FY-2C-derived diurnal features of clouds in the southern contiguous China

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2012jd018125, 2012 FY-2C-derived diurnal features of clouds in the southern contiguous China Haoming Chen, 1 Rucong Yu, 1 and Bingyi Wu 1 Received 16 May 2012; revised 25 July 2012; accepted 3 August 2012; published 18 September [1] Hourly infrared (IR) brightness temperature (BT) derived from China s first operational geostationary meteorological satellite Feng-Yun (FY)-2C is analyzed over the southern contiguous China (20 N 33 N, 100 E 122 E) during The main focus is to investigate the diurnal variation of clouds and compare the different features between the southwestern and southeastern China. The clouds are first divided into three categories by the cloud top temperature (CTT, derived from IR BT) according to their diurnal features in summer. The cold cloud (CC, defined as CTT lower than 30 C) occurs most frequently in the late afternoon in most regions over southern China. The frequency of the middle cloud (MC, defined as CTT between 30 C and 0 C) reaches its diurnal maxima in the early evening to the midnight. The frequency of warm cloud (WC, defined as CTT warmer than 0 C) peaks around noon. There are also distinct regional differences of the diurnal variation of summer CC and MC frequencies. The diurnal variation of CC, MC and WC frequencies also show evident seasonal changes. The late afternoon peak of CC frequency appears in almost all months, and in the cold seasons, there is also a midnight to late evening secondary peak. The MC frequency reaches its diurnal peak in the early evening in summer and in the late evening in other seasons. The diurnal amplitude of WC frequency is smaller in warmer seasons and larger in cold seasons, whereas its diurnal phase shows no evident seasonal changes. Citation: Chen, H., R. Yu, and B. Wu (2012), FY-2C-derived diurnal features of clouds in the southern contiguous China, J. Geophys. Res., 117,, doi: /2012jd Introduction [2] Because of its significant influences on global weather and climate, the diurnal cycle of clouds and precipitation has been of special interest to climate scientists for several decades [Ramage, 1952; Wallace, 1975; Dai et al., 1999a, 1999b; Trenberth et al., 2003; Yang and Smith, 2008]. Detailed analysis on diurnal variation is helpful in enhancing the understanding of local climate and validating certain parameterizations in numerical models [Trenberth et al., 2003]. The global and regional distributions of rainfall diurnal variations have been investigated with ground-based meteorological observations [Wallace, 1975; Dai et al., 1999a; Yu et al., 2007b] and observations from meteorological satellites carrying infrared radiometers [Sui et al., 1997; Yang and Slingo, 2001], microwave radiometers [Sorooshian et al., 2002] or precipitation radar [Nesbitt and Zipser, 2003; Takayabu, 2006; Xu and Zipser, 2011]. These studies indicate that the characteristics and formation mechanisms of diurnal 1 Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing, China. Corresponding author: H. Chen, Chinese Academy of Meteorological Sciences, China Meteorological Administration, 46 Zhongguancun South St., Beijing , China. (chenhm@cams.cma.gov.cn) American Geophysical Union. All Rights Reserved /12/2012JD variations of precipitation are location dependent, which is greatly controlled by different cloud types. [3] The southern contiguous China is located to east of the Tibetan Plateau and north of South China Sea and is probably affected by complex terrain, land sea contrast and monsoon flow [Chen et al., 2009]. Several studies have analyzed the observed diurnal variation of rainfall and revealed a distinctive diurnal variation in this region, with the nocturnal rainfall prevailing southwestern and late afternoon peak dominating the southeastern China [Yu et al., 2007a, 2007b; Zhou et al., 2008]. Li et al. [2008] further examined seasonal variation of the station-observed rainfall diurnal cycle in this region and pointed out that the east-west discrepancy of diurnal variation was both region and duration (defined as the length of lifetime) dependent. The differences of rainfall diurnal variation between the southeastern (the east of the dotted line in Figure 1) and southwestern (the west of the dotted line in Figure 1) China are evident in warm seasons, which is found more distinct in long-duration rainfall events. They discussed the possible reasons and suggested that the different cloud radiative forcing could play important roles. Yu et al. [2010] explored the diurnal variation of rain rate and precipitation profile based on data from Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and revealed the robust diurnal features of summer stratiform and convective precipitation over southern China. They demonstrated that the diurnal phase of summer rainfall depends on the location, 1of11

