The possible impact of urbanization on a heavy rainfall event in Beijing

Transcription

1 PUBLICATIONS Journal of Geophysical Research: Atmospheres RESEARCH ARTICLE Key Points: Urbanization influences the accumulated rainfall amount by 30% The start time of the precipitation event is advanced by urbanization Urbanization plays a significant role in frontal-type rainfall Correspondence to: Y. Liu, lym@lasg.iap.ac.cn Citation: Yu, M., and Y. Liu (2015), The possible impact of urbanization on a heavy rainfall event in Beijing, J. Geophys. Res. Atmos., 120, , doi: / 2015JD Received 4 MAR 2015 Accepted 17 JUL 2015 Accepted article online 22 JUL 2015 Published online 21 AUG American Geophysical Union. All Rights Reserved. The possible impact of urbanization on a heavy rainfall event in Beijing Miao Yu 1,2 and Yimin Liu 1,3 1 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China, 2 Institute of Urban Meteorology, China Meteorological Administration, Beijing, China, 3 Joint Center for Global Change Studies, Beijing, China Abstract The impact of urbanization on a heavy rainfall event that occurred in Beijing on 21 July 2012 was investigated using version of the Weather Research and Forecasting Model coupled with a multilayer urban canopy model. High-resolution land use data for Beijing in 2010 with modified urban parameterization were introduced into the model. Evaluation showed that the simulation result generally agreed well with observations. Two sensitivity tests with different urban high-resolution land use scenarios were employed to analyze the impact of urban expansion on this rainfall event. The simulation results confirmed that urbanization expansion played an important role in the distribution and intensity of precipitation for this extreme event. Urbanization led to total precipitation increasing in upstream and downstream directions. The start time of the precipitation process was advanced by 1 h, and the duration became longer due to the influence of urbanization. Moreover, urbanization caused the spatial distribution of precipitation to become more concentrated. The total precipitation amount above 250 mm and the frequency of precipitation intensity above 40 h 1 mm are both increased. The results of this study show that urbanization plays a significant role in frontal-type rainfall. 1. Introduction An extreme rainstorm hit Beijing from 21 to 22 July The duration of this rainfall event was about 20 h. The average precipitation in Beijing was about 170 mm (24 h) 1, which was the largest rate since 1951 [Sun et al., 2013]. The rainstorm was centered over Fangshan in the southwest of Beijing, and the 24 h accumulated precipitation reached 460 mm. The maximum hourly precipitation amount occurred in the Pinggu Guajia valley, where the peak value was mm. Record-breaking precipitation amounts were recorded at five meteorological stations (Xiayunling, Fangshan, Haidian, Shijingshan, and Mentougou) in southwest Beijing, and 90% of the administrative area experienced more than 100 mm (24 h) 1 of precipitation. As a result of the heavy rain, severe urban waterlogging occurred in the city. The event had a major impact on traffic and caused severe economic loss and human casualties. Therefore, this severe meteorological event, named Rainstorm 721, was (and remains) of great concern. Beijing is situated in the northern part of the North China Plain at (39.9 N, E) and at an elevation of m. Mountains to the north, northwest, and west shield the city. Beijing has a rather dry, monsoon-influenced humid continental climate, characterized by hot and humid summers due to the East Asian monsoon, and generally cold, windy, and dry winters that reflect the influence of the vast Siberian anticyclone. Annual rainfall in Beijing has generally shown a downward trend since 1951, but the frequency of extreme precipitation events has increased significantly under global climate warming [Zheng and Liu, 2008; Zhu et al., 2012]. But to what extent is this phenomenon caused by Beijing s rapid urbanization and land use change? A few studies have examined the causes and predictability of Rainstorm 721 [e.g., Zhang et al., 2013; Zhou et al., 2013; Sun et al., 2013], which together suggest that the convective cells were triggered by local topography and propagated with the large-scale weather system [Zhang et al., 2013; Sun et al., 2013]. These studies, however, have paid little attention to the possible impact of urbanization on this rainfall event. Urbanization can affect precipitation through the following mechanisms: (1) The surface roughness is increased by urbanization, leading to enhanced convergence [Changnon et al., 1981; Bornstein and Lin, 2000; YU AND LIU HEAVY RAINFALL ON 21 JULY

