The urban heat island effect is a phenomenon observed worldwide, i.e. evening and nocturnal temperatures in cities are usually several degrees higher than in the surrounding countryside. In contrast, cities are sometimes found to be cooler than their rural surroundings in the morning and early afternoon. Here, a general physical explanation for this so-called daytime urban cool island (UCI) effect is presented and validated for the cloud-free days in the BUBBLE campaign in Basel, Switzerland. Simulations with a widely evaluated conceptual atmospheric boundary-layer model coupled to a land-surface model, reveal that the UCI can form due to differences between the early morning mixed-layer depth over the city (deeper) and over the countryside (shallower). The magnitude of the UCI is estimated for various types of urban morphology, categorized by their respective local climate zones.
Meteorology and Air Quality, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands;Meteorology and Air Quality, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands;Meteorology and Air Quality, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands;Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innsbruck, Austria;Meteorology and Air Quality, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands
Recommended Citation:
Natalie E Theeuwes,Gert-Jan Steeneveld,Reinder J Ronda,et al. Cool city mornings by urban heat[J]. Environmental Research Letters,2015-01-01,10(11)