Convection Initiation of a Heavy Rainfall Event in the Coastal Metropolitan Region of Shanghai on the South Side of the Meiyu Front

Urban areas where over half the world’s population now resides (Grimm et al., 2008) are vulnerable to heavy rainfall-induced flooding (Yang et al., 2019). Recently, some statistical studies have shown increased tendencies in the intensity and frequency of short-term heavy rainfall in urban areas with the rapid urban expansion in China (Song et al., 2014; Liang and Ding, 2017; Yang et al., 2017a; Wu et al., 2019), especially in city clusters, such as the Yangtze River Delta (YRD), the Pearl River Delta, and the Beijing–Tianjin–Hebei region (Fu et al., 2019; Jiang et al., 2020).

Heavy rainfall in urban areas, which is usually produced by mesoscale convective systems (MCSs), is characterized by its abrupt occurrence and local enhancement. Although some field campaigns such as METROpolitan Meteorological Experiment (METROMEX; Changnon et al., 1971), Beijing City Atmospheric Pollution Observation Field Experiment (BECAPEX; Xu et al., 2004), Shanghai’s Urban Integrated Meteorological Observation Network (SUIMON; Tan et al., 2015), and Study of Urban Impacts on Rainfall and Fog/Haze (SURF; Liang et al., 2018) have been conducted to improve the understanding of urban weather and precipitation, the ability to predict the precise timing and location of the convection initiation (CI) in urban areas is still limited because the relevant physical processes at different scales affecting the CI in urban areas are still not well understood due to the highly complex heterogeneous underlying surfaces where the CI occurs.

The urban heat island (UHI) effect plays an important role in the CI and local enhancement of convective storms in urban areas by destabilizing the low-level atmosphere and enhancing low-level convergence due to the thermal perturbations on the near-surface temperature and pressure fields (Bornstein and Lin, 2000; Rozoff et al., 2003; Niyogi et al., 2011; Zhang et al., 2017; Huang et al., 2019; Yin et al., 2020). However, UHI-induced lifting alone is not usually strong enough to trigger or enhance deep moist convection. In addition, some observational and numerical studies noted that the urban underlying surface contributes negatively to the humidity by decreasing evaporation, as a consequence, decreasing the convective instability (Guo et al., 2006; Liu et al., 2009; Wang et al., 2015; Yang et al., 2017b). This is unfavorable for the CI. Thus, other external factors, such as the synoptic-scale and mesoscale environments (Trier et al., 2006; Huang et al., 2019), topographic forcing (Yang et al., 2019; Wu et al., 2021), sea–land circulation (Gero and Pitman, 2006; Yang et al., 2014; Sun et al., 2021), and forcing from preexisting precipitation systems (Wu and Luo, 2016; Li et al., 2017a) can act as compensating effects of cities on the initiation or intensification of convection (Zhang, 2020). For example, Li et al. (2017a) showed that a northerly flow from a mesohigh within a northern MCS interacting with a prevailing southerly flow provided sustained low-level convergence during the formation of a cloud layer induced by the UHI effect. This was conducive to a CI event near the central metropolitan region of Beijing in the afternoon. Wu et al. (2021) investigated the mechanisms of the CI in the first stage of a record-breaking heavy rainfall event in the coastal metropolitan region of Guangzhou by using reanalysis from the four-dimensional Variational Doppler Radar Analysis System (VDRAS). They found that the local-scale orography played critical roles in the CI by orographic blocking and nighttime cooling, while the UHI contributed to the enhancement of low-level convergence. For the same rainfall event, another modeling study revealed that the cold pool generated by the MCS in the first stage triggered new convection in the second stage and induced subsequent local extreme rainfall through interactions between the UHI and the orography (Yin et al., 2020).

During the early summer (June–July), eastern China is often influenced by a quasi-stationary front called the Meiyu front. In contrast with a classic cold front, the Meiyu front is characterized by a strong low-level equivalent potential temperature (θ_e) gradient and a weak temperature gradient. Previous studies have shown that under favorable large-scale dynamic and thermodynamic forcing dominated by the Meiyu front, successive MCSs may be triggered within or along the frontal zone, resulting in extreme rainfall (Ding, 1992; Luo et al., 2014; He et al., 2017, 2018; Guan et al., 2020). The planetary boundary layer (PBL) within the Meiyu front is relatively moist. The mesoscale convergence forcing generated by the cold outflows from the Meiyu front precipitation system (MFPS) is also an effective mechanism for the CI, especially through interactions with other PBL processes, such as horizontal convective rolls (Luo et al., 2018) and orographic effect (Wang et al., 2021). Wang et al. (2021) showed that the nonuniform interaction between the cold outflows from the Meiyu front MCS within the MFPS and the southerly warm and moist airflow modified by the local-scale orography from the south side of the Meiyu front led to the initiation of parallel convective lines.

In contrast to the extensively discussed CI within or along the Meiyu front, the CI that occurs in urban areas on the south side of the Meiyu front or slightly away from the Meiyu front has received less attention. The role of the MFPS in the CI in the coastal cities is still not well understood. Thus, it is important to illustrate how the interaction of the MFPS and urban effect affects the CI. On the other hand, to what extent urban-induced perturbation affects the CI associated with multi-scale processes is also an important issue considering the urban complexity itself in terms of dynamics and thermodynamics.

