Influence of Underlying Surface on Distribution of Hourly Heavy Rainfall over the Middle Yangtze River Valley

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HHR is a result of the interaction among multiscale systems [13][14][15]. Scientists have analyzed the afecting factors of HHR events from diferent perspectives. For example, Luo et al. [16] classifed the extreme hourly heavy precipitation into four categories according to the synoptic situations in central and eastern China. Shaw et al. [17] and Wang et al. [18] analyzed the efects of surface temperature and humidity on heavy precipitation. Liang and Ding [19] studied the long-term variation of extreme heavy precipitation by the urbanization efect in Shanghai during 1916 to 2014. However, the fne-grained forecast of HHR is still difcult currently since HHR is not only related closely to evolution of multiscale synoptic systems themselves but afected by the boundary forcing from complex terrain or various underlying surfaces. Lock and Houston [20] pointed out that the convection initiation mostly occurred near signifcant terrain and waters. Guo and Sun [21] analyzed three types of convective systems with diferent organizational forms in Hubei Province and found that a large number of nonlinear convective systems may be formed during the movement of isolated convective storms triggered earlier in mountains and hills to plain areas. A large number of studies have shown that the convection triggering and development evolution under diferent underlying surface conditions are signifcantly diferent [22][23][24][25][26][27][28][29][30][31]. Terefore, the study on the triggering mechanism of HHR events near multiscale mountains or giant lakes has become an important branch of investigating heavy rainfall.
Te diurnal variation of convection triggering and evolution is caused by the diference of underlying surfaces [32]. For example, the bimodal diurnal variation characteristics of HHR frequency in the Yangtze River Valley are a phenomenon diferent from the unimodal characteristics in most regions of China. As for the formation of bimodal diurnal variation characteristics, it is generally believed that the sunset peak is mainly related to solar radiation heating [33], but there are fve views on the causes of the sunrise peak. Tey are (1) the eastward propagation of the convective system from the Qinghai-Tibet Plateau. However, not all nocturnal convective systems in the Yangtze River Valley move eastward from the east side of the plateau [34,35]. (2) Te mountain-plains solenoid efect [36]. Te mountain-plains solenoid efect is a large-scale thermal circulation caused by the three order topography in China, and the solenoid efect of mesoscale mountains on it is lack of investigation. (3) Te nocturnal strengthening of low-level jets or boundary layer jets [34,[37][38][39]. Generally, the appearance time of the strongest jet is mostly earlier than that of the precipitation peak in the early morning, but it is usually considered that early morning was corresponding with the strongest water vapor transport [38]. (4) Locally thermal circulation strengthens the precipitation at night. Generally, the sea-land or lake-land breeze circulation has a more signifcant enhancement efect on night precipitation, such as the coastal areas of South China [40][41][42]. In addition, Xue et al. [38] believed that the infuence of thermal circulation is not as signifcant as inertial oscillation in Central China on the early morning HHR events. (5) Convection enhancement was caused by radiation cooling over the cloud top at night [43,44], but Yin et al. [44] pointed out that it mainly induced night rains in the west of 110°E in China.
To sum up, the reasons for high-frequency HHR at sunrise can be summarized from three perspectives. First, it is caused by movement or propagation of the convection system, as in viewpoint 1. Second, it is induced by the thermal efect, such as viewpoints 2, 4, and 5. Tird, it is conducted by the dynamic efect, as in viewpoint 3. Te dynamic efect is usually referred to the enhanced convergence and water vapor transport by low-level jets. Tere are the following views on the reasons for the enhancement of the nocturnal low-level jets: inertial oscillation [45], thermal forcing [46], and a combination of both [47]. In fact, the dynamic efect actually is also closely related to the thermal efect. Du and Chen [48] pointed out that the general lowlevel jet referred to the jet near 850 hPa, and the 925 hPa or lower jet is called the boundary layer jet. Boundary layer jets can interact with the terrain, so the dynamic and thermal efects of the boundary layer are extremely important for the increase of HHR [49,50]. Focusing on the complicated mesoscale underlying surfaces around the twain-lake basins, this study will analyze the basic features of the HHR event diurnal variation and the possible efect mechanism. Te previous analysis on the causes of high-frequency HHR at sunrise has been relatively comprehensive, but there is no much concern about the causes and evolution of high-frequency HHR at sunset. It is generally believed that it is related to thermal instability. However, the twain-lake basins in the middle YRV (TLB-YRV) have a complex underlying surface, including water bodies, mountains, and plains. Te two lakes, Dongting Lake and Poyang Lake, are the largest freshwater lakes in China. Between the two lakes is Mufu-Jiuling Mountain, and Jianghan Plain lies to the northwest of the two lakes. Te corresponding HHR characteristics under complex terrain are not consistent. How does the underlying surface afect the distribution and evolution of HHR? Te surrounding area of the TLB-YRV is an important food production region and the key transportation hubs of Central China. Terefore, the study of the distribution characteristics and infuencing factors of HHR on TLB-YRV underlying surfaces is the basic work to improve the precision forecast of the heavy rain.
Te paper is organized as follows: the data and processing methods are introduced in Section 2. Section 3 introduces the characteristics of HHR events around the twain-lake basins, especially the characteristics of diurnal variation. Te possible mechanism of afecting temporalspatial distribution of HHR events around the twain-lake basins are analyzed in Section 4. Section 5 discusses the infuence mechanism of the underlying surface. Conclusions are given in Section 6.

