Diurnal Variations of Different Cloud Types and the Relationship

atmosphere

Article Diurnal Variations of Diﬀerent Cloud Types and the Relationship between the Diurnal Variations of Clouds and Precipitation in Central and East China

Cuicui Gao 1,2, Yunying Li 1,* and Haowei Chen 2

1 College of Meteorology and Oceanography, National University of Defense Technology, Nanjing 211101, China; [email protected] 2 Shaoguan Meteorological Oﬃce, Shaoguan, Guangdong 512000, China; [email protected] * Correspondence: [email protected]

Received: 19 April 2019; Accepted: 25 May 2019; Published: 3 June 2019

Abstract: In this paper, the diurnal variations of various clouds are analyzed using hourly cloud observations at weather stations in China from 1985 to 2011. In combination with merged hourly precipitation data, the relationship between the diurnal variations of clouds and precipitation in the summers from 2008 to 2011 are studied. The results show that the occurrence frequencies of total cloud and various cloud types exhibit significant diurnal variations. The diurnal variations of the occurrence frequencies of altocumulus and stratocumulus show a bimodal pattern, with peaks appearing in the early morning and late afternoon. The early morning peaks of altocumulus and stratocumulus appear earlier in the summer than in the other seasons, while the late afternoon maxima show an opposite trend. The occurrence frequency of nimbostratus peaks in the morning between 07 and 12 LST (local solar time), and the peak value lags 2 to 3 h from west to east along the Yangtze River valley; meanwhile, the diurnal variation shows no clear differences caused by changes in the latitude or seasons. Cumulus shows an afternoon (14 LST) maximum, while cumulonimbus peaks in the late afternoon during 16–20 LST, and both of them present a great diurnal range. Cirrus usually reaches its peak at 17–18 LST, and it differs by 1 to 2 h with a change in the latitude. The results of the study first show that the diurnal variations of precipitation among different regions are dominated by different clouds. The upper reaches of the Yangtze River valley present a midnight precipitation maximum that is mainly dominated by cumulonimbus. For the middle reaches of the Yangtze River valley impacted by nimbostratus, the precipitation peaks in the early morning. In South and Northeast China, the precipitation peaks in the afternoon and is determined by the diurnal variations of convective clouds. In the region between the Yangtze River valley and Yellow River valley, the precipitation peaks in the early morning and afternoon; the early morning peak is mainly determined by stratiform clouds, while the afternoon peak is closely related to convective clouds.

Keywords: clouds; precipitation; diurnal variation

1. Introduction Diurnal variations are the most basic period of change in the Earth’s climate system, the most important driving force of which is solar radiation. Various meteorological parameters, such as the temperature, wind, pressure and precipitation, all show significant diurnal variations. Rutledge noted that stratiform and convective precipitation are produced by different cloud microphysical processes [1]. The formation mechanism of precipitation is complex, but it ultimately originates from clouds. Clouds are the external manifestation of dynamic and thermodynamic processes in the atmosphere, and there are many differences in the nature of precipitation sourced from different types of clouds. The

Atmosphere 2019, 10, 304; doi:10.3390/atmos10060304 www.mdpi.com/journal/atmosphere Atmosphere 2019, 10, 304 2 of 16 diurnal variation of clouds reflects the changes in the internal movements throughout the atmosphere. However, the diurnal cycle of clouds also plays a feedback role in the movements of the atmosphere in addition to the radiation process and the water cycle [2]. Furthermore, the diurnal variation of clouds also reflects the atmospheric stability and changes in the weather. Therefore, studying the diurnal variation of clouds is of significance for understanding the diurnal cycle of the internal movements throughout the atmosphere. In addition, understanding the relationship between the diurnal variations of clouds and precipitation is beneficial to provide a physical basis and observe relevant facts for cloud parameterization schemes in weather and climate models, which is helpful for improving the ability to simulate weather and climate models. Precipitation exhibits significant diurnal variation characteristics. Consequently, many scholars have carried out a great deal of research [3–9]. Yu et al. noted that the summer precipitation over contiguous China has large diurnal variations with considerable regional features [10]. Over southern inland China and northeastern China, the summer precipitation peaks in the late afternoon, while the precipitation in some regions peaks between midnight and the early hours of the morning. Zhou et al. verified the above results by comparing satellite to rain gauge observations, showing that the diurnal phase of rainfall frequency and intensity were similar to those of the rainfall amount in southern China [11]. A study by Li et al. showed the clear differences in the diurnal cycle of precipitation between southwestern China and southeastern China [12]. The diurnal cycle of the annual mean precipitation in southwestern China tends to reach a maximum either at midnight or during the early morning, while the precipitation in eastern China peaks in the late afternoon. Yuan et al. analyzed the sub-seasonal characteristics of the diurnal variation of summer monsoon rainfall over eastern central China and found that the early-morning diurnal peaks experience sub-seasonal movements similar to those of the monsoon rain belt [13]. The aforementioned scholars have obtained many meaningful conclusions about the diurnal variation of precipitation. Moreover, since precipitation originates from clouds, the diurnal variation of precipitation is closely related to the diurnal variations of different types of clouds, and therefore, studying the relationships between the diurnal variations of clouds and precipitation could be helpful for further understanding the mechanism of precipitation. William et al. presents observational evidence in support of the existence of a large diurnal cycle of oceanic, tropical, deep cumulus convections [14]. Zheng et al. used satellite infrared temperature of black body (TBB) data to explore the climatological characteristics of deep convection over southern China and the adjacent seas during the June-August periods of 1966–2007 [15]. They found that sea-land and mountain-valley breezes accounted for the propagation of deep convection from sea to land in the afternoon and from land to sea after midnight. Based on hourly infrared brightness temperature data acquired by the FY-2C satellite to classify clouds into cold clouds, middle clouds and warm clouds according to the cloud top temperature, Chen et al. analyzed the diurnal variations of three types of clouds in southern China during the summer periods from 2005 to 2008 and the seasonal changes in the diurnal variations of those clouds [16]. Fujinami et al. studied the diurnal variations of high clouds and precipitation over the Tibetan Plateau, where observation stations are sparse, using satellite observations. He noted that the cloud cover frequency for high clouds increased in the afternoon along the ridges and reached a maximum near 18 LST, after which high cloud cover frequencies moved over the valley and persisted until early morning [17]. In addition, a similar evolution occurred in the frequency of rainfall. Zhao et al. based on MODIS observations, showed the statistical characteristics of cloud properties, along with the difference between morning and afternoon [18]. These conclusions have promoted the understanding of the diurnal variations of clouds; however, the corresponding research is relatively fragmented, and the relationship between the diurnal variations of clouds and precipitation has not yet been established. Therefore, this paper aims to analyze the climatic characteristics of the diurnal changes of clouds over China in addition to the relationship between the diurnal variations of clouds and precipitation to deepen the existing understanding of the correlation between clouds and precipitation and to provide the basis for a simulation of the diurnal variations of clouds and precipitation in weather and climate models. Atmosphere 2019, 10, 304 3 of 16

