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- Wiley
- ISSN
- 1687-9309
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- 1687-9317
- DOI
- 10.1155/2022/9696174
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Abstract
Hindawi Advances in Meteorology Volume 2022, Article ID 9696174, 14 pages https://doi.org/10.1155/2022/9696174 Research Article Influence of Underlying Surface on Distribution of Hourly Heavy Rainfall over the Middle Yangtze River Valley 1,2 2,3 1 1 4 Yinglian Guo , Jisong Sun , Guirong Xu, Zhiming Zhou , and Jizhu Wang Hubei Key Laboratory for Heavy Rain Monitoring and Warning Research, Institute of Heavy Rain, China Meteorological Administration, Wuhan 430205, China State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081, China Nanjing Joint Institute for Atmospheric Sciences, Chinese Academy of Meteorological Sciences, Beijing 210000, China Wuhan Central Meteorological Observatory, Wuhan 430074, China Correspondence should be addressed to Jisong Sun; [email protected] Received 14 October 2022; Accepted 19 November 2022; Published 21 December 2022 Academic Editor: Hiroyuki Hashiguchi Copyright © 2022 Yinglian Guo et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Te variation of boundary layer circulation caused by the infuence of complex underlying surface is one of the reasons why it is difcult to forecast hourly heavy rainfall (HHR) in the middle Yangtze River Valley (YRV). Based on the statistics of high- resolution observation data, it is found that the low resolution data underestimate the frequency of HHR in the mountain that are between the twain-lake basins in the middle YRV (TLB-YRV). Te HHR frequency of mountainous area in the TLB-YRV is much higher than that of Dongting Lake on its left and is equivalent to the HHR frequency of Poyang Lake on its right. Te hourly reanalysis data of ERA5 were used to study the variation of boundary layer circulation when HHR occurred. It can be found that the boundary layer circulation corresponding to diferent underlying surfaces changed under the infuence of the weather system. Firstly, the strengthening of the weather system in the early morning resulted in the strengthening of the southwest low-level air fow, which intensifed the uplift of the windward slope air fow on the west and south slopes of the mountainous areas in the TLB- YRV. As a result, the sunrise HHR gradually increases from the foot of the mountain. Te high-frequency HHR period of sunrise occurs when the supergeostrophic efect is weakened, the low-level vorticity and frontal forcing are strengthened, and the water vapor fux convergence begins to weaken. Secondly, the high-frequency HHR period of the sunset is caused by stronger local uplift and more unstable atmospheric stratifcation, but the enhanced local uplift is caused by the coupling of the terrain forcing of the underlying surface and the enhanced northern subgeostrophic fow, which causes the HHR to start closer to the mountain top at sunset than at sunrise. situations in central and eastern China. Shaw et al. [17] and 1. Introduction Wang et al. [18] analyzed the efects of surface temperature Hourly heavy rainfall (HHR), especially extreme HHR, will and humidity on heavy precipitation. Liang and Ding [19] cause food, landslide, urban waterlogging, and other di- studied the long-term variation of extreme heavy precipi- sastrous events [1, 2]. A large number of studies have pointed tation by the urbanization efect in Shanghai during 1916 to out that the extreme precipitation event around the world 2014. However, the fne-grained forecast of HHR is still has been increasing with global warming [3–5], including difcult currently since HHR is not only related closely to the middle Yangtze River Valley (YRV) in China [6–12]. evolution of multiscale synoptic systems themselves but HHR is a result of the interaction among multiscale afected by the boundary forcing from complex terrain or systems [13–15]. Scientists have analyzed the afecting fac- various underlying surfaces. Lock and Houston [20] pointed tors of HHR events from diferent perspectives. For example, out that the convection initiation mostly occurred near Luo et al. [16] classifed the extreme hourly heavy precipi- signifcant terrain and waters. Guo and Sun [21] analyzed tation into four categories according to the synoptic three types of convective systems with diferent 2 Advances in Meteorology organizational forms in Hubei Province and found that a efects of the boundary layer are extremely important for the large number of nonlinear convective systems may be increase of HHR [49, 50]. Focusing on the complicated mesoscale underlying surfaces around the twain-lake basins, formed during the movement of isolated convective storms triggered earlier in mountains and hills to plain areas. A this study will analyze the basic features of the HHR event large number of studies have shown that the convection diurnal variation and the possible efect mechanism. Te triggering and development evolution under diferent un- previous analysis on the causes of high-frequency HHR at derlying surface conditions are signifcantly diferent sunrise has been relatively comprehensive, but there is no [22–31]. Terefore, the study on the triggering mechanism of much concern about the causes and evolution of high-fre- HHR events near multiscale mountains or giant lakes has quency HHR at sunset. It is generally believed that it is become an important branch of investigating heavy rainfall. related to thermal instability. However, the twain-lake basins Te diurnal variation of convection triggering and in the middle YRV (TLB-YRV) have a complex underlying evolution is caused by the diference of underlying