The diurnal variations of precipitation and clouds are important factors for the local weather and climate (Zhou et al., 2008). The CRM output provides an opportunity to examine the diurnal variations of precipitation and cloud concurrently. The precipitation intensity (PI) is an important characteristic of the cloud system. For example, deep cloud systems primarily produce the greatest PI. Here, rainfall events in each region are classified into five types based on the domain-averaged precipitation intensity of the three regions (Table 1). These five types (I-V) are successively defined as little, small, middle, strong, and heavy rain events. Based on this classification, the frequencies of all five types of rainfall events for all three regions were obtained using the 15-minute interval model output, as shown in Fig. 6. Heavy rainfall (type V) events in the SEC can account for almost 15% of all precipitation in summer but account for less than 3% of the total in winter, which corresponds to the seasonal variation of deep clouds (Fig. 5). The frequencies of rainfall events with a PI below 2.5 mm h−1 do not show dramatic seasonal variations in the SEC (Figs. 6a-d), indicating that these clouds, except deep clouds, occur at similar frequencies in different seasons. By contrast, in the CEC, each rainfall type shows seasonal frequency variations. The frequency of heavy rainfall (type V) events reaches its maximum in the warm season in the CEC, where the frequency of heavy rainfall is approximately 10% in summer, which is lower than that in the SEC (Figs. 6b and 6f). Dramatic seasonal variations are detected for all the rainfall types in the ETP (Figs. 6i-k). Type V rainfall, which is produced primarily by deep convective clouds in the ETP, is most frequent in summer, followed by spring and autumn, whereas the strongest rainfall type (type V) events rarely occur in winter, suggesting that clouds with a great precipitation capacity seldom occur during this season. These results agree with the frequency of deep clouds shown in Figs. 5i-l.
Type I II III IV V PI (mm h−1) PI ≤ 0.2 0.2 < PI ≤ 1 1 < PI ≤ 2.5 2.5 < PI ≤ 5 PI > 5
Table 1. Classification of the rainfall events based on the domain-averaged precipitation intensity (PI).
Figure 6. Diurnal cycle of the frequency of different precipitation intensities (mm h−1) in four seasons for the SEC (a−d), the CEC (e−h), and the ETP (i−l).
The frequency of heavy rainfall shows a diurnal variations pattern with a noon peak in spring and a late afternoon peak in summer over the SEC (Figs. 6a-b). These results suggest that rainfall events with the greatest PI (type V) tend to occur around noon (late afternoon) in spring (summer) in the SEC, which agrees with the observational results (Zhou et al., 2008). Chen et al. (2016, 2018a) also found that the SEC rainfall usually shows an afternoon peak of rainfall in summer, which strongly indicates that deep convection is highly active. However, the other types of precipitation do not show significant diurnal variation patterns in spring, autumn, or winter in the SEC (Fig. 6d). Compared with spring and winter, summer and autumn are characterized by a pronounced diurnal precipitation cycle in the CEC (Figs. 6e-h). The frequency of little rainfall (type I) in the warm season shows a diurnal cycle structure with a peak at night in the CEC (Fig. 6f). The diurnal cycle of warm-season precipitation is also seen by the ground-based observational datasets in the Middle Yangtze River Valley (Yu et al., 2007; Zhou et al., 2008). In contrast, the other types of rainfall events in the CEC during the warm season show frequency of occurrence peaks around noon. The greater the PI, the more obvious the solitary peak structure (Fig. 6f). The precipitation, which is estimated by Artificial Neural Networks and Tropical Rainfall Measuring Mission 3B42 (TRMM) (Zhou et al., 2008), failed to capture the early morning peak in the diurnal precipitation frequency cycle in the Middle Yangtze River valley (Zhou et al., 2008). The early morning precipitation frequency peak determined in this study is composed mostly of the little rainfall events (type I, Fig. 6f), which implies that the TRMM dataset may underestimate the frequency of little rain events in the CEC. In contrast, the diurnal cycle signals of type II-V rainfall events weaken in autumn and disappear in winter in the CEC (Figs. 6g-h). Similarly, the frequencies of strong and heavy rainfall events (type IV and V) in the CEC decrease in autumn and winter. However, the CEC small rainfall events (type II) frequency reaches its maximum values in winter with no dramatic diurnal variations. Under the influence of the East Asia summer monsoon, both the CEC and SEC experience the most rainfall in summer; rainfall events with PI greater than 2.5 mm h−1 reach a noon peak in the CEC and an afternoon peak in the SEC. According to Xu and Zipser (2011), deep convective clouds have peaks around noon in the CEC and the late afternoon in the SEC, which implies that deep convective clouds make important contributions to the frequency peak of warm-season strong and heavy rainfall events at approximately noon (afternoon) in the CEC (SEC).