Data and Methods
2.1. Regional Division. Te regional division in this paper is based on the underlying surface attributes. As shown in Figure 1(a), the brown line boxes 1 and 4 are, respectively, Dongting Lake (DT_L) and Jianghan Plain (JH_P) areas, and the corresponding underlying surfaces are, respectively, lake and plain areas. Te boundary is mainly based on the lake area and terrain height. Brown line boxes 2 and 3 are Mufu Mountain (MJ_M) and Poyang Lake (PY_L) areas, respectively, and the corresponding underlying surfaces are mountain areas and lake areas, respectively. Based on the terrain height, the junction of mountain areas and lake areas is taken as the boundary between the two areas. Te northern boundary of the two areas is the junction of mountain areas and plains, and the southern boundary is the boundary of the middle YRV.
In Figure 1

Processing of Ground Observation Data.
Tis study collected hourly ground observation data with higher spatial resolution, including national-level and regional-level automatic stations in the middle YRV (28°N-32.5°N, 110°E-117°E) from April to October during 2012-2017. All data are quality controlled using the method of [51], and doubtful data are eliminated. Te station whose arrival rate (the ratio of the available record hours to the total observation hours) reaching more than 90% was labeled as an efective observation. In this study, the number of efective rainfall stations reaches to 3109 (Figure 1(b)). Te hourly precipitation at any station that reached to 20 mm is recorded as an HHR event [2,50,52].

Defnition of HHR Day and Non-HHR Day.
During the period from 00:00 to 23:00 (local standard time-LST, the 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 day of the subregion. For Non-HHR days, it is required that the stations in the four subregions in Figure 2 do not have HHR within 24 hours on a certain day, hereinafter referred to as NONE.

Processing of Atmospheric Background Data.
Te ERA5 global reanalysis data (horizontal resolution 0.25°× 0.25°, with hourly temporal resolution) were used to investigate atmospheric evolution. Te hourly average felds of the HHR days in each subregion are called JH_P: ave, DT_L: ave, MJ_M: ave, and PY_L: ave. Te hourly average felds of the HHR days in all subregions (excluding the same date in each subregion) are called as ALL: ave. In order to analyze the relevant characteristics of the two peak periods of HHR daily variation: sunrise (05:00 to 09:00) and sunset (16:00 to 18: 00), the composite mean feld of the corresponding periods 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, MJ_ M: sunrise, PY_ L: sunrise, JH_ P:sunset, DT_ L: sunset, MJ_ M: sunset, and PY_ L: sunset.

