The effects of horizontal advection on the urban heat island in Birmingham and the West Midlands, United Kingdom during a heatwave

Birmingham is the second most populous city in the United Kingdom and observations indicate that it has a pronounced urban heat island (UHI), i.e. higher ambient temperatures in the city centre compared with surrounding suburban and rural areas, particularly at night. The effects of UHIs are often amplified during anticyclonic summer weather conditions, which can cause or exacerbate heatwaves. Enhanced temperatures in highly populated regions can mean that significant numbers of people are at risk from heat-related illness during hot weather. Climate change projections often do not include the effects of the UHI, which can mean that assessments of heat-related health effects using these projections underestimate the actual magnitude of future health impacts. We present numerical simulations of the UHI in Birmingham and the West Midlands Metropolitan region during the heatwave of August 2003 using the high-resolution, regional meteorological Weather Research and Forecasting (WRF) model, with an urban canopy scheme. We evaluated the model using local air-temperature observations, and found good model performance in capturing the temporal and spatial signature of the UHI. We performed a sensitivity test, replacing urban land categories with rural ones, and found that the difference in temperature between the two model runs throughout the heatwave period (2-11 August 2003) was around 3 degrees C on average, and reached a maximum of 7 degrees C. Finally, we present a novel generic methodology to enable the examination of the extent of horizontal advection of warm air downwind of the conurbation area. We found that during the heatwave, temperatures downwind of Birmingham were up to 2.5 degrees C warmer than those upwind. This methodology has the potential for improvements to, or parametrizations for, diagnostic models that do not explicitly include dynamics and where local conditions are driven largely by land-surface type.