2 Figure 1. Domain for the southern contiguous China for the analyses in this study and the height of topography (shading, units: 100 m). The black dashed line labels the two distinct regions. The location of Yangtze River is drawn as a gray line. precipitation type, and duration time. The maximum rain rate and the highest profile of stratiform precipitation occur in the late afternoon (late night) over the southeastern (southwestern) China, whereas most of the stratiform short-duration rain rate tends to present late afternoon peaks over southern China. The maximum rain rate and the highest profile of convective precipitation occur in the late afternoon over most regions of southern China. [4] Results of previous studies already mentioned that the diurnal variation of rainfall is closely related to the particular characteristics of clouds in southern China, over which the optically thick continental stratus clouds prevail [Yu et al., 2004]. To comprehensively understand the fundamental controlling physical mechanisms of the diurnal variation of rainfall, the investigation of corresponding cloud variation should be taken into consideration. However, owing to the limited data, the detailed diurnal variation of cloud features in this region is still unclear. The satellite-observed infrared (IR) brightness temperature (BT) can be used to investigate cloud features. Although the BT is not usually well correlated with rainfall amount and does not always indicate active deep convection, it has long been used in cloud identification and precipitation estimation, especially in the tropics [Yang and Slingo, 2001; Li et al., 2007; Aves and Johnson, 2008]. Wang et al. [2004] used the hourly infrared BT observed by the Geostationary Meteorological Satellite (GMS) over East Asia to analyze the characteristics of warm season convection and the behavior of those episodes. They indicated that GMS IR observations can be used to compile climatology of warm season cloud and precipitation episodes over East Asia and provide useful information, as does the radar reflectivity. [5] The simulation of the rainfall diurnal variation in numerical models has been proven to be a challenging issue [Dai et al., 1999a; Trenberth et al., 2003; Dai, 2006]. The poor simulation may be closely related to the cloud-related physics [Cui, 2008; Sato et al., 2008; Takayabu and Kimoto, 2008]. However, it is difficult to validate the cloudiness simulated by large-scale atmospheric models because of problems inherent in the retrieval of an observed cloudiness from satellite observations of radiances [Slingo, 1987]. The BT derived from satellite observations provides an opportunity to directly validate the clouds and their optical properties in atmospheric models [Morcrette, 1991; Sun and Rikus, 2004]. Morcrette [1991] adopted a model-to-satellite approach, in which satellite BT images were directly compared with BTs computed from predicted model fields. Chaboureau et al. [2002] further demonstrated that this approach was especially powerful in identifying discrepancies of cloud cover forecasts. Sun and Rikus [2004] used the model fields to generate radiance as seen by the satellite. The high temporal and spatial resolution data from FY series satellites can be used to validate the weather and climate models in East Asia. However, to efficiently apply the satellite data to validate numerical models, it is the first step to reveal the reliable observational characteristics. [6] In this study, the derived BT data form FengYun (FY)- 2C, the first operational geostationary meteorological satellite of China, is used to investigate the cloud features in southern contiguous China. Our attention is devoted to the characteristics of cloud distributions and diurnal variations of clouds with different BTs at top, including the differences between the southwestern and southeastern China. The results could not only broaden our knowledge of the diurnal variations of clouds in southern China but also provide observational basis for future model validation and improvement. [7] The rest of the paper is organized as follows. Section 2 describes the FY-2C satellite data sets and analysis methods. The spatial variation of the cloud patterns as reflected by cloud top temperature (CTT, derived from IR BT at cloud tops) is