The impact of urban areas on weather

Abstract

SUMMARYThe industrial revolution led to a rapid development of urban areas. This has continued unremittingly over the last 200 years or so. In most urban areas the surface properties are heterogeneous, which has significant implications for energy budgets, water budgets and weather phenomena within the part of the earth's atmosphere that humans live. In this paper I discuss the structure of the planetary boundary layer, confining our analysis to the region above the rooftops (canopy layer) up to around the level where clouds form. It is in this part of the atmosphere that most of the weather impacting our lives occurs, and where the buildings of our cities impact the weather.In this review, observations of the structure of the urban atmospheric boundary layer are discussed. In particular the use of Doppler lidar provides measurements above the canopy layer. The impact of high-rise buildings is considered.Urban morphology impacts energy fluxes and airflow leading to phenomena such as the urban heat island and convective rainfall initiation. I discuss in situ surface-based remote sensing and satellite measurements of these effects. Measurements have been used with simple and complex numerical models to understand the complexity and balance of the interactions involved. Cities have been found to be sometimes up to 10 degC warmer than the surrounding rural areas, and to cause large increases in rainfall amounts. However, there are situations in which urban aerosol may suppress precipitation.Although much progress has been made in understanding these impacts, our knowledge remains incomplete. These limitations are identified. As city living becomes even more the norm for large numbers of people, it is imperative that we ensure that urban effects on the weather are included in development plans for the built environment of the future.

The role of urban boundary layer investigated with high-resolution models and ground-based observations in Rome area: a step towards understanding parameterization potentialities

“…Featuring the urban boundary layer adds a further complexity that is usually resolved by coupling PBL schemes with urban canopy parameterizations inside models. Collier (2006) clearly stated the need for a better understanding of the PBL of the urban areas because of their impact on the weather.…”

Water vapour variability induced by urban/rural surface heterogeneities during convective conditions

“…Heatwaves, heavy rain and flooding produce more harmful effects in urban areas than in rural areas. Moreover, the peculiarity of urban areas may enhance convective precipitation (Thielen et al, 2000;Shepherd et al, 2002;Collier, 2006). The enhancement of convergence and convection above large urban area (Atlanta, New York, Saint Louis, Seoul) have indeed been used by several authors to explain the enhancement of precipitation (Baik et al, 2001;Rozoff et al, 2002;Dixon and Mote, 2003).…”

Modelling the impact of urbanisation on regional climate in the Greater London Area

“…Changes in regional scale flow may result from a thermodynamically driven circulation pattern (Oke, 1995) caused by the UHI modifying the local pressure field and the stability. If the synoptic winds are weak, and the temperature gradients are strong, then there can be a closed circulation pattern associated with the UHI, which is characterised by a strong updraft motion over the city centre, convergent flow near the surface and divergent flow aloft (Collier, 2006). However, quantifying the urban effect on climate (especially at a regional scale) is challenging.…”