Summertime nocturnal Low-Level Jet in Paris and its interactions with urban heat and topography

The Urban Boundary Layer (UBL) is characterized by higher temperatures and weaker airflow than its rural counterpart. This unique microclimate drives the formation of the Urban Heat Island (UHI) effect, which exacerbates the urban heat risks, particularly at night when the human body requires rest. Despite extensive research on the canopy layer UHI, the impact of mesoscale Atmospheric Boundary Layer (ABL) dynamics on the UHI intensity remains poorly understood. 

The mesoscale Low-Level Jet (LLJ) is a common nocturnal ABL phenomenon that influences the transport of moisture, atmospheric pollutants, and heat. However, experimental evidence of LLJ interactions with the UBL is still scarce, leaving a gap in understanding its impact on UHI development. Hence, this work presents a novel wind and turbulence data set that provides insights into the links between these two phenomena.

This study quantifies the interactions between mesoscale LLJs and the UBL in the Paris region (France) using two years of Doppler Wind Lidar (DWL) observations at urban and suburban sites. Results show that LLJs are frequent in summer, with characteristics varying by wind direction. Clear interactions between the LLJ and the UBL are identified. The mechanical vertical mixing generated by the LLJ influences the UHI intensity, while the urban buoyancy can increase the LLJ core height for certain conditions. In addition, the three-dimensional variability of the LLJ and its interaction with the topography and atmospheric stability is assessed through numerical simulations.

These findings highlight the role of ABL dynamics and mesoscale flows in modifying the near-surface processes of the UBL, i.e., UHI intensity and evolution. They also emphasize the impact of turbulent mixing on heat distribution and public health.