Spatial Distribution of HHR Frequency.
From the annual average frequency of HHR events in the middle YRV from 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 fewer in the northwest. PY_L and MJ_M are the two regions with the highest-frequency HHR events. Tis is slightly diferent from the results of Chen et al. [53] only using the national stations analysis. Chen et al. [53] indicated that DT_L and PY_L were high-frequency areas of HHR events, and the frequency in MJ_M was not only lower than in PY_L but was even lower than in DT_L. Tis should be caused by scattering data used in their investigation, and the mesoscale features of HHR events in the complex terrain region were impossible to be exposed well by scattering national stations. 0  125  250  375  500  625  750  875  1000  1125  1250  1375  1500  1625  1750  1875  2000 Unit: m

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(2) Tere were signifcant diferences in the frequency of HHR events between the two giant lakes (Dongting Lake (DT_L) and Poyang Lake (PY_L)), which are the two largest lakes with similar latitude in the middle YRV. Most HHR activities appear around PY_L, and the lowest frequency of HHR events occurs in DT_L area. Tis may be closely related to diferences in topographic distribution around the two lakes. Te surrounding area of DT_L is relatively fat, its west and south sides are relatively far away from the mountains, and its north side is adjacent to Jianghan Plain. However, the surrounding terrain of PY_L is much more complex, similar to a basin, with some small hills scattered around it. (3) Te frequency distribution in MJ_M was closely related to the mountains. Te two high-frequency bands in MJ_M were located, respectively, in the western side of Mufu-Jiuling Mountain and along the Jiuling Mountain. (4) Te frequency 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 in the early morning of the Dabie Mountains and the lowlevel jet in the boundary layer and pointed out that the highfrequency HHR was related to the inertial oscillation of lowlevel jets. Te terrain of the Dabie Mountain is along northwest-southeast direction, almost perpendicular to the southwest jet. Te lifting mechanism in the windward slope was considered to play an important role in the early morning HHR of the Dabie Mountains. However, the terrain of Mufu Mountain is along the southwest-northeast direction, parallel to the southwest jet, and the infuence of the underlying surface increases the infuence of the water body. Are the temporal and spatial distribution and diurnal variation features of HHR events afected by local circulation? Is there any interaction between local circulation and weather system?