Shanghai (see Figs. 1, 2a for its location), surrounded by sea on three sides, is situated in the YRD in eastern China. Its rapid urbanization process in recent years has resulted in a substantial increase in urban heavy rainfall (Liang and Ding, 2017). On 28 July 2020, a short-term heavy rainfall event (> 70 mm within 3 h as shown in Fig. 1a) caused by a localized linear MCS, occurred in the coastal urban area of the Shanghai metropolitan region (SMR, see Fig. 2b for its location), where frequent occurrence of the severe convective precipitation events was recorded (Tan et al., 2015). The commonly used operational numerical weather prediction (NWP) models including the 9-km regional modeling system developed and operated at the Shanghai Meteorological Service [referred to as SMS-WARMSv2 (SMS-WRF-ADAS Real-Time Modeling System), see Zhang et al. (2021) for a detailed description] predicted little precipitation for this event (Fig. 1b). The MCS of interest was initiated in the late morning (at about 1130 LT, LT = UTC + 8 h) on the warm side to the south of the Meiyu front. The UHI and cold outflows from the MFPS were observed before and during the CI. Thus, this hard-to-be-predicted urban heavy rainfall event provides a good opportunity to study the CI associated with the combined effects of the MFPS and UHI.

Figure 1.  The 3-h accumulated precipitation (shading; mm) from 1100 to 1400 LT 28 July 2020 of (a) the observations and (b) the SMS-WARMSv2 modeling system.

Figure 2.  (a) Topographic height (shading; m) and model domain used for the WRF simulation with a grid resolution of 3 km. The red box displayed in (a) shows the domain in (b) and (c). Dominant land use around Shanghai for the (b) CTRL and (c) NOURBAN simulations.

The main objectives of this study are to (1) reveal the CI mechanism of this heavy rainfall event, (2) investigate the linkage between the MFPS and CI, and (3) explore the role of the urban effect in the CI. To achieve these objectives, observational analyses and convection-permitting simulations are employed. The next section presents an overview of this rainfall event and the observational analyses, including both the synoptic-scale and mesoscale environments. Section 3 describes the design and verification of the numerical experiments. The simulated CI mechanism is discussed in Section 4. Section 5 presents two sensitivity experiments demonstrating the roles of the UHI and MFPS in the CI. The conclusions of this study are given in Section 6.

At 0800 LT 28 July 2020, the northeast–southwest-oriented MFPS was located in East China (Fig. 3). The weak stratiform precipitation within the MFPS stayed on the southeast coast of Jiangsu (Fig. 3a). From 0800 to 1100 LT, the severe convective precipitation produced by the Meiyu front MCS (> 40 dBZ) along the MFPS moved eastward from the southwest inland to the southeast coast of Jiangsu (Figs. 3a, b). About 0.5 h later (Fig. 3c), several convective cells were abruptly initiated in the coastal urban area of the SMR approximately 150 km to the south of the MFPS. These cells then intensified and organized locally into a strong linear MCS, parallel to the MFPS, which lasted for about 3 h in the SMR (Figs. 3d–f). This MCS produced heavy rainfall exceeding 70 mm in the SMR (Fig. 1) with the maximum hourly rainfall rate of 50 mm h⁻¹ from 1200 to 1300 LT.

Figure 3.  Composite radar reflectivity (shading; dBZ) at (a) 0800, (b) 1100, (c) 1130, (d) 1200, (e) 1300, and (f) 1500 LT 28 July. The red cross in (a) indicates the location of the Baoshan sounding station.

Figure 4 shows the variations of the synoptic-scale environments at 850 hPa and the surface from the ECMWF Reanalysis version 5 (ERA5; Hersbach et al., 2020). The 345-K θ_e contour at 850 hPa could represent the approximate location of the Meiyu front as widely used in previous studies (Zhang and Zhang, 2012; Luo and Chen, 2015). Shanghai was located on the warm side to the south of the Meiyu front and was occupied by the southwesterly winds at 850 hPa. The high θ_e sector on the south side of the Meiyu front created favorable thermal conditions for the CI. As shown in the sounding profile of Baoshan station (see Fig. 3a for its location) at 0800 LT (Fig. 5a), there was high relative humidity from approximately 850 to 700 hPa (1500–3000 m) and the convective available potential energy (CAPE) exceed 2000 J kg⁻¹ about 3.5 h prior to the CI. Although the Baoshan sounding also revealed a low-level temperature inversion owing to radiative cooling in the early morning, the increase in the surface temperature around the CI region by about 4°C at 1100 LT represented the breakup of the temperature inversion prior to the CI (Figs. 4c, d).

Figure 4.  Synoptic circulations of (a, b) equivalent potential temperature (shading; K) and wind barbs at 850 hPa (full barb = 4 m s⁻¹), and (c, d) mean sea level pressure (black contour; hPa), air temperature (shading; °C), and wind barbs at the surface (full barb = 4 m s⁻¹) at (a, c) 0800 LT and (b, d) 1100 LT 28 July. The blue 345-K contours of θ_e and the red 5880-gpm contours of geopotential height in (a, b) represent the locations of the Meiyu front and the subtropical high, respectively. The bold black contours in (c, d) represent the 1008-hPa sea level pressure. The red stars represent the CI location. The blue box displayed in (d) shows the domain in Fig. 6.

Figure 5.  Skew T–logp diagram of (a) the Baoshan sounding (the red cross in Fig. 3a) and (b) the simulated sounding around the CI location (the red cross in Fig. 8a) derived from the control simulation at 0800 LT 28 July. The ambient temperature and dew point are represented by the black and blue lines, respectively. The ascending curve of surface air parcel is represented by a dashed red line.

Dominated by the northwestward-extending subtropical high (Figs. 4a, b), there was an inland warm low pressure, represented by the 1008-hPa isobar, at the surface over eastern China (Figs. 4c, d). From 0800 to 1100 LT, the warm low pressure strengthened and moved southward and the CI occurred on its northeastern periphery, where the prevailing southerly winds turned to the easterly winds. At the same time, the northeasterly sea winds at the southeast coast of Jiangsu became stronger due to the increase in the temperature contrast between the East China Sea and the warm low pressure. Besides, it could be noted that an inland cold center caused by precipitation and evaporative cooling within the MFPS was located on the north of the warm low pressure. The increase in the temperature contrast between the cold center and the warm low pressure was favorable for the development of the northerly outflows. However, the global reanalysis, with a coarse-resolution of about 31 km, poorly represented the evolution of the cold outflows from the MFPS.

