Evaluating ERA5-Land air temperature and dew point biases in built-up environments in the UK

ERA5-Land reanalysis provides spatiotemporally continuous meteorological data that can serve as a practical proxy in observation-scarce regions. However, its known biases must be understood before it is used to support decisions. Here, we comprehensively evaluate ERA5-Land surface air temperature and dew point against hourly observations across the UK, and examine how biases vary with built-up areas. We then quantify how these biases propagate into representative downstream urban climate studies and impact applications. Our analysis reveals that ERA5-Land does not fully capture the extreme values of air temperature and dew point, with daily maximum air temperature underestimated by 0.66 °C and daily minimum air temperature overestimated by 0.44 °C. The performance also degrades with increasing impervious surface area ratio, leading to stronger underestimations of air temperature in more built settings. Despite these limitations, ERA5-Land usefully informs the onset of heat events, but it tends to underestimate the total annual hours of extreme heat in both urban and rural areas. The mortality risk and building energy analysis are presented as downstream case studies to illustrate how ERA5-Land temperature and dew point biases may propagate into applied modeling frameworks. Overall, this study clarifies where ERA5-Land can inform analyses and decisions when better local observations are unavailable, and where caution or bias-aware adjustments are needed to avoid underestimating the heat exposure profile, particularly in urban areas.