4.6 Article

Understanding London's summertime cloud cover

Journal

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY
Volume 148, Issue 742, Pages 454-465

Publisher

WILEY
DOI: 10.1002/qj.4214

Keywords

atmospheric boundary layer; clouds; moisture transport; urban

Funding

  1. NWO Rubicon grant Clouds above the city [019.161LW.026]
  2. UK-China Research & Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund

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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.
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.

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