Downstream and upstream effects of urban chains on precipitation in Beijing

A summer convective rainfall event in Beijing on June 26, 2018 was investigated though a series of numerical simulations. The system moved from northwest to southeast, passing through Yanqing (YQ), Changping (CP), and the central urban area (CUA) of Beijing. The combined influences of the multiple urban areas and the mechanisms of the mutual impacts of upstream and downstream cities on precipitation were investigated. Results showed that the urban surfaces led to an increase in near-surface temperature, a decrease in lower-level wind speed, and airflow triggered convergence and updrafts. The resulting upward moisture transport and latent heat release in the cloud were promoted during the subsequent precipitation. The thermal and dynamic impacts of CUA and CP urban areas propagated downstream along the low-level southeasterly flow, resulting in increased temperatures, strengthened updrafts, and latent heat release in the cloud along YQ-CP-CUA region. The urban effects propagated downstream under the southeasterly wind, leading to enhanced precipitation over YQ and CP. The YQ also, however, contributed to the increased precipitation in CUA. The "city chain" comprised of YQ, CP, and CUA thus promoted the progression and advancement of the precipitation system. This study highlights the importance of the correct identification of the "upwind" and "downwind" directions when studying urban impacts on precipitation.