Evaluation of microclimatic variations and adaptation effects in a central European city during the most excessive heat wave in summer 2022 by ENVI-met modelling

Extreme heat events are becoming more frequent in urban areas, and the magnitude of the urban heat island effect is increasing. Thus, adaptation of cities to climate change is a major challenge in urban planning. The summer of 2022 was the hottest summer in Germany on record. It was characterized by prolonged drought periods and low water levels in many rivers like the Rhine. For our urban study area in the city of Cologne (Germany), temperatures of more than 40°C were measured on several days, which is associated with excessive heat stress and health risks for the affected population. Significant small-scale temperature differences can be determined within cities for such heat events showing local effects and potentials for local adaptation and participation in climate change mitigation. As heat waves predominantly occur during radiation intensive and low-exchange weather conditions with limited or no advective air flow, microscale temperature differences can be traced back to the following most relevant processes: (1) differences in radiation absorption due to the albedo of urban surfaces, (2) shading effects by vegetation or buildings, (3 ) heat storage capacity and emissivity of materials, and (4) cooling effects through evapotranspiration of green infrastructures, urban water bodies, green facades or roofs. The aim of this study is to identify and explain small-scale microclimatic differences within a 16-hectare research area in the city of Cologne. Air temperature differences in a pedestrian level for two parallel streets with the same orientation and significant differences in terms of street width and greenery are analysed. We used the 3D ENVI-met model to simulate the urban microclimate of our study area with a spatial resolution of 1m² for the three hottest consecutive days which show the maximum 72-hour mean temperature in 2022 (July 18^th-20^th). The simulation results are validated using a densely distributed microclimate measurement network of 36 NETATMO low-cost sensors. The accuracy of these citizen science measurements is checked by three recalibration runs under laboratory conditions and direct comparisons with research-grade meteorological sensors in the field. The sensors show a high long-term stability and consistency with a measurement error less than one tenth of a Kelvin. ENVI-met model outputs and measurements are in very good agreement and show a high correlation. Thus, cause and effect relationships explaining the microclimatic conditions and its local deviations between the two selected streets can be made with high confidence during this heat wave in July 2022. Significant temperature differences of several Kelvin were identified when comparing the narrow, vegetation-free street canyon with the parallel broader street characterized by green front gardens, a double avenue of street trees and several facade greenings. Measured and modelled results show that local climate change adaptation measures can be highly effective in mitigating urban heat stress. In further steps, model scenarios will be developed to simulate and assess the potentials of various heat mitigation strategies in our study area in order to improve thermal outdoor comfort in cities with an increasing frequency of heat waves with over 40°C due to global warming.