2 Figure 1. Domain configuration and location of the study area. White dotted line is the location of Beijing ( N, E) and the white point is the center of Beijing city (39.80 N, E). Thielen et al., 2000; Wang et al., 2012]; (2) Destabilization is increased in the boundary layer [Shepherd et al., 2002; Shepherd and Burian, 2003;Yang et al., 2012]; (3) Increases in aerosols associated with urban land use lead to change in cloud condensation nuclei (CCN) sources [Diem and Brown, 2003; Molders and Olson, 2004]; and (4) Precipitating systems are bifurcated or diverted by the urban canopy or related processes [Loose and Bornstein, 1977; Bornstein and Lin, 2000]. Horton [1921] found that thunderstorms appear to be more frequent in urban than rural areas. Results from the Metropolitan Meteorological Experiment (METROMEX) have shown that precipitation increases in association with urbanization in the summer. A downstream effect was also revealed by METROMEX, with precipitation over downwind cities increasing by 5% 25% compared with the average value [Changnon, 1979; Changnon et al., 1981; Braham et al., 1981; Changnon et al., 1991]. The size of the urban area can also determine the magnitude and pattern of downwind precipitation [Changnon, 1992]. With the rapid urbanization that has, and is still, taking place in Beijing, it is becoming increasingly important to assess the role of this process in high-impact weather. The aim of the present paper is to establish the extent of the influence of urban-induced land use change and anthropogenic heat on the Rainstorm 721 extreme rainfall event by analyzing the results of numerical experiments. Different from previous study that was mainly based on with and without urban, we conduct the urban expansion experiments over 10 years and 20 years. 2. Data, Model Description, and Experimental Design The observed rainfall data used in this study are the gridded hourly rainfall grid data of a China automatic meteorological station combined with the CMORPH (the Climate Prediction Center morphing method) precipitation product from The model used in this study is version of the Weather Research and Forecasting (WRF) model. The multilayer urban canopy model (BEP) was switched on in the model to represent urban boundary layer physics. The BEP developed by Martill et al. [2002] can recognize the three-dimensional nature of urban surfaces and the process that the buildings vertically distribute sources and sinks of heat, moisture, and momentum through the whole urban canopy layer. Compared to the single-layer urban canopy model (embedded only in the first model layer), the BEP allows a direct interaction with the planetary boundary layer [Chen et al., 2011]. The simulation domain was centered at (39.0 N,116.0 E) and implemented with triple-nested grids at resolutions of 27, 9 and 3 km for three domains (D1, D2, and D3, respectively). The number of grid cells was for D1, for D2, and for D3 in the east-west and south-north directions (Figure 1). The model was initialized with National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis data (resolution: 1 1 ). The version of WRF that we employed incorporates the urban land cover parameterization of Miao et al. [2012], which includes the following: YU AND LIU HEAVY RAINFALL ON 21 JULY

3 Figure 2. High-resolution land use map in WRF/BEP: (a) 2010; (b) 2000; and (c) The building heights are 15 m (10%), 20 m (25%), 40 m (40%), and 30 m (25%) in industrial and commercial; 10 m (10%), 15 m (60%), and 20 m (30%) in high-intensity residential; and 5 m (15%), 10 m (70%), and 15 m (15%) in low-intensity residential. 2. Fraction of the urban landscape. The urban fractions are 0.95 in industrial and commercial, 0.9 in highintensity residential, and 0.5 in low-intensity residential in the default setting which does not include the natural vegetation; we reduce the fraction of urban landscape to in high-intensity residential according to the remote sensing data. In the control run (hereafter CTL) we used high-resolution (30 m) remote sensing data for the year 2010 in Beijing [Yu et al., 2013] to replace the Moderate Resolution Imaging Spectroradiometer (MODIS) land use data in WRF/BEP. The interpretation of the Beijing 2010 remote sensing data set was based mainly on the Landsat Thematic Mapper/Enhanced Thematic Mapper Plus sensors. We performed an atmospheric correction and used supervised classification combined with an artificial method to obtain the high-resolution (30 m) land cover data for 2010 in Beijing (Figure 2a). To evaluate the impact of urban expansion on the precipitation system over Beijing, we employed two other artificial scenarios in sensitivity experiments (Figures 2b and 2c). These tests were designed to analyze the contribution from urbanization. The first test (hereafter referred to as the 2000 test ) replaced the MODIS land use with the high-resolution (30 m) scenario of the year 2000 in Beijing, using the same method as in the CTL scenario. The second test was similar to the first, but for the year 1990 (hereafter referred to as the 1990 test ). The Beijing urban area is 923 km 2 in 1990, 1136 km 2 in 2000, and 1891 km 2 in The increase ratios are 23.07% from 1990 to 2000 and 66.46% from 2000 to To reduce the model uncertainty, the CTL and the two sensitivity tests began at three different times: at 1800 LST 19 July 2012, 0000 LST 20 July 2012, and 1200 LST 20 July 2012, respectively. Table 1 shows the model settings. 3. Observed Rainfall Characteristics On 21 July 2012, there were two heavy rain centers of more than 50 mm detected by satellite in China. One was in North China, in the Beijing-Tianjin-Hebei region, and the other was in the Sichuan Basin. The rainfall in Beijing City began at 1400 LST 21 July and finished at 0400 LST 22 July. This storm occurred under a weather pattern involving high pressure to the east and low pressure to the west and was affected by synoptic systems including vortices, troughs, cold fronts, and low-level jets. The Bay of Bengal, Bohai Sea, and Yellow Sea provided sufficient water vapor to the city [Sun et al., 2012]. There were three stages to this heavy rainfall process [Chen et al., 2012]. In the first stage (from 1000 to 1600 LST 20 July), cold air invasion and terrain triggered a mesoscale convective system (MCS), called warm-area precipitation, which was located in the southwest of downtown Beijing [Guo et al., 2015]. However, during the second stage (from 1700 to 1900 LST YU AND LIU HEAVY RAINFALL ON 21 JULY