presented in section 3. Section 4 analyzes the diurnal variation of the summer clouds with different CTTs and compares the discernible diurnal features between southwestern and southeastern China. Section 5 further investigates the seasonal variation of clouds. Summary and concluding remarks are given in section Data and Methods [8] The Feng-Yun (FY)-2 series of geo-stationary meteorological satellites are operated by China Meteorological Administration (CMA). As the first operational geostationary meteorological satellite of China, the FY-2C provides an overview of cloud systems at synoptic scale, which is very useful for weather monitoring and long-term climatology construction. It was launched on 19 October 2004 and became fully operational in The FY-2C is located 35,800 km above the equator at longitude 105 E. The objective of the mission is to monitor the temperature and the clouds above China and neighboring areas and also to provide meteorological information for the Asia-Pacific region. The upgraded stretched-visible and infrared spin-scan radiometer (S-VISSR) is one of the major payloads onboard the FY-2C. This optical imaging radiometer consists of four infrared channels and one visible channel, and it can acquire a full disc image covering the Earth surface from 60 Nto60 S in latitude and from 45 E to 165 E in longitude with high time resolution (acquisition per 30 min for flood seasons and per hour for other seasons) with the sensor s original resolution of 0.04 latitude 0.04 longitude [Xu et al., 2002, 2004; Liu et al., 2009]. [9] This work aims to analyze the BT product derived from the first infrared (IR1) channel (with spectrum range of mm) for the summers (June August, JJA) of 2of11

3 Figure 2. Summer mean (June August) percentage of frequency distributions of cloud top temperature (CTT, unit: C) derived from IR brightness temperature (BT) for each 5 C bins derived from FY2C IR1 channel averaged over the southern China during 2005 to The data set covers the same region as the scan area of FY-2C with precision of 1 C. The BT product is averaged to 0.1 latitude 0.1 longitude. As one of the motivations is to compare the cloud features in accordance with distinct diurnal rainfall variation in southern China, the cloud classification (CLC) product during the same periods is also applied to remove the grid without cloud before the analyses. An algorithm [Yang et al., 2008] similar to that of International Satellite Cloud Climatology Project (ISCCP) [Rossow and Schiffer, 1991, 1999] is used for satellite IR radiance data collected at IR1 channel to recognize the pixels with cloud and clear sky pixels. Jin et al. [2009] have already shown that the FY-2C cloud mask performs consistently with other cloud mask products such as ISCCP, the moderate resolution imaging spectroradiometer on board Terra and Aqua satellites, and conventional ground observations over Southeast Asia. The hourly data sets are calibrated and released by the National Satellite Meteorological Center of CMA and available online at gov.cn/arssen/ord/satellite.aspx?seriescode=fy2x. [10] The IR BT at cloud top (hereinafter defined as CTT) is first extracted by removing clear-sky data using the CLC product. To facilitate the examination of the long-term characteristics of the cloud features, the occurrence frequency of CTTs is calculated. As shown in Figure 3a, clouds with different CTTs show different diurnal variations in the contiguous southern China. Based on the diurnal features, the clouds are classified into three categories. The cold cloud (CC), which have late afternoon maximum, is defined as clouds with CTT lower than 30 C. It mainly reflects the convective activities [Wang et al., 2004]. The warm cloud (WC), dominated by noon maximum, is defined as clouds with CTT higher than 0 C, which presents the liquid content. The cloud with CTT between 30 Cand0 C, which have no dominant phase, is defined as middle cloud (MC). The division of cloud is only according to the diurnal features of clouds with different CTTs, as the diurnal variation of cloud is the main focus in this study. A series of tests show that our conclusions of diurnal variation of clouds are insensitive to this choice of threshold, but the climatology of the cloud distribution differs. It should be noted that the definitions of the clouds here do not exactly consist of that in cloud micro-physics. [11] The frequency of each cloud is calculated before analyzing the diurnal features. The frequency is defined as the percentage of all hours having the clouds. At each grid box and for each hour, the JJA averages of frequency of