Compared to those in the lowland regions, the frequencies of all rainfall types in the ETP (except small rainfall (type II) events, which do not display a pronounced diurnal frequency in spring, summer, and autumn) show diurnal variations in spring, summer, and autumn (Figs. 6i-k). In this region, types III-V rainfall events are collectively considered great rainfall events. Great rainfall events rarely occur in winter in the ETP (Fig. 6l), leading to a nearly rainless winter, as noted in the TRMM dataset (Fig. 2c). Great rainfall events in the ETP being to appear in spring with a frequency peak around noon (Fig. 6i). When summer arrives, great rainfall events become more frequent (Fig. 6j) and show a diurnal frequency variation with a bimodal structure. The main peak is in the afternoon, accounting for over 20% of all rainfall events; there is also a weak secondary peak in the early morning, accounting for less than 10% of the total. Using the TRMM datasets, Fu et al. (2006) found that a strong diurnal cycle of precipitation occurring over the central TP with a peak in the late afternoon and a minimum at approximately 0500 LST (local standard time), confirming the diurnal cycle structure depicted in Fig. 6j and indirectly indicating that most rainfall events on the plateau are convective in summer (Fu et al., 2006).
Deep convection signifies a type of cloud with great precipitation capacity. Here, deep convective clouds are distinguished by the following two criteria: (1) the cloud depth is greater than 10 km over lowland regions (5.5 km over the ETP), and (2) the cloud top is greater than 12 km AGL over lowland regions (6 km AGL over the ETP). This definition considers differences in the cloud depth between the ETP and the lowland regions (e.g., Fig. 6i). The basic idea behind this definition is that deep convection normally extends vertically through most of the troposphere, and the atmospheric column is much thinner over the ETP than over the lowlands. Compared to adopting a uniform definition for all the regions, our definition can provide more deep convection samples in the ETP. This other method just considers the cloud depth and cloud top, which may regard the thick stratiform cloud as DCC and cause some uncertainties.
Figures 7 and 8 show the diurnal cycles of the averaged profiles of the frequency and TCWC profiles of the DCCs, respectively, for each season over the SEC, CEC, and ETP. The TCWC can be used to measure the intensity of deep convection and rainfall (Chakraborty et al., 2016), which helps understand the diurnal precipitation variations, as shown in Fig. 6. The averaged profiles of the TCWC and frequency of deep convective clouds show seasonal variations corresponding to great rainfall events (Figs. 6, 7 and 8). Both the SEC and CEC feature DCCs have a high TCWC and high frequency in summer (Figs. 7b, 7f and 8b, 8f), which agrees with the seasonal variations of great rainfall events (type III-V, Figs. 6b and 6f). It is worth noting that DCCs over the CEC show a winter frequency peak. However, the TCWC of the CEC winter deep convection is smaller than those in summer. Considering that DCCs are filtered by cloud depth and cloud top, the CEC winter DCCs may include some thick stratiform clouds. In contrast, the DCCs over the SEC show a maximum frequency and TCWC around noon in summer (Figs. 7b and 8b), while great rainfall (type III-V) events show a high frequency in the afternoon (Fig. 6b). The different peak time of the DCC’s frequency and TCWC and rainfall implies that other deep cloud systems may contribute to the afternoon rainfall peak. Unfortunately, the DCC definition may raise some uncertainties regarding the DCC peak time. The TCWC of the summer deep convection reaches its maximum around noon at approximately 8 km AGL in the CEC, whereas the maximum TCWC can extend to 10 km AGL between noon and late afternoon in the SEC (Figs. 8b and 8f). However, a previous study reported that midsummer rainfall in the CEC peaks in the later afternoon (approximately 1600 LST) (Chen et al., 2009). Figure 7f indicates high-frequency DCCs in the late afternoon, suggesting that the late afternoon peak of rainfall in the CEC (Chen et al., 2009) is related to these high-frequency DCCs. Furthermore, the DCCs over the SEC show a higher maximum TCWC level than those over the CEC in all seasons (Figs. 8a-h), suggesting that the DCCs in the SEC are more energetic than those in the CEC. The TCWC of convective clouds over the lowland regions (e.g., SEC and CEC) shows a diurnal cycle in spring, summer, and autumn but no significant diurnal cycle in winter (Figs. 8d and 8h). However, the frequencies of DCCs in the lowland regions exhibit diurnal variations in the cold season (Figs. 7d and 7h), which could be a possible reason for the cold-season diurnal precipitation cycle, as reported by Huang and Chan (2012).
