Unique Seasonal Variation in Rainfall Diurnal Features on the Yunnan–Guizhou Plateau

Abstract

Based on hourly precipitation observations, the diurnal variation in precipitation and its seasonal evolution over the Yunnan–Guizhou Plateau (YGP) were analyzed. The results indicate that the seasonal variation in hourly rainfall in the western part of the YGP is unique. The rainfall reaches its hourly maximum during the late afternoon in spring (March–April) and during nighttime in summer (July–August), which contrasts with the pattern in most of eastern China. By further classifying the rainfall into short-duration (1–3 h) and long-duration (more than 6 h) events, the unique seasonal variations in the western YGP are found to mainly be comprised of short-duration rainfall. The long-duration rainfall shares similar diurnal peaks year-round for both the western and eastern parts of the YGP. The short-duration rainfall in the western part of the YGP shows a year-round afternoon peak, which is different from that of the eastern YGP, which peaks from midnight to early morning in spring and in the late afternoon in summer. The surface maximum daily temperature and low-level instabilities reach their annual maximum in spring over the western YGP and are also higher than in other parts of the YGP, together providing favorable conditions for convection to be triggered in spring afternoons over the western YGP.

1. Introduction

Precipitation, one of the atmospheric variables that gains great attention, has been a research hotspot for many years, especially in the context of global warming and the frequent occurrence of extreme weather events [1,2]. Variations in precipitation have attracted more extensive attention. Topography is one of the most important factors affecting precipitation [3], influencing not only the spatial distribution of precipitation but also the process of precipitation initiation and evolution. The Yunnan–Guizhou Plateau (YGP) is located in southwestern China at the intersection of two groups of mountain ranges with north–south and northeast–southwest axes. The altitude in the region ranges from 400 to 3500 m [4]. Under the influence of different topographic and atmospheric system interactions [5], precipitation in the YGP is highly varied spatially [6,7]. In terms of spatial distribution, some researchers [8] found that the spatial distribution of precipitation in the YGP was generally higher in southern Yunnan, with relatively less precipitation in northeastern Yunnan and northwestern Guizhou [7]. In terms of seasonal evolution, the overall precipitation in the YGP is low in winter and spring [1]: the central region is prone to spring droughts, while the eastern part of the YGP receives relatively more precipitation under the influence of the Yunnan–Guizhou quasi-stationary fronts [9]. In recent decades, precipitation in the YGP has shown a trend of increasing in spring and decreasing in summer [1,10].

Diurnal variability is the most basic cycle of the Earth’s climate system, with solar radiation as the main driving force [11], and it is an important indicator of the characteristics of the weather and climate system [12,13]. The diurnal variation in precipitation is the result of the combined influence of atmospheric thermal and dynamical processes on the water cycle [14]. The time of the peak of the diurnal variation in precipitation reflects that precipitation periodically starts and reaches its maximum at a certain time of the day, which can be used to study precipitation mechanisms. Therefore, studying the time of peak precipitation can further improve our understanding of the mechanism of different types of precipitation, and the resulting conclusions can also be used to evaluate the results of numerical models [15]. In recent decades, an increasing number of studies have focused on hourly-scale precipitation refinement characteristics, and these studies have identified obvious nocturnal rainfall characteristics in the YGP. The diurnal variation in the annual average precipitation at most stations shows the main peak from midnight to early morning and the sub-peak in the late afternoon. Only in local areas of southern Yunnan and northwestern Guangxi is there some obvious daytime precipitation, and the peak of precipitation occurs in the late afternoon [15,16]. In addition, Li et al. [17] analyzed the seasonal evolution of the diurnal variation in rainfall in southern China and found that the annual average hourly rainfall in most areas of southern China peaks from midnight to early morning in spring and in the late afternoon in summer. In the southwestern region, rainfall peaks from midnight to early morning throughout the year, with a sub-peak in the late afternoon only in summer. Yu et al. [15] found that the eastern part of the YGP is in the nocturnal peak zone, and its precipitation in the warm season (May–October) has a prominent nocturnal peak without a secondary peak. The peak time of precipitation in the warm season of the YGP occurs at night in most regions, except for the southern part of the YGP where precipitation peaks in the late afternoon. For the spring season, the peak precipitation time in most areas of the YGP is also concentrated from midnight to early morning, and the afternoon is the least precipitation-prone time, with the exception of some stations along the Yunnan–Sichuan border, where precipitation is concentrated in the late afternoon [18].

