The `urban meteorology island': a multi-model ensemble analysis

Cities and urban areas are well-known for their impact on meteorological variables and thereby modification of the local climate. Our study aims to generalize the urban-induced changes of specific meteorological variables by introducing a single phenomenon – the urban meteorology island (UMI). A wide ensemble of 24 model simulations with the WRF and RegCM regional climate models on European domain was performed to investigate various urban-induced modifications as individual components of the UMI. The results show that such an approach is meaningful, because nearly in all meteorological variables considered, statistically significant changes occur in cities. Besides previously documented urban-induced changes of temperature, wind speed and boundary-layer height, the study is focused also on changes of cloud cover, precipitation and humidity. An increase of cloud cover in cities, together with a higher amount of sub-grid scale precipitation is detected in summer afternoons. Specific humidity is significantly lower in cities. Further, the study shows that different models and parameterizations can have a strong impact on discussed components of UMI. Multi-layer urban scheme with anthropogenic heat considered increases winter temperatures by more than 2 °C and reduces wind speed more strongly than other urban models. Also the selection of planetary boundary-layer scheme influences the urban wind speed reduction, as well as boundary-layer height with the greatest extent. Finally, urban changes in cloud cover and precipitation are mostly sensitive to the parameterization of convection.

The effects of anthropogenic heat fluxes on regional meteorology and air quality in typical city clusters of China

<p>Anthropogenic heat (AH) can affect regional meteorology and air quality. The spatial distributions of AH fluxes in the typical city clusters of China are estimated. Moreover, in order to study their impacts on regional atmospheric environment, these heat fluxes are incorporated into the modified WRF/Chem with the seasonal and the diurnal variation. The modeling results show that AH fluxes over YRD and PRD have been growing in recent years. The high values of AH can reach 113.5 W/m<sup>2</sup> in YRD and 60 W/m<sup>2</sup> in PRD, respectively. AH fluxes can significantly change the urban meteorology. In YRD, 2-m air temperature (T<sub>2</sub>) increases by 1.6 °С in January and 1.4°С in July, the planetary boundary layer height (PBLH) rises up by 140m in January and 160m in July, and 10-m wind speed (W<sub>10</sub>) is intensified by 0.7 m/s in January and 0.5 m/s in July. More moisture can be transported to higher levels, and increase the accumulative precipitation by 15-30% in July of YRD. In PRD, T<sub>2</sub> rises up by 1.1°С in January and over 0.5°С in July, the PBLH increases by 120m in January and 90m in July, W<sub>10</sub> is enhanced over 0.35 m/s in January and 0.3 m/s in July, and the accumulative precipitation is intensified by 20-40% in July. These changes in meteorology can influence the distribution of air pollutants as well. Due to the increase of PBLH, surface wind speed and upward movement, the concentrations of primary air pollutants decrease near surface and increase at the upper layers over the cities. Chemical effects can play a significant role in ozone changes over the urban areas of YRD, so ozone concentrations increase at surface and decrease at the upper layers. In PRD cities, however, the chemical effects play a significant role in ozone changes in winter, while the vertical movement can be the dominant effect in summer. Thus, ozone concentrations in big cities increase in January, but decrease at the lower layers and increase at the upper layers in July. In all, AH fluxes should not be ignored in urban meteorology and air quality assessments.</p>

Urban Meteorology and Climatology

Urban areas tend to form their own weather and climate that significantly relates to effects of buildings, paved surfaces and materials, the water landscape, and emissions of waste heat and pollution into the atmosphere by industry, transportation systems, and human metabolism. A city gives rise to regional and local urban heat islands, altered river flows, changes in precipitation and humidity, elevated pollution levels, and altered airflow within, over, and across the built landscape. A myriad of microenvironments and microclimates exist across a metropolitan region as a result. These differences are important for a number of reasons, among them the fact that populations are increasingly moving to and living in cities, and the fact that there are several impacts of these changes that affect communities, businesses, urban planning, transportation systems, public health, energy and water use, security, and emergency responses. It is well known that cities have altered the atmosphere, producing changes in air temperature, humidity, precipitation, wind flow, sunshine, and pollution levels. A first formal study is Luke Howard’s The Climate of London. Since the 19th century, several developments led to modern-day practices and applications by urban meteorologists and climatologists. These are outlined in a seminal book Urban Climates and include four periods: (a) prior to 1930, pioneering climatographies of selected cities and weather elements, (b) 1930–1965, growth of micro- and local climatology, (c) 1965–2000, rapid increase in research with links to meteorology and physically based models, and (d) in the 21st century, the maturing of urban meteorology and climatology into a predictive science is occurring. This selective article cites literature on various areas of urban meteorology and climatology. The books, reports, and articles that are listed, with some exceptions, are recent ones. The maturing phase has created an enormous literature from many related disciplines focusing on urban weather and climate. Not every aspect is covered in detail, but books and references will lead to uncovering the vast literature related to this subject. Although it is a selective bibliography, certainly further in-depth searches, it is hoped, can be made by the reader.