The Effects of Heat Advection on UK Weather and Climate Observations in the Vicinity of Small Urbanized Areas

Abstract

Weather and climate networks traditionally follow rigorous siting guidelines, with individual stations located away from frost hollows, trees or urban areas. However, the diverse nature of the UK landscape suggests that the feasibility of siting stations that are truly representative of regional climate and free from distorting local effects is increasingly difficult. Whilst the urban heat island is a well-studied phenomenon and usually accounted for, the effect of warm urban air advected downwind is rarely considered, particularly at rural stations adjacent to urban areas. Until recently, urban heat advection (UHA) was viewed as an urban boundary-layer process through the formation of an urban plume that rises above the surface as it is advected. However, these dynamic UHA effects are shown to also have an impact on surface observations. Results show a significant difference in temperatures anomalies ( p < 0.001) between observations taken downwind of urban and rural areas. For example, urban heat advection from small urbanized areas (∼1 km 2 ) under low cloud cover and wind speeds of 2-3 m s −1 is found to increase mean nocturnal air temperatures by 0.6 • C at a horizontal distance of 0.5 km. Fundamentally, these UHA results highlight the importance of careful interpretation of long-term temperature data taken near small urban areas. Keywords Temperature observations · Urban heat advection · Urban heat island · UrbanizationElectronic supplementary material The online version of this article

Semi‐idealized urban heat advection simulations using the Weather Research and Forecasting mesoscale model

“…Results from previous studies (Heaviside et al ., ; Bassett et al ., ; Bassett et al ., ; ) showed UHAI () to be negative upwind and positive downwind, a product of the methodology. A negative UHAI component at a location upwind of the city does not refer to cooling, but reflects the advected heat which is induced by the wind flow from the opposite direction and is hidden inside ; once Equation is applied to this location, because upwind contains zero temperature elevation but upwind contains a positive elevated temperature, this gives a negative value for .…”

“…Techniques developed through observational (Bassett et al, 2016;Bassett et al, 2017a) and modelling studies (Heaviside et al, 2015) have shown that the temperature at a given location was a combination of (a) heat generated locally (i.e., determined by the underlying land use) and (b) heat advected from upstream urban sources. Further analysis by Bassett et al (2017b) showed that the derived UHAI results contained RHA.…”

“…Nevertheless, recent observational (Bassett et al ., ; Bassett et al ., ) and modelling (Heaviside et al ., ; Bassett et al ., ) studies have overcome some of these challenges by developing and refining a methodology to separate UHA from the background UHI data. Detailed in the methodology, a time‐mean temperature field (where all wind directions are considered) represents the background temperature, and a time‐mean temperature field (for a chosen wind direction) represents the departure from the background temperature.…”

“…This methodology was tested on a high-density urban observation network where a UHA intensity (UHAI) up to 1.2 C was found for the city of Birmingham, UK (Bassett et al, 2016; details of the high-density urban observation network can be found in Warren et al, 2016). The UHA methodology was also shown to hold at a smaller (village) urban scale (Bassett et al, 2017a). However, although the UHA methodology was suitable to decompose temperatures into local and advected components at a local scale, it failed to account for regional heat advection (RHA).…”

The State-of-the-Art of Urban Climate Change Modeling and Observations

“…Zhang et al (2009) reported that upstream urbanization exacerbates UHI effects along the Washington-Baltimore corridor in the US. Similar effect was found in the Suzhou-Wuxi area (China) by Zhang and Chen (2014) and more recently over the UK by Bassett et al (2017) where the heat advection from small urban centers increases the mean nocturnal air temperature by 0.6 °C up to a horizontal distance of 0.5 km. There are also examples of interaction between sea-breeze front penetration and urban areas, either enhancing the sea-breeze front (Kusaka et al 2000(Kusaka et al , 2019Li et al 2015) or decelerating its penetration inland (Yoshikado and Kondo 1989;Kusaka et al 2000Kusaka et al , 2019Hamdi et al 2012a;Rojas et al 2018) and therefore impacting the spatial distribution of the urban heat island.…”