The Influence of Cities on Boundary-Layer Clouds

Abstract:

Urban areas are a hotspot for interactions between the built environment, its inhabitants, and weather. Unlike the impact on temperatures through the well-known urban heat island effect, urban impacts on cloud formation remains largely unknown. This talk explores whether and to what extent cloud formation is different over cities compared to their surroundings using observations and numerical modelling.

A study using satellite measurements shows a systematic enhancement of cloud cover in the afternoon and evening over two large metropolitan areas in Europe (Paris and London). Ground-based remote sensing measurements in London confirm these findings. Numerical weather forecasts tend to underpredict the magnitude of the cloud enhancement over the city. These simulations also show that during low wind-speed conditions, cloud enhancement over the city is strongest and is linked to an urban-induced thermal circulation, caused by spatial differences in the surface energy balance partitioning. The spatial difference in cloud cover is hypothesised to cause differences in the radiative forcing, especially during night-time. A higher radiative forcing over the city could intensify the nocturnal urban heat island.