Influence of urban green infrastructure on thermal stress for humans

In urban climatology green infrastructure is considered as useful measure to mitigate extreme temperatures during hot summer days. The presented research investigates the effect of green infrastructure on the air temperature and on the thermal stress of humans, quantified as Universal Thermal Climate Index (UTCI). The main study site is within the premises of the Botanical Garden of the university in Dresden, Germany. It comprises a mix of different types of tall and low vegetation, and non-vegetated surfaces covered with gravel and buildings. The influence of green infrastructure on ambient meteorological conditions was measured using a mobile and a stationary system. The stationary system measured the energy balance based on the eddy-covariance method on a lawn area. An instrumented backpack measured global radiation, air temperature and humidity and the radiative surface temperature on a predefined route, which was sampled several times over the course of a cloudless day with high global radiation. The route crosses sections with different vegetation types and densities, impervious surfaces and shaded areas. Background data of urban and rural meteorological stations is used to assess the heat mitigation potential of the urban green infrastructure within the Botanical Garden. During the day (7:00 to 18:00 UTC) a mean UTCI of 26.6 °C is measured at the stationary system in the Botanical Garden. At the background stations, the mean UTCI is 3 °C lower for rural surroundings and 3.8 °C higher within the urban environment of Dresden. Inside the Botanical Garden, the low and medium-tall vegetation reduces the UTCI by in average 0.08 to 0.7 °C compared to the stationary measurements. This small reduction is probably due to the radiation emitted by the warm gravel path, which was heated by occasional patches of sunshine. The maximum heat mitigation is observed in the shade of dense and tall vegetation, where the UTCI is reduced by 9.8 °C. Based on the mobile measurements, data of the various green infrastructure arrangements with different micrometeorological characteristics are compared with standardized measurements of a meteorological station located along the measurement route. The comparison with background stations exemplifies the urban heat mitigation potential of parks such as the Botanical Garden in Dresden.