In order to better understand the physical mechanism of different types of precipitation, Yu et al. [19] divided precipitation into short and long precipitation events according to their duration. Li et al. [17] found that short-duration precipitation in southern China showed the characteristics of afternoon peak in summer and midnight to early morning peak in spring, while long-duration precipitation events showed similar characteristics in southeast China, but in the southwest, short- and long-duration precipitation peaks from midnight to early morning throughout the year. Yuan et al. [20] analyzed the precipitation in central eastern China from May to October, and found that the long-duration precipitation peaked in the early morning, while the short-duration precipitation in this region peaked in the afternoon. For the physical mechanism of precipitation of different durations, short-duration precipitation is often related to the influence of surface temperature and low-level instability. The diurnal peak from midnight to early morning in spring is mainly affected by the effect of radiation cooling at night [17]. The long-duration precipitation is often related to the large-scale circulation background and is significantly affected by the topography. Li et al. [21] analyzed the cold season precipitation of the YGP and found that the northward- and eastward-facing slopes of the YGP uplift the shallow, cold air carried by the northerly and easterly winds, and the terrain effects trigger the precipitation process.

Due to the influence of complex topography, precipitation in the YGP has regional patterns, and there are large seasonal differences in the diurnal variation in precipitation. However, most studies on precipitation in the YGP focus on the warm season, and few studies have considered the characteristics of spring precipitation and the seasonal evolution of diurnal precipitation in the YGP. Therefore, this paper focuses on the seasonal characteristics of the diurnal variation in precipitation in the YGP and the possible precipitation mechanics. The results could improve our understanding of the seasonal evolution of the diurnal variation in precipitation in complex terrain.

The rest of this paper is organized as follows. Section 2 describes the datasets and analysis methods. Section 3 shows separately the seasonal variation in diurnal features of precipitation in the YGP, the seasonal variation in precipitation duration, and the possible reasons for the unique seasonal variations in diurnal precipitation in the YGP. Discussion and conclusions are provided in Section 4.

2. Materials and Methods

In this study, the hourly precipitation and temperature from 362 stations over the YGP are analyzed (dots in Figure 1) during 2001–2015 using the quality-controlled hourly rain gauge records provided by the National Meteorological Information Center of the China Meteorological Administration. Quality controls consisted of a climatological limit value test, station extreme value test, and internal consistency test. An effective precipitation hour is defined as an hour with ≥0.1 mm of hourly precipitation, and the number of hours from the beginning to the end of the precipitation event is used as the duration of the precipitation event [22]. The hourly temperatures in the ERA5 reanalysis data for 500 hPa and 700 hPa for 2001–2015 (0.25° × 0.25°) provided by the European Centre for Medium-Range Weather Forecasts [23] are used to analyze the possible reasons for the unique seasonal differences in the region.

In this paper, March–April is considered spring, and summer is considered July–August. All times are local solar time (LST hereafter). The rainfall diurnal variation is normalized as D(h) = R(h)/Rmn, where D(h) is the rainfall after the normalization, R(h) is the original rainfall, and Rmn is the daily mean.

3. Results

3.1. Seasonal Variation in the Diurnal Variation in Precipitation in the YGP

The spatial distribution of the diurnal precipitation peak in spring and summer in the YGP is given in Figure 1, and most stations in the YGP have their maximum hourly rainfall in both seasons from midnight (22:00–3:00 LST) to early morning (4:00–10:00 LST). The eastern part of the YGP is distinguished by a unique nocturnal rainfall pattern in spring (Figure 1a), with nearly all stations east of 104°E having their peak diurnal rainfall at midnight, with a few scattered stations in the eastern and southern regions having their peak in the early morning. Simultaneously, in the western part of the plateau, there are some stations in a relatively concentrated area that reach their peak in the late afternoon (15:00–21:00 LST). In summer (Figure 1b), the fraction of stations with an early morning peak increases, and most plateau stations still exhibit a midnight to early morning peak. An obvious feature is that in the western part of the YGP, the stations with a concentrated late-afternoon peak in spring shift to a midnight to early morning peak in summer, which differs significantly from the results in southern China, as noted by Li et al. [17]. Thus, this paper focuses on two regions of the YGP: western YGP (the spring rainfall peak is concentrated in the late afternoon and the summer rainfall peak occurs between midnight and early morning) and eastern YGP (the precipitation characteristics are consistent with previous research), comparing the seasonal variations in diurnal precipitation in two regions.

