Understanding London's summertime cloud cover

Abstract

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.

An analytical approximation of urban heat and dry islands and their impact on convection triggering

“…Hence, changes in α and γ θ , can increase the chance of the occurrence of the ABL-LCL crossing. This supports the conclusion that moisture advection and atmospheric stability may play a key role in regulating cloud formation over urban areas (Theeuwes et al, 2022). Note, however, that the vertical profile of humidity and the moisture flux in the upper boundary layer can be major controls of cloud formation mechanisms (Theeuwes et al, 2022) but they cannot be properly resolved by the slab representation of the ABL employed here.…”

Local drying climate magnified by urbanization in West Africa

“…It has been discovered that the water vapour levels in Beijing's surrounding rural and urban areas differ significantly (Luan et al, 2022). According to some researchers, low relative humidity levels have also been observed across other regions (Hass et al, 2016;Theeuwes et al, 2022). Even though changes in humidity patterns in urban areas have significant impact on building energy use, people's health, and urban climate; however, local-scale research on annual and seasonal trends of UDI magnitudes are lacking.…”

Urban Effects on Cloud Base Height and Cloud Persistence over Sofia, Bulgaria

“…Confirming the sought-after urban effects on cloudiness is hindered by the constantly moving atmosphere, which may lead to cloud detection that is far away from the cloud's origin. Often, cities are located around the borders of natural landscapes with contrasting physical properties (e.g., sea-land, mountain-valley); this leads to natural inhomogeneities in cloudiness that overlap with urban ones [7].…”