Diurnal Variation of the HHR Frequency.
Te diference of temperature, humidity, and turbulence caused by solar radiation on diferent underlying surfaces results in the local circulation of the boundary layer and its diurnal variation. By statistical analysis of the diurnal variation characteristics of the HHR events in the four subregions ( Figure 3), it is found that there are bimodal diurnal variation characteristics of HHR frequency in each subregion. Te diurnal variation of HHR event frequency over the PY_L and MJ_M presents obviously bimodal characteristics, and the two peaks appear at 17:00 at sunset and about 06:00 at sunrise. Tis is consistent with the diurnal characteristics of the middle-lower reaches of the Yangtze River Valley investigated by Yu et al. [11,12]. Generally, the most unstable atmospheric stratifcation appears in the afternoon, so the peak time of HHR frequency at sunset is essentially the same as the thunderstorm or gale event frequency in this region [21]. Te peak of HHR event frequency at sunrise is related to the interaction between local circulations and the synoptic system. However, DT_L and PY_L are, respectively, located in the east and west of MJ_M. Te bimodal diurnal variation around PY_L is signifcant, while the bimodal feature around DT_L is relatively insignifcant. Te bimodal characteristics around JH_P are not signifcant as DT_L. Tis is related to the lower HHR frequency in these two regions.
In order to clarify the relationship between the frequency of HHR events and the underlying surface environments, Figure 4 shows the hourly distribution and topography of the HHR events around the TLB-YRV. At MJ_M area, the spatial distribution and evolution of high-frequency HHR events (red dots) at sunrise are diferent from those at sunset. At sunrise, high-frequency HHR frst occurs on the northwest slope and then gradually extends to the whole MJ_M area. At sunset, high-frequency HHR erupts along the ridge of Mufu Mountain and usually spreads to the surrounding areas with a fast decreasing frequency. As shown in Figure 4, the number of stations with high-frequency HHR 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. At 08:00-09:00, the high-frequency stations are relatively well distributed over Mufu Mountain and Jiuling Mountain. After 10:00, the number of high-frequency stations is signifcantly reduced. At 16:00, the number of high-frequency HHR stations in the western Mufu Mountain increased rapidly again. Te high-frequency stations were more concentrated and closer to the ridge of Mufu Mountain than at 06:00. After 18:00, the number of high-frequency stations decreases obviously, and the distribution gradually tends to disperse. In the late evening (until 23:00), the number of high-frequency stations decreased more slowly than in the morning, which seems inconsistent with Figure 3. Tis is because the frequency of HHR events is much higher during 16:00-17:00 than that in later evening over the MJ_M area.
In PY_L area, the evolution of high-frequency HHR events during sunrise and sunset is in good agreement with the lake-land breeze circulation; that is, the high-frequency HHR at sunrise extends outward from the center of the lake, and the high-frequency HHR at sunset spreads from mountains to the center lake. As shown in Figure 4, during 06:00-10:00, stations with the high-frequency HHR events are mainly distributed on the northeast and southwest of the Poyang Lake. Most of the stations with the high-frequency HHR events on the southwest are closer to Poyang Lake than to Jiuling Mountain. It is easy to fnd from Figure 4 that the distribution evolution of the stations with high-frequency HHR events initially appears near the lake center (during 05: 00-06:00) and then gradually expanded outward. Another frequency peak of HHR events over PY_L appears in the afternoon to evening. Te stations with high-frequency HHR events over PY_L frst appear along the northwest bank of the Poyang Lake (15:00)   then spread around the lake and locate away from mountains. Te number of the stations with high-frequency HHR events reaches the peak at 16:00-17:00, which is consistent with Figure 3. Comparing the evolution trends of the two high-frequency HHR periods of sunrise (05:00∼08:00) and sunset (15:00∼17:00), it can be found that the stations with high-frequency HHR events during the sunset period are stably distributed around the lake, and there is no highfrequency station similar to the sunrise near the central lake.
In DT_L, diferent from PY_L, the stations with the high-frequency HHR events are mainly concentrated on the south side of the Dongting Lake whether in morning or afternoon-evening. Te number and density of the stations with high-frequency HHR events over the DT_L region are much less than over PY_L, and the diurnal variation amplitude of HHR event frequency is also the lowest in the three regions.

Possible Mechanism of Underlying Surfaces on HHR Events
Te previous analysis shows that the frequency of HHR events around the TLB-YRV exhibits a bimodal diurnal variation, but amplitudes of the diurnal variation in the DT_L and JH_P are much smaller than that in MJ_M and PY_L. In mountain and lake areas, the location and movement trend of high-frequency HHR events in the morning and evening are also diferent. Te previous phenomena may be related to the following mechanisms: (1) diurnal variation of the HHR weather system and (2) local circulation of the boundary layer caused by diference of the underlying surface.