To examine the finescale environments, Fig. 6 shows the variations of the surface wind and temperature fields in a smaller area (the blue box in Fig. 4b) from the observations of the intensive automated weather stations (AWSs). At 0800 LT, there were three key flows: northeasterly sea winds (indicated by red arrows in Fig. 6) at the southeast coast of Jiangsu; the southerly winds at the south coast of Shanghai; and the northwesterly outflows from the MFPS (Fig. 6a). The patterns of the first two flows shown by the surface observations from the AWSs were similar to the surface analysis from the ERA5 (Fig. 4c). By contrast, the weak northwesterly outflows from the MFPS at this time were found in the AWSs, but were missed in the ERA5. The MFPS intensified with the eastward movement of the Meiyu front MCS (Figs. 3a, b) and therefore, the northwesterly outflows strengthened and advanced to the CI region (Figs. 6b, c). Influenced by the enhanced northwesterly outflows, the northeasterly sea winds at the southeast coast of Jiangsu gradually shifted to the northwesterly. At the same time, a significant zone of enhancement of the northeasterly sea winds (indicated by the purple arrows in Figs. 6c, d) appeared at the northeast coast of the SMR indicating the southward extension of the northeasterly sea winds, such mesoscale variation of the northeasterly sea winds was also missed in the ERA5. With the northwesterly outflows and northeasterly sea winds converging with the southerly–southeasterly winds, a significant wind convergence zone appeared near the CI region (Figs. 6c, d).

Figure 6.  Air temperature (shading; °C), wind barbs (full barb = 4 m s⁻¹, wind speeds less than 0.2 m s⁻¹ are not shown) at the surface from intensive automated weather stations, and the contours of composite radar reflectivity exceeding 35 dBZ (magenta contours, interval at 10 dBZ) at (a) 0800, (b) 1000, (c) 1100, and (d) 1130 LT 28 July. The wind barbs in black represent the easterly component and the wind barbs in white represent the westerly component. The red and purple arrows indicate the general directions of the sea winds. The blue dotted lines indicate the surface convergence lines.

A rapid increase in temperature in Shanghai, especially in the SMR after 0800 LT due to the radiative heating (Figs. 6b–d), and the peak temperature in the SMR at 1130 LT was over 4°C higher than that in the coastal region (Fig. 6d), indicating the notable UHI in this CI event. The CI occurred on the edge of the peak temperature region accompanied by the wind convergence zone, which indicated the possible impacts of the UHI combined with the external converging flows on the CI.

Observational analyses suggested that the CI of this urban heavy rainfall event was associated with the northwesterly outflows from the MFPS, northeasterly sea winds, and UHI. However, due to the limitation of the sparse and irregular two-dimensional observational data, it is unclear how the mesoscale dynamic and thermal processes associated with the interaction of the UHI-induced circulation, the northwesterly outflows, and the northeasterly sea winds affected the CI. Neither is the contribution of the urban effect to the CI clear. To answer these questions, the high-resolution numerical simulations were performed to further examine the mesoscale processes that accounted for the CI.

In this study, the Advanced Research version of the Weather Research and Forecasting (WRF-ARW) version 4.2 (Skamarock et al., 2021), a non-hydrostatic atmospheric model, was used to conduct the 24-h convective-permitting numerical simulations from 2000 LT 27 to 2000 LT 28 July with two one-way nested domains at horizontal grid spacings of 9 and 3 km (Fig. 2a) and 51 vertical levels.

The Thompson microphysics scheme (Thompson et al., 2008), Yonsei University PBL scheme (Hong et al., 2006), rapid radiative transfer model longwave radiation scheme (Mlawer et al., 1997), Dudhia shortwave radiation scheme (Dudhia, 1989), and Noah-MP land surface scheme (Niu et al., 2011) were used for both two domains. The Kain–Fritsch cumulus scheme (Kain, 2004) was used in the outer domain and the cumulus parameterization scheme was not activated in the inner domain because deep convection can be explicitly simulated at a convection-permitting resolution (Clark et al., 2016). The land use fields used in the model were interpolated from the Moderate Resolution Imaging Spectroradiometer (MODIS) datasets.

Due to the limited deterministic predictability of the CI processes (Zhang et al., 2019) and the uncertainties in the initial conditions (Nystrom and Zhang, 2019), one single numerical simulation using the ERA5 reanalysis as the initial and lateral boundary conditions poorly captured the CI location and timing as well as near-CI environment (omitted) and as a consequence had a weak simulating ability for heavy rainfall (Fig. 7v). We therefore performed a 21-member ensemble forecast initialized with the Global Ensemble Forecast System (GEFS) data (Toth and Kalnay, 1993; Wei et al., 2008; Zhou et al., 2017) with the same model configuration mentioned above to select the best simulation result as the control simulation (CTRL).

Figure 7.  Simulated accumulated precipitation (shading; mm) from 1100 to 1400 LT 28 July for the mem00–mem20 runs and ERA5, NOURBAN, and FAKEDRY simulations. Mem07 is indicated by red fonts as the control simulation (CTRL). The gray contours indicate the urban borders.

To investigate the effects and relative roles of the UHI and MFPS in the CI, two additional sensitivity experiments based on the CTRL were conducted. A sensitivity experiment called FAKEDRY with turning off the latent heating and cooling from the microphysics scheme on the north of the SMR after 0200 LT 28 July was conducted to suppress the development of the MFPS. This sensitivity experiment also was called “fake-dry” simulation and was used in Luo et al. (2020) to suppress the upstream convective system. Another sensitivity experiment, NOURBAN, in which urban land use of the SMR was replaced by cropland (Fig. 2c) was conducted to explore the relative role of the UHI in the CI. The method has also been used in many previous studies (Miao et al., 2011; Yu and Liu, 2015) to remove the urban effect. The other model settings in the two sensitivity experiments were identical to those in the CTRL. Table 1 shows detailed description of all numerical experiments.

