6036 J O U R N A L O F C L I M A T E VOLUME 21 NOTES AND CORRESPONDENCE Seasonal Variation of the Diurnal Cycle of Rainfall in Southern Contiguous China JIAN LI LaSW, Chinese Academy of Meteorological Sciences, China Meteorological Administration, and LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China RUCONG YU LaSW, Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing, China TIANJUN ZHOU LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China (Manuscript received 14 August 2007, in final form 28 April 2008) ABSTRACT Hourly station rain gauge data are employed to study the seasonal variation of the diurnal cycle of rainfall in southern contiguous China. The results show a robust seasonal variation of the rainfall diurnal cycle, which is dependent both on region and duration. Difference in the diurnal cycle of rainfall is found in the following two neighboring regions: southwestern China (region A) and southeastern contiguous China (region B). The diurnal cycle of annual mean precipitation in region A tends to reach the maximum in either midnight or early morning, while precipitation in region B has a late-afternoon peak. In contrast with the weak seasonal variation of the diurnal phases of precipitation in region A, the rainfall peak in region B shifts sharply from late afternoon in warm seasons to early morning in cold seasons. Rainfall events in south China are classified into short- (1 3 h) and long-duration (more than 6 h) events. Short-duration precipitation in both regions reaches the maximum in late afternoon in warm seasons and peaks in either midnight or early morning in cold seasons, but the late-afternoon peak in region B exists during February October, while that in region A only exists during May September. More distinct differences between regions A and B are found in the long-duration rainfall events. The long-duration events in region A show dominant midnight or early morning peaks in all seasons. But in region B, the late-afternoon peak exists during July September. Possible reasons for the difference in the diurnal cycle of rainfall between the two regions are discussed. The different cloud radiative forcing over regions A and B might contribute to this difference. 1. Introduction The diurnal cycle of precipitation has been of special interest to climate scientists for several decades (Wallace 1975; Gray and Jacobson 1977; Dai et al. 1999; Yang and Slingo 2001; Dai 2001; Liang et al. 2004; Wang et al. 2007). Detailed analysis on diurnal variation is helpful to enhance the understanding of local climate and to validate certain parameterizations in numerical models. Using hourly rain gauge records, Yu et Corresponding author address: Jian Li, Chinese Academy of Meteorological Sciences, Beijing 100081, China. E-mail: lijian@cams.cma.gov.cn al. (2007b) described the unusual characteristics of the diurnal variation of summer precipitation over contiguous China. They found that precipitation peaks around midnight over the eastern periphery of the Tibetan Plateau and the rainfall amount reaches the maximum in early morning over the middle reaches of the Yangtze River Valley. To understand the physical mechanisms behind the diurnal cycle of East Asian summer precipitation, Yu et al. (2007a) classified rainfall events over eastern China based on different duration times. They concluded that the long-duration rainfall events have the maximum around early morning and the shortduration rainfall events have the maximum in late afternoon. DOI: 10.1175/2008JCLI2188.1 2008 American Meteorological Society
15 NOVEMBER 2008 N O T E S A N D C O R R E S P O N D E N C E 6037 Most of the previous studies on the diurnal variation of rainfall in China have focused on warm seasons, during which a majority of rainfall events occur. Although there are fewer rainfall events and less rainfall amount in cold seasons, analyzing the diurnal cycle of rainfall in cold seasons is helpful in understanding the physical mechanisms of different kinds of precipitation. Influenced by the East Asian monsoon, there are significant contrasts in the prevailing wind and water vapor supply between warm and cold seasons, in addition to the seasonal variation of solar radiation. Specific analysis and a comparison of the diurnal cycle in different seasons can offer clues for understanding the precipitation processes. In this