each type of cloud are computed for each year. The multiyear ( ) mean states of JJA frequency are derived by averaging the hourly frequency. The JJA mean hourly data are averaged over the years to derive a composite diurnal cycle of each cloud type. The spatial distribution of the time when the maximum frequency occurs is displayed to show the spatial feature of diurnal phase. The significance of the diurnal cycle is expressed qualitatively by comparing the amplitude with the daily mean using normalized amplitude. In addition, the southern contiguous China (20 N 33 N, 100 E 120 E,seeFigure1for the domain) is divided into west and east subregions [Li et al., 2008; Yu et al., 2010] to compare the diurnal variation of clouds and discuss its relations with rainfall characteristics. 3. Summer Cloud Patterns as Reflected by CTT Frequency [12] The regional averaged frequency of the CTT in each 5 C interval over southern contiguous China (the whole domain in Figure 1) is presented in Figure 2. The frequency of cloud with CTT lower than 0 C contributes to 77.1% of all the observational hours in FY-2C in summers during The CC accounts for 24.2% of the observational hours, the MC accounts for 52.9%, and the WC accounts for 22.9%. [13] The diurnal variation of the summer CTT occurrence frequency (normalized by the daily mean) in each 5 C bin averaged over southern contiguous China is shown in Figure 3a. The frequencies of CC, MC, and WC show different diurnal features. The diurnal amplitudes of frequency of clouds either CTT colder than 30 C and warmer than 0 C are larger than that between 30 C and 0 C. The cloud with CTT colder than 30 C occurs more frequently in the afternoon to the early evening. The frequency of cloud with CTT between 30 C and 0 C is larger in the nocturnal hours, whereas in the daytime, the frequency is smaller. For the cloud with CTT warmer than 0 C, it occurs more frequently around the noon to early afternoon. [14] The diurnal curves of summer mean CC, MC, and WC frequencies averaged over southern China are compared in Figure 3b. The frequency of CC reaches its diurnal minima at 10 LST and then increases quickly in the afternoon and gets its maxima at 17 LST. The frequency of CC is generally small in the nocturnal hours. In contrast, the frequency of MC is larger in the late afternoon to the midnight. It decreases from the midnight to the noon and reaches its diurnal minimum at 15 LST and then increases in the afternoon. The frequency of WC peaks at 12 LST, and it is relatively large during the daytime and small in nocturnal hours. [15] The spatial distributions of JJA mean occurrence frequency of CC, MC, and WC are shown in Figure 4. The distribution of occurrence frequency of CC shows no evident west-east contrast in FY-2C data (Figure 4a). The CC occurs most frequently in the eastern periphery of the Tibetan Plateau (25 N 33 N, 100 E 103 E) and southern coastal regions, where more than 18% of the summer clouds are colder than 30 C at top. A large value center (greater than 21%) is also found in the lower reaches of the Yangtze River valley (26 N 30 N, 117 E 120 E). The relatively small frequency was 3of11

4 Figure 3. (a) The diurnal variations of the normalized CTT frequencies (normalized by the daily mean) averaged over the southern China for each 5 C bins from FY2C IR1channels. The x axis is the Local Solar Time (LST), and the y axis is the CTT (units: C). Values greater than 0.2 are shaded in gray. Negative contours are drawn by dashes. (b) The diurnal curves of summer normalized frequencies for CC (solid black line, left y axis), MC (gray dashed line, right y axis) and WC (black dotted line, left y axis). found in the middle inland regions (24 N 33 N, 105 E 115 E), where the occurrence frequency is less than 15%. The MC occurs more frequently in the southwestern than the southeastern China. In the west region, the frequency is larger than 48% in most areas, with the high center in the eastern periphery of the Tibetan Plateau. A relatively small frequency of MC occurs in the east of Sichuan basin (27 N 32 N, 105 E 108 E). In the east region, the occurrence frequency shows a relatively uniform pattern, with the values smaller than 44%. The distribution of WC frequency shows opposite pattern as compared with that of MC. The WC occurs more frequently in the southeastern China, with the frequency larger than 40% in most areas. In the western country, it occurs less frequently, especially in the eastern periphery of the Tibetan Plateau, which may be correlated with the higher topography in this region. 