Performance analysis of the urban climate model MUKLIMO_3 for three extreme heatwave events in Bern

Abstract

Extreme heatwaves represent a health hazard that is expected to increase in the future, and which particularly affects urban populations worldwide due to intensification by urban heat islands. To analyze the impact of such extreme heatwaves on urban areas, urban climate models are a valuable tool. This study examines the performance of the urban climate model MUKLIMO_3 in modelling spatial air temperature patterns in the greater urban area of Bern, Switzerland, a city in complex topography, during three distinct extreme heatwaves in 2018 and 2019 over a total of 23 days. The model is validated using low-cost air temperature data from 79 (2018) and 84 (2019) measurement sites. The intercomparison of the three extreme heatwaves shows that during the first extreme heatwave 2019 at lower elevation regions in the outskirts of the city, modelled air temperature was higher than observation, which was likely due to pronounced mesoscale cold air advection. During calm and dry days, the air temperature distribution was modelled realistically over all three extreme heatwaves investigated. During daytime, modelled air temperatures were lower across all evaluation sites and all extreme heatwaves when compared to the measured values, with highest median air temperature differences of −3.7 K to −4.8 K found in the late afternoon. At night, MUKLIMO_3 generally shows a slowed cooling, so that higher air temperatures were modelled when compared to measured values, with median air temperature biases of +1.5 K to +2.8 K at midnight. By sunrise, the model biases continuously decreased, so that the lowest air temperatures at 7 a.m. were modelled with a bias of +0.2 K to +0.7 K. Peak biases exceed 7 K both during day and night. In sum, our results show that MUKLIMO_3 allows to realistically model the urban air temperature distributions during the peaks of the heatwaves investigated with the highest day and night air temperatures, which may assist in the development of heat mitigation measures to reduce the impacts of heat extremes and improve public health in cities with complex topography.