关键词: 城市下垫面, 农田下垫面, 对流性降水, 热岛效应, 地表粗糙度

Abstract:

In order to study the impact of the underlying surface on convective precipitation in Beijing and improve the convective precipitation prediction ability, simulations from RMAPS−Urban model and observations from the Beijing station were used to study a specific case that occurred on August 16, 2021. The simulation results showed that the RMAPS−Urban model was able to well model the temporal and spatial distribution of precipitation, with the heavy precipitation area located in the western part of the city and the main precipitation period occurring around 22:00 on 16^th. In the early period of heavy precipitation, the urban heat island (UHI) was obvious in the urban area of Beijing, with UHI arriving 1.0−2.0℃. When the urban underlying surface was replaced by crop, the UHI was weakened below 1.0℃, the extreme precipitation decreased from 117.1mm to 56.6mm, and the rainstorm area in the western part of the urban area disappeared. Those indicated that the occurrence and development of convective precipitation was closely related to the urban underlying surface. Moreover, the sensible heat flux over the urban underlying surface was significantly higher than that over the surrounding regions, up to 100W·m⁻², which enhanced the convergence over the urban areas and provided the uplift for the formation of the convection. In addition, the perturbations in the potential temperature of the lower atmosphere (below 600m) were also enhanced by the urban underlying surfaces, offering unstable stratification conditions. Finally, larger roughness over the urban underlying surface caused more precipitation in the windward side of the city of the low level surface for the lower water vapor and energy can be transported up with the intensity of the convergence reaching −30×10⁻⁴·s⁻¹ and the rainwater mixing ratio exceeding 3g·kg⁻¹, which was more favorable for the occurrence of convective precipitation. 

Key words: Urban underlying surface, Crop underlying surface, Convective precipitation, Urban heat island, Surface roughness