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 Using hourly station rain-gauge data during 1991–2004, we studied the relation between rainfall duration and diurnal variation during the warm season (May to September) over the central eastern China (105–120°E, 26–36°N). Our result shows that more than 60% of the total rainfall in the central eastern China comes from long-duration rainfall events (an event that lasts longer than 6 hours). The long-duration rainfall events tend to have their maximum hourly rainfall around early morning, while short-duration rainfall events (an event of 1–3 hours in duration) have their peak rainfall around late afternoon. This contrast explains the two peaks in the diurnal variation of averaged precipitation over the central eastern China shown in previous studies.
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 Scientists are always interested in advancing their knowledge of the characteristics of precipitation, because of the synthetic integrated signals involved in the precipitation processes and the important relation with human being. Characterizing diurnal precipitation features will help us to understand not only the mechanism of rain formation but also the mechanism of the local climate. Therefore, the diurnal variation of precipitation has always been a hot topic [Brier, 1965; Wallace, 1975; Dai et al., 1999; Sorooshian et al., 2002; Liang et al., 2004; Wang et al., 2007; Yu et al., 2007]. The problem is that in these studies the diurnal precipitation is generally presented by averaged hourly precipitation over the entire data record or over all of the rainy days, which could downplay the significance of diurnal cycle or misrepresent the mechanism behind different types of rainfall events. Multi-phases of diurnal precipitation were seen in many regions, especially the semidiurnal harmonic variation of precipitation [Dai, 2001; Ramesh Kumar et al., 2006; Yu et al., 2007]. The diurnal variations with multi-phases usually present no significant diurnal cycle. Yu et al.  showed that in the central eastern China, especially in the area between the Yangtze and Yellow Rivers, there exist two peaks in the averaged diurnal variation of precipitation and that the significance of the diurnal cycle, expressed by the ratio of the amplitude to the daily mean, is relatively weaker than that in the southern, northern and western China. This study tries to separate out two kinds of rainfall events which are hopefully with single phase respectively in diurnal rainfall variation. It is found that the key is the rainfall duration.
 We introduce the data and analysis method in section 2. We then present the relation between rainfall duration and diurnal variation in section 3. A brief discussion is given in section 4.
2. Data and Analysis Method
 The precipitation data set used in this study was obtained from the National Meteorological Information Centre (NMIC) of China Meteorological Administration. It consists of quality-controlled hourly rain-gauge records during 1991–2004 [Yu et al., 2007]. We classify rainfall events according to their durations without any intermittence or at most one-hour intermittence during a single rainfall event. After a rainfall event begins, if its intermittence lasts for two hours we deem that the rainfall after the intermittence belongs to a new rainfall event. The duration of a rainfall event is defined by the hours between the beginning and the end of one event. The key region (105–120°E, 26–36°N) of the central eastern China is marked by white dashed lines in Figure 1.
 The diurnal analysis is performed on different types of rainfall events according to their durations. For each type of rainfall events, let Ra(h) represent the amount of hourly rainfall at time h. The normalized diurnal variation of precipitation (or rainfall), Da(h), is calculated by
The Ra(h) is resulted from an average of the rainfall events with a specific duration. For example, for the rainfall events with 1–3 hours of duration a rainfall event has at most three hours of rain at time i1, i2 and i3 in terms of local solar time (LST), with hourly rainfall p1 (>0), p2 (≥0) and p3 (≥0). This rainfall event will be registered as 24-hour diurnal precipitation with hourly rainfalls p1, p2 and p3 at time i1, i2 and i3, respectively, and zero rainfall at all other hours. To increase the sample size for a more reliable estimation, the analysis is done during warm season (May–September) over 1991–2004.
 To gain information about the diurnal cycle related to the rainfall duration, we classify the rainfall events according to different durations as defined above. Figure 1a shows that over 50% of rainfall events have a duration time of 1–3 hours, mostly in the northeastern China and south inland of China. A small percentage of short-duration rainfall is seen in the central eastern China. Figure 1b reveals that long-duration rainfall events (longer than 6 hours) occur most frequently (over 20%) and contribute more than 60% to the total rainfall in the central eastern China. Comparing Figure 1 with Figures 1 and 2 of Yu et al. , the marked region with long-duration rainfall events in this study corresponds well to the regions studied by Yu et al.  that have insignificant diurnal amplitude and less coherent diurnal phase. The diurnal variation of precipitation given by Yu et al.  have two peaks in late afternoon and early morning, while the region dominated by short-duration precipitation corresponds well with the regions that have a single late-afternoon peak, suggesting that the peak of short-duration rainfall events usually occur around late afternoon.
 To verify whether the late-afternoon peak is mainly resulted from short-duration precipitation and whether the short-duration rainfall events are separable from the rainfall events with early morning peak, the diurnal variation of precipitation is examined according to their durations. Figure 2 shows the spatial distributions of the mean diurnal phase of precipitation using arrows, as given by Yu et al. , at each station along the Yangtze and Yellow river valleys for short-duration (Figure 2a) and long-duration rainfall (Figure 2b) events. It is found that the short-duration rainfall events exhibit a nearly uniform late-afternoon maximum and the long-duration rainfall events have consistent nocturnal or early morning diurnal precipitation phases. There are 173 stations in the central eastern China, the key region for this study. For short-duration rainfall events, there are 130 (75%) stations that have the maximum rainfall around 1500–1800 LST. For long-duration rainfall events, there are 119 (69%) stations that have the maximum rainfall around 0300–0900 LST. To be noted is that the eastward delayed diurnal phases downstream of the Tibetan Plateau, stressed by Yu et al. , can be found only in Figure 2b, which indicates that the eastward delayed diurnal phases occur mainly in the long-duration rainfall events.