Figure 7 shows the simulated 3-h accumulated precipitation from 1100 to 1400 LT 28 July for the 21-member ensemble forecast. Compared with the observed precipitation (Fig. 1a), only the seventh member (mem07) in all ensemble members was able to reproduce the rainfall belt with the maximum rainfall amount of over 50 mm in the SMR (Fig. 7h). Although the simulated maximum rainfall amount of 51 mm in the mem07 was less than the observed maximum rainfall amount of 75 mm, which was partly attributed to the weaker CI intensity (Figs. 3, 8), given that we focused on the physical processes governing the CI rather than the quantitative precipitation forecast. Thus, the mem07 was considered to be the best member of all simulations to capture the intensity and distribution of heavy rainfall in the SMR well, and we then will examine the ability of the mem07 to reproduce the CI process by comparing the simulated composite radar reflectivity with the observations.

Figure 8.  Simulated composite radar reflectivity (shading; dBZ) in mem07 at (a) 0800, (b) 1000, (c) 1100, (d) 1130, (e) 1230, and (f) 1300 LT 28 July. The red cross in (a) represents the location of the simulated sounding. The black lines displayed in (d) indicate the cross-sections in Figs. 11, 13.

Figure 8 shows the evolution of the simulated composite radar reflectivity in the mem07. Compared with the observed composite radar reflectivity (Fig. 3), the mem07 successfully captured the general features of the CI and its subsequent development in the SMR, apart from the latter CI over the sea (Fig. 3e). Herein, CI was defined when and where the radar echo first exceed 35 dBZ (Weckwerth, 2000; Du et al., 2020). In the mem07, discrete convective cells with the radar echo of exceeding 35 dBZ occurred in the SMR at 1130 LT and then intensified locally into a northeast–southwest-oriented linear MCS with a contiguous band of over 40 dBZ (Figs. 8e, f). The CI location, timing, and pattern were similar to those in the observations (Figs. 3c, 8d). The process was what we focused on. In addition to the CI in the SMR, the simulation also reproduced the development of the MFPS including the stratiform precipitation that stayed on the southeast coast of Jiangsu at 0800 LT and the convective precipitation produced by the eastward-moving Meiyu front MCS (Figs. 8a–c). But the speed of movement of the Meiyu front MCS was slightly slower than that in the observations. The location of the Meiyu front MCS at 1130 LT in the simulation (Fig. 8d) was closer to the counterpart at 1100 LT in the observations (Fig. 3b).

We further examine the observed and simulated convection environment prior to the CI by comparing the sounding profile around the CI location and the near-surface condition. As shown in Fig. 5, the simulated sounding reproduced the low-level temperature inversion and lapse rate as well as the horizontal winds at 0800 LT (Fig. 5b), which closely resembled the Baoshan sounding (Fig. 5a). Despite slightly underestimating the amount of moisture below 850 hPa, the simulation reasonably captured the general humidity vertical profile where there was relatively dry from approximately 700 to 500 hPa and relatively humid from approximately 850 to 700 hPa.

Figure 9 shows the evolution of the simulated surface wind and temperature fields. The simulation reproduced the convergence zone in the SMR, which formed between the northwesterly outflows from the MFPS and the enhanced northeasterly sea winds at the northeast coast of the SMR (indicated by the purple arrows in Figs. 9c, d) as well as the southerly–southeasterly winds at the south coast of Shanghai. Consistent with the observations, the sea winds at the southeast coast of Jiangsu shifted from the southeasterly to the northwesterly (indicated by the red arrows in Fig. 9) with the intensification of the northwesterly outflows. Besides, the simulation also reproduced the main temperature pattern with warmer air in the SMR. Similar to the observations, the simulated CI occurred within the strong UHI. It occurred on the edge of the high temperature region, not in the peak temperature region because the colder external flows (including the outflows and sea winds) cooled the near-surface atmosphere around the CI location. We have also evaluated the near-CI environment in other members and SMS-WARMSv2, the northwesterly outflows produced by all the other simulation results were too weak to intrude into the SMR (omitted).

Figure 9.  Simulated air temperature (shading; °C) and wind barbs (full barb = 4 m s⁻¹) at the surface, and the contours of composite radar reflectivity exceeding 35 dBZ (magenta contours, interval at 10 dBZ) in mem07 at (a) 0800, (b) 1000, (c) 1100, and (d) 1130 LT 28 July. The wind barbs in black represent the easterly component and the wind barbs in white represent the westerly component. The red and purple arrows indicate the general directions of the sea winds.

Overall, the simulation result of the mem07 could capture the CI of interest and the near-CI environment reasonably well. Thus, we considered the mem07 as the CTRL to investigate the processes governing the CI. In the following sections, we will use the model results to address the issues raised in Section 2.

In this section, the interaction of the UHI-induced circulation, the northwesterly outflows, and the northeasterly sea winds in generating the variations of the mesoscale thermal and dynamic conditions before the CI is further examined.

Figure 10 shows the evolutions of the perturbation pressure, horizontal divergence, and wind field at 150 m. Due to the earlier rapid warming in the SMR than that in the surrounding region, a thermal mesolow caused by the UHI appeared in the SMR, while two cold high pressure centers developed within the MFPS and over the East China Sea, respectively (Figs. 10a–d). Such horizontal pressure configuration provided very favorable conditions for the formation of the converging flows in the SMR. The cold high pressure center within the MFPS was primarily caused by precipitation and evaporative cooling. The eastward-moving Meiyu front MCS contributed to the increase in the high pressure perturbation within the MFPS. The cold high pressure center over the East China Sea became apparent in the late morning due to the increase in the land–sea temperature difference (Figs. 10c, d).

Figure 10.  Distributions of (a–d) the perturbation pressure from the domain mean (shading; hPa) and (e–h) the horizontal divergence field (shading; s⁻¹) overlaid by the horizontal wind vector at 150 m at (a, e) 1000, (b, f) 1030, (c, g) 1100, and (d, h) 1130 LT 28 July. The area used for averaging is the coverage of Fig. 10.

