Influence of urbanization on pre-monsoon precipitation driven by southeastward-advancing synoptic shear line over South China

Twelve precipitation events over the Pearl River Delta (PRD) in South China, occurring with low-level southeastward-moving shear lines prior to the monsoon onset over the South China Sea, were identified and analyzed using multiple observations. The impacts of the PRD urban agglomeration on precipitation were investigated using convection-permitting ensemble simulations with the WRF model. These simulations incorporated the synoptic background averaged over the twelve events and compared scenarios with and without the cities in the PRD. Results reveal that these events are characterized by prevailing westerly flows in the mid-troposphere and low-level southwesterly flows south of the shear lines, which transport air with high equivalent potential temperature to the PRD. Accumulated precipitation from 0600 local solar time (LST) to 0000 LST (+1d) is predominantly located to the north of the city cluster and near its northeastern border. Simulations indicate that the existence of urban heat islands (UHI) in megacities significantly enhances downstream convection initiation and precipitation along the northeastern urban boundaries. This UHI-induced local convection, combined with urban dynamic effects, hinders further inland transport of warm, moist air by impeding low-level southwesterly flows and depleting moisture resources. Consequently, the shear line-associated precipitation north of the urban cluster is reduced.