study, the diurnal cycle of precipitation over southern contiguous China in all of the seasons are investigated. Two key regions and two kinds of rainfall events are singled out for a comparison of the seasonal variation. It is found that distinct responses of rainfall events to diurnal solar radiative heating might be a major reason for regional differences in the diurnal cycle. The rest of this paper is organized as follows: section 2 describes the datasets and analysis method; the diurnal peak of rainfall over southern China is presented in section 3. The seasonal variation of the rainfall diurnal cycle in the two key regions is described in section 4. Section 5 discusses the possible reasons for the difference in the diurnal cycle of rainfall in the two regions. A summary is given in section 6. 2. Data and method In this study, automatically recorded hourly precipitation data over southern China is analyzed. This dataset is collected and compiled by the National Meteorological Information Center (NMIC) of the China Meteorological Administration (Yu et al. 2007b). It covers the period of 1991 2004. Because of the behavior of the instruments (siphon or tipping-bucket rain gauges), solid-phase aqueous particles falling on the surface are not recorded. The number of stations with data available in the cold seasons is less than that in warm seasons, with 125 versus 137 stations, respectively, in southern contiguous China. Only the records from stations in southern China are used in this study, because in northern China most rainfall events occur in warm seasons and there are no data available in winter. To exhibit the seasonal variation of diurnal phases, the hourly rainfall amount in each month (January December) during 1991 2004 has been standardized. To show the spatial distribution of the phase of the diurnal cycle, the time when the maximum precipitation occurs is represented by a vector on a dial clock. To discuss the possible mechanisms of the rainfall diurnal cycle in southern China, the following datasets are used: 1) The station-observed hourly air temperature and relative humidity data from 1991 to 2004, which are collected and compiled by the China Meteorological Administration, are used. 2) The station-observed total cloud amount data, collected and compiled by the China Meteorological Administration, which has been interpolated onto a 0.5 latitude 0.5 longitude grid by averaging the station data with weights proportional to the inverse of the squared distance between the center of the grid box, and the stations within a radius of 1 are used. It covers the period of 1984 2004. 3) The cloud optical thickness from the International Satellite Cloud Climatology Project (ISCCP) products (Rossow and Schiffer 1991, 1999) dataset from 1984 to 2004 is used. 4) The Earth Radiation Budget Experiment (ERBE; Barkstrom 1984) data from 1985 to 1989 are used. The shortwave cloud radiative forcing at the top of the atmosphere is derived from this dataset. 5) The National Centers for Environmental Prediction (NCEP) Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP)-II reanalysis (hereafter NCEP-2) data (Kanamitsu et al. 2002) from 1991 to 2004 are used. Specifically, the 6-hourly air temperature and relative humidity from this dataset are used in this study. The periods of these datasets are different because of the data availability, which might lead to biases in analysis related to interannual variations. This, however, would not have any significant effect on the climatological seasonal variation. 3. Diurnal peak of rainfall over southern China The diurnal peaks of rainfall over southern contiguous China are shown in Fig. 1a. Two clusters of diurnal cycle can be identified from Fig. 1a. For one group of stations, the maximum hourly rainfall occurs at midnight and in early morning, compared to the lateafternoon precipitation peaks in the other group of stations. These two clusters are spatially separable. The precipitation over southeastern China has a lateafternoon peak. The midnight and early morning peaks are found over southwestern China. Thus, southern China is divided into the following two key regions: southwestern and southeastern China. These two regions, between which striking contrasts in the rainfall peak clock exist, are marked by A and B on Fig. 1a,