The pressure contrast between the East China Sea and the UHI mesolow could further enhance the sea–land circulation that the SMR was controlled by the synoptic-scale warm low pressure (Figs. 4c, d). It could be noted that the local-scale northeasterly sea winds near the northeast coast of the SMR developed earlier, which was attributed to the larger local thermally-induced pressure gradient between the water body and coastal urban area of the SMR (Figs. 10a, b, e, f), which is consistent with the observations (Fig. 6). Besides, the heterogeneity underlying surface on the northeast of the SMR might slow down the regional-scale sea winds when they propagated to the northeast of the SMR from the East China Sea. Coastal convergence with an intensity of −8 × 10⁻⁴ s⁻¹ was found near the CI region at 1000 LT (Fig. 10e). Thereafter, the northeasterly sea winds gradually strengthened while the northwesterly outflows advanced to the CI region and convergence increased (Figs. 10f–h).

In addition to accelerating the sea winds, the UHI mesolow caused the local airflow to be pumped into the UHI center inducing and enhancing the local-scale low-level convergence near the CI region. This “heat pumping” effect associated with the UHI was also shown and described in previous studies (Li et al., 2017b; Sun et al., 2021). The strong low-level convergence with a maximum intensity of −13 × 10⁻⁴ s⁻¹ induced by the cooperation of the northwesterly outflows, northeasterly sea winds, and UHI-induced circulation led to the CI eventually (Fig. 10h). Next, the impacts of the UHI-induced circulation interacting with the northwesterly outflows and northeasterly sea winds on the CI will be discussed in detail.

Figure 11 shows the evolutions of the wind field, perturbation potential temperature (the deviation from the mean averaged over 0200–1200 LT), water vapor mixing ratio, and divergence in the vertical cross-section through the initiating cell along the northwest–southeast transect. At 1000 LT (Figs. 11a, e), the air below about 400 m in the urban region was warmer and drier than that in the surrounding region as a consequence of the increase in solar radiative absorption and the decrease in evaporation in the urban underlying surface (Zhang et al., 2009). Adiabatic cooling and the vertical transport of water vapor resulting from the UHI-induced lifting in the urban region were favorable for the formation of the cloud or saturated layer prior to the CI. Due to the advection and cooling by the weak northwesterly outflows from the stratiform precipitation at this time, the UHI center (indicated by the peak thermal perturbation) shifted to the southeast. The southeasterly flows blowing to the UHI center were enhanced by the UHI-induced inflows and collided with the northwesterly outflows to enhance the low-level convergence and form a strong ascending branch near the CI region. With the increase in the peak thermal perturbation in the urban region, the southeasterly flows were further enhanced, especially at the surface (Figs. 9b, c). The low-level convergence was enhanced, which produced a stronger updraft near the CI region, with an obvious local secondary circulation established (Figs. 11b, c, f, g). Meanwhile, another strong convergence center with an ascending branch formed on the northwest side when the relatively strong northwesterly outflows converged to the UHI.

Figure 11.  Vertical cross-sections along the northwest–southeast transect in Fig. 8d of (a–d) the perturbation potential temperature (shading; K), the horizontal velocity (blue contours, starting from 1 m s⁻¹ with 1 m s⁻¹ intervals), and the wind vector (vertical winds are multiplied by 10); and (e–h) the divergence (shading; s⁻¹) and the water vapor mixing ratio (black contours; intervals of 1 g kg⁻¹) in the CTRL at (a, e) 1000, (b, f) 1030, (c, g) 1100, and (d, f) 1130 LT 28 July. The green contours show the cloud water mixing ratio of 0.01 g kg⁻¹. The dots on the x-axis indicate the land use of each grid and have the same colors as those used in Fig. 2b. The annotation SE and NW are abbreviations of southeast and northwest, respectively.

Although the enhanced frictional effect in the urban region due to the increase in roughness and vertical mixing (Wong and Dirks, 1978) may have contributed to the increase in the convergence by decelerating the northwesterly outflows, the northwesterly outflows (indicated by the blue contours) were stronger as they advanced to the urban region (Figs. 11a, c), suggesting that the frictional effect was not the main factor enhancing the convergence. Besides, noted that the northwesterly outflows presented different variation patterns when they converged to the UHI passing through different types of underlying surface. When a northwesterly flow passed through the local water body to the UHI, its northerly component increased by merging with the stronger local northeasterly sea winds enhanced by the UHI effect near the north coastal urban area of the SMR while another northwesterly flow passing through the inland to the UHI veered to nearly westerly (Figs. 10a–d). It could be inferred that the variation of the northwesterly flows was a result of the coupling of the northwesterly outflows and the UHI-induced circulation, which was the main reason for the formation of the northwest convergence center. The role of the UHI in regulating and accelerating the northwesterly outflows was indeed confirmed in the following section by a sensitivity experiment with the urban area of the SMR removed.

As the upward motion intensified near the CI region, the adiabatic cooling and moist layers became deeper (Figs. 11c, g). A saturated layer (indicated by the cloud water mixing ratio of 0.01 g kg⁻¹) between 1 and 3 km near the CI region formed at 1100 LT prior to the CI. The simulated sounding at 1100 LT (Fig. 12a) exhibited a moist absolute unstable layer (MAUL) from approximately 875 to 775 hPa where the lapse rate within the saturated layer was greater than the moist adiabatic rate (Bryan and Fritsch, 2000). The formation of the MAUL meant strong convective instability and favored to the CI (Schumacher and Johnson, 2008; Zhang et al., 2019; Hua et al., 2020). About 30 min later, the enhanced northwesterly outflows approached the UHI center leading to the merge of the two ascending branches and the enhancement of the low-level convergence which promoted the CI (Figs. 11d, h). Meanwhile, the UHI center slightly moved southeastward due to the cooling caused by the strong northwesterly outflows. The decrease in the absorption of solar radiative due to the growth of the cloud water was another possible reason for the cooling near the CI region. Thus, the CI did not occur in the peak temperature region.