4 Table 1. WRF/BEP Model Setting WRF3.6.1 D1 D2 D3 Horizontal grids Grid interval (km) Vertical layers 32 Time step (s) Microphysics Lin et al. scheme Cumulus Kain-Fritsch (new Eta) None LW radiation RRTM SW radiation Dudhia shortwave radiation PBL MYJ PBL LSM Noah LSM + BEP Building heights in industrial and commercial 15 m (10%), 20 m (25%), 40 m (40%), 30 m (25%) The maximum number of vertical levels is 18 in the urban grid. Building heights in high-intensity residential 10 m (10%), 15 m (60%), 20 m (30%) Building heights in low-intensity residential 5 m (15%), 10 m (70%), 15 m (15%) 20 July), a cold front primarily forced and intensified the MCS, called frontal-type precipitation [Sun et al., 2013]. The results of Sun et al. [2013] showed that the cold front had already influenced the Beijing area at 1000 LST 21 July, and the propagation speed of the rain band stretching from west to east increased significantly from 1400 LST 21 July to 0600 LST 22 July The precipitation amount in the second stage was much greater than that in the first stage: the rainfall amount that accumulated in 3 h during the second stage was about 128 mm, accounting for 46.7% of the total precipitation amount. The third stage took place after the frontal-type precipitation. The frontal process played a decisive role in the storm, despite lasting only for 3 h in downtown Beijing. Thus, whether or not urbanization can influence the frontal-type precipitation stage is the key to determining its influence on the storm as a whole. 4. Results 4.1. Precipitation Simulated in CTL Compared to the observation (Figure 3a), the CTL simulation result is basically consistent. In the CTL run, total precipitation is above 100 mm in most parts of Beijing. One extreme precipitation area is located in the southwest of Beijing and another smaller one is in the northeast (Figure 3a). However, there are some deviations in the simulated precipitation patterns of northeast Beijing. The area of total precipitation above 230 mm is larger in the observation than in CTL. But overall, the CTL simulated the distribution of precipitation reasonably well, except that the northeastern peak precipitation simulated in CTL (Figure 3b) is smaller than the observed (Figure 3a). Therefore, the performance of the model in simulating the total precipitation is satisfactory. To further verify the model s performance with respect to this heavy rainfall process, we also analyze the CTL hourly precipitation during 1400 LST 21 July to 1000 LST 22 July The start time of the precipitation defined as hourly precipitation is larger than 10 mm (1400 LST 21 July) in Beijing City. Start time in CTL (Figure 4b) is delayed by about 2 h compared to the observation (Figure 4a). The hourly precipitation of CTL is slightly less than observed, and the area over which hourly precipitation is greater than 50 mm in CTL is smaller than observed. Nevertheless, the distribution of the simulated hourly precipitation (Figure 4) is similar to the observation. The 2000 test and 1990 test were also able to simulate the complete precipitation process relatively well, but the start time of the precipitation was further delayed by 1 h and the duration of the precipitation was also shorter (not shown). The above results suggest that the performance of WRF/BEP can be improved by using accurate land use data and urban parameters Influence of Urbanization on Precipitation To reveal the impact of urbanization on total precipitation, we next compare the total precipitation in the two sensitivity tests to those of CTL. As shown in Figure 5a, urbanization over the 20 years, from 1990 to 2010, leads to a total precipitation increase in downtown Beijing and its vicinity. Since the dominant flow is southwesterly wind, precipitation increases in the urban area and its upstream and downstream areas. YU AND LIU HEAVY RAINFALL ON 21 JULY