6038 J O U R N A L O F C L I M A T E VOLUME 21 respectively. Vectors denoting the peak hour of precipitation in warm seasons (May September) show similar results with Fig. 1a (figure omitted). Only five stations in region A and two stations in region B exhibit notable peak hour differences (greater than 3 h) between allyear and warm-season precipitation. The local solar time of the maximum precipitation in cold seasons (November March) is shown in Fig. 1b. Inspection of Figs. 1a,b reveals similar distribution patterns over region A, suggesting a weak seasonal variation of the diurnal phase. In contrast, the condition over region B is much more complex. All but one station in region B have different maximum clocks in all-year and cold-season rainfall. Large shift of the direction of vectors (greater than 6 h) is found in 42 stations out of the 54 stations in the southeastern region. Late-afternoon peaks disappear in cold seasons and the rainfall in most stations (72.2%) of region B reaches the maximum in midnight and early morning (0000 0800 LST). Only in four stations of region B does the maximum rainfall appear in the afternoon and evening (1300 2100 LST) two of them are located in mountain regions and the other two are located in southern coastal regions. As implied by the results of Zhou and Wang (2006), the diurnal variation of rainfall in these stations might be affected by the orographic forcing and land sea breeze. 4. Seasonal variation of rainfall diurnal cycle in two key regions FIG. 1. Spatial distributions of the phase of the 1991 2004 mean diurnal cycle of hourly precipitation (a) all year (January December) and (b) for cold seasons (November March). The unit vectors denote the phase clock of the maximum precipitation in local solar time (LST). Two distinct regions are labeled: southwestern and southeastern China. Seasonal changes of the diurnal cycle in the two key regions are shown in Fig. 2. In cold seasons, diurnal phases in both regions exhibit an early morning peak. From November to March, the peaks of hourly rainfall amount in region A appear at 0400 0500 LST and the precipitation in region B reaches the maximum around 0200 0400 LST. Distinct differences are found in warm seasons. In region A, during warm seasons the maximum rainfall amount still occurs in early morning, and a secondary afternoon (1500 LST) peak only can be found in August. The maximum hourly rainfall in region B, in contrast, occurs at 1600 1700 LST in all warm seasons. In April, precipitation in region B also peaks in the late afternoon (1900 LST). The difference of seasonal variation of the precipitation diurnal cycle in the two key regions during warm seasons is consistent with the sharp contrast of the time of the maximum rainfall shown in Fig. 1a. Rainfall in warm seasons plays a dominant role in determining the annual diurnal characteristics because of the smaller percentage contribution of the cold-season precipitation. The area-averaged total rainfall amount over regions A and B are 1195.9 and 1469.7 mm yr 1, respectively. In region A, only 25.1% of the rainfall events occur during November March and they account for 13.4% of the total rainfall amount. The percentage contribution of cold-season precipitation is also small in region B, both in frequency (32.8%) and amount (21.8%). The shifts of the maximum rainfall clock between the two key regions are reflections of the differences in warm-season rainfall characteristics. Yu et al. (2007a) revealed the relationship between rainfall duration and its diurnal variation. They proposed that analyses of the diurnal variation of rainfall events with different durations are helpful to the physical understanding of corresponding mechanisms. In this study, rainfall events are categorized into two groups according to their duration. Following Yu et al. (2007a),
