The impact of COVID-19 confinement measures on the canopy urban heat island intensity of Ghent (Belgium)

In the context of the COVID-19 outbreak, a strict lockdown was ordered by Belgian authorities from 18/03/2020 till 04/05/2020. This led to a limitation of industrial production, human activities and transport use where only essential motorized transport were permitted. This research is an attempt to study the impact of these measures on the canopy layer urban heat island intensity in the city of Ghent. We used the high-accuracy observational MOCCA (MOnitoring the City’s Climate and Atmosphere) network. This network is monitoring the urban climate of the city of Ghent since July 2016. The network consists of six weather stations in the Ghent region and provides a database of hourly observation including 2m temperature at six locations (including dense urban, industrial and suburban). Only clear-sky days with an average wind speed lower than 3 m/s were selected for both the confinement period in 2020 and for similar periods in the reference years 2017, 2018 and 2019. For the years 2017, 2018 and 2019 respectively 3, 3 and 7 reference days were retained to compare with 9 selected days of the 2020 confinement period. Results indicate a lower UHI intensity during the day for 2020 compared to the reference years for the dense, industrial and suburban site. A statistically significant difference was found at 15h, 16h, and 17h for the dense urban site (Provinciehuis). The statistical test did not give significant difference for the suburban site (Wondelgem). Human activities in the urban dense areas release a large amount of heat, which can directly heat the air and during the daytime around 16h when the storage heat flux switch from positive to negative values with weak value of the net radiation fluxes, the external source of energy due to the anthropogenic heat can drive the surrounding hot air to mix with local air and further warm near‐surface air temperature (2 m above ground level). However, during the lockdown period this external contribution to the surface energy balance was absent inducing a cooling gradient of the temperature in the dense urban site (Provinciehuis) up to 0.4°C/h around 18h-19h stronger in 2020 compared to the references years. During nighttime the UHI intensity becomes larger mainly driven by the release of energy stored during the day and the UHI intensities are similar for 2020 and the reference years indicating that the lockdown measures will not have had an impact on the UHI intensity during the night.