Impact of Kuala Lumpur as a tropical city on cloud formation: Numerical experiments using the WRF-UCM model

Abstract

Previous satellite-based studies have shown that Greater Kuala Lumpur (GKL) consistently exhibits a higher cloud frequency than surrounding rural areas. To investigate the underlying mechanisms, numerical experiments were conducted using the Weather Research and Forecasting (WRF) model for 227 hot days without daytime precipitation during the wet season (DJF) and 141 hot days without daytime precipitation during the dry season (JJA). The results show that urbanised areas within GKL exhibit higher sensible heat fluxes and deeper boundary layers than surrounding rural regions. Despite lower latent heat fluxes and reduced near-surface relative humidity, which would normally suppress cloud formation by raising the lifting condensation level (LCL), cloud frequency is enhanced over GKL. This enhancement can be explained by two mechanisms. First, the deeper urban boundary layer largely offsets the negative effect of an elevated LCL. Second, urban heat island and modified sea-breeze circulations enhance moisture advection from surrounding rural areas and adjacent oceans, increasing moisture within the urban boundary layer. The spatial distribution of urban-induced cloud frequency exhibits a clear seasonal dependence. During DJF, the enhancement extends across the GKL region, whereas during JJA it is mainly confined to inland urban areas. Areas of enhanced horizontal moisture flux convergence closely correspond to increased cloud frequency. These seasonal differences suggest that urbanisation modifies interactions among the background monsoonal flow, sea-breeze circulation, and urban heat island circulation, thereby altering the location and intensity of moisture convergence and the associated cloud response. These findings provide new insights into urban-induced cloud formation in tropical cities.