Land and atmospheric conditions regulating urban heat and dry islands and their impact on convective cloud formation

Urbanization modifies heat, moisture and energy budgets at the land surface, resulting in significant urban-rural differences. A consequence of land conversion to the built environment is the higher air and surface temperatures in cities compared to their rural surroundings, the so-called urban heat island (UHI) effect.  A few studies have also analysed the impact of cities on atmospheric humidity, the so-called urban dry island (UDI) effect, and observational evidence have revealed enhanced cloud cover and intensified rainfall events over large metropolitan areas. However, the impact of UHI and UDI on convection triggering is still a matter of enquiry. The understanding of how urban-induced change in the surface energy budget affects the diurnal evolution of the boundary layer temperature and humidity profiles is crucial to investigate the formation of convective clouds over cities.

We propose an analytical zero-order model of the Atmospheric Boundary Layer (ABL) to quantify the impact of surface and free atmosphere conditions on UHI, UDI, and convection triggering. The model is shown to reproduce field observations from the BUBBLE experiment in Basel (Switzerland) and is used to investigate the crossing between the ABL height and the lifting condensation level (LCL) as a proxy for the triggering of convective clouds. Our results confirm that urban areas are generally warmer and drier compared to rural counterparts, thus increasing both ABL and LCL heights. There is a range of free atmosphere conditions for which changes in urban imperviousness can impact convection triggering but surface warming alone cannot explain the observed enhancement of cloud cover over cities.