surfaces surface, including water bodies, mountains, and plains. Te [32]. For example, the bimodal diurnal variation charac- two lakes, Dongting Lake and Poyang Lake, are the largest freshwater lakes in China. Between the two lakes is Mufu- teristics of HHR frequency in the Yangtze River Valley are a phenomenon diferent from the unimodal characteristics in Jiuling Mountain, and Jianghan Plain lies to the northwest of most regions of China. As for the formation of bimodal the two lakes. Te corresponding HHR characteristics under diurnal variation characteristics, it is generally believed that complex terrain are not consistent. How does the underlying the sunset peak is mainly related to solar radiation heating surface afect the distribution and evolution of HHR? Te [33], but there are fve views on the causes of the sunrise surrounding area of the TLB-YRV is an important food peak. Tey are (1) the eastward propagation of the con- production region and the key transportation hubs of vective system from the Qinghai-Tibet Plateau. However, Central China. Terefore, the study of the distribution not all nocturnal convective systems in the Yangtze River characteristics and infuencing factors of HHR on TLB-YRV Valley move eastward from the east side of the plateau underlying surfaces is the basic work to improve the pre- [34, 35]. (2) Te mountain-plains solenoid efect [36]. Te cision forecast of the heavy rain. Te paper is organized as follows: the data and pro- mountain-plains solenoid efect is a large-scale thermal circulation caused by the three order topography in China, cessing methods are introduced in Section 2. Section 3 and the solenoid efect of mesoscale mountains on it is lack introduces the characteristics of HHR events around the of investigation. (3) Te nocturnal strengthening of low-level twain-lake basins, especially the characteristics of diurnal jets or boundary layer jets [34, 37–39]. Generally, the ap- variation. Te possible mechanism of afecting temporal- pearance time of the strongest jet is mostly earlier than that spatial distribution of HHR events around the twain-lake of the precipitation peak in the early morning, but it is basins are analyzed in Section 4. Section 5 discusses the usually considered that early morning was corresponding infuence mechanism of the underlying surface. Conclusions with the strongest water vapor transport [38]. (4) Locally are given in Section 6. thermal circulation strengthens the precipitation at night. Generally, the sea-land or lake-land breeze circulation has a 2. Data and Methods more signifcant enhancement efect on night precipitation, such as the coastal areas of South China [40–42]. In addition, 2.1. Regional Division. Te regional division in this paper is Xue et al. [38] believed that the infuence of thermal cir- based on the underlying surface attributes. As shown in culation is not as signifcant as inertial oscillation in Central Figure 1(a), the brown line boxes 1 and 4 are, respectively, China on the early morning HHR events. (5) Convection Dongting Lake (DT_L) and Jianghan Plain (JH_P) areas, and enhancement was caused by radiation cooling over the cloud the corresponding underlying surfaces are, respectively, lake top at night [43, 44], but Yin et al. [44] pointed out that it and plain areas. Te boundary is mainly based on the lake mainly induced night rains in the west of 110 E in China. area and terrain height. Brown line boxes 2 and 3 are Mufu To sum up, the reasons for high-frequency HHR at Mountain (MJ_M) and Poyang Lake (PY_L) areas, re- sunrise can be summarized from three perspectives. First, it spectively, and the corresponding underlying surfaces are is caused by movement or propagation of the convection mountain areas and lake areas, respectively. Based on the system, as in viewpoint 1. Second, it is induced by the terrain height, the junction of mountain areas and lake areas thermal efect, such as viewpoints 2, 4, and 5. Tird, it is is taken as the boundary between the two areas. Te conducted by the dynamic efect, as in viewpoint 3. Te northern boundary of the two areas is the junction of dynamic efect is usually referred to the enhanced conver- mountain areas and plains, and the southern boundary is the gence and water vapor transport by low-level jets. Tere are boundary of the middle YRV. the following views on the reasons for the enhancement of In Figure 1(a), the flled area is the terrain, the blue solid the nocturnal low-level jets: inertial oscillation [45], thermal lines are the rivers, the black solid line is the provincial forcing [46], and a combination of both [47]. In fact, the boundary, the brown lines and black numbers indicate dynamic efect actually is also closely related to the thermal subregions (1 �Dongting Lake Area [DT_L], 2 �Mufu- efect. Du and Chen [48] pointed out that the general low- Jiuling Mountainous Area [MJ_M], 3 �Poyang Lake Area level jet referred to the jet near 850hPa, and the 925hPa or [PY_L], and 4 �Jianghan Plain Area[JH_P]), and the red lower jet is called the boundary layer jet. Boundary layer jets thick solid lines and circled numbers represent the moun- can interact with the terrain, so the dynamic and thermal tains (① �Mufu Mountain and ② �Jiuling Mountain). In Advances in Meteorology 3 Unit: m 32N 32N 31N 1375 31N 30N 875 30N 29N 29N 2 250 12 3 28N 28N 110E 111E 112E 113E 114E 115E 116E 117E 110E 111E 112E 113E 114E 115E 116E 117E (a) (b) Figure 1: Topography and the location of subregions (a); distribution of ground observation stations (b). Figure 1(b), black points represent rainfall stations, red points are stations together with rainfall and temperature 32°N observation, green points are stations together with rainfall, temperature, and wind observation, and the gray solid line is 31°N the provincial boundary. 