Urban heat load assessment in Zagreb, Croatia: a multi-scale analysis using mobile measurements and satellite imagery

There is often a lack of meteorological stations in cities, which makes it difficult to examine their microclimate. Alternatively, it is possible to get around this by fixing measuring instruments on bicycles and traversing through the city to observe spatial patterns in the urban canopy layer. The "traverse" approach enables insight into the spatial variation of air temperature in relation to urban form.

In this research, air temperature measurements were carried out using bicycles and an IoT MF-300 instrument with a synchronized GPS receiver and a temperature probe under different weather conditions throughout 2021 and 2022 in Zagreb, Croatia. The GPS receiver's high sensitivity and position accuracy allowed measurements in places like urban canyons and dense foliage environments. The routes were carefully designed to pass through morphologically diverse parts of Zagreb to emphasize the heat characteristics of the city's microclimate. We specifically analysed the spatial distribution of air temperature and land surface temperature (LST) during a heat wave event on the 24^th of June 2021. On that day mobile measurements were conducted between 10:30 and 11:15 AM local time to match the LST measurements of the Landsat-8 satellite that overpassed Zagreb at approximately 10:45 AM. Additional measurements were carried out in other seasons and at different times of the day.

Results show thermal differences between surface types and urban forms. During a heat wave event, air temperatures reached up to 35 °C, and LST was above 40 °C, which are high temperatures considering the time of measurement. However, mobile measurements showed that city parks can be even 3 °C cooler compared to densely built-up city areas. Due to the high response time of the instrument, the effect of microscale city properties, such as tree lines, was also observed. These results indicate the cooling effect of green areas in Zagreb and the importance of their preservation for heat load reduction and mitigation of the negative effects of the urban heat island.