Methodology to separate urban from regional heat advection by use of the Weather Research and Forecasting mesoscale model

Recent studies have identified the significance of urban heat advection (UHA) as the process whereby heat, originally generated through urban modifications to the Earth's surface, is transported downwind of urban areas. Current techniques to separate UHA from local heat signals do not exclude the additional potential impacts of regional heat advection (RHA). For example, large-scale coastal effects, in addition to latitude and longitude variations, could cause downwind temperature gradients to exist. In this study, the numerical Weather Research and Forecasting (WRF) model coupled with the Building Effect Parameterization (BEP) urban scheme is used to simulate meteorological fields for Birmingham, UK, at a high horizontal resolution (1 km²). The model is run over six case-studies to provide over 1600 h of simulations (called ‘urban-case’), and evaluated using a unique high-resolution dataset from 32 weather stations across Birmingham. The UHA component is decomposed from RHA by conducting a second set of simulations (called ‘rural-case’), where all urban land-use is replaced with vegetation. Simulated directional ‘rural-case’ time-mean temperature fields, that show RHA, are then subtracted from the equivalent ‘urban-case’ time-mean fields. This effectively separates UHA from RHA and shows that a significant portion of heat, previously attributed to UHA in mesoscale modelling, is found to be due to RHA. Using the new methodology, a UHA signal up to 1.9 °C is found largely confined to within, and several kilometres downwind of, the urban areas. These UHA effects highlight the importance of using wind direction segmentation when determining local climate.