30°N 2.2. Processing of Ground Observation Data. Tis study collected hourly ground observation data with higher spatial 29°N resolution, including national-level and regional-level au- ° ° tomatic stations in the middle YRV (28 N–32.5 N, ° ° 110 E–117 E) from April to October during 2012–2017. All 28°N data are quality controlled using the method of [51], and 110°E 111°E 112°E 113°E 114°E 115°E 116°E 117°E doubtful data are eliminated. Te station whose arrival rate (the ratio of the available record hours to the total obser- 123456789 10 11 12 vation hours) reaching more than 90% was labeled as an Figure 2: Annual average distribution of the HHR frequency in the efective observation. In this study, the number of efective middle YRV from April to October 2012-2017. Te brown lines rainfall stations reaches to 3109 (Figure 1(b)). Te hourly indicate partitions, the same as Figure 1(a). precipitation at any station that reached to 20mm is recorded as an HHR event [2, 50, 52]. of HHR days in each subregion (JH_P, DT_L, MJ_M, and PY_L) is calculated and called JH_ P:sunrise, DT_ L: sunrise, 2.3. Defnition of HHR Day and Non-HHR Day. During the MJ_ M: sunrise, PY_ L: sunrise, JH_ P:sunset, DT_ L: sunset, period from 00:00 to 23:00 (local standard time–LST, the MJ_ M: sunset, and PY_ L: sunset. same as follows) of a day, there is at least one HHR at any station in a single subregion. Tis day is defned as the HHR 3. Characteristics of HHR Frequency day of the subregion. For Non-HHR days, it is required that the stations in the four subregions in Figure 2 do not have 3.1. Spatial Distribution of HHR Frequency. From the annual HHR within 24 hours on a certain day, hereinafter referred average frequency of HHR events in the middle YRV from to as NONE. April to October during 2012–2017 (Figure 2), the following facts can be found: (1) the frequency distribution in the middle YRV showed more HHR events in the southeast and 2.4. Processing of Atmospheric Background Data. Te ERA5 ° ° fewer in the northwest. PY_L and MJ_M are the two regions global reanalysis data (horizontal resolution 0.25 ×0.25 , with the highest-frequency HHR events. Tis is slightly with hourly temporal resolution) were used to investigate diferent from the results of Chen et al. [53] only using the atmospheric evolution. Te hourly average felds of the HHR national stations analysis. Chen et al. [53] indicated that days in each subregion are called JH_P: ave, DT_L: ave, DT_L and PY_L were high-frequency areas of HHR events, MJ_M: ave, and PY_L: ave. Te hourly average felds of the and the frequency in MJ_M was not only lower than in PY_L HHR days in all subregions (excluding the same date in each but was even lower than in DT_L. Tis should be caused by subregion) are called as ALL: ave. In order to analyze the scattering data used in their investigation, and the mesoscale relevant characteristics of the two peak periods of HHR daily features of HHR events in the complex terrain region were variation: sunrise (05:00 to 09:00) and sunset (16:00 to 18: impossible to be exposed well by scattering national stations. 00), the composite mean feld of the corresponding periods 4 Advances in Meteorology 2.5 (2) Tere were signifcant diferences in the frequency of HHR events between the two giant lakes (Dongting Lake 2.0 (DT_L) and Poyang Lake (PY_L)), which are the two largest 1.5 lakes with similar latitude in the middle YRV. Most HHR activities appear around PY_L, and the lowest frequency of 1.0 HHR events occurs in DT_L area. Tis may be closely related 0.5 to diferences in topographic distribution around the two lakes. Te surrounding area of DT_L is relatively fat, its west 0.0 0 123456789 1011121314151617181920212223 and south sides are relatively far away from the mountains, LST and its north side is adjacent to Jianghan Plain. However, the surrounding terrain of PY_L is much more complex, similar JH_P MJ_M DT_L PY_L to a basin, with some small hills scattered around it. (3) Te frequency distribution in MJ_M was closely related to the Figure 3: Diurnal variation of HHR event frequency over the four mountains. Te two high-frequency bands in MJ_M were regions (JH_P, DT_L, MJ_M, and PY_L) around the TLB-YRV. located, respectively, in the western side of Mufu-Jiuling Te ordinate axis represents the ratio between the total number of Mountain and along the Jiuling Mountain. (4) Te frequency HHR events in the subregion and the number of stations in the subregion, and the abscissa axis represents hours in a day. of HHR events in JH_P is much lower than that in PY_L and MJ_M, but the Dabie Mountain on the north side of JH_P and PY_L is a high-frequency area of HHR events. Fu et al. [34] studied the relationship between high-frequency HHR around DT_L is relatively insignifcant. Te bimodal char- in the early morning of the Dabie Mountains and the low- acteristics around JH_P are not signifcant as DT_L. Tis is level jet in the boundary layer and pointed out that the high- related to the lower HHR frequency in these two regions. In order to clarify the relationship between the frequency frequency HHR was related to the inertial oscillation of low- level jets. Te terrain of the Dabie Mountain is along of HHR events and the underlying surface environments, Figure 4 shows the hourly distribution and topography of northwest-southeast direction, almost perpendicular to the southwest jet. Te lifting mechanism in the windward slope the HHR events around the TLB-YRV. At MJ_M area, the was considered to play an important role in the early spatial distribution and evolution of high-frequency HHR morning HHR of the Dabie