4. Regional Features of Diurnal Variation of the Cloud Frequencies in Summer [16] From the analyses above, it is shown that the clouds with different CTTs not only present distinct diurnal variations, but also exhibit evident spatial differences. To further demonstrate the diurnal features of the clouds and its regional differences, the distribution of diurnal phase (the hours when the frequency of clouds reaching the diurnal peaks) from FY-2C observation are shown in Figure 5. Consistent with the diurnal curves shown in Figure 3b, the diurnal phases of CC, MC, and WC frequency show different distributions with evident regional contrasts. The CC exhibits a late afternoon to early evening (15 21 LST) diurnal peak over most of southern China (Figure 5a). In the southern edge of the Tibetan Plateau (26 N 28 N, 106 E 108 E) and Sichuan basin (28 N 33 N, 103 E 107 E), the CC shows a midnight to early morning peak (03 09 LST), which is in accordance with the prevailing nocturnal rainfall during summer in these regions [Yu et al., 2007b; Zhou et al., 2008], indicating the contribution of the convective activities to the nocturnal rainfall maxima. The diurnal phase of MC shows obvious westeast differences. In most of the eastern region, the frequency of MC reaches the daily maxima in the midnight to the early morning (00 06 LST). However, in most of the western region, the frequency of MC reaches the diurnal maxima in the evening to the midnight (18 00 LST), which is more than 3 h earlier than that in the eastern region. In the eastern periphery of the Tibetan Plateau, the frequency of MC peaks in the late afternoon to the early morning, which is different from other regions in both west and east of southern China. The WC frequency presents a noon to afternoon (12 15 LST) peak in most areas of southern China, whereas only in some southeastern coastal regions, the diurnal peak occurs in the midnight to early morning. [17] The above analyses indicate that the diurnal phase of MC frequency shows west-east contrast in southern China, and the diurnal phase of CC shows a uniform pattern in the southwestern China as in the southeastern China. To further 4of11

5 Figure 4. The spatial distribution of the percentage for (a) CC, (b) MC and (c) WC frequencies in summer from the FY-2C IR1 channel. Figure 5. Spatial distribution of the diurnal phase (LST) of the occurrence frequencies of (a) CC, (b) MC, and (c) WC derived from the FY-2C IR1 channel. The white polygons in Figure 5a mark two distinct subregions in southwestern China. The black dashed line in Figure 5b marks the west and east of the southern contiguous China. 5of11

6 Figure 6. Same as Figure 3 but for the average of (a and c) west and (b and d) east of the southern China. reveal the regional features of diurnal variation of clouds with different CTTs, the normalized frequency in each 5 C CTT bin averaged over the west and east of southern contiguous China is compared in Figures 6a and 6b. In the west region, the CC occurs more frequently in the late afternoon to the midnight, and generally, the diurnal maxima of frequency occur later when CTT is colder. For example, the cloud with CTT between 65 C and 30 C occurs most frequently in the late afternoon around 18 LST, whereas the frequency of much colder cloud (CTT < 65 C) reaches its daily peak in the early evening to midnight. The diurnal amplitude of MC ( 30 C 0 C) frequency is smaller than that of the CC. The MC occurs more frequently in the early morning. The cloud with CTT warmer than 0 C occurs more frequently around noon. To show the different diurnal variations of cloud frequencies in southwestern China, the diurnal curves of the normalized frequencies of CC, MC, and WC are shown in Figure 6c. For the CC, the frequency reaches the diurnal peak at 18 LST. The frequency decreases from the late afternoon to the midnight and reaches a second diurnal maximum around 02 LST, which is mainly contributed by CTT colder than 65 C (Figure 6a). The frequency reaches its daily minimum before noon and then begins to increase in the afternoon. The diurnal curve of MC frequency shows a dominant midnight to early morning peak, which is about 6 8 h later than that of CC. The frequency decreases after sunrise and falls to the daily minimum in the late afternoon when CC occurs most frequently during the day. The WC frequency is larger during the day and smaller in the nocturnal hours. It reaches the daily maxima at 12 LST and minima at 04 LST in the southwestern China. [18] In the southeastern China, the different diurnal variations of occurrence frequencies among the CC, MC, and WC can also be found (Figures 6b and 6d). A dominant