15 NOVEMBER 2008 N O T E S A N D C O R R E S P O N D E N C E 6039 TABLE 1. The percentages of total rainfall in frequency and amount for rainfall events of 1 3-h duration (short) and more than 6-h duration (long). Short duration Long duration Frequency Amount Frequency Amount Annual mean Region A 56.9 15.0 23.8 65.7 Region B 56.4 15.5 24.5 66.2 Warm seasons (May September) Region A 59.0 15.9 22.2 64.8 Region B 60.6 18.5 20.7 61.9 Cold seasons (November March) Region A 54.9 12.0 25.9 70.3 Region B 51.2 8.6 29.8 77.3 FIG. 2. The diurnal cycle of precipitation, standardized in each month for regions (a) A and (b) B. The x axis signifies the change of local solar time and the y axis signifies the annual cycle. Negative contours are drawn by dashes. the short-duration rainfall events are defined as the precipitation events lasting for 1 3 h, compared to the long-duration rainfall events defined by the criterion of above 6 h. Table 1 shows the contribution of short- and long-duration rainfall events to total precipitation. In both regions A and B, more than half of the rainfall events have a duration time of 1 3 h, while these shortduration rainfall events account for a small percentage (less than 20%) of total rainfall amount. In contrast, the long-duration rainfall events occur less frequently (20% 30%), but they account for 60% of the total rainfall amount. As shown in Table 1, a notable difference between the two regions is that the precipitation in region B experiences much stronger seasonal variation than that in region A. For short-duration rainfall events, when the seasons shift from warm to cold, the frequency (amount) decreases about 4.1% ( 3.9%) in region A, while a much more significant decrease is seen in region B, which is 9.4% ( 9.9%) for frequency (amount). The percentage contribution of long-duration rainfall events to the total rainfall during cold seasons is higher than that of warm seasons. When the seasons shift from warm to cold, the change of long-duration rainfall events in region B is 9.1% (15.4%) for frequency (amount). The corresponding change in region A is comparatively small, which is 3.7% (5.5%) for frequency (amount). The difference between region A and B is also evident in Fig. 3, which shows the distribution of rainfall percentages against duration time. Larger differences between warm and cold seasons are found in region B for both short- and long-duration rainfall events. Diurnal changes of the two kinds of precipitation in each month are presented in Fig. 4. For rainfall events lasting for 1 3 h, the diurnal phases in both regions show a dominant peak in the afternoon in warm seasons. The afternoon peak in region B exists from February to October. During February and March, the diurnal precipitation peaks at 1400 LST in region B. The afternoon peak then is postponed to 1500 1700 LST in April September and 1900 LST in October. A morning peak ranging from 0600 to 0800 LST develops during November January. However, in region A, the afternoon maximum only occurs from May to September, with the peak clock varying from 1500 to 1700 LST. For rainfall events lasting more than 6 h, southwest-
6040 J O U R N A L O F C L I M A T E VOLUME 21 FIG. 3. The rainfall percentage of precipitation events classified by duration for regions (a) A and (b) B. Precipitation in warm (cold) seasons is shown by a solid (dashed) line. The x axis signifies the duration time in hours; the y axis signifies the percentage of rainfall. ern China experiences a negligible annual change in diurnal phases, as shown in Fig. 4c. The rainfall diurnal cycle is dominated by an early morning peak throughout the year. In spring (March May), the diurnal peak comes earlier at either 0200 or 0300 LST. During other seasons, the maximum hourly rainfall occurs around 0400 0600 LST. In contrast to the coherent diurnal phases in each season in region A, a prominent seasonal variation of long-duration precipitation is seen over region B. From January to June, the diurnal peak is postponed gradually from 0200 to 0900 LST. From July to September, the maximum rainfall amount appears in the late afternoon. An early morning peak at 0500 LST is evident in October. From November to December, the diurnal peak appears at 0400 and 0200 LST, respectively. 5. Discussion In cold months, both short- and long-duration rainfall events in the two regions exhibit similar diurnal changes. The diurnal rainfall maxima appear at midnight or early morning. This indicates that the nocturnal radiative cooling of the continental stratus might play an important role in the seasons when the solar heating reaching the ground is not strong enough. In warm seasons, short-duration events in both regions reach their peaks in late afternoon. It is understandable that the surface air temperature over both regions peaks in the afternoon and the low-level atmospheric instability is favorable for the short-duration convection. The