Figure 12.  Skew T–logp diagrams of soundings around the CI location derived from the (a) CTRL, (b) NOURBAN, and (c) FAKEDRY simulations at 0800 LT (blue lines and wind barbs) and 1100 LT (red lines and wind barbs) 28 July. The ambient temperature and dew point are represented by the solid and dashed lines, respectively.

There were also two ascending branches along the southwest–northeast transect at 1000 LT (Figs. 13a, e). The ascending branch near the CI region was mainly associated with the UHI-induced lifting and convergence. Besides, the local northeasterly sea winds were enhanced by the UHI-induced low pressure perturbation, which induced another ascending branch near the coastal area. With the stronger regional-scale northeasterly sea winds expanding to the urban region and the increase in the peak thermal perturbation in the urban region, a strong sea–urban circulation established and propagated farther inland leading to the merge of the two ascending branches and the enhancement of the low-level convergence near the CI region (Figs. 13b, c, f, g). At 1100 LT, the deep MAUL formed due to adiabatic cooling and the vertical transport of water vapor induced by the strong updraft (Figs. 13c, g). About 10 min later, the convection was triggered (omitted). At 1130 LT, the coastal cell produced convective precipitation with reflectivity exceeding 45 dBZ (Fig. 8d). Due to the cooling and downdraft induced by the precipitation, the UHI center moved out of the CI region (Figs. 13d, h).

Figure 13.  As in Fig. 11, but along the northeast–southwest transect. The annotation NE and SW are abbreviations of northeast and southwest, respectively.

In summary, the above analysis suggested that the UHI effect played an important role in the CI through interaction with the northwesterly outflows and the northeasterly sea winds. In the early morning, the intrusion of the weak northwesterly outflows into the SMR caused the UHI center to move to the CI region. Prior to the arrival of the relatively strong northwesterly outflows and the northeasterly sea winds, the UHI created a lifting condition producing adiabatic cooling and the vertical moisture transport in the urban region. The mesolow generated by the UHI induced the local low-level convergence near the CI region. Besides, The UHI-induced circulation variation and low pressure perturbation accelerated the northwesterly outflows and the northeasterly sea winds as they converged to the UHI. In the late morning, the convection was triggered when the enhanced northwesterly outflows and northeasterly sea winds approached the updraft zone caused by the UHI center.

Both the observations and control simulation showed the important roles of the UHI, the northwesterly outflows from the MFPS, and northeasterly sea winds in the CI. Besides, the UHI influenced the development of the northwesterly outflows and northeasterly sea winds at local scale. In this section, we will discuss the results of the NOURBAN and FAKEDRY runs to gain insight into the effects and relative roles of the UHI and MFPS in the CI.

Figure 14 shows the evolution of the simulated composite radar reflectivity in the two sensitivity experiments and the CTRL. The experiment NOURBAN captured the evolution of the MFPS (Figs. 14a2, b2), which was similar to the CTRL (Figs. 14a1, b1), but the CI in the SMR occurred about 1 h later than that in the CTRL (Figs. 14c2, b1). Although the radar echo exceeding 35 dBZ occurred in the coastal area of the SMR at about 1230 LT, it then exhibited weak and less-organized structure (Fig. 14d2) and developed further eastward than that in the CTRL leading to less precipitation during 1100–1400 LT (Fig. 7w).

Figure 14.  Simulated composite radar reflectivity (shading; dBZ) for the (a1–d1) CTRL, (a2–d2) NOURBAN, and (a3–d3) FAKEDRY simulations at (a1–a3) 1100, (b1–b3) 1130, (c1–c3) 1230, and (d1–d3) 1300 LT 28 July.

As shown in Fig. 15, the development of the northeasterly sea winds over the East China Sea in the NOURBAN was similar to that in the CTRL. However, in the NOURBAN, the northeasterly sea winds at the northeast coast of the SMR became weaker (Figs. 15b, 17e–h) due to the decrease in the local land–sea pressure and thermal contrasts near the coast of the SMR (Figs. 15a, b, j, k). Besides, due to the absence of the UHI-induced circulation, the northwesterly outflows from the MFPS became weaker and extended more to the southeast when they advanced to the CI region (Figs. 15b, 17a–d); thus, a weaker convergence zone formed near the CI region (Figs. 15d, e), which confirmed the role of the UHI in inducing and enhancing the local-scale converging flows.

Figure 15.  (a–c) Perturbation pressure from the domain mean (shading; hPa), (d–f) divergence (shading; s⁻¹), (g–i) water vapor mixing ratio (shading; g kg⁻¹), and (j–l) temperature (shading; °C) overlaid by the wind vector at 150 m for the CTRL, NOURBAN, and FAKEDRY simulations at 1130 LT 28 July. The area used for averaging is the coverage of Fig. 15.

Figure 16.  Differences in the wind vector and zonal wind speed (shading; m s⁻¹) (CTRL minus NOURBAN) at 150 m at (a) 1000, (b) 1100, and (c) 1130 LT 28 July.

Figure 17.  Vertical cross-sections along the (a–d) NW–SE and (e–h) NE–SW transects in Fig. 8d of the perturbation potential temperature (shading; K), horizontal velocity (blue contours, starting from 1 m s⁻¹ with 1 m s⁻¹ intervals), wind vector (vertical winds are multiplied by 10), and cloud water mixing ratio (0.01 g kg⁻¹ contour in green) in the NOURBAN at (a, b) 1000, (c, d) 1030, (e, f) 1100, and (g, h) 1130 LT 28 July.