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 weather system and the underlying surface is mainly under 850 hPa. Figure 5 shows the 925 hPa synthetic wind felds and θ se when HHR events occur in four subregions (JH_P, DT_L, MJ_M, and PY_L), respectively. It is easy to be found that their boundary layer weather control systems are similar, but the HHR events in each subregion occur in diferent positions of the system. Tis means that most HHR events in the middle YRV are caused by medium-α-scale vortex or horizontal wind shear. Tis is consistent with the development of mesoscale vortex systems or wind shears in the lower troposphere during Mei-Yu period in the YRV [54][55][56][57], and the monthly average frequency of HHR events over the twain-lake basins indicated that most of HHR events happened during June to July (the fgure omitted) and were corresponding with the Mei-Yu season. Te boundary layer circulation of non-HHR roughly represents the local climate characteristics in warm seasons. Te HHR diurnal variation minus the non-HHR diurnal variation can remove the regular diurnal variation of the boundary layer fow feld so as to better reveal the diurnal variation of the weather system with HHR. Of course, it still includes the infuence of the weather system on the boundary layer circulation. Figure 6 shows the regional average diurnal variation of vorticity and vapor-fux divergence on HHR days minus non-HHR days, where the regional average range is 28°N-30.7°N, 112°E-117°E. It includes the three key subregions (DT_L, MJ_M, and PY_L) of TLB-YRV. Te maximum value of vapor fux convergence (− 1.1 × 10 − 7 g kg − 1 s − 1 ) in Figure 6 appears during 01:00-06: 00 under 950 hPa with the warm-wet low-level jet strengthening after midnight, and vorticity of the synoptic system reaches maximum (more than 1.7 × 10 − 5 s − 1 ) during 07:00-10:00 at 900 hPa. In the afternoon, the subgeostrophic efect causes reducing of the vortex system and vapor fux convergence at low levels in the afternoon, and the central value of vapor fux convergence goes down to − 6 × 10 − 8 g kg − 1 s − 1 during 13:00-16:00, and the convergence stars to enhance slightly again in the evening (about 17:00) at this level.
Te previous analysis shows that the HHR boundary weather system presents diurnal variation features: HHR events at sunrise are related with vorticity strengthening and appear after vapor fux convergence up to maximum. However, the high-frequency HHR events at sunset are impossible to be well explained by the diurnal variation of the synoptic system itself since the highest-frequency HHR events are corresponding with only slight increase of vapor fux convergence and no obviously enhancement of vorticity at low levels. On the other hand, the diurnal variation of the weather system cannot explain the diferent diurnal variation behaviors of the HHR events in mountain and lake areas. Tis implies that the local circulations caused by the complex underlying surface may play another important role in afecting the diurnal variation of the subregions HHR events around TLB-YRV.

Impact of the Underlying Surface.
First, from the average divergence/convergence at 925 hPa and surface fow on the non-HHR days (Figure 7), it can be found that the mountain-valley breeze caused by the MJ_M is the clearest boundary local circulation in the TLB-YRV. Te mountain breeze at sunrise corresponds to the divergence at 925 hPa, and the valley breeze at sunset corresponds to the convergence at 925 hPa. Te height of the lake-land breeze near the lake area is lower than that of the mountain-valley breeze because the land breeze in the morning (converging to the lake center) and the lake breeze in the evening (divergent to the lake shore) are clear near the surface, but the divergence of 925 hPa above the twain-lake basins is always greater than 0, especially at DT_L. At sunrise (Figure 7(a)), the convergence zone by mountain breeze is located along the south-eastern and western slopes of MJ_M over 925 hPa, and the strongest convergence center (− 1.5 × 10 − 5 s − 1 ) is along the western slope of MJ_M or the eastern side of DT_L. Tis indicates that the mountain breeze is more conducive to strengthening the convergence and uplifting movement on the west side of MJ_M, and its position is consistent with the location of the high-frequency HHR events in the early morning. At sunset, the thin lake breeze, which is guided by DT_L and PY_L on both sides of MJ_M, is favorable for intensifying the valley breeze and induces stronger convergence toward the mountain top. Te superposition efect of mountain-valley breeze and lakeland breeze leads to the convergence intensity of the mountain top at sunset (the maximum convergent