Figure 2 displays the seasonal variations the diurnal variation in precipitation in the eastern and western parts of the YGP. In summer, the maximum hourly rainfall in the eastern YGP occurs between midnight and early morning, with a secondary peak occurring around 14:00–16:00 LST. The western YGP exhibits characteristics similar to those in the eastern YGP, but with an earlier peak occurring between 4:00 and 5:00 LST, and the secondary peak in the late afternoon is relatively weaker. The difference in diurnal features between the two regions is more prominent in spring. The rainfall in the eastern YGP still shows a midnight to early morning peak, and the diurnal peak occurs around 1:00–5:00 LST, which is consistent with the findings of previous studies. However, the precipitation in the western YGP is more prominent at 16:00–18:00 LST, even exceeding the rainfall amount from midnight to early morning. The diurnal peak in the western YGP shifts from nighttime to afternoon, exhibiting significant seasonal differences when compared with that in summer. It is worth noting that the afternoon peak in the western YGP only appears in spring, and the peak in the other months of the year still appears from midnight to early morning.

Figure 3 further shows the diurnal variation in precipitation averaged for the eastern and western parts of the YGP for each month of spring. The eastern YGP shows a single peak at 2:00–4:00 LST in March and April and reaches a minimum at 19:00 LST. For the western YGP, in March, the late-afternoon precipitation (16:00–18:00 LST) is significant, but the heavy rain remains during nighttime. There is still a sub-peak in the early morning, and the early-morning peak is delayed when compared with the main peak in the eastern YGP. In April, the late-afternoon precipitation is further enhanced, showing a prominent single peak at 17:00 LST. The nocturnal precipitation is relatively weak, and the secondary early-morning precipitation in March is nearly nonexistent.

In summary, there is a unique seasonal variation in the western YGP, where rainfall exhibits a late-afternoon peak in spring but a midnight to morning peak in summer. These diurnal rainfall features are different from the eastern YGP.

3.2. Seasonal Variations in the Diurnal Features of Precipitation of Different Durations

Yu et al. [19] revealed the relationship between rainfall duration and its diurnal variation, and they found that analyzing the diurnal variation in rainfall events of different durations was helpful to understand the physical mechanisms corresponding to different types of rainfall. Following the methods of Yu et al., the rainfall events herein are further classified based on their duration. The diurnal cycle of precipitation amounts of different durations for the YGP is given in Figure 4. As shown in Figure 4, rainfall events in the YGP lasting 1–3 h exhibit an afternoon peak, while events lasting more than 6 h show a dominant early-morning peak. For rainfall events lasting 4–6 h, two comparable diurnal peaks in the late afternoon and early morning are found, so they are not discussed herein. The short-duration rainfall events are thus defined as events with a duration of 1–3 h, and a long-duration event is defined as an event with a duration of more than 6 h.

Figure 5 shows the ratio of rainfall amounts of events of different durations to the total rainfall in spring and summer averaged over the eastern and western parts of the YGP. The precipitation characteristics in the western YGP differ more between spring and summer than in the eastern YGP, especially the short-duration events. From spring to summer, the proportion of precipitation lasting less than 3 h in the eastern YGP increases, and the proportion of precipitation lasting 3–10 h in spring is higher than that in summer, but the proportion of long-duration events lasting over 11 h is higher in summer. For the western YGP, from spring to summer, the contribution of precipitation events lasting more than 6 h to the total precipitation increased, but the short-duration rainfall events are proportionally larger in spring.

Table 1. The percentages (unit: %) of short-duration and long-duration rainfall events to total rainfall in frequency and amount.