2. Data and Analysis Method

2.1. Data In this study, hourly cloud data during 1985–2011 are acquired from 114 surface observation stations after a quality-control procedure is performed. Since the distribution of stations to the west of 105◦ E is sparse, this study mainly analyzes the diurnal variations of clouds over eastern and central China. The dataset includes the information of total cloud (TC), and 10 cloud genera (cirrus (Ci), cirrocumulus (Cc), cirrostratus (Cs), altocumulus (Ac), altostratus (As), nimbostratus (Ns), stratocumulus (Sc), stratus (St), cumulus (Cu), and cumulonimbus (Cb)). Due to the light intensity and visual reasons, artificial observations on the ground will produce some random observation errors, however, many years of climatological normal results can significantly reduce the instabilities of artificial observations. In addition, the reliability of the dataset has been tested and verified [19]. From the perspective of surface observations, the diurnal variation of low clouds is relatively accurate. High clouds and middle clouds are sometimes blocked by low clouds, and the article therefore analyzes the diurnal variation of a certain type of high (middle) cloud when it appears. Hourly merged precipitation data during 2008–2011 were obtained from the National Meteorological Information Center of the China Meteorological Administration with a spatial resolution of 0.1 0.1 . These hourly precipitation data were merged with more than 30,000 automatic weather ◦ × ◦ stations over China in addition to global Climate Prediction Center morphing technique (CMORPH) precipitation estimates with a temporal interval of 30 min and a resolution of 8 km provided by the American Climate Prediction Center. The merged precipitation product effectively combines the advantages of surface precipitation observations with satellite retrieved precipitation products. The overall error of the product is less than 10%, which is better than the same type of product worldwide [20]. The quality of the merged precipitation products in the sparse station areas still needs improvement. However, the stations in eastern and central China studied in this paper are densely and effectively distributed, and the precipitation values are accurate since station data were utilized as the main data source in the fusion process. Since 56.5 percent of the precipitation falls during the summertime throughout most of China [21], this paper mainly analyzes the relationship between the diurnal variations of clouds and precipitation during the summer. The diurnal variation of clouds is the average of the data from the stations from 1985 to 2011. Since the precipitation data range from 2008 to 2011, to match the precipitation data, the cloud data are truncated from 2008 to 2011, when the relationship between the diurnal variations of clouds and precipitation is analyzed.

2.2. Analysis Method The diurnal variations of the meteorological parameters are the changes in the meteorological parameters with the local solar time (LST) [5,22]. Therefore, statistical work on the diurnal variations of clouds and precipitation should be conducted according to the local solar time. The time system for the hourly cloud data used in this paper corresponds to China Standard Time (CST), and the method used to convert CST into LST is as follows: lon + 7.5 T1 = 8 + T (1) 15 −   T1 0 < T1 24  ≤ LST =  24 + T1 T1 0 (2)  ≤  T1 24 T1 > 24 − where lon represents the longitude of the station, and the unit of lon is degree, T denotes CST, LST is the local solar time, and [] indicates an operation to change the number into an integer. Atmosphere 2019, 10, x FOR PEER REVIEW 4 of 16

at a certain time within the analysis period, rec is the number of observations at a certain time, and the definition of the occurrence frequency is suitable for all types of cloud, including TC. The maximum occurrence frequency of a cloud from 01 to 24 LST in a day is the daily peak. The amplitude of the diurnal variation of clouds reflects the magnitude of the maximum and minimum occurrence frequency of cloud in 24 h and is defined as A = cldmax/cldavg, where A is the amplitude, cldmax is the Atmospheredaily2019 peak, 10 ,of 304 the cloud occurrence frequency, and cldavg is the daily occurrence frequency of the4 of 16 cloud. When analyzing the diurnal variation of clouds in different seasons, to eliminate the differences in the occurrence frequencies of cloud among the different seasons, this paper calculates Thethe anomalies time system in the for occurrence the precipitation frequencies data of clouds used in each this papermonth from corresponds 01 to 24 LST. to Greenwich Mean Time (GMT), and the method used to convert GMT into LST is similar to CST, and it just don’t need to subtract3. Climatic the time Characteristic zone in the of first Diurnal step. TheVariation time usedof Clouds hereafter in this study is LST. The occurrence frequency of a cloud is defined as F = (reccld/rec) 100%, where F represents the 3.1. Diurnal Variation of Different Types of Clouds × occurrence frequency of a cloud at a certain time, reccld represents the number of cloud occurrences at a certain timeTo obtain within an overall the analysis understanding period, recof the is thediurnal number variations of observations of the total clouds at a certain (TC) and time, of the and the definitionvarious of types the occurrence of clouds over frequency central and is suitableeastern China, for all this types paper of first cloud, analyzes including the diurnal TC. The variations maximum occurrenceof the average frequency occurrence of a cloud frequencies from 01 of tothe 24 TC LST and in various a day clouds is the dailyat 114 peak.stations The over amplitude central and of the eastern China. The diurnal variation of the frequency of TC shows a bimodal pattern; the main peak diurnal variation of clouds reflects the magnitude of the maximum and minimum occurrence frequency appears in the late afternoon, and the second peak appears in the early morning (Figure 1a). During of cloud in 24 h and is defined as A = cld /cld , where A is the amplitude, cld is the daily the day, the occurrence frequency of the maxTC is signavgificantly greater than that of nocturnalmax clouds. peakAfter of the 04 cloudLST, the occurrence occurrence frequency,frequency of and the TC cld increasesavg is the rapidly daily occurrenceand reaches a frequency peak at 07 LST. of the The cloud. Whenfrequency analyzing of thethe diurnalTC changes variation slightly of during clouds 07–09 in di LST,fferent and seasons, it begins to to eliminate increase again the di afterfferences 09 LST in the occurrenceand reaches frequencies the main of peak cloud at among 14–15 LST. the diAcff erentalso shows seasons, a diurnal this paper variation calculates with a thebimodal anomalies pattern in the occurrencewith peaks frequencies at 07 LST of and clouds 18 LST, in each and the month morning from peak 01 to is 24 larger. LST. As occurs at a frequency of less than 3.5% but at a relatively high frequency in the afternoon. 3. ClimaticThe Characteristic diurnal cycle of of the Diurnal occurrence Variation frequency of Cloudsof Ns is unimodal, and it occurs at approximately 08 LST. Ns is a component of frontal clouds, which are mainly formed as deep, moist air rises along 3.1. Diurnalthe front Variation surface; the of Direlativefferent humidi Typesty of of Clouds the air is larger in the morning and is saturated more easily and condensed, and thus, Ns appears more frequently in the early morning. The diurnal variation of To obtain an overall understanding of the diurnal variations of the total clouds (TC) and of the St is consistent with that of Ns, and its diurnal peak appears at 08 LST. St, whose formation various types of clouds over central and eastern China, this paper first analyzes the diurnal variations mechanism is the same as that of fog, is generally formed by fog rising. Fog usually appears during of thethe average night until occurrence 08 LST, and frequencies it especially of occurs the TC thro andughout various most clouds of the morning; at 114 stations St also shows over central such a and easternpattern China. in the The morning. diurnal The variation diurnal of variation the frequency of the occurrence of TC shows frequency a bimodal of Sc pattern;presents thea bimodal main peak appearspattern in the with late a main afternoon, diurnal andpeak the at 06 second LST and peak a second appears peak in at the 17 LST. early As morning a typical (Figureboundary1a). layer During the day,cloud, the the occurrence diurnal variation frequency of Sc of reflects the TC the is significantlydiurnal variation greater of the than boundary that of layer. nocturnal The low clouds. Aftertemperature 04 LST, the in occurrence the boundary frequency layer and of the the TC stable increases atmospheric rapidly stratification and reaches in athe peak morning at 07 LST.are The frequencybeneficial of the for TC water changes vapor slightlybelow the during top of 07–09the boun LST,dary and layer it begins to accumulate to increase and againform Sc. after When 09 LSTthe and reachesatmosphere the main stability peak at deteriorates, 14–15 LST. the Ac conditions also shows are no a diurnallonger favorable variation to the with formation a bimodal of stratiform pattern with clouds and the occurrence of Sc. As the solar radiation weakens in the evening and the atmosphere peaks at 07 LST and 18 LST, and the morning peak is larger. As occurs at a frequency of less than 3.5% stratification tends to become stable, the frequency of Sc starts to increase again and reaches the but at a relatively high frequency in the afternoon. second peak at approximately 19 LST.

