| The Pearl River Delta urban agglomeration is located in southern China,bordering the South China Sea and surrounded by mountains on the other three sides.As one of the earliest regions to implement reform and opening-up policy in China,this area has experienced rapid urbanization since the 1990s,gradually forming a"π"-shaped urban agglomeration centered around the mouth of the Pearl River.The urban expansion has brought not only a dense population and a developed economy,but also significant changes to the natural environment,resulting in an increase in surface heat storage capacity and friction,which alter the thermodynamic properties of the atmospheric boundary layer and affect convection and precipitation;the decreased permeability of the urban underlying surface also makes it more vulnerable to the adverse effects of short-term,heavy rainfall events that have become more frequent since rapid urbanization.Therefore,strengthening research on the evolution and causation mechanisms of heavy rainfall in the Pearl River Delta region,especially the possible impacts of the urban underlying surface,is not only of great scientific value,but also crucial for preventing and reducing urban waterlogging disasters in the region.This work focuses on two different weather backgrounds（weak weather forcing and strong weather forcing）and investigates the occurrence of heavy rainfall events in the Pearl River Delta urban agglomeration.Using the radar mosaics,high-resolution observations from the radar network and automatic weather stations over South China and the reanalysis data,the first part of the study examines 24 local rainfall events in the warm seasons of 2011-2016,revealing their weather background characteristics and statistically significant convective evolution features.Furthermore,a semi-idealized convection-permitting（3 km）ensemble simulation using the WRF model coupled with a multi-layer urban canopy model is conducted to explore the individual and cooperative effects of the urban thermodynamic effects and the complex underlying surfaces such as coastal boundaries and mountainous terrain on these local heavy rainfall events.In the second part of this study,12 rainfall events in 2011 to 2020 that passed over the Pearl River Delta region from the northwest are examined,i.e.,the northwest-moving-in events.These events occurred before the outbreak of the South China Sea monsoon and under the influence of a low-level synoptic-scale shear line.The synthetic synoptic background fields are used to drive the WRF model to simulate this type of rainfall events.Sensitivity experiments are also conducted by changing the underlying surface types to investigate the potential impacts of urbanization.The main conclusions are as follows:（1）The localized heavy rainfall events(>60 mm h-1)occurring during the warm season in the Pearl River Delta urban agglomeration are influenced by weak synoptic forcing,featured with abundant water vapor,low wind speed throughout the troposphere,weak dynamic and thermal gradients,moderate instability,and prevailing southwesterly flows in the boundary layer.Hourly rainfall exceeding 60 mm h-1 is mostly distributed over the urban agglomeration and within about 40 km on its downwind side,which is mainly generated by short-duration,slow-moving,non-linear-shaped convective systems at theγ-toβ-meso-scale.（2）Strong urban heat island effects lead to formation of a"warm bubble"and a decrease in surface pressure over the city agglomeration around noon,with strong turbulent mixing and deepening of the boundary layer.As a result,the air near the surface converges towards the urban agglomeration,forming a vertically coupled structure with lower-level convergence and upper-level divergence within the boundary layer.The rising air reaches the lifting condensation level and forms clouds,with latent heat released during condensation facilitating further upward motion to the free convection level,thereby triggering convection.The presence of mountains to the north of the urban agglomeration leads to the formation of a plain-mountain circulation in the afternoon,which enhances the low-level southerly airflow and,together with the sea breeze under the influence of the herringbone coastline,strengthens water vapor transport.Consequently,the convection over the city agglomeration intensifies and produces more intense rainfall.The shape of coastline also affects the timing of heavy rainfall.The presence of small topographic features surrounding the urban agglomeration can affect the fine-scale temporal and spatial distribution of rainfall,while changes in vegetation types in the southern China region have little impact on this type of heavy rainfall events in the Pearl River Delta.（3）The northwest-moving-in rainfall events are influenced by the synoptic-scale shear lines and the southwesterly flows to the sooth,resulting in widespread rainfall primarily in the mountains north of the urban agglomeration.Meanwhile,localized convective processes associated with the urban heat island effects also generate rainfall.The downdrafts to the north of the localized convective rainfall alter the low-level wind field,causing the rainfall over the northern mountains to shift towards the urban cluster to some extent and merge with the localized rainfall in the northeast of the urban agglomeration.The urban agglomeration and the rainfall on its northeast hinder the transport of warm and humid airflow from the southwest,leading to decreased rainfall in the areas further downwind. |