Location	Short-Duration Rainfall		Long-Duration Rainfall
	Frequency (%)	Amount (%)	Frequency (%)	Amount (%)
	Annual mean
Western YGP	47.41	32.62	29.12	41.40
Eastern YGP	43.69	28.94	31.44	43.51
	Summer (July–August)
Western YGP	46.59	33.07	29.51	39.93
Eastern YGP	50.55	37.44	26.96	37.94
	Spring (March–April)
Western YGP	62.36	55.19	14.65	18.07
Eastern YGP	43.98	31.02	28.34	35.15

shows the contribution of the frequency and amount of short-duration and long-duration rainfall events to the total precipitation within the eastern and western parts of the YGP. For short-duration events, from summer to spring, the frequency (amount of rainfall) decreases by about 6.57% (6.42%) in the eastern YGP, while the frequency (amount of rainfall) within the western YGP increases by about 15.77% (22.12%). The two regions of long-duration rainfall events decrease from summer to spring in terms of the amount percentage of rainfall, but the difference is greater in the western YGP. In addition, one of the more striking features in Table 1 is that both the frequency and amount of short-duration rainfall events in spring in the western YGP contribute more than 55% to the total rainfall, which is quite different from their contributions in other periods or regions in which the frequency of short-duration events is high and the contribution to the total rainfall is small, while the long-duration events occur at a low frequency but contribute substantially to the total rainfall. Therefore, the spring precipitation at the western YGP can be considered to come primarily from the contribution of short-duration rainfall events.

Figure 6 shows the diurnal variation in precipitation for short-duration and long-duration rainfall events in the eastern and western parts of the YGP. For the long-duration rainfall events, rainfall in the eastern YGP and the western YGP shows a dominant nocturnal diurnal peak year-round. In most months, the long-duration events achieve their hourly maximum at about 4:00–6:00 LST and their minimum around 19:00–20:00 LST. However, in the eastern YGP, the diurnal peak of long-duration events advances to around 4:00–5:00 LST in spring, and the amplitudes of the diurnal cycle also become strongest in spring. In the western YGP, a similar situation occurs during the summer. For the short-duration rainfall events, the peak time of precipitation occurs around 1:00–4:00 LST in November–April for the eastern YGP, and the peak time shifts to 16:00–17:00 LST in May–October, showing significant seasonal variations with a peak in the late afternoon in summer and a peak from midnight to early morning in spring. In contrast, the short-duration rainfall in the western YGP reaches its diurnal peak in the afternoon almost year-round, especially in March–May. There is a small tendency for the peak time of precipitation to advance in summer relative to the spring. In the coldest months (December and January), the amplitudes of the diurnal cycle of short-duration rainfall are small.

By analyzing the diurnal variation in precipitation events of different durations and their seasonal variation characteristics, it is found that the characteristics of the long-duration rainfall events in the western YGP in spring and summer are more consistent with the overall characteristics of the eastern YGP. The seasonal variation in the short-duration events, however, is weak, exhibiting an almost year-round late-afternoon peak with a stronger amplitude in spring. Meanwhile, the ratio of the short-duration rainfall amount to the total rainfall can reach 55.19%, which is about twice that in the eastern YGP. Therefore, it appears that the spring afternoon peak of the total rainfall in the western YGP is mainly comprised of the short-duration events. The discussion hereafter will focus on the factors affecting the short-duration rainfall to explore the possible reasons for the strong spring afternoon rainfall in this area.

3.3. Thermal Conditions in the Yunnan–Guizhou Plateau in Spring

Previous studies have noted that precipitation peaks in the late afternoon are mainly due to the influence of solar radiation. Higher surface temperatures in the late afternoon combined with lower atmospheric instability provide favorable conditions for locally triggered convection to produce precipitation [17,19]. Figure 7 displays the time at which the annual maximum of the 10-day mean daily maximum temperature occurs in the YGP. A notable feature is that the daily maximum temperature reaches the annual maximum in spring in the western YGP, and some stations even reached their annual maximum in April. This contrasts with most of the regions in the eastern YGP, where the annual maximum temperature is reached in summer.

Figure 8 further displays the monthly averaged daily maximum temperatures. For the daily maximum temperatures, the eastern YGP reaches its annual maximum in July, and the curve rises and falls distinctly before and after the height of summer. In contrast, the western YGP reaches its annual maximum in May, with little change in the daily maximum temperature from April to August. There is even a slight decrease in temperature from June to July in summer. After August, the temperature begins to drop significantly and rises in January in the following year. Compared with the eastern YGP, the daily maximum temperature in the western YGP is low in summer, but it is significantly high in spring. The daily maximum temperature in the western YGP reaches the annual maximum earlier than it does in the eastern YGP, which reaches its annual maximum in spring, which is conducive to the triggering of convection and increases the likelihood of short-duration events in spring afternoons at these areas.