lateafternoon peak of the frequency is evident for CC, whereas the daily maximum of MC frequency appears in the midnight. The WC frequency reaches the diurnal maxima around noon in the east region, similar to that in the west region. The diurnal curves of the frequencies of CC, MC, and WC all show single diurnal peaks in the late afternoon, noon, and midnight, respectively. Different from the west region, the CC presents no secondary nocturnal maxima in the east region. For the MC, the diurnal peak occurs in the midnight, earlier than that in the east region, and then decreases quickly from the early morning. [19] As illustrated in Figure 5, the spatial distribution of diurnal phase exhibits evident regional differences. Meanwhile, it is also found the averaged diurnal curves of CC and MC over the southwestern and southeastern China show different shapes (Figures 6c and 6d). These features suggest that temporal distribution of the clouds during the day may also differ in the west and east regions. Figure 7 shows the distribution of late afternoon (14 20 LST) and early morning (02 08 LST) fractions in the daily cloud frequency of the CC from FY-2C observation, respectively. The two periods are selected because the diurnal curves averaged in the west and east regions show large differences during such periods, whereas in the other hours of the day, the fraction in the daily frequency shows much smaller differences (Figures omitted). Although the diurnal phase of CC shows no apparent discrepancy between most regions of the southwestern and southeastern China, the temporal distribution of the fraction in the daily frequency does not resemble each other between the two regions. Corresponding to the differences of the diurnal curves between the two regions, the CC occurs more frequently in the east region in the late afternoon, which is about 10% more than that in the west region. In the west region, the CC occurs more frequently in the early morning, with the centers in the southern edge of the Tibetan Plateau and Sichuan basin, which coincides with the distribution of its diurnal phase (Figure 5). 6of11

7 summer CC and MC occur most frequently in southern China (Figures 3a and 3b), the diurnal feature of CC frequency is similar to that of the regional mean in southern China (Figure 3a), and its diurnal curve shows no secondary diurnal peak in this region (figure not shown). The MC and WC frequencies in S1 show similar diurnal variations, with the peak around noon. In the downstream of the Tibetan Plateau (i.e., Sichuan basin and the southern edge of the Tibetan Plateau, S2), the clouds with CTT colder than 10 C present a midnight to early morning peak (Figure 9b). It is also noted that the cloud with CTT colder than 60 C and between 30 C and 10 C, a nocturnal maximum dominates the diurnal variation of the frequency, whereas the frequency of clouds with CTT between 60 C and 30 C has a weak late afternoon to early evening peak. The clouds with CTT warmer than 10 C still show a noon peak, which shows few regional differences between S1 and S2. 5. Seasonal Variations of the Diurnal Cycle of Cloud Frequency [22] Seasonal changes of the diurnal cycle of CC, MC, and WC frequencies averaged over southern China are shown in Figure 7. The (a) late afternoon (14 20 LST) and (b) early morning (02 08 LST) fraction in the daily CC frequency from FY2C IR1 channels. The contour interval is 4%. Values greater than 36% and 28% are shaded in Figures 7a and 7b, respectively. [20] Because the frequency of MC presents weaker diurnal variations than the CC (Figure 6), the differences of the temporal distribution are also not evident for MC, but the spatial distribution still shows differences between the west and east of southern China (Figure 8). More than 26% of the MC occurs in the early evening to midnight (20 02 LST) over most of the southern contiguous China, except the eastern periphery of the Tibetan Plateau. Generally, the MC occurs more frequently in the southeastern China in the early evening. In the southwestern China, the MC occurs more frequently in the late morning (08 14 LST), especially in the eastern periphery of the Tibetan Plateau. Nevertheless, because of the relatively weak diurnal amplitude (Figure 3b), the differences among the time slides are not as large as that of the CC. [21] Careful examination of the diurnal features of CC and MC frequency also indicates that in the southeastern China, the diurnal phase show relatively uniform distributions, whereas in the southwestern China where the topography is more complicated, the regional features are distinct (Figure 5a). To further reveal the detailed regional diurnal features of cloud frequencies in southwestern China, two subregions (as marked in Figure 5a) are selected. In the eastern periphery of the Tibetan Plateau (S1) where both the Figure 8. Same as Figure 7, but for the (a) early evening (20 02 LST) and (b) late morning (08 14 LST) fraction in the daily MC frequency, respectively. The contour interval is 2%. Values greater than 28% and 24% are shaded in Figures 8a and 8b, respectively. 7of11