Figure 1. The diurnal variation of the occurrence frequencies (%) of various cloud types in central and east China during 1981–2011: (a) TC, Ac, As, (b) Ns, Sc, St, (c) Cu, Cb, Ci(the horizontal axis represents the local solar time, and the vertical axis represents the occurrence frequency of clouds).

The diurnal cycle of the occurrence frequency of Ns is unimodal, and it occurs at approximately 08 LST. Ns is a component of frontal clouds, which are mainly formed as deep, moist air rises along the front surface; the relative humidity of the air is larger in the morning and is saturated more easily and condensed, and thus, Ns appears more frequently in the early morning. The diurnal variation of St is consistent with that of Ns, and its diurnal peak appears at 08 LST. St, whose formation mechanism is the same as that of fog, is generally formed by fog rising. Fog usually appears during the night until Atmosphere 2019, 10, 304 5 of 16

08 LST, and it especially occurs throughout most of the morning; St also shows such a pattern in the morning. The diurnal variation of the occurrence frequency of Sc presents a bimodal pattern with a main diurnal peak at 06 LST and a second peak at 17 LST. As a typical boundary layer cloud, the diurnal variation of Sc reflects the diurnal variation of the boundary layer. The low temperature in the boundary layer and the stable atmospheric stratification in the morning are beneficial for water vapor below the top of the boundary layer to accumulate and form Sc. When the atmosphere stability deteriorates,Atmosphere 2019 the, conditions10, x FOR PEER areREVIEW no longer favorable to the formation of stratiform clouds5 of and 16 the occurrence of Sc. As the solar radiation weakens in the evening and the atmosphere stratification tends to becomeFigure 1. The stable, diurnal the variation frequency of the ofoccurrence Sc starts frequenc to increaseies (%) of again various and cloud reaches types in the central second and peak at approximatelyeast China 19 LST.during 1981–2011: (a) TC, Ac, As, (b) Ns, Sc, St, (c) Cu, Cb, Ci(the horizontal axis represents The diurnalthe local variationsolar time, and of Cu the isvertical alsounimodal axis represents (Figure the occurrence1c). During frequency the daytime, of clouds). as the solar radiation graduallyThe enhances, diurnal the variation atmosphere of Cu temperatureis also unimodal around (Figure and above1c). During the ground the daytime, surface as gradually the solar rises whileradiation the instability gradually of theenhances, atmospheric the atmosphere stratification temperature increases, around and and thus, above the the occurrence ground surface frequency of Cugradually begins to rises increase while beginning the instability in the of early the morningatmospheric and stratification reaches a peak increases, at 14 LST,and afterthus, whichthe it decreases.occurrence The diurnalfrequency variation of Cu begins of the to occurrenceincrease beginning frequency in the of early Cb shows morning a unimodal and reaches pattern a peak asat well. The occurrence14 LST, after frequency which it decreases. of Cb is high The atdiurnal 16–20 va LST,riation and of its the main occurrence diurnal frequency peak appears of Cb atshows 16–17 a LST, whichunimodal is 4–6 h behindpattern as the well. time The when occurrence Cu reaches frequency its diurnal of Cb is peak, high indicatingat 16–20 LST, that and it its takes main some diurnal time for shallowpeak convection appears at clouds16–17 LST, to develop which is into 4–6 deep h behind clouds. the Thetime frequency when Cu ofreaches Ci increases its diurnal from peak, 04 to 05 LST continuously,indicating that andit takes it reaches some time a maximum for shallow at co 17nvection LST, after clouds which to develop it rapidly into decreases deep clouds. after The 18 LST. frequency of Ci increases from 04 to 05 LST continuously, and it reaches a maximum at 17 LST, after Sometimes, Ci acts as an anvil to the development of Cb to a mature stage, and thus, there is a lag in which it rapidly decreases after 18 LST. Sometimes, Ci acts as an anvil to the development of Cb to a the timemature required stage, and for Cu,thus, Cb there to Ciis a to lag reach in the their time diurnalrequired peaks.for Cu, Cb to Ci to reach their diurnal peaks. FigureFigure2 shows 2 shows the the times times of theof the diurnal diurnal peaks peaks ofof didifferentfferent clouds clouds and and the the spatial spatial distribution distribution of of the amplitudesthe amplitudes of their of their diurnal diurnal variations variations over over centralcentral and eastern eastern China. China. The The figure figure shows shows the time the time whenwhen the occurrence the occurrence frequency frequency peaks peaks at at each each station station in the the form form of of a aclock, clock, and and the thenumber number indicates indicates the amplitudethe amplitude of the of the diurnal diurnal variation variation of of the the cloudcloud occurrence frequency. frequency. Figure Figure 2a 2showsa shows that thatthe the diurnaldiurnal peak peak of the of the TC TC appears appears from from the the afternoon afternoon to the the late late afternoon, afternoon, but but it appears it appears in the in early the early morning at some stations. For example, at most of the stations to the north of 32° N in China, the morning at some stations. For example, at most of the stations to the north of 32◦ N in China, the diurnal peak appears at 15–18 LST; meanwhile, over the Liaodong Peninsula and Shandong diurnal peak appears at 15–18 LST; meanwhile, over the Liaodong Peninsula and Shandong Peninsula, Peninsula, the diurnal peak appears a little earlier at 13–14 LST. However, to the south of 32° N, the the diurnal peak appears a little earlier at 13–14 LST. However, to the south of 32 N, the diurnal peak diurnal peak appears in two periods at 14–18 LST and 06–07 LST. For instance, the◦ diurnal peak of appearsthe TC in twoin the periods southeastern at 14–18 coastal LST areas and 06–07of China LST. mainly For instance,occurs in the the early diurnal morning peak at of 06–07 the TCLST. in the southeasternThe amplitude coastal of areasthe occurrence of China frequency mainly occurs of the TC in theis large early in morningnorthern China at 06–07 (approximately LST. The amplitude 1.2– of the1.3), occurrence while that frequency at most stations of the in TC southern is large China in northern is approximately China (approximately 1.1, indicating that 1.2–1.3), the diurnal while that at mostvariations stations of in the southern occurrence China frequencies is approximately of clouds 1.1,in northern indicating China that are the greater diurnal than variations those in of the occurrencesouthern frequencies China. of clouds in northern China are greater than those in southern China.

