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

Abstract

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 km 2 ). 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 landuse 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.

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

“…While it was not possible to directly evaluate simulated 2‐m temperatures for the semi‐idealized cases, a WRF‐BEP configuration has been run for a real urban case and evaluated (Bassett et al ., ). A mean root‐mean‐square error (RMSE) of 1.68 °C and mean bias error (MBE) of −0.33 °C across 32 stations (both urban and rural) were found in this previous study.…”

“…Further analysis by Bassett et al . () showed that the derived UHAI results contained RHA. A methodology to separate UHA from RHA was consequently developed and contained two stages: (Stage I) total urban heat (UHI and UHA) was separated from background regional temperature field (i.e., the rural simulation), and (Stage II) UHA was separated from the UHI.…”

“…The model was centred over Birmingham, UK (52.5°N, 1.9°W), the configuration already evaluated by Bassett et al . (). The outermost domain covered northwest Europe at 27‐km resolution and the innermost domain was set at 1‐km resolution.…”

“…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 .…”

“…Due to the island nature of the UK where the methodology was developed, RHA may be particularly pronounced (i.e., large-scale coastal effects and topography). However, for modelling, this was overcome by conducting a second set of rural simulations that effectively removed any RHA effects (Bassett et al, 2017b).…”