Advances in Meteorology 7
intensity in 925 hPa reaches − 2.0 × 10 − 5 s − 1 ) is stronger than that along the slope at sunrise (convergent intensity is down to − 1.5 × 10 − 5 s − 1 ). Ten, we analyze the divergence/convergence at 925 hPa and surface fow on HHR days (Figure 8). Considering that 02:00 and 14:00 are the least active periods of HHR events (Figure 3), the low-level fow and divergence at these two time points can maximally avoid the feedback efect of HHR events on local circulation and can better reveal the characteristics of boundary layer circulation under the action of HHR weather systems. It can be seen from Figure 8 that the patterns of average surface fow and divergence in 925 hPa at 02:00 of the HHR days (Figure 8(a)) are roughly similar to that of Figure 7(a). Te land breeze convergence over DT_L region is clearer. Tere is a stronger convergence (the minimum value is less than − 2.0 × 10 − 5 s − 1 ) around MJ_M slopes, and there is a more intense divergence (the maximum value reaches up to 2.5 × 10 − 5 s − 1 ) at mountaintop over 925 hPa. Figure 8(b) is similar to Figure 7(b), MJ_M is covered by local convergence in 925 hPa at 14:00 (Figure 8(b)), and their intensities are roughly similar. However, due to the infuence of the HHR weather system, northerly wind prevails on the ground in the middle YRV. Te surface fow feld on the top of the mountain is even dominated by downslope wind (similar to mountain wind), but the convergence area of MJ_M at 925 hPa is obviously larger than that in Figure 7. It can be seen that the weather system conducive to HHR events has not fundamentally changed the pattern of divergence and convergence around MJ_M at sunrise and sunset, but they are signifcantly strengthened.
Last, Figure 9 shows the deviation (HHR days of each subregion minus non-HHR days) vertical sections (along 29°N) of θ se and divergence. It can be found that no matter which subregion is hit by the HHR event, the deviation patterns of θ se and divergence in the vertical sections along 29°N are slightly similar. Corresponding with the peak of HHR events frequency at sunrise, a clearly and distinguishably slant front under 850 hPa distributes from west (cold) to east (warm) with a high θ se zone over the slant front. On the contrary, there is not obviously a horizontal gradient of θ se in the low-level troposphere around MJ_M at sunset. It is furtherly confrmed that the high-frequency HHR events at sunrise are guided by the intensifying nocturnal synoptic system, which not only presents vorticity and vapor-fux convergence strengthening but also represents front forcing to be intensifed. In addition, the west side of MJ_M, as the windward slope of the southwest low-level jet, is afected by the supergeostrophic efect before sunrise, which enhances the convergence enhancement of the west side of MJ_M, resulting in stronger rising than that of non-HHR days. Te deviation of the conditional static stability ( zθ se ′ /zp > 0) of the troposphere below 750 hPa in the MJ_M region shows that it is more unstable at sunset than at sunrise, indicating that local uplift plays a decisive role in triggering the convective high-resolution high-frequency HHR event at sunset. It shows that the local lifting plays a decisive role in triggering the high-frequency HHR events at sunset. Te diferences of the stratifcation status and trigger mechanism induce the variation of convective intensity at sunrise and sunset. It can be confrmed by some features of radar refective intensity, which correspond to the two peak periods of HHR events in MJ_M, respectively (Figure omitted). For example, the average height of maximum radar refective intensity of high-frequency HHR events at sunset is higher than that at sunrise (3.01 km vs 2.76 km), and the average thickness of radar refective intensity above 40 dBZ at sunset is thicker than that at sunrise (1.55 km vs 1.26 km). In conclusion, the strong convergence movement at sunset cannot be directly caused by the weak weather system itself but may be caused by the topographic forcing caused by the underlying surface and the thermal circulation in the boundary layer. In addition, it is worth noting that (1) when HHR occurs in any subregion, the slant convergence is well matched from the eastern side of MJ_M to the western shore of PY_L, but their intensities present obviously  (2) Below 800 hPa, the conditional static stability between PY_L and MJ_M is more unstable than that between DT_L and MJ_M. It implies that corresponding to the HHR synoptic system, the local convergence intensity and unstable stratifcation in the east of MJ_M are more remarkable than those in the west, and more convective HHR events are induced near the top of MJ_M and along the lakeshore of PY_L at sunset.