Figure 2. Cont. Atmosphere 2019, 10, 304 6 of 16

Atmosphere 2019, 10, x FOR PEER REVIEW 6 of 16

FigureFigure 2. Spatial 2. Spatial distributions distributions of theof the phases phases and and amplitudes amplitudes ofof thethe diurnaldiurnal cycles of of various various clouds: clouds: (a) (a) TC, (b) Ci, (c) Ns, (d) Sc, (e) Cu, and (f) Cb. The directions of the vectors denote the local solar time TC, (b) Ci, (c) Ns, (d) Sc, (e) Cu, and (f) Cb. The directions of the vectors denote the local solar time (LST) of the maximum cloud occurrence frequency (phase clock in Figure 2b), while the length of the (LST) of the maximum cloud occurrence frequency (phase clock in (b)), while the length of the vectors vectors represent the amplitude of the diurnal cycle, the reference arrow represents the amplitude of represent the amplitude of the diurnal cycle, the reference arrow represents the amplitude of diurnal diurnal cycle is 1.0. cycle is 1.0. According to this study, the diurnal peak of Ci appears at 17–18 LST consistently at each station According(Figure 2b), toand this the study,diurnal the variation diurnal amplitude peak of Ciis larger appears in South at 17–18 China LST and consistently in the Sichuan at Basin. each station (Figure2b),The and diurnal the diurnal peak of variationNs occurs in amplitude the morning is larger(08–09 inLST) South (Figure China 2c), andbut those in the of Sichuansome stations Basin. Thein Shanxi diurnal Province, peak of western Ns occurs Henan in theProvince, morning and (08–09 Hubei LST)Province (Figure and2 inc), the but Sichuan those of basin some occur stations in Shanxiaround Province, noon (11–14 western LST). The Henan Meiyu Province, front near and the Yangtze Hubei ProvinceRiver valley and is also in the a frequent Sichuan area basin of Ns occur aroundformation, noon (11–14 yet the LST). diurnal The variation Meiyufront amplitude near theof Ns Yangtze in this Riverarea is valley small. isSc also is the a frequentcloud with area the of Ns formation,highest yet occurrence the diurnal frequency variation over amplitudeChina. Its diurnal of Ns peak in this over area most is small.stations Scoccurs is the at cloud06–07 withLST the (Figure 2d), but those in western Inner Mongolia and North China usually occur at 18–19 LST. highest occurrence frequency over China. Its diurnal peak over most stations occurs at 06–07 LST The diurnal peak of Cu over all stations appears in the afternoon (12–14 LST) (Figure 2e). The (Figure2d), but those in western Inner Mongolia and North China usually occur at 18–19 LST. diurnal variation of Cu in most areas throughout China is large, and its amplitudes are basically Theabove diurnal 2.9, while peak the of diurnal Cu over variation all stations of Cu appears in South in China the afternoon is relatively (12–14 small LST) with (Figure amplitudes2e). The diurnalbetween variation 1.9 and of Cu 3.1. in The most frequency areas throughout of Cb usually China reaches is large, a peak and value its amplitudes at 16–20 LST are (Figure basically 2f); above 2.9, whilehowever, the diurnalat very few variation stations, of the Cu peak in South occurs China at 02–04 is relativelyLST. In addition, small the with diurnal amplitudes variation between of Cb 1.9 and 3.1.is large The with frequency the second-largest of Cb usually amplitude reaches (approximately a peak value 1.5 at to 16–20 2.7) following LST (Figure that2 off); Cu. however, at very few stations, the peak occurs at 02–04 LST. In addition, the diurnal variation of Cb is large with the second-largest amplitude (approximately 1.5 to 2.7) following that of Cu. Atmosphere 2019, 10, 304 7 of 16

TheAtmosphere analysis 2019, 10 above, x FOR shows PEER REVIEW that there are regional differences among the diurnal peaks of7 Ac of 16 and Sc that are mainly due to the frequent occurrences of both cloud types in the early morning and evening The analysis above shows that there are regional differences among the diurnal peaks of Ac and withSc di ffthaterent are peak mainly times due at to di thefferent frequent stations. occurrenc As Aces of and both Sc occurcloud types more in frequently, the early morning they have and a great impactevening on the with diurnal different variation peak times of at the different TC occurrence stations. As frequency, Ac and Sc occur and thus,more frequently, the TC shows they have a similar diurnala great changing impact patternon the diurnal with these variation two typesof the ofTCclouds. occurrence The frequency, diurnal peak and thus, of Ns the mainly TC shows appears a in the morningsimilar diurnal (07–12 changing LST), and pattern the time with di fftheseerence two in ty thepes diurnal of clouds. peaks The betweendiurnal peak different of Ns stations mainly does not exceedappears 5 in h. the The morning diurnal (07–12 peak LST), of Ci and usually the time appears difference in the in eveningthe diurnal (17–18 peaks LST)between while different that of Cu appearsstations in the does afternoon not exceed but 5 showsh. The diurnal no clear peak regional of Ci usually difference. appears Finally, in the theevening diurnal (17–18 peak LST) of while Cb mainly appearsthatat of 16–20Cu appears LST with in the slight afternoon differences but shows among no clear different regional regions. difference. The diurnalFinally, the variations diurnal ofpeak Cu and of Cb mainly appears at 16–20 LST with slight differences among different regions. The diurnal Cb are both relatively enormous. variations of Cu and Cb are both relatively enormous. 3.2. Diurnal Variation Cycles of Clouds with the Latitude and Longitude 3.2. Diurnal Variation Cycles of Clouds with the Latitude and Longitude FigureFigure3 shows 3 shows the the diurnal diurnal variations variations of the the TC TC and and of various of various cloud cloud types typeswith the with latitude; the latitude; the the zonalzonal averageaverage of 110–120110–120°◦ EE represents represents the the occurre occurrencence frequency frequency of cloud of cloud types types at each at eachlatitude. latitude. FigureFigure3a illustrates 3a illustrates that that the the diurnal diurnal peak peak of of the the TCTC toto the south south of of 20° 20 N◦ Nappears appears in the in afternoon the afternoon (12–15(12–15 LST), LST), which which is closelyis closely related related to to the the diurnal diurnal variation of of convective convective clouds. clouds. Meanwhile, Meanwhile, the the diurnaldiurnal peak peak of theof the TC TC at at 20–26 20–26°◦ N N (i.e., (i.e., southernsouthern China) appears appears in intwo two periods periods during during the early the early morningmorning (06–08 (06–08 LST) LST) and and afternoon afternoon (11–16 (11–16 LST).LST). This is is probably probably due due to tothe the diurnal diurnal peaks peaks of Ac of and Ac and Sc inSc these in these areas areas that that occur occur in in the the early early morning morning and evening evening and and have have a great a great impact impact on the on diurnal the diurnal variation of the TC occurrence frequency. The diurnal peak of the TC at 26–36° N appears mainly in variation of the TC occurrence frequency. The diurnal peak of the TC at 26–36◦ N appears mainly in the afternoon. The occurrence frequency of the TC to the north of 36° N increases with the latitude the afternoon. The occurrence frequency of the TC to the north of 36 N increases with the latitude with a maximum at 16 LST. ◦ with a maximum at 16 LST.