Mountains. However, the terrain events (red dots) at sunrise are diferent from those at sunset. of Mufu Mountain is along the southwest-northeast di- At sunrise, high-frequency HHR frst occurs on the rection, parallel to the southwest jet, and the infuence of the northwest slope and then gradually extends to the whole underlying surface increases the infuence of the water body. MJ_M area. At sunset, high-frequency HHR erupts along the Are the temporal and spatial distribution and diurnal var- ridge of Mufu Mountain and usually spreads to the sur- iation features of HHR events afected by local circulation? Is rounding areas with a fast decreasing frequency. As shown in Figure 4, the number of stations with high-frequency HHR there any interaction between local circulation and weather system? events increased rapidly around 05:00 on the northwest slope of Mufu Mountain and reached the maximum at 06:00, which corresponds to the sunrise peak at 06:00 in Figure 3. 3.2. Diurnal Variation of the HHR Frequency. Te diference At 08:00–09:00, the high-frequency stations are relatively of temperature, humidity, and turbulence caused by solar well distributed over Mufu Mountain and Jiuling Mountain. radiation on diferent underlying surfaces results in the local After 10:00, the number of high-frequency stations is sig- circulation of the boundary layer and its diurnal variation. nifcantly reduced. At 16:00, the number of high-frequency By statistical analysis of the diurnal variation characteristics HHR stations in the western Mufu Mountain increased of the HHR events in the four subregions (Figure 3), it is rapidly again. Te high-frequency stations were more found that there are bimodal diurnal variation character- concentrated and closer to the ridge of Mufu Mountain than istics of HHR frequency in each subregion. Te diurnal at 06:00. After 18:00, the number of high-frequency stations variation of HHR event frequency over the PY_L and MJ_M decreases obviously, and the distribution gradually tends to presents obviously bimodal characteristics, and the two disperse. In the late evening (until 23:00), the number of peaks appear at 17:00 at sunset and about 06:00 at sunrise. high-frequency stations decreased more slowly than in the Tis is consistent with the diurnal characteristics of the morning, which seems inconsistent with Figure 3. Tis is middle-lower reaches of the Yangtze River Valley investi- because the frequency of HHR events is much higher during gated by Yu et al. [11, 12]. Generally, the most unstable 16:00–17:00 than that in later evening over the MJ_M area. atmospheric stratifcation appears in the afternoon, so the In PY_L area, the evolution of high-frequency HHR peak time of HHR frequency at sunset is essentially the same events during sunrise and sunset is in good agreement with as the thunderstorm or gale event frequency in this region the lake-land breeze circulation; that is, the high-frequency [21]. Te peak of HHR event frequency at sunrise is related HHR at sunrise extends outward from the center of the lake, to the interaction between local circulations and the synoptic and the high-frequency HHR at sunset spreads from system. However, DT_L and PY_L are, respectively, located mountains to the center lake. As shown in Figure 4, during in the east and west of MJ_M. Te bimodal diurnal variation 06:00–10:00, stations with the high-frequency HHR events around PY_L is signifcant, while the bimodal feature are mainly distributed on the northeast and southwest of the Total number of HHR events (Numbers of station) Advances in Meteorology 5 00 16 30N 30N 30N 29N 29N 29N 28N 28N 28N 01 09 17 30N 30N 30N 29N 29N 29N 28N 28N 28N 02 10 18 30N 30N 30N 29N 29N 29N 28N 28N 28N 03 11 19 30N 30N 30N 29N 29N 29N 28N 28N 28N 04 12 20 30N 30N 30N 29N 29N 29N 28N 28N 28N 05 13 21 30N 30N 30N 29N 29N 29N 28N 28N 28N 14 22 30N 30N 30N 29N 29N 29N 28N 28N 28N 07 15 23 30N 30N 30N 29N 29N 29N 28N 28N 28N 112E 113E 114E 115E 116E 117E 112E 113E 114E 115E 116E 117E 112E 113E 114E 115E 116E 117E 2 <= N < 4 0 200 400 600 800 1000 1200 N >= 4 Figure 4: Hourly distribution of HHR event frequency of each station (the number in the upper left corner is local standard time, the flling is terrain, the blue line is river and lakes, and the color dots represent the HHR event frequency of each station, where the red dots represent high-frequency events). Poyang Lake. Most of the stations with the high-frequency 00–06:00) and then gradually expanded outward. Another HHR events on the southwest are closer to Poyang Lake than frequency peak of HHR events over PY_L appears in the to Jiuling Mountain. It is easy to fnd from Figure 4 that the afternoon to evening. Te stations with high-frequency distribution evolution of the stations with high-frequency HHR events over PY_L frst appear along the northwest HHR events initially appears near the lake center (during 05: bank of the Poyang Lake (15:00) near Jiuling Mountain and 6 Advances in Meteorology then spread around the lake and locate away from moun- includes the infuence of the weather system on the tains. Te number of the stations with high-frequency HHR boundary layer circulation. Figure 6 shows the regional average diurnal variation of vorticity and vapor-fux di- events reaches the peak at 16:00–17:00, which is consistent with Figure 3. Comparing the evolution trends of the two vergence on HHR days minus non-HHR days, where the ° ° ° ° high-frequency HHR periods of sunrise (05:00∼08:00) and regional average range is 28 N–30.7 N, 112 E–117 E. It in- sunset (15:00∼17:00), it can be found that the stations with cludes the three key subregions (DT_L, MJ_M, and PY_L) of high-frequency HHR events during the sunset period are TLB-YRV. Te