5 Figure 3. The observed and simulated 20 h accumulated precipitation from 1000 LST 21 July 2012 to 0600 LST 22 July 2012: (a) observation, the black line shows the urban area of CTL test; (b) CTL, the black line shows the urban area of CTL test; (c) 2000 test, the black line shows the urban area of 2000 test; (d) 1990 test; the black line shows the urban area of 1990 test (units: mm). Similar results have been previously reported for Beijing [Guo et al., 2006], Houston [Burian and Shepherd, 2005], Atlanta [Shem and Shepherd, 2009], and Baltimore [Zhang et al., 2009]. Besides the magnitude change, urbanization may also induce shift of the rain belt location [e.g,. Wan et al., 2013; Wan and Zhong, 2014] The effect of urban expansion from 1990 to 2000 on total precipitation (Figure 5b) is similar (but weaker) to that during 1990 to 2010 (Figure 5a). This result presents an interesting phenomenon that there is a positive correlation between the size of the urban area and the precipitation amount of this heavy rainfall event. The first stage of the rainfall process was located in the southwest of Beijing, while the frontal-type rainfall (second stage) occurred in downtown Beijing and downstream of the city [Chen et al., 2012]. Urbanization obviously increased the precipitation in Beijing City and its downstream area, where frontal-type rainfall (second stage) occurred. So we infer that urbanization mainly influenced the frontal-type rainfall process in this event. To understand how urbanization affected the front, we analyze the moving path of the front. According to Sun et al. [2012], the temperature gradient around this type of front is about 4 5 C (100 km) 1. We calculated the temperature gradient, and thus, we chose the moving of the 25 C isotherm where the temperature gradient is about 4 5 C (100 km) 1 to approximately represent the moving of the cold front. The CTL-simulated start time of the frontal-type rainfall in the urban area is about 1700 LST 21 July 2012, but in the 1990 test this changes to 1800 LST 21 July 2012 (Figure 6). Furthermore, the duration of precipitation in CTL was 13 h, which was about 1 h longer than in the two sensitivity tests (not shown). At 1400 LST, when the front has not yet moved to the downtown area, the CTL result is a little ahead of the 2000 and 1990 tests, leading to the appearance of precipitation occurring earlier in the CTL simulation compared to the sensitivity tests. However, at 1600 LST the front simulated in CTL and the 2000 test fall YU AND LIU HEAVY RAINFALL ON 21 JULY

6 Figure 4. The (a) observed and (b) CTL-simulated hourly precipitation from 1100 LST 21 July 2012 to 0600 LST 22 July 2012 (units: mm). YU AND LIU HEAVY RAINFALL ON 21 JULY

7 Figure 5. The 20 h accumulated rainfall change percentage refers to that in the 1990 test (units: %): (a) difference between the 2010 test and 1990 test, the green line shows the urban area of CTL test; (b) difference between the 2000 test and 1990 test, the green line shows the urban area of 2000 test;. obviously behind that of the 1990 test. This indicates that urbanization leads to an earlier start time and extended duration of the rainfall process, i.e., the city slows down the precipitation process due to the barrier effect of its buildings. A similar phenomenon has been reported for large urban clusters in the Yangtze River Valley [Wan and Zhong, 2014]. The rainstorm led to severe waterlogging in downtown Beijing. But was this related to urbanization causing an increase in the concentration of precipitation? We use the results of a probability density function (PDF) analysis to address this question (Figure 7). The PDF of the three tests is close to the normal distribution. The chance of total precipitation being outside the range of mm is 8% in the CTL test. The total precipitation in most areas of Beijing is mm in the 1990 test, mm in the 2000 test, and mm in CTL. The important point is that the chance of the precipitation amount being greater than 200 mm in the 1990 test is significantly less than in CTL and the 2000 test. This suggests that urbanization increases the chance of extreme precipitation. The PDF of precipitation intensity (hourly precipitation) shows a similar phenomenon (Figure 7b). The process of urbanization increases the probability of extreme precipitation (precipitation intensity 40 mm hr 1 ) in a short time. According to the above analysis, the likelihood of Figure 6. Simulated horizontal temperature distribution at 2 m (shading) (units: C) and the 25 C isotherm in the CTL (black line), 2000 (purple line), and 1990 (green line) runs: (a) 2100 LST; (b) 2200 LST. The black line shows the urban area of CTL test. YU AND LIU HEAVY RAINFALL ON 21 JULY