 To exhibit the relation between the diurnal variation and the rainfall duration, the averaged diurnal variations of precipitation over the central eastern China are presented in Figure 3 according to their rainfall durations. The averaged diurnal variation of rainfall events with a duration of 1–3 hours shows a dominant late-afternoon maximum (see dash-dotted line marked with filled triangle in Figure 3), the amplitude is about twice of the mean. When the duration increases, a secondary early morning peak appears, as indicated by the dotted line marked with filled square in Figure 3, which shows the diurnal variation of rainfall events lasting for 4–6 hours. When the duration increases further, the primary peak of rainfall occurs around early morning. The diurnal variation of precipitation that lasts for 7–12 hours (the dashed line marked with open circle in Figure 3) has a weak secondary peak in late afternoon. For rainfall events that last for more than 12 hours (the solid line marked with filled circle in Figure 3), the diurnal variation exhibits a harmonic sinusoidal evolution with the maximum around 0600 LST and the minimum around 1900 LST.
 Our results present two interesting characteristics of precipitation in the central eastern China. First, the long-duration rainfall events dominate the total rainfall. Second, the diurnal phases are highly determined by the duration of rainfall events. The rainfall events that last for 1–3 hours always exhibit the maxima around later afternoon, while the rainfall events that last for more than 6 hours usually occur during mid-night to noon with the maxima around early morning.
 To have a much clearer picture of the relation between the rainfall duration and diurnal characteristics, three stations (marked in Figure 2) are selected to examine their diurnal features individually. These three stations locate at different latitudes and represent three distinct diurnal phases of precipitation. The averaged diurnal precipitation at Shangqiu station in Henan Province (the northernmost dot in Figure 2) presents typical two-peak rainfall as shown by grey dashed line in Figure 4a, and the two amplitudes are comparable, about 125% of the mean. Figure 4a exhibits significant difference of diurnal cycles for rainfall events that last for 1–3 hours and 9–12 hours respectively, representing the features seen in Figure 2. The rainfall events that last for 1–3 hours reach their maximum mostly around 1600 LST (cf. Figure 2a), having an amplitude more than 180% of the mean. The rainfall events that last for 9–12 hours present their maximum mostly around 0600 LST and their amplitude is about 260% of the mean. As shown by grey dashed line in Figure 4b, the averaged diurnal variation of precipitation in Wuhan station of Hubei Province (the middle dot in Figure 2) presents a typical early-morning maximum, and the amplitude is about 150% of the mean. But the diurnal phase of precipitation for short-duration rainfall events exhibits a peak around late afternoon, and the amplitude is about 190% of the mean (filled circles in Figure 4b). The rainfall events with early morning maximum mostly have duration of 13–18 hours and amplitude more than twice of the mean (filled squares in Figure 4b). A late-afternoon precipitation prevails in large part of southeastern inland China [Yu et al., 2007]. As shown in Figure 2b, even for the rainfall events of more than 6 hours, the diurnal maximum still appears in late afternoon. For example, Ganzhou station is located in the southern Jiangxi Province (the southernmost dot marked in Figure 2), the dominant maximum rainfall occurs around 1800 LST, having an amplitude of about 170% of the mean (see grey dashed line in Figure 4c). However, the diurnal phase of rainfall events of 13–18 hours duration exhibits a peak around 0300 LST, having an amplitude about 170% of the mean (filled squares in Figure 4c). For the rainfall events of 1–3 hours duration observed in Ganzhou station, most of them exhibit peaks around late afternoon, and the amplitude are more than 220% of the mean (filled circles in Figure 4c).
 Our analyses reveal the relation between rainfall duration and characteristics of diurnal precipitation in the central eastern China during the warm season, which should be helpful to the physical understanding on the diurnal precipitation variation with double-peaks or multi-peaks. The long-duration rainfall events mostly have the maximum around early morning, while the short-duration events have the maximum around late afternoon. The late-afternoon peak of short-duration rainfall events may be explained by the diurnal variation of surface solar heating, which has a great influence on the diurnal variation of low-level atmospheric stability. The mechanism of the prevailing nocturnal or early morning precipitation, however, is more complex.
Lin et al.  suggested that the nocturnal maximum is resulted from stratiform rain enhanced by instability due to nocturnal radiative cooling at cloud top. Kubota and Nitta  mentioned that the water-vapor accumulation at low levels in the evening contributes significantly to the development of nocturnal convection. Nesbitt and Zipser  suggested that the nocturnal rain is often caused by meso-scale convective systems (MCS) rather than isolated convection, and the MCSs is the strongest after midnight, presumably from the upscale growth of late-afternoon convection. As mentioned by Chen et al.  and Sun et al. , the heavy rainfall of the summer Meiyu Front is mostly resulted from well organized MCSs overlapped on the distinctive stratus cloud. Previous studies also pointed out that nocturnal precipitation is very sensitive to the moisture accumulation in the boundary layer in upper valley of Yangtze River [Zeng et al., 1994]. For either moisture accumulation or convection growth or organization, a certain period of time is needed. A long duration may also be needed as a necessary condition for moisture accumulation and for each isolated convection cell to grow into well organized MCSs before the maximum rainfall occurs. In addition, the continental stratus cloud downstream of the Tibetan Plateau is unique [Yu et al., 2004]. Whether the nocturnal radiative cooling of the continental stratus partly contributes to the nocturnal or early morning diurnal phase remains an open question. Additional study is needed to understand the physical processes behind the long-duration rainfall.
 This work was jointly supported by the Major State Basic Research Development Program of China (973 Program) under grant 2004CB418304 and the National Natural Science Foundation of China under grants 40625014, 40523001, and 40221503.