maximum value of vapor fux convergence − 7 − 1 − 1 stably distributed around the lake, and there is no high- (− 1.1 ×10 g kg s ) in Figure 6 appears during 01:00–06: frequency station similar to the sunrise near the central lake. 00 under 950hPa with the warm-wet low-level jet In DT_L, diferent from PY_L, the stations with the strengthening after midnight, and vorticity of the synoptic − 5 − 1 high-frequency HHR events are mainly concentrated on the system reaches maximum (more than 1.7 ×10 s ) during south side of the Dongting Lake whether in morning or 07:00–10:00 at 900hPa. In the afternoon, the subgeostrophic afternoon-evening. Te number and density of the stations efect causes reducing of the vortex system and vapor fux with high-frequency HHR events over the DT_L region are convergence at low levels in the afternoon, and the central much less than over PY_L, and the diurnal variation am- value of vapor fux convergence goes down to − 8 − 1 − 1 plitude of HHR event frequency is also the lowest in the − 6 ×10 g kg s during 13:00–16:00, and the convergence three regions. stars to enhance slightly again in the evening (about 17:00) at this level. Te previous analysis shows that the HHR boundary 4. Possible Mechanism of Underlying weather system presents diurnal variation features: HHR Surfaces on HHR Events events at sunrise are related with vorticity strengthening and appear after vapor fux convergence up to maximum. Te previous analysis shows that the frequency of HHR However, the high-frequency HHR events at sunset are events around the TLB-YRV exhibits a bimodal diurnal impossible to be well explained by the diurnal variation of variation, but amplitudes of the diurnal variation in the the synoptic system itself since the highest-frequency HHR DT_L and JH_P are much smaller than that in MJ_M and events are corresponding with only slight increase of vapor PY_L. In mountain and lake areas, the location and fux convergence and no obviously enhancement of vorticity movement trend of high-frequency HHR events in the at low levels. On the other hand, the diurnal variation of the morning and evening are also diferent. Te previous weather system cannot explain the diferent diurnal varia- phenomena may be related to the following mechanisms: (1) tion behaviors of the HHR events in mountain and lake diurnal variation of the HHR weather system and (2) local areas. Tis implies that the local circulations caused by the circulation of the boundary layer caused by diference of the complex underlying surface may play another important underlying surface. role in afecting the diurnal variation of the subregions HHR events around TLB-YRV. 4.1. Diurnal Variation of the Boundary Weather System. Te highest altitude of Mufu-Jiuling Mountains is lower than 1500 m, and the direct interaction between the 4.2. Impact of the Underlying Surface. First, from the average weather system and the underlying surface is mainly divergence/convergence at 925hPa and surface fow on the non-HHR days (Figure 7), it can be found that the under 850 hPa. Figure 5 shows the 925 hPa synthetic wind felds and θ mountain-valley breeze caused by the MJ_M is the clearest when HHR events occur in four subregions se boundary local circulation in the TLB-YRV. Te mountain (JH_P, DT_L, MJ_M, and PY_L), respectively. It is easy to be found that their boundary layer weather control sys- breeze at sunrise corresponds to the divergence at 925hPa, and the valley breeze at sunset corresponds to the conver- tems are similar, but the HHR events in each subregion occur in diferent positions of the system. Tis means that gence at 925hPa. Te height of the lake-land breeze near the lake area is lower than that of the mountain-valley breeze most HHR events in the middle YRV are caused by medium-α-scale vortex or horizontal wind shear. Tis is because the land breeze in the morning (converging to the lake center) and the lake breeze in the evening (divergent to consistent with the development of mesoscale vortex the lake shore) are clear near the surface, but the divergence systems or wind shears in the lower troposphere during of 925hPa above the twain-lake basins is always greater than Mei-Yu period in the YRV [54–57], and the monthly 0, especially at DT_L. At sunrise (Figure 7(a)), the con- average frequency of HHR events over the twain-lake vergence zone by mountain breeze is located along the basins indicated that most of HHR events happened south-eastern and western slopes of MJ_M over 925hPa, during June to July (the fgure omitted) and were cor- − 5 − 1 responding with the Mei-Yu season. and the strongest convergence center (− 1.5 ×10 s ) is along the western slope of MJ_M or the eastern side of Te boundary layer circulation of non-HHR roughly represents the local climate characteristics in warm seasons. DT_L. Tis indicates that the mountain breeze is more conducive to strengthening the convergence and uplifting Te HHR diurnal variation minus the non-HHR diurnal variation can remove the regular diurnal variation of the movement on the west side of MJ_M, and its position is consistent with the location of the high-frequency HHR boundary layer fow feld so as to better reveal the diurnal variation of the weather system with HHR. Of course, it still events in the early morning. At sunset, the thin lake breeze, Advances in Meteorology 7 JH_P DT_L 925 hPa 925 hPa 32N 32N 2000 2000 1900 1900 1700 31N 31N 30N 30N 1100 1100 1000 1000 900 900 29N 29N 800 800 700 700 600 600 500 500 28N 28N 400 400 300 300 200 200 100 100 27N 27N 110E 111E 112E 113E 114E 115E 116E 117E 118E 110E 111E 112E 113E 114E 115E 116E 117E 118E (a) (b) MJ_M 925 hPa PY_L 925 hPa 32N 32N 2000 2000 1900 1900 1800 1800 1700 1700 31N 31N 