Investigating Thermal and Dynamic Effects of Urban Areas on Cumulus Cloud Formation Using the City-LES Model

It is well established that the frequency of cumulus cloud formation over the leeward areas of central Tokyo, as well as cumulonimbus development and precipitation, is higher than in surrounding areas. Urban-induced cloud and precipitation generation is generally attributed to two primary mechanisms: thermal and dynamic effects. Here, the dynamic effects involve airflow modification, such as the inhibition of thermals due to obstacles like buildings, which force air parcels to ascend or divert around them. However, the dynamic effect has not been fully investigated using building-resolving models; instead, it has primarily been studied with mesoscale models incorporating urban canopy parameterization schemes. The purpose of this study is to disentangle the thermal and dynamic effects of Tokyo on cloud formation using the building-resolving City-LES model (Kusaka et al., 2024). The study focuses on central Tokyo, with simulations conducted for a clear-sky summer day when a daytime sea breeze reaches the area. The key findings are as follows: (1) Thermals form and are transported by southerly winds, generating roll convection. (2) The thermals reach the lifting condensation level (LCL). The atmospheric boundary layer is well mixed from the surface to the upper planetary boundary layer. Furthermore, sensitivity experiments were conducted to isolate the thermal and dynamic effects of urban areas on cloud formation. The results indicate that: The thermal effect of the city enhances thermal generation, thereby promoting cloud formation. The local-scale dynamic effect suppresses thermal development and inhibits cloud formation, which may differ from the mesoscale dynamic effect. These findings provide new insights into the mechanisms of urban-induced cloud formation and highlight the importance of resolving both thermal and dynamic processes in high-resolution urban meteorological models.