The difference between the middle- and lower-layer potential temperatures can reflect the instability of the atmospheric stratification. The greater the potential temperature difference between the middle- and lower-layer potential temperatures, the more stable the stratification, and vice versa [24]. To further consider the spring convective instability in the western YGP, the differences in potential temperatures between 500 hPa and 700 hPa in spring are given in Figure 9. A notable feature of Figure 9 is that the instability of the lower troposphere is higher in the western part of the YGP than in the eastern YGP, and the center of the low values is in the southeastern part of the Tibetan Plateau. The instability of the lower troposphere in the western YGP decreases from northwest to southeast, but the instability is still smaller than that in the eastern YGP.

4. Conclusions and Discussion

Based on hourly precipitation and temperature data, as well as ERA5 reanalysis data, the seasonal variation in diurnal characteristics of precipitation in the YGP were explored. A unique late-afternoon diurnal peak in spring was found in the western part of the YGP, and the possible causes were discussed. The main results are as follows.

(1) There is a unique seasonal diurnal variation in precipitation in the western part of the YGP, where spring rainfall has a diurnal peak in the late afternoon but a peak from midnight to early morning in summer. Over the eastern YGP, more rainfall is apparent in the late afternoon in summer.

(2) The late-afternoon diurnal peak in the western YGP in spring is mainly comprised of short-duration rainfall events (lasting less than 3 h). In the western YGP, the short-duration rainfall events peak in the late afternoon year-round, but the diurnal amplitude of these events in spring is much stronger. The short-duration rainfall can account for more than 55% of the total rainfall amount in the western YGP in spring, while it accounts for only 33.07% of the total rainfall in summer. Over the eastern YGP, only the short-duration summer rainfall achieves its hourly maximum in the late afternoon, and the short-duration spring rainfall only represents 31.02% of the total rainfall. The seasonal variations in the diurnal features of long-duration events are similar in the western YGP and the eastern YGP.

(3) By analyzing surface temperatures and middle- and lower-layer potential temperature differences, it can be found that the daily maximum temperature in the western YGP tends to reach the annual maximum earlier than in the eastern YGP. Compared with summer, the higher surface temperatures at the western part combined with the unstable stratification in the afternoon provide favorable conditions for the triggering of convection in the spring over the western YGP.

Researchers have analyzed the diurnal variation in precipitation in China and found that the central eastern regions show the coexistence of a double peak in the morning and afternoon in summer and a peak in the early morning in spring [25]. The northeast also shows the characteristics of an afternoon peak in summer and midnight to early morning peak in spring [26]. In general, the diurnal variation in precipitation in most regions of China presents the characteristics of midnight to early morning peak in spring and summer or midnight to early morning peak in spring and afternoon peak in summer. Li et al. found that the precipitation peak in southwest China throughout the year was midnight to early morning [17]. In this study, it is found that the peak time of spring total precipitation in the western region of YGP occurs in the afternoon, which is different from most studies. In addition, different from Li et al. pointed out that long-duration precipitation was the main source of total precipitation in southern China [17], the short-duration precipitation in the western YGP contributed more to the total rainfall. After dividing precipitation events into short-duration and long-duration events, it was found that the precipitation characteristics in the eastern YGP are consistent with previous studies, while the main difference in the western region is that the spring peak time of short-duration precipitation shifts from midnight to early morning to early afternoon. Therefore, the differences in the western YGP may be mainly influenced by short-duration precipitation. Yu et al. [25] point out that the occurrence of short-duration precipitation is mainly related to the triggering of convection, which is triggered by higher surface temperature and an unstable atmospheric environment [27]. Some studies have pointed out that spring precipitation is mainly affected by radiation cooling at night, and can occur from midnight to early morning. Summer is significantly affected by solar radiation, and the surface temperature is higher in the afternoon, which triggers short-duration convection [19]. In addition, the potential temperature difference between the high and low layers further reflects the relative instability of stratification in the western part of the YGP. Under the combined influence of the two, convection is triggered in the western part of the YGP in spring afternoons, producing short-duration precipitation and reaching the diurnal peak.

Through the classification of precipitation events according to their duration, the diurnal variation in and unique seasonal variation characteristics of precipitation in the YGP can be revealed. In addition, the possible reason for the western YGP’s unique change has been discussed in combination with the surface temperatures and the upper- and lower-layer potential temperatures. This is helpful to further understand the precipitation characteristics of the Yunnan–Guizhou Plateau’s complex topography.