To see the distribution of the UHI-induced circulation more clearly, Fig. 16 shows the difference in the wind vector and zonal wind speed fields. At 1000 LT (Fig. 16a), the UHI-induced circulation exhibited obvious local confluent features, including southerly–southwesterly flows and easterly–northeasterly flows. The transition zone of the wind directions agreed well with the subsequent strong convergence zone (Figs. 10e–h). The northeasterly sea winds were enhanced when they met the UHI-induced northeasterly inflows near the northeast coast of the SMR while the UHI-induced southerly–southwesterly inflows near the west of the SMR veered the northwesterly outflows to the nearly westerly. From 1000 to 1100 LT (Figs. 16b, c), there were obvious westerly–northwesterly flows near the north coast of the SMR which indicated the role of the UHI in regulating and accelerating the northwesterly outflows. These results were consistent with our analysis in the previous section.

The upward motion in the NOURBAN induced by the weak northwesterly outflows and northeasterly sea winds became much weaker than that in the CTRL and was unable to trigger convection at 1130 LT (Figs. 17d, h). From the thermodynamic aspect, the low-level temperature inversion became weak but still persisted at 1100 LT, as indicated by the simulated sounding in the NOURBAN (Fig. 12b). The MAUL formed at a lower level than that in the CTRL and exhibited a shallow structure due to the temperature inversion and the weak upward motion (Figs. 12a, b, 17). It was concluded that the enhancement of the low-level convergence near the CI region was mainly contributed by the UHI effect.

In the FAKEDRY experiment, by turning off latent heating and cooling from the microphysics scheme on the north of the SMR (the north of the black lines shown in Figs. 15a3–d3), the development of the convection within the MFPS was suppressed. The CI in the SMR occurred at about 1200 LT (figure omitted), which was somewhat later than in the CTRL. Convection in the SMR then exhibited a less-organized structure and developed further westward than that in the CTRL (Figs. 14a1–d1, a3–d3) causing less precipitation during 1100–1400 LT (Fig. 7x).

The low-level flow patterns in the FAKEDRY exhibited obvious differences from those in the CTRL and NOURBAN (Figs. 15a–c, 12). In the absence of the high pressure perturbation induced by precipitation and evaporative cooling within the MFPS in the FAKEDRY, the zonal land–sea pressure contrast increased and the meridional land–sea pressure contrast decreased on the north of the SMR, which enhanced the easterly winds instead of the northerly winds. Thus, the northeasterly sea winds developed at the southeast coast of Jiangsu and were influenced by the synoptic-scale warm low pressure instead of extending southward in the CTRL and NOURBAN. However, local-scale northeasterly sea winds at the coast of the SMR were enhanced by the UHI-induced low pressure perturbation and converged with the enhanced southerly winds in the SMR (Figs. 15c, 18e–h), which led to the formation of the strong northwest–southeast-oriented convergence line (Fig. 15f).

Figure 18.  As in Fig. 17, but for the FAKEDRY.

These results further suggested that the development of the northeasterly sea winds was influenced by multi-scale pressure perturbations. First, the northeasterly sea winds at the southeast coast of Jiangsu were enhanced in relation to the synoptic-scale warm low pressure; besides, the high pressure perturbation within the MFPS contributed to the enhancement of the northerly winds by increasing the meridional land–sea pressure contrast and decreasing the zonal land–sea pressure contrast on the north of the SMR, which led to the southward extension of the northeasterly sea winds. Meanwhile, the UHI-induced low pressure perturbation directly enhanced the local-scale northeasterly sea winds at the coast of the SMR and played a major role in the enhancement of the coastal convergence.

Because the ERA5 reanalysis poorly depicted the northerly outflows from the MFPS and UHI-induced circulation (Fig. 4c), it could not capture the mesoscale variation of the northeasterly sea winds as shown in the AWS observation (Figs. 6c, d). Besides, noted that without the obvious increment of the regional-scale northerly winds, the prevailing southerly winds in the FAKEDRY became stronger and were further enhanced by merging with the UHI-induced southerly inflows (Figs. 15a–c).

The outflows were removed by turning off the evaporative cooling in the FAKEDRY. Without the cooling and advection caused by the northwesterly outflows, on the one hand, the FAKEDRY failed to reproduce the northeast–southwest-oriented convergence line (Fig. 15f) and thereafter the development of the northeast–southwest-oriented linear MCS (Figs. 14c3–d3); on the other hand, the UHI mesolow was shifted further westward than that in the CTRL, and therefore, the UHI-induced easterly inflows extended more to the west correspondingly (Figs. 15c, 18a–d), which resulted in the deviation of the CI location in the FAKEDRY. These results suggested that the northwesterly outflows from the MFPS changed the horizontal distribution of the low-level convergence induced by the UHI and influenced the location of the UHI mesolow, which determined the CI location.

The isolated urban effect also could be examined from the FAKEDRY in which there were no the outflows and the enhancement of the northeasterly sea winds near the coast of the SMR was mainly attributed to the UHI-induced low pressure perturbation. In the FAKEDRY, the single urban forcing still induced strong low-level convergence, which produced a stronger updraft in the urban region than that in the NOURBAN (Figs. 17, 18). However, in addition to producing higher temperature, the urban effect reduced the moisture significantly because there was less evaporation in the urban region. Without the transport of additional cold and moist air to the SMR by the northwesterly outflows and the regional-scale northeasterly sea winds in the FAKEDRY, the low-level atmosphere in the urban region became warmer and drier than that in the other two experiments (Figs. 15g–l) resulting in lower relative humidity in the urban region also indicated by the temperature dew point difference (Fig. 12), which caused higher cloud base (Williams et al., 2015), unfavorable for the development of the deep MAUL in the FAKEDRY because a stronger updraft is needed for the air parcel to pass through its lifting condensation level. During 1100–1130 LT, the MAUL in the FAKEDRY appeared at a higher level than in the NOURBAN but exhibited a shallower structure than in the CTRL due to shallower adiabatic cooling and moist air layers (Figs. 12, 18). The CI was therefore delayed in the FAKEDRY.