Figure 3. Diurnal variation cycles of occurrence frequency (%) of clouds with the latitude averaged

over 110–120◦ E: (a) TC, (b) Ci, (c) Ac, (d) Ns, (e) Sc, and (f) Cb (the horizontal axis is the local solar time in hours, and the vertical axis represents the latitude). Atmosphere 2019, 10, x FOR PEER REVIEW 8 of 16 Atmosphere 2019, 10, 304 8 of 16 Figure 3. Diurnal variation cycles of occurrence frequency (%) of clouds with the latitude averaged over 110–120° E: (a) TC, (b) Ci, (c) Ac, (d) Ns, (e) Sc, and (f) Cb (the horizontal axis is the local solar

Thetime occurrence in hours, and frequency the vertical of axis Ci atrepresents 20–32◦ Nthe over latitude). China is small, but the occurrence frequencies over Hainan Province and to the north of 32◦ N are large, and the frequency of the latter area increases The occurrence frequency of Ci at 20–32° N over China is small, but the occurrence frequencies with the latitude (Figure3b). Moreover, there is a slight di fference in the diurnal peak of Ci between the over Hainan Province and to the north of 32° N are large, and the frequency of the latter area increases two areas. The occurrence frequency of Ci over Hainan Province is large (approximately 35%), and the with the latitude (Figure 3b). Moreover, there is a slight difference in the diurnal peak of Ci between peak appears at 06 LST and 18 LST. The diurnal peak of Ci to the north of 32 N occurs at 16–17 LST, the two areas. The occurrence frequency of Ci over Hainan Province is large◦ (approximately 35%), and the occurrence frequency of Ci near 45 N reaches 40% at 16 LST. The occurrence frequency of Ac and the peak appears at 06 LST and 18 LST.◦ The diurnal peak of Ci to the north of 32° N occurs at 16– increases with the latitude from south to north (Figure3c); it reaches a maximum at 28–36 N and then 17 LST, and the occurrence frequency of Ci near 45° N reaches 40% at 16 LST. The occurrence◦ decreases.frequency At of 28–36 Ac increases◦ N, the with diurnal the peaklatitude of Acfrom appears south to at north 07 LST (Figure and 18 3c); LST, it reaches while in a maximum other areas at the diurnal28–36° peak N and of then Ac appears decreases. at approximatelyAt 28–36° N, the 08 diurnal LST, indicating peak of Ac that appears its maximum at 07 LST and daily 18 peakLST, appearswhile at ain di otherfference areas of the 1 to diurnal 2 h with peak achange of Ac appears in the latitude.at approximately 08 LST, indicating that its maximum dailyFigure peak3d appears shows aat higha difference occurrence of 1 frequencyto 2 h with ofa change Ns within in the 26–29 latitude.◦ N. After 04 LST, the high value of the frequencyFigure 3d shows of Ns beginsa high occurrence to expand tofrequency the south of andNs within to the 26 north.–29° N. In After addition, 04 LST, a frequency the high value of 10% expandsof the frequency to the south of Ns to 22begins◦ N at to 08 expand LST and to the to thesout northh and to to 32 the◦ N north. at 09 In LST. addition, After 09a frequency LST, the frequency of 10% of Nsexpands begins to tothe shrink, south to as 22° the N maximum at 08 LST and frequency to the no ofrth Ns to occurs 32° N at 09 LST. LST. After The diurnal 09 LST, peakthe frequency of Ns does notof change Ns begins significantly to shrink, withas the the maximum latitude. frequency The diurnal of Ns peak occurs of Sc at occurs09 LST. in The two diurnal periods peak during of Ns the earlydoes morning not change (approximately significantly 06 with LST) the and latitude. the late The afternoon diurnal peak (18–19 of LST)Sc occurs (Figure in 3twoe). Theperiods diurnal during peak – ofthe Sc over early Hainan morning Province (approximately appears 06 at LST) 05 LST and and the 18late LST afternoon while that(18 19 at 20–28LST) (Figure◦ N over 3e). China The diurnal occurs at peak of Sc over Hainan Province appears at 05 LST and 18 LST while that at 20–28° N over China 06 LST and 19 LST. Moreover, the diurnal peak of Sc at 18–25◦ N lags behind by approximately 1 h. occurs at 06 LST and 19 LST. Moreover, the diurnal peak of Sc at 18–25° N lags behind by The diurnal peak of Cb over Hainan Province is reached at 09 LST while that of Cb to the north of 20◦ Napproximately mainly occurs 1 in h. the The period diurnal of peak 16–20 of LST,Cb over and Hainan its appearance Province timeis reached lags behindat 09 LST with while the that latitude of – (FigureCb to3 thef). Therenorth of is 20° no diN mainlyfference occurs in the in diurnal the period peak of of16 Cu20 withLST, and the latitude,its appearance as it consistently time lags behind occurs with the latitude (Figure 3f). There is no difference in the diurnal peak of Cu with the latitude, as it at 13–14 LST (figure omitted). consistently occurs at 13–14 LST (figure omitted). The diurnal variation cycles of clouds with the longitude are not as clear as that with the latitude. The diurnal variation cycles of clouds with the longitude are not as clear as that with the latitude. Ns shows clear zonal difference over the Yangtze River valley and its south side, where the occurrence Ns shows clear zonal difference over the Yangtze River valley and its south side, where the frequency of Ns is the largest. In the upper reaches of the Yangtze River, the diurnal peak of Ns occurs occurrence frequency of Ns is the largest. In the upper reaches of the Yangtze River, the diurnal peak at 06–09of Ns occurs LST, while at 06–09 the diurnalLST, while peaks the overdiurnal the peak middles over and the lower middle reaches and lower of the reaches Yangtze of River the Yangtze appear at 07–13River LST appear (Figure at 07–134a). The LST occurrence (Figure 4a). frequency The occurrence of Cu isfrequency the largest of Cu in southis the largest China. in And south the China. diurnal variationAnd the of diurnal the occurrence variation frequency of the occurrence of Cu does freque notncy change of Cu with does the not longitude change with (Figure the4 longitudeb) in south China.(Figure The 4b) peak in south occurrence China. frequency The peak appears occurrence in the frequency afternoon appears (13–14 in LST); the however,afternoon at (13–14 22–26 ◦LST);N, the frequencyhowever, of at Cu 22–26° decreases N, the frequency clearly from of Cu west decreases to east. clearly The diurnal from west variations to east. The of the diurnal TC and variations the other typesof the of TC clouds and showthe other no significanttypes of clouds change show with no thesignificant longitude. change with the longitude.

