Thermally driven mesoscale circulations and their impact on the urban boundary layer and turbulence in Madrid

During the summer of 2025 (23 June–13 July), an intensive meteorological and turbulence observation campaign was conducted in central Madrid within the framework of the AIRTEC2-CM and MULTIURBAN-II projects. Measurements combined data from a permanent meteorological station and a portable high-frequency eddy-covariance system (IRGASON) from the GuMNet network, both installed on a rooftop at 27 m above ground level. Standard meteorological variables, radiative fluxes, and key turbulence parameters, including friction velocity, turbulent kinetic energy, and sensible heat flux, were recorded.

The observational period was dominated by persistent anticyclonic conditions over the Iberian Peninsula, leading to strong atmospheric stability, weak synoptic forcing, and positive geopotential height anomalies at 500 hPa. These conditions favoured the development of thermally driven mesoscale circulations, particularly nocturnal breezes, which interacted with the urban boundary layer and modulated turbulence and mixing processes. Diurnal cycles of meteorological and turbulent variables are analysed with particular emphasis on the evening transition and the nocturnal stable boundary layer.

Several episodes characterized by very stable conditions and elevated NO₂ concentrations (exceeding 100 μg m⁻³) were observed. The onset of nocturnal breezes was associated with enhanced turbulent mixing and a rapid decrease in pollutant concentrations. High-resolution simulations with the WRF mesoscale model are also presented to evaluate its ability to reproduce the observed thermally driven circulations and their impact on the nocturnal urban boundary layer. Overall, the results highlight the key role of mesoscale thermally driven flows in regulating turbulence, mixing, and scalar transport in urban environments under weak synoptic forcing.