1600 1600 1500 1500 1400 1400 1300 1300 30N 30N 1200 1200 1100 1100 1000 1000 900 900 29N 29N 800 800 700 700 600 600 500 500 28N 28N 400 400 300 300 200 200 100 100 27N 27N 110E 111E 112E 113E 114E 115E 116E 117E 118E 110E 111E 112E 113E 114E 115E 116E 117E 118E (c) (d) − 1 − 1 Figure 5: Synthesized wind (barb; m·s ; each bar for 1m·s ) and θ (red solid line; K) at 925hPa when HHR events occur in diferent se subregions (a) JH_P, (b) DT_L, (c) MJ_M, and (d) PY_L, respectively. Te black dashed line box is the respective HHR area, and the gray fll is the terrain altitude (m). ALL:ave-NONE 0123456789 1011121314151617181920212223 -10 -8 -6 -4 -2 0 2 4 − 8 − 1 − 1 ° ° ° ° Figure 6: Regional (28 N–30.7 N, 112 E–117 E) average diurnal variation of vapor-fux convergence (color, 10 g kg s ) and vorticity − 6 − 1 (black line, 10 s ). which is guided by DT_L and PY_L on both sides of MJ_M, superposition efect of mountain-valley breeze and lake- is favorable for intensifying the valley breeze and induces land breeze leads to the convergence intensity of the stronger convergence toward the mountain top. Te mountain top at sunset (the maximum convergent 8 Advances in Meteorology NONE TIME:06LST NONE TIME:17LST 32N 32N 31N 31N 30N 30N 29N 29N 28N 28N 111E 112E 113E 114E 115E 116E 117E 111E 112E 113E 114E 115E 116E 117E (a) (b) Figure 7: Te average divergence at 925hPa (the red line represents convergence, the blue line represents divergence, and the unit is − 6 − 1 10 s ) and surface fow (stream) at 06:00 (a) and 17:00 (b) on non-HHR days. − 5 − 1 intensity in 925 hPa reaches − 2.0 ×10 s ) is stronger distinguishably slant front under 850hPa distributes from than that along the slope at sunrise (convergent intensity west (cold) to east (warm) with a high θ zone over the slant se − 5 − 1 is down to − 1.5 ×10 s ). front. On the contrary, there is not obviously a horizontal Ten, we analyze the divergence/convergence at 925hPa gradient of θ in the low-level troposphere around MJ_M at se and surface fow on HHR days (Figure 8). Considering that sunset. It is furtherly confrmed that the high-frequency 02:00 and 14:00 are the least active periods of HHR events HHR events at sunrise are guided by the intensifying (Figure 3), the low-level fow and divergence at these two nocturnal synoptic system, which not only presents vorticity time points can maximally avoid the feedback efect of HHR and vapor-fux convergence strengthening but also repre- events on local circulation and can better reveal the char- sents front forcing to be intensifed. In addition, the west side acteristics of boundary layer circulation under the action of of MJ_M, as the windward slope of the southwest low-level HHR weather systems. It can be seen from Figure 8 that the jet, is afected by the supergeostrophic efect before sunrise, patterns of average surface fow and divergence in 925hPa at which enhances the convergence enhancement of the west 02:00 of the HHR days (Figure 8(a)) are roughly similar to side of MJ_M, resulting in stronger rising than that of non- that of Figure 7(a). Te land breeze convergence over DT_L HHR days. Te deviation of the conditional static stability ( region is clearer. Tere is a stronger convergence (the zθ /zp>0) of the troposphere below 750hPa in the MJ_M se − 5 − 1 minimum value is less than − 2.0 ×10 s ) around MJ_M region shows that it is more unstable at sunset than at slopes, and there is a more intense divergence (the maxi- sunrise, indicating that local uplift plays a decisive role in − 5 − 1 mum value reaches up to 2.5 ×10 s ) at mountaintop over triggering the convective high-resolution high-frequency 925hPa. Figure 8(b) is similar to Figure 7(b), MJ_M is HHR event at sunset. It shows that the local lifting plays a covered by local convergence in 925hPa at 14:00 decisive role in triggering the high-frequency HHR events at (Figure 8(b)), and their intensities are roughly similar. sunset. Te diferences of the stratifcation status and trigger However, due to the infuence of the HHR weather system, mechanism induce the variation of convective intensity at northerly wind prevails on the ground in the middle YRV. sunrise and sunset. It can be confrmed by some features of Te surface fow feld on the top of the mountain is even radar refective intensity, which correspond to the two peak dominated by downslope wind (similar to mountain wind), periods of HHR events in MJ_M, respectively (Figure but the convergence area of MJ_M at 925hPa is obviously omitted). For example, the average height of maximum larger than that in Figure 7. It can be seen that the weather radar refective intensity of high-frequency HHR events at system conducive to HHR events has not fundamentally sunset is higher than that at sunrise (3.01km vs 2.76km), changed the pattern of divergence and convergence around and the average thickness of radar refective intensity above MJ_M at sunrise and sunset, but they are signifcantly 40dBZ at sunset is thicker than that at sunrise (1.55km vs strengthened. 