8 in the southwestern and southeastern China. In the southwestern China (Figure 11c), the MC frequency exhibits a noon peak in winter, whereas in other seasons, the diurnal peaks appear in the late night, close to the phase of the secondary diurnal speak of the CC frequency in these seasons. In summer, the diurnal amplitude of MC frequency is generally smaller than that in other seasons. In the southeastern China (Figure 11d), the frequency of MC exhibits a late night peak in cold seasons, whereas early evening peak in warm seasons. The frequency of WC in the west and east of southern China show no evident regional contrasts and seasonal changes (Figures 11e and 11f). The diurnal phase of the WC frequencies appears around noon in both west and east regions during the year. The seasonal change of diurnal amplitudes is also similar in the two regions, with the amplitude smaller in warm seasons and larger in cold seasons. Figure 9. The diurnal variations of the normalized CTT occurrence frequency averaged over the two distinct regions (as marked in Figure 5a) over the southwestern China for each 5 C bins, for regions (a) S1 and (b) S2. Negative contours are drawn by dashes. Figure 10. The late afternoon diurnal peak is evident for CC frequency throughout the year (Figure 10a). In the warm seasons (May to October), the late afternoon peak dominates the diurnal variation of the CC frequency. In the cold seasons, a comparable midnight peak is also found, and the diurnal amplitude is relatively small compared with that in warm seasons. The diurnal cycle of MC frequency in southern China also shows evident seasonal changes, and its diurnal amplitude is weaker than that of CC in all months (Figure 10b). In summer, the diurnal peak of the MC frequency appears in the early evening around 21 LST, whereas in other seasons, the midnight to early morning peak dominates the diurnal variation of the MC frequency. The diurnal phase of the WC frequency shows no evident seasonal variations (Figure 10c). It exhibits a noon peak in all the months. In the warmer seasons, the diurnal amplitude of the WC frequency is smaller than that in cold seasons. [23] The seasonal changes of the diurnal cycle of cloud frequencies averaged over the southwestern and southeastern China are further compared in Figure 11. The dominant lateafternoon diurnal peak of the CC frequency in warm seasons is obvious in both west and east of southern China. In the southwestern China (Figure 11a), the single late afternoon peak only appears in the warmest July to August, whereas the midnight peak of the CC frequency exists in other months during the year. In the southeastern China (Figure 11b), the single late afternoon peak of the CC frequency dominates most months of the year, whereas in the coldest November to February, there is a weak late night peak. The diurnal variation of the MC frequencies exhibit different seasonal changes Figure 10. The diurnal cycles of (a) CC, (b) MC, and (c) WC frequencies, normalized in each month for the southern contiguous China. The x axis signifies the local solar time (LST), and the y axis signifies the annual cycle. Negative contours are drawn by dashes. 8of11

9 Figure 11. Same as Figure 10 but for the (a, c, and e) southwestern and (b, d, and f) southeastern China. Meanwhile, the diurnal amplitude of the WC frequency in the west region is larger than that in the east region. 6. Summary and Concluding Remarks [24] The hourly CTT frequency observed using the FY-2C satellite IR1 channel is analyzed over southern contiguous China. The climatology and the diurnal variation of the occurrence frequency of CC (CTT < 30 C at top), MC (CTT between 30 C and 0 C at top), and WC (CTT > 0 C at top) are investigated, and the differences between the southwestern and southeastern China are compared. The major conclusions are summarized below. [25] 1. The summer CC occurs more frequently in the plateau and coastal regions without obvious zonal differences over southern China. However, summer mean frequency of MC and WC show evident west-east contrast. The summer MC occurs more frequently in the southwestern