long-duration events in region B also reach their maximum rainfall in late afternoon. This signifies the significant influences of the diurnal solar heating over region B. However, in region A, the rainfall events lasting for more than 6 h exhibit no responses to the solar radiation changes. Even in midsummer, a strong diurnal cycle of solar radiation cannot affect the diurnal phases of the long-duration precipitation in southwestern China. It seems that the differences between regions A and B shown in Fig. 1 might originate from the distinct responses of long-duration rainfall events to the diurnal solar radiative heating. Generated by the Tibetan Plateau, deep continental stratus cloud over southwestern China hinders the solar radiation from reaching the ground (Yu et al. 2004; Li et al. 2005). Figure 5a shows the distribution of the station-observed warm-season total cloud amount in southern contiguous China. The maximum cloud amount center is located over the eastern periphery of the Tibetan Plateau. More cloud is found in southwestern than in southeastern China. Figure 5b shows the cloud optical thickness in warm seasons. There is a steep decrease of the cloud optical thickness across the border between regions A and B. The shortwave cloud radiative forcing during the warm seasons is shown in Fig. 5c. Large cloud amount and large cloud optical thickness produce extremely strong negative shortwave cloud radiative forcing over region A, which might lead to negligible influences of the solar radiation changes on the long-duration rainfall events. Compared with region A, smaller cloud optical thickness and weaker
15 NOVEMBER 2008 N O T E S A N D C O R R E S P O N D E N C E 6041 FIG. 4. The seasonal variation of the diurnal cycle of precipitation, standardized in each month for regions (a), (c) A and (b), (d) B. (a), (b) Rainfall events of 1 3-h duration. (c), (d) The seasonal variation of long-duration rainfall events (more than 6 h) are shown also. The x axis signifies the change of local solar time and the y axis signifies the annual cycle. Negative contours are drawn by dashes. shortwave cloud radiative forcing are found in region B. Figure 6a shows the station-observed surface air temperature at 1400 LST. Consistent with the distribution of shortwave cloud radiative forcing (Fig. 5c), the temperature over region B is 2.1 C higher than that over region A. The static stability at 1400 Beijing, China, time (BJT), defined as the difference of potential temperature between 500 and 850 hpa, is shown in Fig. 6b. Controlled by a large potential temperature difference center, the lower troposphere of region A is more stable than that of region B. At 1400 LST, the area-averaged specific humidity of region B (calculated by station-observed air temperature and relative humidity) is 1 g kg 1 higher than that of region A. Weak stability and plenty of water vapor in region B contributes to the high frequency of convective activity in the afternoon. The warm-season total rainfall (average for 1991 2004) in the afternoon (1400 2000 LST) over region B is 364.3 mm, while that over region A is much lower (205.5 mm). Compared with region B, lower surface air temperature and more stable stratification in region A are relatively unfavorable for the occurrence of the afternoon convection. Thus, the occurrence of some convection is delayed. At night, longwave radiative cooling at the cloud top leads to instability and precipitation. With a longer moisture-accumulating
6042 J O U R N A L O F C L I M A T E VOLUME 21 FIG. 6. (a) The 1991 2004 station observed surface air temperature at 1400 LST in warm seasons, with a contour interval 0.5 C. (b) The 1991 2004 potential temperature difference between 500 and 850 hpa at 1400 BJT in warm seasons, with a contour interval 0.5 K. The potential temperature is derived from NCEP-2 data. FIG. 5. (a) The station-observed cloud amount in the warm seasons for the period of 1984 2004, with a contour interval 2.5%. (b) The 1984 2004 warm-season mean cloud optical thickness derived from ISCCP, with a contour interval 0.5. (c) The 1985 89 warm-season mean ERBE shortwave cloud radiative forcing at the top of the atmosphere with contour interval 5 W m 2. period, this precipitation tends to be a long-duration event and reaches the maximum rainfall late at night or in early morning. 