8 Figure 7. PDF analysis of precipitation in the CTL, 2000, and 1990 experiments accumulated from 1000 LST 21 July 2012 to 0600 LST 22 July 2012: (a) total precipitation; (b) precipitation intensity. waterlogging increases with the development of the city despite the contribution of urbanization to the regional mean precipitation amount not being significant Potential Underlying Mechanism To understand exactly how urbanization affects the distribution of rainfall we next analyze the differences between the results of the CTL and sensitivity experiments to diagnose the underlying physical processes involved. The simulated surface-forcing sensible heat fluxes change is accompanied by a change of temperature, i.e., the urban heat island (UHI) effect (not shown), up until the beginning of the rainfall process. The expansion of the city induces a rise in the sensible heat flux within the area of expansion (Figure 8). The change in sensible heat flux is not only located in the urban area but also mainly increases in the land areas adjacent to the urban area. Such change induces a significant difference of heat flux in the urban fringe and hence induces in situ convergence and cyclonic circulation in the downtown and upwind and downwind areas (Figure 8). Figure 8. Mean surface sensible heat fluxes (shading; units: W m 2 ) from 0800 LST 21 July 2012 to 1200 LST 21 July 2012 and wind (vectors; units: m s 1 ) at 850 hpa at 2300 LST 21 July 2012: (a) difference between CTL and the 1990 test, the green line shows the urban area of CTL test; (b) difference between the 2000 test and 1990 test, the green line shows the urban area of 2000 test. YU AND LIU HEAVY RAINFALL ON 21 JULY

9 Figure 9. Mean surface entropy from 2200 LST 21 July 2012 to 2300 LST 21 July 2012 (units: K): (a) CTL, the green line show the urban area of CTL test; (b) difference between the 2010 test and 1990 test, the green line shows the urban area of CTL test; (c) difference between the 2000 test and 1990 test, the green line shows the urban area of 2000 test. Near-surface moist entropy, measured in terms of the equivalent potential temperature, represents the availability of a system s thermal energy for conversion into mechanical work. The surface entropy is larger in the southwestern and downtown Beijing when the strongest rainfall occurs (Figure 9a). The 20 year urban expansion results in larger entropy change in the downtown area and the urban border regions (Figure 9b) due to the UHI and corresponding water vapor advection and moisture convergence (Figure 8a). Such impact can also be found in the case with less urban expansion, but the change in entropy is smaller (Figure 9c) because of the weaker UHI effect from 1990 to 2000 (Figure 8b). In the vertical direction, the UHI effect builds up so that the isentropic surfaces intersect with the ground. This process is strongest in the CTL run (Figure 10a), and weakest in the 1990 test (Figure 10c). Accordingly, strong updrafts occur in the small vertical gradients of the isentropic surfaces. Thus, in agreement with Yu et al. [2013], urbanization enhances the instability in the boundary layer in the downtown area and urban border regions. The foregoing analysis indicated that urbanization develops the convergence in the upstream and downstream of Beijing City. It contributes the extra convergence of water vapor flux in the same area (Figure 11). Urban expansion causes the enhancement of water vapor flux convergence, which plays a role in increasing the precipitation in the upstream and downstream of Beijing City. Previous case studies have shown that urbanization can lead to convective available potential energy (CAPE) increase [Miao et al., 2011; Wan and Zhong, 2014; Yang et al., 2014; Zhong and Yang, 2015]. The impact of urbanization on maximum CAPE in this case is shown in Figure 12. CAPE is obviously increasing (about J kg -1 ) in urban area, YU AND LIU HEAVY RAINFALL ON 21 JULY