FigureFigure 4. 4.Diurnal Diurnal variation variation cycles cycles ofof cloudsclouds withwith the longitude: ( (aa) )Ns Ns (averaged (averaged over over 26–30° 26–30 N)◦ N) and and (b)(b Cu) Cu (averaged (averaged over over 22–26 22–26°◦ N) N) (the (the horizontalhorizontal axis re representspresents the the longitude, longitude, an andd the the vertical vertical axis axis is is thethe local local solar solar time time in in hours). hours).

3.3.3.3. Diurnal Diurnal Variation Variation Cycles Cycles of of Clouds Clouds withwith thethe SeasonSeason TheThe diurnal diurnal peak peak of of the the TC TC over over China China during during the th springe spring occurs occurs at at approximately approximately 16 16 LST LST (Figure (Figure5 a). In5a). addition, In addition, there there is a clear is a clear diurnal diurnal variation variation of of the the TC TC in in the the summersummer and autumn autumn (i.e., (i.e., from from June June to November). The maximum occurrence frequency occurs at 14 LST while the minimum occurs at 23 LST, and the diﬀerence between them reaches 28%. In addition, the amplitude of the diurnal Atmosphere 2019, 10, x FOR PEER REVIEW 9 of 16

to November). The maximum occurrence frequency occurs at 14 LST while the minimum occurs at 23 LST, and the difference between them reaches 28%. In addition, the amplitude of the diurnal variation of the TC in the winter is small; at 13–15 LST, however, the occurrence frequency is relatively large. In contrast, the occurrence frequency of the TC during the summer reaches a peak time 1–2 h earlier than that during the spring, which may occur because the mean temperature increases more rapidly in the summer than in the spring, and thus, the conditions that are unstable for cloud formation are reached earlier. The diurnal peak of Ci also changes with the season (Figure 5b). Ci reaches its maximum at 16– 17 LST in December and at 18–19 LST in July, thereby lagging 1–2 h between the winter and summer.

AtmosphereThe changes2019 ,in10 ,the 304 peak times of Ac are the most clear from June to September (Figure 5c). The 9peak of 16 times of the two peaks appear from early winter to summer, while the early morning peak appears earlier and the evening peak appears later in the summertime. The diurnal peak times of Ns and Cu variationdo not change of the with TC in the the seasons winter (Figure is small; 5d, at 13–155f), but LST, both however, of the amplitudes the occurrence of their frequency diurnal is variations relatively large.are relatively In contrast, large the in occurrencethe summer. frequency The diurnal of the variation TC during of Sc the with summer the seasons reaches is somewhat a peak time similar 1–2 h earlierto that thanof Ac that (Figure during 5e), the but spring, the minimum whichmay occurrence occur because frequency the always mean temperature appears at 14 increases LST andmore does rapidlynot change in the with summer the seasons. than in the The spring, occurrence and thus, frequenc the conditionsy of Cb thatin the are winter unstable is forvery cloud small, formation and its areseasonal reached variation earlier. is also not clear and therefore is not discussed here.

Figure 5.5. Seasonal variationsvariations inin thethe diurnal diurnal cycle cycle of of anomalous anomalous values values of of the the occurrence occurrence frequencies frequencies of clouds:of clouds: (a )(a TC,) TC, (b ()b Ci,) Ci, (c )(c Ac,) Ac, (d ()d Ns,) Ns, (e ()e Sc,) Sc, (f ()f) Cu Cu (the (the horizontal horizontal axis axis is is thethe locallocal solarsolar timetime in hours, and the vertical axisaxis representsrepresents thethe month).month).

4. TheThe Relationship diurnal peak between of Ci also the changesDiurnal withVariations the season of Clouds (Figure and5b). Precipitation Ci reaches its maximum at 16–17 LST in December and at 18–19 LST in July, thereby lagging 1–2 h between the winter and summer. The4.1. Diurnal changes Variation in the peak of Precipitation times of Ac are the most clear from June to September (Figure5c). The peak timesThe of thestudy two of peaks Yu et appear al. showed from that early the winter regional to summer, differences while in the earlydiurnal morning variation peak of summer appears earlierprecipitation and the is evening significant peak across appears the later Chinese in the ma summertime.inland [10]. The Therefore, diurnal peakin order times to of analyze Ns and the Cu do not change with the seasons (Figure5d,f), but both of the amplitudes of their diurnal variations are relatively large in the summer. The diurnal variation of Sc with the seasons is somewhat similar to that of Ac (Figure5e), but the minimum occurrence frequency always appears at 14 LST and does not change with the seasons. The occurrence frequency of Cb in the winter is very small, and its seasonal variation is also not clear and therefore is not discussed here. Atmosphere 2019, 10, 304 10 of 16

4. The Relationship between the Diurnal Variations of Clouds and Precipitation

4.1. Diurnal Variation of Precipitation The study of Yu et al. showed that the regional differences in the diurnal variation of summer precipitation is significant across the Chinese mainland [10]. Therefore, in order to analyze the relationship between the diurnal variations of clouds and precipitation, the following analysis is based on the regional division of central and eastern China into five sub-regions (Table1) by Yu et al. , who studied the diurnal variation of precipitation.

Table 1. The information of ﬁve regions in eastern China.

Region 1 Region 2 Region 3 Region 4 Region 5

Latitude Range 27–32◦ N 27–30◦ N 23–26◦ N 40–50◦ N 30–40◦ N Longitude 100–107 E 108–113 E 110–117 E 110–130 E 110–120 E Range ◦ ◦ ◦ ◦ ◦ The upper The area between The middle reaches of the Northeast the Yangtze River Region reaches of the South China Yangtze River China and the Yellow Yangtze River (Sichuan Basin) River

The reliability of the hourly merged precipitation data has been verified by Shen et al. [23]. To compare the error of the hourly merged precipitation data and rain-gauge data, we first analyzed the diurnal variation of the precipitation in each of the five regions by using the hourly merged precipitation data from 2008 to 2011 in summer (June-August). The diurnal variation of precipitation in each of the five regions is the average of grid data of each regions. Figure6a shows that the maximum precipitation in the upper reaches of the Yangtze River occurs at midnight (24–02 LST), while the minimum precipitation falls at noon (12 LST). Meanwhile, the precipitation in the middle reaches of the Yangtze River reaches a diurnal peak in the early morning (06–07 LST) and a sub-peak in the afternoon (14–16 LST) (Figure6b). The diurnal peaks of precipitation in South and Northeast China appear in the afternoon (16–17 LST) (Figure6c,d). Although the peak precipitation between the Yangtze River valley and the Yellow River valley (region 5) differs among different stations (figure omitted), the diurnal variation of precipitation in this region is generally bimodal with peaks in the early morning (06–08 LST) and afternoon (16–18 LST). By comparing our results with the diurnal variation characteristics of the precipitation in the five regions from 1991 to 2004 reported by Yu et al. (Figure2 in Yu’s article), it is found that the hourly merged precipitation data can e ffectively reflect the diurnal variation of the precipitation in each region. However, the amounts of precipitation in these data are smaller than those observed by the stations. Therefore, in the following discussion, we mainly analyze the corresponding relationship between the diurnal cycles of clouds and precipitation regardless of the diurnal variation of the precipitation amount. Atmosphere 2019, 10, 304 11 of 16 Atmosphere 2019, 10, x FOR PEER REVIEW 11 of 16