Discussion
Generally, the intensifying low-level airfow caused by inertial oscillation is considered to be a signifcant forcing factor to induce nocturnal heavy rainfall [38,40]. Te oscillation nature of low-level airfow is thought as a result of the diurnal variation of the turbulence intensity [41,45]. Te supergeostrophic wind appears after midnight with turbulence weakening under the boundary layer, and the wind vector rotates clockwise and induces the vorticity and low-level jet to be intensifed. On the contrary, the enhancing turbulence induces the subgeostrophic efect in the afternoon. Hourly ERA5 data can well show the diurnal variation characteristics of inertial oscillation and have a certain reproducibility ability for the mountain-valley breeze or lake-land breeze in the study area of this paper. However, it is still difcult to quantitatively separate the diurnal variation of wind feld caused by inertial oscillation from the thermal circulation in the boundary layer, so geostrophic wind is used to analyze them together. Te ageostrophic fow can well represent divergence/convergence, so the analysis of the ageostrophic fow on diferent underlying surfaces can explain the action mechanism of the topographic forcing caused by the underlying surface and the boundary layer thermal circulation on HHR. Figures 10(a)-10(d) shows the diurnal variation (hourly value minus daily average value) of the ageostrophic wind when HHR occurs at sunrise or sunset in each region, and Figure 10(f) shows the diurnal variation of the ageostrophic wind on non-HHR days. It is easy to fnd that whether on HHR days or non-HHR days, the diurnal variation direction of the low troposphere ageostrophic wind has obvious inertial oscillation characteristics (clockwise rotation with time). Te diurnal variation amplitude of the ageostrophic wind in non-HHR days is weaker than that in any subregion with HHR events. In non-HHR days, the maximum supergeostrophic wind (southwestern wind less than 1.5 m·s − 1 ) from midnight to early morning occurs near 875 hPa, and the strongest subgeostrophic wind (northern wind less than 1.5 m·s − 1 ) appears near 950 hPa from afternoon to evening. Compared with the regional ageostrophic wind of the non-HHR days, the average ageostrophic wind of HHR days in MJ_M and PY_L shows bigger amplitude of diurnal variation either HHR events at sunrise or sunset. Te strongest supergeostrophic phenomenon induces more a powerful southwest low-level jet at midnight, and the strongest subgeostrophic (northern wind) that appears in the evening signifcantly reduces the prevailing southern fow. Corresponding to HHR days, the subregional average velocity of the maximum ageostrophic (regardless of supergeostrophic or subgeostrophic) wind over MJ_M is stronger than that over PY_L, and their diference is about 0.2 m/s. It indicates that the terrain is not only conducive to the strengthening low-level jet at midnight but also its friction efect is more favorable for producing a subgeostrophic efect in the evening. It is also worth noting that the strong subgeostrophic wind occurs from 18:00 to 19:00, which is about 1-2 hours later than the sunset peak time (16: 00-17:00) of HHR frequency. Tis means that more HHR events in evening occur during the stage of the subgeostrophic wind strengthening, instead of the strongest subgeostrophic period. More HHR events in the early morning (05:00∼09:00) occur in period of the low-level supergeostrophic wind weakening (Figure 10) or the lowlevel vertical vortex gradually intensifying ( Figure 6). Te mechanism may be related to the convergent forcing caused by the ageostrophic efect.
Te vertical vorticity tendency equation without the advection term is as follows:
Te role of the topographic friction will be briefy discussed frst. On the northeast side of the Dongting Lake, the west-east direction is the transition zone from lake area to mountainous area (zF x /zx) > 0), and its north is from the mountainous area to the plain (zF y /zy) < 0); meanwhile, the south is from the mountainous area (north slope of Xuefeng Mountain) to the lake area ((zF y /zy) < 0), so the topographic friction efect is favorable for the local vorticity  to be enhanced over the DT_L area. Tis may be the reason about why the average location of the low-level vorticity center is generally located near DT_L, regardless of HHR events happening in any subregion ( Figure 5). Te topographic friction efect is an important forcing mechanism on more HHR events either in early morning or evening to likely start on the south of DT_L and northwest slope of MJ_M. Te diurnal variation of the low-level divergence by ageostrophic wind is another important mechanism which  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 1 m/s. induces the diurnal variation of the HHR events. In the early morning v ′ > 0, the enhancement of vorticity always follows the stronger convergent movement in low levels. Tis can be confrmed by the sequential evolution of the vapor fux convergence and vorticity ( Figure 6). In the evening, v ′ < 0 with the negative ageostrophic vorticity (ζ ′ < 0), the enhancement of the low-level convergence induces the positive vorticity of assembling wind speed to weaken slightly with quickly increasing the HHR station numbers during 16: 00-17:00 (Figures 6 and 4). On the other hand, in correspondence with HHR events at sunset either in MJ_M or PY_L, the maximum absolute value of the average subgeostrophic wind in low levels is larger than that at sunrise (the diferences is about 0.2∼0.4 m/s), and it indicates that the stronger subgeostrophic acceleration is favorable for triggering HHR events in the evening.