1.26km). In conclusion, the strong convergence movement Last, Figure 9 shows the deviation (HHR days of each at sunset cannot be directly caused by the weak weather subregion minus non-HHR days) vertical sections (along system itself but may be caused by the topographic forcing 29 N) of θ and divergence. It can be found that no matter caused by the underlying surface and the thermal circulation se in the boundary layer. In addition, it is worth noting that (1) which subregion is hit by the HHR event, the deviation patterns of θ and divergence in the vertical sections along when HHR occurs in any subregion, the slant convergence is se 29 N are slightly similar. Corresponding with the peak of well matched from the eastern side of MJ_M to the western HHR events frequency at sunrise, a clearly and shore of PY_L, but their intensities present obviously Advances in Meteorology 9 MJ_M:ave TIME:02LST MJ_M:ave TIME:14LST 32N 32N 31N 31N 30N 30N 29N 29N 28N 28N 111E 112E 113E 114E 115E 116E 117E 111E 112E 113E 114E 115E 116E 117E (a) (b) Figure 8: Te same as Figure 7 but at 02:00 (a) and 14:00 (b) on the HHR days in MJ_M. diferent in the western slope of MJ_M when the HHR HHR days or non-HHR days, the diurnal variation direction events happen in diferent subregions. (2) Below 800hPa, the of the low troposphere ageostrophic wind has obvious in- conditional static stability between PY_L and MJ_M is more ertial oscillation characteristics (clockwise rotation with unstable than that between DT_L and MJ_M. It implies that time). Te diurnal variation amplitude of the ageostrophic corresponding to the HHR synoptic system, the local wind in non-HHR days is weaker than that in any subregion convergence intensity and unstable stratifcation in the east with HHR events. In non-HHR days, the maximum of MJ_M are more remarkable than those in the west, and supergeostrophic wind (southwestern wind less than − 1 more convective HHR events are induced near the top of 1.5m·s ) from midnight to early morning occurs near MJ_M and along the lakeshore of PY_L at sunset. 875hPa, and the strongest subgeostrophic wind (northern − 1 wind less than 1.5m·s ) appears near 950hPa from after- noon to evening. Compared with the regional ageostrophic 5. Discussion wind of the non-HHR days, the average ageostrophic wind of HHR days in MJ_M and PY_L shows bigger amplitude of Generally, the intensifying low-level airfow caused by inertial diurnal variation either HHR events at sunrise or sunset. Te oscillation is considered to be a signifcant forcing factor to strongest supergeostrophic phenomenon induces more a induce nocturnal heavy rainfall [38, 40]. Te oscillation nature powerful southwest low-level jet at midnight, and the oflow-levelairfow isthoughtasaresultofthediurnalvariation strongest subgeostrophic (northern wind) that appears in of the turbulence intensity [41, 45]. Te supergeostrophic wind the evening signifcantly reduces the prevailing southern appears after midnight with turbulence weakening under the fow. Corresponding to HHR days, the subregional average boundary layer, and the wind vector rotates clockwise and velocity of the maximum ageostrophic (regardless of induces the vorticity and low-level jet to be intensifed. On the supergeostrophic or subgeostrophic) wind over MJ_M is contrary, the enhancing turbulence induces the subgeostrophic stronger than that over PY_L, and their diference is about efect in the afternoon. Hourly ERA5 data can well show the 0.2m/s. It indicates that the terrain is not only conducive to diurnal variation characteristics of inertial oscillation and have the strengthening low-level jet at midnight but also its a certain reproducibility ability for the mountain-valley breeze friction efect is more favorable for producing a sub- or lake-land breeze in the study area of this paper. However, it geostrophic efect in the evening. It is also worth noting that is still difcult to quantitatively separate the diurnal variation of the strong subgeostrophic wind occurs from 18:00 to 19:00, wind feld caused by inertial oscillation from the thermal which is about 1–2 hours later than the sunset peak time (16: circulation in the boundary layer, so geostrophic wind is used 00–17:00) of HHR frequency. Tis means that more HHR to analyze them together. Te ageostrophic fow can well events in evening occur during the stage of the sub- represent divergence/convergence, so the analysis of the geostrophic wind strengthening, instead of the strongest ageostrophic fow on diferent underlying surfaces can explain subgeostrophic period. More HHR events in the early the action mechanism of the topographic forcing caused by the morning (05:00∼09:00) occur in period of the low-level underlying surface and the boundary layer thermal circulation supergeostrophic wind weakening (Figure 10) or the low- on HHR. level vertical vortex gradually intensifying (Figure 6). Te Figures 10(a)–10(d) shows the diurnal variation (hourly mechanism may be related to the convergent forcing caused value minus daily average value) of the ageostrophic wind by the ageostrophic efect. when HHR occurs at sunrise or sunset in each region, and Te vertical vorticity tendency equation without the Figure 10(f) shows the diurnal variation of the ageostrophic advection term is as follows: wind on non-HHR days. It is easy to fnd that whether on 10 Advances in Meteorology DT_L:sunrise TIME:06LST DT_L:sunset TIME:17LST 500 500 550 550 600 600 650 650 700 700 750 750 800 800 850 850 900 900 950 950 1000 1000 111E 112E 113E 114E 115E 116E 117E 111E 112E 113E 114E 115E 116E 117E (a) (b) MJ_M:sunrise TIME:06LST MJ_M:sunset TIME:17LST 500 500 550 550 600 600 650 650 700 700 750 750 800 800 850 850 900 900 950 950 1000 1000 111E 112E 113E 114E 115E 116E 117E 111E 112E 113E 114E 115E 116E 117E (c) (d) PY_L:sunrise TIME:06LST PY_L:sunset TIME:17LST 500 500 550 550 600 600 650 650 700 700 750 750 800 800 850 850 900 900 950 950 1000 1000 111E 112E 113E 114E 115E 116E 117E 111E 112E 113E 114E 115E 116E 117E (e) (f) Figure 9: Te deviation (HHR days of each subregion minus non-HHR days) vertical sections (along 29 N) of divergence (blue line; − 6 − 1 10 s ) and θ (red line; K). se Te role of the topographic friction will be briefy dis- zF zζ zF � − fD + − . (1) cussed frst. On the northeast side of the Dongting Lake, the zt zx zy west-east direction is the transition zone from lake area to mountainous area (zF /zx)>0), and