Figure 6.6. DiurnalDiurnal variation variation of of the the 2008–2011 2008–2011 mean mean summer summer precipitation precipitation averaged averaged over the over five the regions five inregions Table 1in (the Table horizontal 1 (the horizontal axis corresponds axis corresponds to the local to solarthe local time, solar units: time, mm units:/h). mm/h). 4.2. The Relationship between the Diurnal Variations of Clouds and Precipitation 4.2. The Relationship between the Diurnal Variations of Clouds and Precipitation The probability occurrence of Ci and Ac with precipitation is relatively small, and thus, they are The probability occurrence of Ci and Ac with precipitation is relatively small, and thus, they are classified as non-precipitation clouds [19]. Therefore, this paper mainly discusses the correspondence classified as non-precipitation clouds [19]. Therefore, this paper mainly discusses the correspondence between the diurnal variations of Ns, Sc, Cu and Cb and that of precipitation. Table2 shows the between the diurnal variations of Ns, Sc, Cu and Cb and that of precipitation. Table 2 shows the correlation coefficients between the diurnal variations of various clouds and precipitation, where P_Ns correlation coefficients between the diurnal variations of various clouds and precipitation, where represents the correlation coefficient between the diurnal variation of the occurrence frequency of Ns P_Ns represents the correlation coefficient between the diurnal variation of the occurrence frequency and that of precipitation, and it has a similar meaning for P_Sc, P_Cu and P_Cb, while the clouds of Ns and that of precipitation, and it has a similar meaning for P_Sc, P_Cu and P_Cb, while the are different. clouds are different. Table 2. Correlation coefficient between the diurnal cycle of precipitation and the occurrence frequency ofTable clouds 2. inCorrelation summer. coefficient between the diurnal cycle of precipitation and the occurrence frequency of clouds in summer. Precipitation_Cloud Region 1 Region 2 Region 3 Region 4 Region 5 Precipitation_Cloud Region 1 Region 2 Region 3 Region 4 Region 5 P_Ns 0.82 0.51 0.16 0.13 0.17 P_Ns − −0.82 0.51 0.16 − −0.13 0.17 P_Sc 0.82 0.42 0.51 0.27 0.13 P_Sc 0.82 0.42 − −0.51 − −0.27 0.13 P_Cu 0.78 0.04 0.70 0.55 0.05 P_Cu − −0.78 0.04 0.70 0.55 0.05 P_Cb 0.90 0.57 0.68 0.88 0.09 P_Cb 0.90 − −0.57 0.68 0.88 0.09 Note: Bold font indicates a 0.05 significance level. Note: Bold font indicates a 0.05 significance level.

Figure7 7a1a1 shows shows that that thethe diurnaldiurnal variationvariation of of CbCb isis inin goodgood agreementagreement withwith thatthat ofof precipitationprecipitation inin thethe upperupper reachesreaches ofof thethe YangtzeYangtze River;River; bothboth reachreach aa peakpeak aroundaround midnightmidnight withwith aa signiﬁcantsignificant positivepositive correlationcorrelation coecoefficientﬃcient of of 0.90 0.90 (Table (Table2). 2). While While there there is a is clear a clear negative negative correlation correlation between between the diurnalthe diurnal variations variations of Cu of and Cu precipitation,and precipitation, there isthere also is a consistentalso a consistent correlation correlation between between the diurnal the variationsdiurnal variations of Sc and of precipitationSc and precipitation (Figure (Figure7a2), but 7a2), the but diurnal the diurnal peak of peak Sc occurs of Sc occurs slightly slightly later than later thatthan of that precipitation. of precipitation. This showsThis shows that the that diurnal the diurnal variation variation of precipitation of precipitation in the in upper the upper reaches reaches of the of the Yangtze River relies upon the diurnal variations of Cb and Sc. Moreover, the correspondence between the diurnal variations of Cb and precipitation is better, and Cb may evolve into Sc after Atmosphere 2019, 10, 304 12 of 16

Yangtze River relies upon the diurnal variations of Cb and Sc. Moreover, the correspondence between the diurnal variations of Cb and precipitation is better, and Cb may evolve into Sc after precipitation falls; thus, the diurnal peak of Sc lags behind that of precipitation by approximately 2 h. Atmosphere 2019, 10, x FOR PEER REVIEW 13 of 16

Figure 7. Diurnal variations ofof thethe standardizedstandardized occurrence occurrence frequencies frequencies of of Cu, Cu, Cb Cb and and precipitation precipitation (in (ina1– Figuree1) and 7a1 of the–7e1 standardized) and of the occurrencestandardized frequencies occurrence of Ns,frequencies Sc and precipitation of Ns, Sc and (in precipitationa2–e2) (the black (in Figuresolid line 7a2 corresponds–7e2) (the black to precipitation, solid line corresponds while the to blue precipitation, dotted line while and red the dashed blue dotted line represent line and red the dashedoccurrence line frequencies represent the of Cuoccurrence and Cb, respectively,frequencies inofa1 Cu–e1 and; meanwhile, Cb, respectively, the blue dottedin Figure line 7a1 and–7e1 red; meanwhile,dashed line representthe blue dotted the occurrence line and red frequencies dashed lin ofe Ns represent and Sc, respectivelythe occurrence in a2 frequencies–e2). of Ns and Sc, respectively in Figure 7a2–7e2). The precipitation in the middle reaches of the Yangtze River mainly occurs in the early morning 5.(Figure Discussions7b1). There and Conclusions is a sub-peak at approximately 15 LST, the occurrence of which corresponds to the sub-peaksBased on ofhourly both Cucloud and observations Cb, indicating at weather that the afternoonstations in rainfall combination in the middlewith merged reaches hourly of the precipitation data, the diurnal climatic features of different types of clouds in central and eastern China are revealed and the relationship between the diurnal variations of clouds and precipitation are analyzed. The primary conclusions are as follows: The TC and the occurrence frequencies of all types of clouds show significant diurnal variations. The diurnal variation of the TC shows a bimodal pattern with a main peak in the late afternoon. Ac, Sc and Ci contribute the most to the diurnal variation of the TC. The diurnal cycles of the occurrence Atmosphere 2019, 10, 304 13 of 16

