Urban boundary layer effects on incoming shortwave and longwave fluxes over and downwind of Paris, France

The urban boundary layer is generally warmer, richer in aerosols, and experiences an increased frequency of convective clouds. Consequently, incoming shortwave and longwave radiation fluxes are expected to be modified over and possibly also downwind of cities. Previous field studies have attempted to quantify this effect, but were limited to urban-rural station-pairs. However, a two-station approach makes it difficult to disentangle regional patterns, urban and plume effects. To more comprehensively isolate urban effects and assess whether there are detectable influences by the thermal and aerosol plume of a large city on incoming radiative fluxes in the downwind area, we established a 10 station network with actively ventilated thermopile pyranometers and pyrgeometers in the Paris metropolitan area. Stations were operated to measure global (GHI) and longwave hemispherical irradiance (LHI) between April 2023 and March 2024. With three sites in the built-up area (on roofs) and seven in a ring 55±7 km from Paris city centre, representative background upwind and downwind locations are sampled. Five sites had sun trackers to quantify diffuse and direct shortwave irradiance. Radiometers were calibrated side-by-side and tested regularly using a roving master calibration system during field operations.

Despite inherent spatio-temporal variability, we find that for all-sky weather conditions, total GHI is reduced over the city, by about 3% in summer and 1 % in winter (average -10 W m-2 at 12:00). During clear-sky midday conditions, decreased direct and increased diffuse irradiance over the city are observed. LHI is elevated over the city, particularly during evenings (average +5 W m-2 at 20:00, differences up to +30 W m-2 during clear sky conditions). There is limited evidence from individual cases at 50 km downwind when LHI is elevated compared to upwind.