China, whereas WC occurs more frequently in the southeastern China. [26] 2. The summer mean frequency of CC, MC and WC shows different diurnal variations in southern contiguous China. The CC exhibits large diurnal amplitude in frequency and occurs most frequently in the late afternoon in most regions. The frequency of MC shows relatively weaker diurnal amplitude and presents a dominant nocturnal maximum during the day. The WC occurs more (less) frequently in the daytime (nocturnal hours) and reaches the peak around noon. [27] 3. Despite the similarity, the diurnal features of summer mean CC and MC frequency in the southwestern and southeastern China present remarkable regional differences, whereas the diurnal variation of WC shows no obvious zonal contrasts over most of southern China. The CC occurs more frequently in the early morning (late afternoon) in the west (east) region. In the west region, the frequency of CC also has a weak secondary diurnal peak. The frequency of MC shows midnight to early morning maxima in the west region and an evening to midnight maxima in the east region. 9of11

10 [28] 4. The diurnal variations of CC and MC frequencies exhibit evident seasonal changes, whereas the frequency of WC reaches the diurnal maxima around noon all over the year. The late afternoon peak of CC frequency appears in almost all months. In the warmer season, the late afternoon diurnal maximum dominates, whereas in the cold seasons, there is also a midnight to late evening secondary peak. The MC frequency reaches the diurnal peak in the early evening in summer and in the late evening in other seasons. The seasonal changes of WC frequency only present in its diurnal amplitude, which is smaller in warmer seasons and larger in cold seasons. [29] The results of this study indicate that the CC and MC greatly contribute to the different diurnal variations of summer rainfall between the southwestern and southeastern China as revealed by both station observation and TRMM products. The MC could be associated with the stratiform clouds, which do not extend highly in the vertical direction. As revealed by Yu et al. [2010], the highest value in the stratiform rainfall rate profiles occurs in the midnight to early morning (late afternoon) in the southwestern (southeastern) China. The analyses of our study support their conclusions that the late-night rainfall peak in the southwestern China could be largely attributed to the stratiform precipitation. For the CC, it occurs most frequently in the late afternoon in both the west and east regions, coinciding with the conclusions of previous studies that these clouds are closely related to convective activities that are strongly modulated by local thermal forcing. However, it is also noted that in southwestern China, especially the Sichuan basin, which locates downstream the Tibetan Plateau, the cold cloud also occurs frequently in the nocturnal hours because the atmospheric stratification is relatively stable during the day. The CC frequency is relatively small in the afternoon in this region (Figure 7a), indicating the repression of the deep thermal convection during the daytime, and it could be related to the stable lower troposphere caused by the large cloud optical depth [Yu et al., 2004; Li et al., 2008]. The other factors, such as large-scale dynamical forcing caused by the clockwise rotation of the low-level wind and the downward upstream cold temperature advection [Kuo and Qian, 1981; Chen et al., 2010], may play important roles in modulating the nocturnal convections in these regions. [30] By comparing our results with those of Yu et al. [2010], it is also noted that the distribution of diurnal variation of the CC (MC) shows great similarity with that of convective (stratiform) rainfall detected by TRMM PR, especially the diurnal variation of precipitation profiles. Because the IR BT is a measure of cloud and does not always indicate the rainfall [Wang et al., 2004], it is understandable that there are also some phase differences between the clouds and rainfall and precipitation profiles. However, the similarity indicates that the BT from FY-2 series is potentially useful for the study of the diurnal variation of cloud and rainfall in southern contiguous China, which may further extend our knowledge of the evolution of the regional cloud and rainfall as the geostationary orbit satellites can provide much more information than the polar orbit satellites such as TRMM PR. Nevertheless, the present study only investigates the diurnal variations of clouds and the seasonal changes. 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