Understanding London's summertime cloud cover

Cities are a source of complex land-atmosphere interactions. Spatial differences in the energy balance and enhanced surface roughness interact with the atmosphere to alter clouds and precipitation. Here, we explore how London (UK) alters cloud formation during the spring and summer. The Met Office's high-resolution operational forecasts predict enhanced cloud cover over the city, as found in observations, but underpredicts the intensity. During low wind speeds, cloud enhancement over the city is strongest and linked to an urban-induced thermal circulation. These circulations advect moist air from the city edge inwards, transporting it upwards with a large moisture convergence over the urban area. At around 1,000 m above the surface, the turbulent moisture flux takes over the moisture transport to the cloud layer. A relative humidity budget shows the moisture flux in the upper boundary layer to be the largest contribution to the urban-rural differences in relative humidity.

Automated summary

The study shows that London alters cloud formation during the spring and summer, with spatial differences in energy balance and enhanced surface roughness interacting with the atmosphere to produce enhanced cloud cover over the city. During low wind speeds, the strongest cloud enhancement over the city is linked to an urban-induced thermal circulation.