its north is from the Among them, ζ � (zv/zx) − (zu/zy) is the vertical mountainous area to the plain (zF /zy)<0); meanwhile, vorticity of horizontal full wind speed; y ′ ′ ′ the south is from the mountainous area (north slope of D � (zu /zx) + (zv /zy) is ageostrophic divergence, where Xuefeng Mountain) to the lake area ((zF /zy)<0), so the ′ ′ v � v − v u � u − u are ageostrophic wind, and F , F is y g, g x y topographic friction efect is favorable for the local vorticity friction. Advances in Meteorology 11 MJ_M:sunrise MJ_M:sunset 500 500 550 550 600 600 650 650 700 700 750 750 800 800 850 850 900 900 950 950 1000 1000 0123456789 1011121314151617181920212223 0123456789 1011121314151617181920212223 (a) (b) PY_L:sunrise PY_L:sunset 500 500 550 550 600 600 650 650 700 700 750 750 800 800 850 850 900 900 950 950 1000 1000 0123456789 1011121314151617181920212223 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 (c) (d) NONE 0123456789 1011121314151617181920212223 (e) Figure 10: Te diurnal variation (hourly value minus daily average value) of the ageostrophic wind when HHR occurs at sunrise or sunset in each region (Figure 10a-d) and the diurnal variation of the ageostrophic wind on non-HHR days (Figure 10e). Te regional average range of ° ° ° ° Figure 10e is (27 N–32 N, 110 E–118 E), and the regional average range of other fgures is marked in the upper right corner. Te contour line represents the wind speed, and one bar of wind bar is 1m/s. to be enhanced over the DT_L area. Tis may be the reason more HHR events either in early morning or evening to about why the average location of the low-level vorticity likely start on the south of DT_L and northwest slope of center is generally located near DT_L, regardless of HHR MJ_M. events happening in any subregion (Figure 5). Te topo- Te diurnal variation of the low-level divergence by graphic friction efect is an important forcing mechanism on ageostrophic wind is another important mechanism which 12 Advances in Meteorology (2) Te ageostrophic efect of air fow in low levels is induces the diurnal variation of the HHR events. In the early morning v >0, the enhancement of vorticity always follows possible to be an important mechanism which in- duces the diurnal variation of the HHR events. Te the stronger convergent movement in low levels. Tis can be confrmed by the sequential evolution of the vapor fux high-frequency HHR events in early morning are convergence and vorticity (Figure 6). In the evening, v <0 guided by nocturnal intensifcation of the favorable with the negative ageostrophic vorticity (ζ <0), the en- synoptic system, and HHR events appear in the stage hancement of the low-level convergence induces the positive of the supergeostrophic efect weakening and the vorticity of assembling wind speed to weaken slightly with vapor fux convergence going down from the quickly increasing the HHR station numbers during 16: maximum, which is in correspondence with vertical 00–17:00 (Figures 6 and 4). On the other hand, in corre- vorticity and front forcing strengthening at low spondence with HHR events at sunset either in MJ_M or levels. On the other hand, the convergent lifting is intensifed in the west and south slope of MJ_M by PY_L, the maximum absolute value of the average sub- geostrophic wind in low levels is larger than that at sunrise strengthening low-level south-westerly with super- geostrophic fow, and their synergic efect induces (the diferences is about 0.2∼0.4m/s), and it indicates that the stronger subgeostrophic acceleration is favorable for more HHR events to start along the two slopes of triggering HHR events in the evening. MJ_M in the early morning. Te high-frequency HHR events in the evening are led by the local lifting trigger with more unstable atmospheric stratifcation 6. Conclusions and the diurnally weakening synoptic system. Strengthening local lifting is conducted by the In this study, the hourly surface observation data with higher coupling efect of terrain forcing and intensifying spatial resolution and ERA5 reanalysis data from April to northern subgeostrophic fow, and it induces more October during 2012–2017 in the middle YRV were used to HHR events to begin near the ridge of MJ_M in the investigate the fundamental characteristics of HHR events evening than in the early morning. around the twain-lake basins and their possible mechanisms, such as the temporal-spatial distribution, diurnal variation, and the complicated interactions among the favorable Data Availability synoptic system, the locally mesoscale topography, and lake- land circulation. Results indicate Te Chinese Academy of Meteorological Science provides the national stations and regional automatic stations’ data in (1) PY_L area and MJ_M area are regions of the high- China. ECMWF datasets provide the ERA5 Global Re- frequency HHR events in the middle YRV. As the analysis Data. (https://cds.climate.copernicus.eu/cdsapp#!/ two largest lakes in the region, DT_L and PY_L dataset/reanalysis-era5-pressure-levels?tab�overview). locate at similar latitude and, respectively, in two sides of MJ_M, but the most active HHR events appear around PY_L and the lowest frequency of Conflicts of Interest HHR events over DT_L in the region. Other high- All authors declare that they have no conficts of interest. frequency HHR events distribute in MJ_M, and this is diferent from the statistic results based only on sparse national stations. Some previous studies Acknowledgments considered that the frequency of HHR events in MJ_M was not only lower than over PY_L but even Tis study was supported by the National Key Research and lower than over DT_L. 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Published: Dec 21, 2022