Impacts of Anthropogenic Heat and Building Height on Urban Precipitation Over the Seoul Metropolitan area in Regional Climate Modeling

Abstract

An urban heat island (UHI) is a well-known urban climatic feature; however, an opposite effect, i.e., an urban cool island (UCI), was recently observed at locations where high-rises were concentrated. Here, we analyzed the impact of two urbanization factors (anthropogenic heat flux (AH) and building height (ZR)), which could affect the formation of UHIs and UCIs, on urban precipitation over the Seoul Metropolitan area. AH caused an increasing precipitation in urban and downwind areas, especially during daytime. AH directly contributed to the sensible heat flux, resulting in surface warming associated with an UHI. Surface heat was transferred within the planetary boundary layer by turbulence, which increased the low-level instability, resulting in increased precipitation. In the experiments with the ZR, both surface temperature and precipitation increased (decreased) during the nighttime (daytime). The diurnal variation of ground heat flux intensified with an increasing ZR, inducing a smaller variation of sensible heat flux for surface energy balance. Changes in the ground heat flux could also explain occurrence of UCIs in cities with high-rises. Tall buildings reduced daytime surface temperature, thereby facilitating atmospheric stabilization, and reducing precipitation. Also, the surface friction increased with a higher ZR, resulting in enhanced convergence in the western part of urban areas where dominant westerlies first met land, which was favorable for increased precipitation. In summary, a study of variations in AH and ZR demonstrated how UHIs and UCIs significantly altered precipitation, respectively. In particular, an UCI alleviated an increment in thermo-driven daytime precipitation caused by an UHI.