

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various transects in southern China show that the related convective systems often propagate in characteristic patterns. MCSs frequently propagate over the Sichuan basin, the region of Hubei and Anhui provinces near 31◦ N, and from the southeastern part of the western Sichuan plateau to the coastal area of Guangxi via the northeast Yunnan-Guizhou Plateau (southwestern Guizhou). The diurnal variations and propagation patterns of SDHR and MCSs are closely related to topography and the distribution of land and sea. The local circulations and convergence caused by differential heating by solar radiation during the daytime and cooling by outgoing long wave radiation during the nighttime are largely consistent with the diurnal variations and propagation of SDHR and MCS systems (although these local circulation systems are typically very shallow in the vertical direction). The influence of multi-scale interactions between these local circulations and large-scale circulation systems on diurnal variations of SDHR and MCS activities will require detailed analysis in future studies. Acknowledgments. The authors would like to thank Ruan Xin of the National Meteorological Information Center of China for providing the hourly precipitation data. The language editor for this manuscript is Dr. Jonathon S. Wright.

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