Yangtze River mainly originates from Cu and Cb. Figure7b2 shows that the corresponding relationship between the diurnal variations of precipitation and stratiform clouds in the middle reaches of the Yangtze River is good, and there is a notable positive correlation between precipitation and Ns with a correlation coefficient of 0.51. These results show that the peak of early morning rainfall in the middle reaches of the Yangtze River is mainly caused by stratiform clouds, while the sub-peak of afternoon rainfall mainly depends upon Cu and Cb. The diurnal peaks of Cu, precipitation and Cb appear sequentially over southern China (Figure7c1). The peak frequency of Cu occurs at 12 LST, while those of precipitation and Cb occur at 16 LST and 20 LST, respectively. In addition, the occurrence frequency of Cu decreases during 12–16 LST, but those of precipitation and Cb rapidly increase during this period, implying that Cu begins to produce precipitation during this period and that a part of Cu develops vigorously into Cb under the favorable conditions. However, the corresponding relationship between the diurnal variations of stratiform clouds and precipitation is poor over South China (Figure7c2). In conclusion, both Cu and Cb over southern China show a good correspondence with the diurnal variation of precipitation, indicating that the diurnal variation of precipitation in southern China is mainly attributed to the diurnal variations of convective clouds. There is a significant positive correlation between the diurnal cycles of Cu and precipitation in Northeast China with a correlation coefficient of 0.55. Cb shows a better correspondence with the diurnal variation of precipitation (Figure7d1), with a significant positive correlation of 0.88. For example, the diurnal peaks of Cb and precipitation are both almost synchronous at 09–16 LST, although the diurnal peak of Cb that occurs during 01–09 LST is a little earlier than that of precipitation, and the occurrence frequency of Cb peaks one hour later than that of precipitation. In addition, the correspondence between the diurnal variations of stratiform clouds and precipitation is also poor in these areas (Figure7d2). This shows that the diurnal variation of precipitation in Northeast China is primarily dominated by convective clouds, especially Cb. There are two peaks in the diurnal variation of precipitation in the region between the Yangtze River and the Yellow River (region 5). In the afternoon, Cu, precipitation and Cb reach their peaks sequentially (Figure7e1). Cu peaks at 14 LST, while the occurrence frequencies of precipitation and Cb increase rapidly after 12 LST. Cb and precipitation both peak at 17 LST, indicating that the afternoon peak of precipitation is determined by convective clouds. Figure7e2 shows that the diurnal peaks of Sc and Ns occur at 05 LST and 09 LST, respectively, while the precipitation peaks at 07 LST, meaning that the morning precipitation peak in the area is dominated by stratiform clouds. Therefore, the results show that the diurnal variation of precipitation in the area is comparatively complicated and is closely related to both convective and stratiform clouds. The morning precipitation is closely related to stratiform clouds, while the afternoon precipitation is dominated by convective clouds. All of the above analysis shows that the precipitation peak in central and eastern China usually occurs in the early morning or afternoon. The precipitation peak in early morning is dominated by stratiform clouds, while the peak in the afternoon is dominated by convective clouds.

5. Discussions and Conclusions Based on hourly cloud observations at weather stations in combination with merged hourly precipitation data, the diurnal climatic features of different types of clouds in central and eastern China are revealed and the relationship between the diurnal variations of clouds and precipitation are analyzed. The primary conclusions are as follows: The TC and the occurrence frequencies of all types of clouds show significant diurnal variations. The diurnal variation of the TC shows a bimodal pattern with a main peak in the late afternoon. Ac, Sc and Ci contribute the most to the diurnal variation of the TC. The diurnal cycles of the occurrence frequencies of Ac and Sc also show a bimodal pattern with peaks appearing in the early morning and late afternoon. The occurrence frequency of nimbostratus peaks in the morning between 07 and 12 LST, and the daily difference is weaker. The diurnal peak of Ci usually appears at 17–18 LST in the late Atmosphere 2019, 10, 304 14 of 16 afternoon, Cu peaks in the afternoon, and Cb peaks in the late afternoon during 16–20 LST, and there are slight differences among different regions. In addition, the diurnal variation amplitudes of Cu and Cb are larger, indicating that they both have significant diurnal variations. The diurnal variations of clouds vary with both the latitude and the longitude. There is a slight difference in the time when the occurrence frequency of the TC reaches its peak value at different latitudes, while there is a difference of 1–2 h with a change in the latitude for both Ci and Ac. The peak times of Ns, Sc, Cu and Cb do not change with the latitude. In addition, the peak appearance time of the occurrence frequency of Ns in the Yangtze River valley lags by 1–2 h from west to east. The diurnal features of clouds change with the seasons. All of the types of clouds show the largest diurnal amplitudes in the summer and autumn, and the time when the TC and Ci reach their maximum values tend to lag by 1–2 h from the spring to the summer. Ac and Sc present bimodal patterns with early morning peaks that appear earlier in the summer, while their late afternoon peaks appear later in the summer than in the other seasons. The maximum peak appearance times of Ns and Cu do not change with the seasons. The diurnal variations of precipitation among the different regions are dominated by convective or stratiform clouds. The upper reaches of the Yangtze River valley present a midnight precipitation maximum that is mainly dominated by Cb. Both precipitation and Cb reach a peak around midnight with a significant positive correlation coefficient of 0.90. In other regions, the precipitation usually peaks in the early morning or in the afternoon, and there is a notable positive correlation between the early morning peak of precipitation and stratiform clouds, while the afternoon peak of precipitation shows positive relationship with convective clouds. The diurnal peak in the Yangtze River valley and the region between the Yangtze River valley and the Yellow River valley is mainly dominated by stratiform clouds, while it is mainly determined by convective clouds in South and Northeast China. The peak appearing in the afternoon over the Yangtze River valley is also dominated by convective clouds. Nesbitt and Zipser suggested that the nocturnal rain is often caused by meso-scale convective systems (MCS) rather than isolated convection, and the MCSs are the strongest systems after midnight [24]. Lin et al. indicated that the nocturnal maximum rainfall is enhanced by instability due to nocturnal radiative cooling at cloud top [25]. This explains that the midnight precipitation maximum is mainly dominated by Cb in the upper reaches of the Yangtze River valley. The afternoon peaks of convective clouds and precipitation can be explained by the highest surface air temperature and the strongest instability in the lower troposphere occurring in the afternoon, which are beneficial to the trigger of the convection [22]. The study points out the relationship of diurnal variations of clouds and precipitation among different regions for the first time. However, the research on the physical mechanism of the diurnal variations of clouds and precipitation is not deep. We will study the diurnal variation of cloud microphysical characteristics, and analyze the influence of terrain, atmospheric stability and atmospheric circulation on the diurnal variation of clouds and precipitation in the next step. This is helpful for understanding the mechanism of clouds and precipitation, and for improving the ability to simulate weather and climate models.

Author Contributions: Conceptualization, C.G.; Data curation, C.G. and H.C.; Formal analysis, C.G.; Funding acquisition, Y.L.; Project administration, Y.L.; Validation, H.C.; Visualization, C.G. and H.C.; Writing – original draft, C.G.; Writing – review and editing, Y.L. Funding: This research was funded by the National Key Research and Development Program of China, Grant /Award Number 2018YFC1507604; National Natural Science Foundation of China, Grant/Award Number 41475069. Conﬂicts of Interest: The authors declare no conﬂict of interest. Atmosphere 2019, 10, 304 15 of 16

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