Conclusions
In this study, the hourly surface observation data with higher spatial resolution and ERA5 reanalysis data from April to October during 2012-2017 in the middle YRV were used to investigate the fundamental characteristics of HHR events around the twain-lake basins and their possible mechanisms, such as the temporal-spatial distribution, diurnal variation, and the complicated interactions among the favorable synoptic system, the locally mesoscale topography, and lakeland circulation. Results indicate (1) PY_L area and MJ_M area are regions of the highfrequency HHR events in the middle YRV. As the two largest lakes in the region, DT_L and PY_L 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 HHR events over DT_L in the region. Other highfrequency HHR events distribute in MJ_M, and this is diferent from the statistic results based only on sparse national stations. Some previous studies considered that the frequency of HHR events in MJ_M was not only lower than over PY_L but even lower than over DT_L. Te investigation fnds that there are two high-frequency bands over MJ_M: one of them is located along the northwest slope of Mufu Mountain, and another presents basically along Jiuling Mountain and extends to the west bank of the PY_L. Te frequencies of HHR events in subregions of the middle YRV almost all present bimodal diurnal variations but with diferent amplitude. PY_L or MJ_M shows a signifcant bimodal characteristic, with two peaks at sunset (about 17:00) and sunrise (about 6:00). Te hourly location evolution of HHR events display that the initial location of the highfrequency HHR events over MJ_M is closer to the mountain top in the evening than in the early morning. Te high-frequency stations of HHR events around PY_L in the evening locate further away from the center of the lake than in the early morning.
(2) Te ageostrophic efect of air fow in low levels is possible to be an important mechanism which induces the diurnal variation of the HHR events. Te high-frequency HHR events in early morning are guided by nocturnal intensifcation of the favorable synoptic system, and HHR events appear in the stage of the supergeostrophic efect weakening and the vapor fux convergence going down from the maximum, which is in correspondence with vertical vorticity and front forcing strengthening at low levels. On the other hand, the convergent lifting is intensifed in the west and south slope of MJ_M by strengthening low-level south-westerly with supergeostrophic fow, and their synergic efect induces more HHR events to start along the two slopes of 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 and the diurnally weakening synoptic system. Strengthening local lifting is conducted by the coupling efect of terrain forcing and intensifying northern subgeostrophic fow, and it induces more HHR events to begin near the ridge of MJ_M in the evening than in the early morning.

Conflicts of Interest
All authors declare that they have no conficts of interest.