The results of the two sensitivity experiments indicated that the combined effect of the UHI and MFPS determined the CI timing, location, and its subsequent development.

A short-term heavy rainfall event occurred in the coastal urban area of the SMR in the late morning on 28 July 2020 on the warm side to the south of the Meiyu front. This event was missed by the 9-km regional operational modeling system at the Shanghai Meteorological Service. Based on observational analyses and convection-permitting (3-km resolution in the innermost domain) numerical simulations with the WRF-ARW model, we investigated the CI mechanism of this urban heavy rainfall event. The control simulation, which was selected from a 21-member ensemble simulation, captured the CI process and near-CI environment reasonably well. It was found that the CI was associated with the UHI, the northwesterly outflows from the MFPS, and the northeasterly sea winds. The synoptic-scale and mesoscale environments and the key physical processes governing the CI are summarized in the conceptual model shown in Fig. 19.

Figure 19.  Conceptual diagram for the convection initiation mechanism near the coastal urban area with the influence of the urban heat island (UHI), outflows from the Meiyu front precipitation system (MFPS), and sea winds.

The CI of this urban heavy rainfall event took place about 150 km south of the MFPS and near the northeastern periphery of the synoptic-scale inland warm low pressure. The thermal mesolow generated by the UHI developed in the SMR. Meanwhile, there were two cold high pressure centers: one center was located on the north of the SMR and was induced by the precipitation and evaporative cooling within the MFPS; the other center was located over the East China Sea and was associated with the increase in the land–sea temperature difference in the late morning. This horizontal pressure configuration provided favorable conditions for the formation of the converging flows in the SMR.

In the early morning, the intrusion of the weak northwesterly outflows into the SMR caused the UHI center to move to the CI region. Prior to the arrival of the relatively strong northwesterly outflows and the northeasterly sea winds, the UHI created a lifting condition producing adiabatic cooling and the vertical moisture transport in the urban region, which was favorable for the formation of the MAUL. Meanwhile, the UHI mesolow induced and enhanced the local low-level convergence near the CI region and accelerated the northwesterly outflows and the northeasterly sea winds as they converged to the UHI. In the late morning, the intensification of the low-level convergence when the enhanced northwesterly outflows and northeasterly sea winds approached the updraft zone caused by the UHI center promoted the CI.

To further examine the contributions of the UHI and MFPS to the CI, the two sensitivity simulations where the urban area of the SMR was replaced by cropland and the MFPS was suppressed by turning off latent heating and cooling from the microphysics scheme were conducted respectively. The results showed that the enhancement of the low-level convergence near the CI region was mainly contributed by the UHI effect. The northwesterly outflows and northeasterly sea winds transported cold and moist air to the CI region to partly offset the negative contribution of the urban drying effect to the low-level relative humidity. This facilitated the development of the deep MAUL during the CI. Furthermore, the high pressure perturbation within the MFPS also contributed to the enhancement of the northeasterly sea winds by increasing the meridional land–sea pressure contrast and decreasing the zonal land–sea pressure contrast on the north of the SMR, leading to the southward extension of the northeasterly sea winds. In both sensitivity experiments, the CI occurred later with the position deviation and then exhibited a less-organized structure, which suggested that the combined effect of the UHI and MFPS determined the CI timing, location, and its subsequent development.

With their coarse-resolution and inaccurate initial conditions, the current operational NWP models still have difficulties in accurately predicting localized CI events. By using convection-permitting ensemble simulations with the WRF model, this study reproduced the CI, which was associated with the interaction of multi-scale processes including the sea–land circulation, urban-induced perturbation, and pressure perturbation within the MFPS. While previous studies have shown that the effect of the coupling between the urban-induced circulation and the sea–land circulation played an important role in the initiation and intensification of convection in the coastal cities (Yang et al., 2014; Ooi et al., 2017; Sun et al., 2021), our study further emphasized the importance of understanding the pressure perturbation within the upstream frontal precipitation system in forecasting the CI in the downstream coastal cities. Besides, in addition to corroborating findings from previous studies (Li et al., 2017a; Yin et al., 2020) that emphasized the positive impact of the UHI on the CI by altering the vertical structure of the PBL and facilitating the formation of the MAUL through the convergence and lifting, our analysis in current study indicated that the rapid increase in temperature and less evaporation in urban underlying surface could exert a negative impact on the CI by decreasing the low-level relative humidity.

Previous studies have suggested that urban expansion has a negative impact on precipitation by aggravating the urban drying effect (Feng et al., 2012; Wang et al., 2015). Wang et al. (2015) showed that after the cities expand to a certain extent in the Beijing–Tianjin–Hebei region, the urban drying effect might negate the positive impacts of urban surface on regional rainfall. Other than the local urban effect, the influence of upstream cities is non-negligible (Zhang et al., 2011). In a recent numerical study on the Zhengzhou “July 20” extreme rainfall event by Luo et al. (2023), they found that the PBL heating, drying, and wind stilling effects by numerous upstream cities/towns could weaken the 24-h rainfall accumulation over the Zhengzhou region through reducing the lateral inflow of water vapor, but the local UHI influence of the Zhengzhou city was largely negligible due to the continuous rainfall. These findings indicated the complexity of the urban effect. How and to what extent the different mechanisms of urbanization contribute to the CI need to be further investigated.

Given the poor predictability of CI events in the coastal cities, the successful simulation of the CI using convection-permitting ensemble simulations with the WRF model in this study demonstrates the necessity to develop a high-resolution ensemble prediction system for providing a valuable reference for CI forecasts in these regions.

The authors are grateful to NCEP for providing the NCEP-GEFS data (https://www.nco.ncep.noaa.gov/pmb/products/gens/) and to ECMWF for providing the ERA5 data (https://cds.climate.copernicus.eu). The authors are thankful to the editor, two anonymous reviewers, and Dr. Qiuping Wang for their help improving the manuscript.