A Numerical Study on the Impact of Urban Land-Use Change over Eastern China on the Onset of the South China Sea Monsoon

The effect of urban land-use change over eastern China on the onset of the South China Sea summer monsoon (SCSSM) is investigated using Community Atmosphere Model Version 5. 1(CAM5. 1). Results show that the seasonal transition characteristics of large-scale atmospheric circulation during the SCSSM onset are reproduced well in CAM5. 1. Comparison of two runs show that urbanization over eastern China may advance the date of SCSSM onset by one pentad. Possible mechanisms are analyzed, revealing that urbanization induces strengthened warming and sensible heating from land to air in the middle to high latitudes over continental eastern China, and the thermal contrast between ocean and land is reduced, causing the advancement of the formation of continental low pressure and leading to the early appearance of zonal wind over the South China Sea. This provides abundant vapor, conducive to precipitation over the South China Sea. The increase in precipitation causes the increase in condensation latent heat release, making the control (CTRL) experiment generate a condensation latent heating center in advance. The related dynamic diagnosis shows that the vertical change in condensation latent heating has an important influence on the vorticity field, causing abnormal circulation that contributes to a northerly jump and strengthening of the South Asian high. This then prompts the western Pacific subtropical high into an eastward withdrawal, and the generation of low-level cyclonic circulation, leading to an advanced SCSSM onset. An earlier SCSSM onset is detected due to the urban land-use change over eastern China, implying that urban land-use change probably plays an important role in the observed earlier SCSSM onset around 1993.