6. Summary Using hourly station rainfall data, the seasonal variation of the diurnal cycle of precipitation over the two key regions in southern contiguous China were investigated. The major conclusions are summarized below, as follows: 1) In southwestern China (region A), the annual averaged hourly rainfall amount reaches the maximum at midnight and in early morning, compared to a lateafternoon peak in southeastern China (region B). 2) In region A, no significant differences in phase clock of the maximum rainfall amount are found between warm and cold seasons. However, most stations in region B shift their peaks from late afternoon to early morning, when seasons convert from warm to cold months. 3) Rainfall events are classified into two groups: short (less than 3 h) and long duration (more than 6 h). For short-duration events, rainfall in both regions peaks at midnight or in early morning during cold seasons and reaches the maximum in late afternoon during warm seasons. The difference between the two regions in short-duration precipitation is that
15 NOVEMBER 2008 N O T E S A N D C O R R E S P O N D E N C E 6043 the late-afternoon peak in region B exists for a much longer period (9 months) than the late-afternoon peak in region A. In the case of long-duration rainfall events, an early morning maximum dominates all year-round in region A, but in region B, except for the midnight and early morning peaks, lateafternoon peaks can be found from July to September. The above results about the characteristics of two kinds of rainfall diurnal cycle offer a more rigorous standard for the validation of parameterizations in numerical models. Acknowledgments. This work was jointly supported by the Major State Basic Research Development Program of China (973 Program) under Grant 2004CB418304, the National Natural Science Foundation of China under Grants 40625014, 40705025, 40523001, and 40221503, and the China Meteorological Administration under Grant GYHY200706010. REFERENCES Barkstrom, B., 1984: The Earth Radiation Budget Experiment (ERBE). Bull. Amer. Meteor. Soc., 65, 1170 1185. Dai, A., 2001: Global precipitation and thunderstorm frequencies. Part II: Diurnal variations. J. Climate, 14, 1112 1128., F. Giorgi, and K. Trenberth, 1999: Observed and modelsimulated diurnal cycles of precipitation over the contiguous United States. J. Geophys. Res., 104 (D6), 6377 6402. Gray, W., and R. Jacobson Jr., 1977: Diurnal variation of deep cumulus convection. Mon. Wea. Rev., 105, 1171 1188. Kanamitsu, M., W. Ebisuzaki, J. Woollen, S. Yang, J. Hnilo, M. Fiorino, and G. Potter, 2002: NCEP-DOE AMIP-II Reanalysis (R-2). Bull. Amer. Meteor. Soc., 83, 1631 1643. Li, J., R. Yu, T. Zhou, and B. Wang, 2005: Why is there an early spring cooling shift downstream of the Tibetan Plateau? J. Climate, 18, 4660 4668. Liang, X.-Z., L. Li, A. Dai, and K. E. Kunkel, 2004: Regional climate model simulation of summer precipitation diurnal cycle over the United States. Geophys. Res. Lett., 31, L24208, doi:10.1029/2004gl021054. Rossow, W., and R. Schiffer, 1991: ISCCP cloud data products. Bull. Amer. Meteor. Soc., 72, 2 20., and, 1999: Advances in understanding clouds from ISCCP. Bull. Amer. Meteor. Soc., 80, 2261 2287. Wallace, J., 1975: Diurnal variations in precipitation and thunderstorm frequency over the conterminous United States. Mon. Wea. Rev., 103, 406 419. Wang, Y., L. Zhou, and K. Hamilton, 2007: Effect of convective entrainment/detrainment on the simulation of the tropical precipitation diurnal cycle. Mon. Wea. Rev., 135, 567 585. Yang, G., and J. Slingo, 2001: The diurnal cycle in the tropics. Mon. Wea. Rev., 129, 784 801. Yu, R., B. Wang, and T. Zhou, 2004: Climate effects of the deep continental stratus clouds generated by Tibetan Plateau. J. Climate, 17, 2702 2713., Y. Xu, T. Zhou, and J. Li, 2007a: Relation between rainfall duration and diurnal variation in the warm season precipitation over central eastern China. Geophys. Res. Lett., 34, L13703, doi:10.1029/2007gl030315., T. Zhou, A. Xiong, Y. Zhu, and J. Li, 2007b: Diurnal variations of summer precipitation over contiguous China. Geophys. Res. Lett., 34, L01704, doi:10.1029/2006gl028129. Zhou, L., and Y. Wang, 2006: Tropical Rainfall Measuring Mission observation and regional model study of precipitation diurnal cycle in the New Guinean region. J. Geophys. Res., 111, D17104, doi:10.1029/2006jd007243.