10 Figure 10. Cross section at 39.9 N of vertical velocity (shading; units: m s 1 ), potential temperature (black contours; units: K) at 2300 LST 21 July The green lines show the urban area. suggesting that urbanization enhances the instability in the boundary layer in the downtown area and the urban border regions [Yu et al., 2013]. The increase of CAPE is conducive to the formation of convective precipitation and modulates the distribution of rainfall. Bornstein and Lin [2000] reported that hot plumes caused by urbanization can trigger cumulus clouds more easily than rural areas. Our result reveals a similar phenomenon. Such hot plumes in the southwest of Beijing are displayed in the CTL and both sensitivity test simulations. However, the largest amount of surface heating provided by urban areas in the CTL run maintains the warm rising air columns, meaning robust hot plumes develop most strongly and result in the heaviest rainfall in the CTL experiment. More sensitive tests by with and without urban type to analyze the impact and mechanism of urbanization on other rainfall with different strengths have conduced [Yu, 2015]. It shows that the influence of urbanization on heavy rain is the most significant. Precipitation of urban and its upstream and downstream areas are obviously increasing, similar to the 721 case in the current study. For moderate-strength rainfall, precipitation Figure 11. Cross section at 39.9 N of water vapor flux divergence (units: ghpa 1 m 2 s 1 ) averaged from 2000 LST to 2300 LST 21 July 2012: (a) difference between CTL and the 1990 test, the green line shows the urban area of CTL test; (b) difference between the 2000 test and 1990 test, the green line shows the urban area of 2000 test. YU AND LIU HEAVY RAINFALL ON 21 JULY

11 Figure 12. Mean maximum CAPE (units: J kg 1 ) from 2000 LST to 2300 LST 21 July 2012: (a) difference between CTL and the 1990 test, the green line shows the urban area of CTL test; (b) difference between the 2000 test and 1990 test, the green line shows the urban area of 2000 test. on the edge of the city is increased by urbanization. But the small rain is hardly affected by the change in land use and anthropogenic heat. 5. Conclusion and Discussion The present reported study used the latest WRF/BEP model with three land use maps to simulate a torrential rainstorm that occurred in Beijing on 21 July The aim was to assess the effect of urbanization on the rainfall event. We analyzed the 10 year and 20 year urban expansion effects, which were scarcely investigated. The major findings of the study can be summarized as follows: 1. The three different land use maps can describe this rainfall process well. However, the CTL result with the current land use map is closest to the observation. This shows that an accurate land use map, urban parameters, and anthropogenic heat parameterization can improve the performance of high-resolution cloudpermitting models. It also suggests that the role of urbanization in triggering precipitation is not dominant. However, for the present case of Rainstorm 721, urbanization did influence the accumulated rainfall amount by 30% and the duration of the precipitation event by 1 h. Urbanization makes the distribution of precipitation more spatially concentrated. This leads to the increased risk of urban waterlogging appearance. 2. Urbanization caused an increase in the accumulated precipitation amount in the urban area, as well as in the regions in its upwind and downwind directions. The start time of the precipitation event in the CTL run was earlier, and the precipitation duration within the urban area longer than in the two sensitivity simulations. Furthermore, urbanization increased the frequency of precipitation above 200 mm and the precipitation intensity of above 40 mm h 1. The impact of urbanization was most significant in the period of frontal-type rainfall. 3. A possible mechanism of urban-induced precipitation change has been proposed. The change in land use type, characteristic parameters, and anthropogenic heat directly cause a local UHI effect and surface heat flux change in the city and suburban districts, generating the cyclone in the upwind and downwind directions of the urban area. When moisture is sufficiently high, such as in the case of Rainstorm 721, the water vapor flux converges in the urban and the upstream and downstream areas. These areas then provide more available potential energy for convection system development, resulting in the generation of extreme precipitation. 4. It is important to acknowledge that the current study did not take into account the impact of aerosols. Previous studies have shown that increased aerosol levels from anthropogenic sources could influence the features of CCN, which play an important role in light precipitation [Van den Heever and Cotton, 2007; Rosenfeld et al., 2007]. But how this aspect influences heavy rainfall requires further investigation. In this regard, a high-resolution and coupled atmospheric chemistry model would be useful to reveal more details with respect to storm development. YU AND LIU HEAVY RAINFALL ON 21 JULY

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