Figure 7. Diurnal cycle of frequency (%) of deep convective clouds in four seasons for the SEC (a−d), the CEC (e−h), and the ETP (i−l). Note that because no deep convective cloud cells could be identified during winter for the ETP, they were removed from the analysis.
Figure 8. Diurnal cycle of total cloud water content (g kg−1) of deep convective clouds in four seasons for the SEC (a−d), the CEC (e−h), and the ETP (i−l). Note that because no deep convective cloud cells could be identified during winter for the ETP, they were removed from the analysis.
Figures 5 and 6 show that the ETP features the most significant seasonal cloud frequency and the most dramatic diurnal precipitation cycle among all three regions. These features also appear in the TCWC and frequency of the DCCs (Figs. 7i-l and 8i-l). The most favorable environment for the triggering and development of DCCs over the ETP occur in summer, followed by spring and autumn (Figs. 7i-k and 8i-k). Previous studies reported that deep convection activity and precipitation over the ETP display an afternoon peak because of the influence of the surface heat flux in the warm season (e.g., Chen et al., 2017a, 2018b; Li, 2018). Figure 9 shows diurnal variations with a noon peak in the surface heat fluxes (including sensible heat flux and latent heat flux) in the ETP in each season. Figures 7j and 8j display frequent summer DCCs with a high afternoon TCWC in the ETP, which greatly contribute to the early-afternoon precipitation peak (Fig. 6j). Besides, warm-season nocturnal rainfall in the ETP has been reported by Chen et al. (2018b). Consistent with this finding, the deep convection frequency shows a weak secondary peak at night in the warm season (Fig. 7j). However, nighttime DCCs have a smaller TCWC than afternoon DCCs (Fig. 8j), which implies that nocturnal rainfall events have relatively weak intensity. The warm-season structures (a strong afternoon peak and a hint of a secondary night mode) of the diurnal cycles of the deep convection frequency and TCWC (Figs. 7j and 8j) correspond to the diurnal structure of rainfall events with a PI greater than 2.5 mm h−1 (Fig. 6j). According to TRMM observations (Singh and Nakamura, 2009), the precipitation falling over the central TP shows a second peak at midnight in the warm season, which is similar to the diurnal structures of the deep convection TCWC and frequency in the ETP (Figs. 7j and 8j). This similarity suggests that DCCs not only are the primary mechanism responsible for the afternoon precipitation peak but also greatly contribute to the nocturnal precipitation peak. A bimodal structure of the deep convection frequency is detected in spring and autumn (Figs. 7i and 7k) and the maximum frequency in autumn appears during the night (Fig. 7k). However, nocturnal DCCs have small TCWCs (Figs. 8i and 8k), which will reduce their precipitation capacity and possibly lead to weak-intensity rainfall events. Winter does not provide conducive conditions for triggering and development of DCCs in the ETP (Figs. 7l and 8l), largely due to the dry and cold climate therein.